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Two anionic asymmetric Fe-Fe hydrogenase model compounds containing a single cyano (CN) and five carboxyl (CO) ligands, [Et4N][Fe2(μ-S2C3H6)(CO)5(CN)1] and [Et4N][Fe2(μ-S2C2H4)(CO)5(CN)1], dissolved in room-temperature acetonitrile, are examined. The molecular asymmetry affects the redox potentials of the central iron atoms, thus changing the photophysics and possible catalytic properties of the compounds. Femtosecond ultraviolet excitation with mid-infrared probe spectroscopy of the model compounds was employed to better understand the ultrafast dynamics of the enzyme-active site. Continuous ultraviolet lamp excitation with Fourier transform infrared (FTIR) spectroscopy was also used to explore stable product formation on the second timescale. For both model compounds, two timescales are observed; a 20-30 ps decay and the formation of a long-lived photoproduct. The picosecond decay is assigned to vibrational cooling and rotational dynamics, while the residual spectra remain for up to 300 ps, suggesting the formation of new photoproducts. Static FTIR spectroscopy yielded a different stable photoproduct than that observed on the ultrafast timescale. Density functional theory calculations simulated photoproducts for CO-loss and CN-loss isomers, and the resulting photoproduct spectra suggest that the picosecond transients arise from a complex mixture of isomerization after CO-loss, while dimerization and formation of a CN-containing Fe-CO-Fe bridged species are also considered.

Preventing the degradation of hybrid perovskites by humid air remains a challenge for their future commercial utilization. 3D/2D perovskites with hierarchical architecture have attracted significant attention due to their promising power conversion efficiency (PCE) and device stability. Here, we report a novel 3D/2D planar bi-layer perovskite obtained by growing a 2D Ruddlesden–Popper layer on top of a 3D rubidium (Rb+)-doped triple-cation perovskite. Rb+ cation incorporation decreases the work function, and 3D/2D films show smaller work function values compared to classic 3D perovskites. X-ray photoemission spectroscopy (XPS) confirms the presence of a 2D perovskite capping layer and observes halide migration. Time-resolved terahertz spectroscopy (TRTS) shows that the average DC carrier mobility for the 3D/2D hierarchical structures and their 3D counterparts is one order of magnitude higher than that for the 2D perovskite. The resulting 3D/2D Rb+-incorporated perovskite solar cells show a peak PCE of >20%, which is slightly higher than their 3D counterparts (19.5%). Benefiting from moisture resistivity, the 3D/2D perovskite photovoltaics show significantly improved long-term stability by retaining 81% of the initial PCE after 60 days of exposure to ambient air (50 ± 10% relative humidity) without encapsulation, highlighting the potential of engineered stable 3D/2D perovskite solar cells for their commercial utilization.

Room temperature Time-Domain Terahertz (TDS) and Time-Resolved Terahertz (TRTS) spectroscopic methods are employed to measure carrier mobility and charge generation efficiency in thin-film semicond...

Mobilities and lifetimes of photogenerated charge carriers are core properties of photovoltaic materials and can both be characterized by contactless terahertz or microwave measurements. Here, the expertise from fifteen laboratories is combined to quantitatively model the current-voltage characteristics of a solar cell from such measurements. To this end, the impact of measurement conditions, alternate interpretations, and experimental inter-laboratory variations are discussed using a (Cs,FA,MA)Pb(I,Br)3 halide perovskite thin-film as a case study. At 1 sun equivalent excitation, neither transport nor recombination is significantly affected by exciton formation or trapping. Terahertz, microwave, and photoluminescence transients for the neat material yield consistent effective lifetimes implying a resistance-free JV-curve with a potential power conversion efficiency of 24.6 %. For grainsizes above ≈20 nm, intra-grain charge transport is characterized by terahertz sum mobilities of ≈32 cm2 V−1 s−1. Drift-diffusion simulations indicate that these intra-grain mobilities can slightly reduce the fill factor of perovskite solar cells to 0.82, in accordance with the best-realized devices in the literature. Beyond perovskites, this work can guide a highly predictive characterization of any emerging semiconductor for photovoltaic or photoelectrochemical energy conversion. A best practice for the interpretation of terahertz and microwave measurements on photovoltaic materials is presented.

Two asymmetrically structured model compounds for the hydrogen-generating [Fe-Fe]-hydrogenase active site were investigated to determine the ultrafast photodynamics, structural intermediates, and photoproducts compared to more common symmetric di-iron species. The bidentate-ligand containing compounds studied were Fe(2)(μ-S(2)C(3)H(6))(CO)(4)(bipy), 1, and Fe(2)(μ-S(2)C(3)H(6))(CO)(4)(phen), 2, in dilute room temperature acetonitrile solution and low-temperature 2Me-THF matrix isolation using static FTIR difference and time-resolved infrared spectroscopic methods (TRIR). Ultraviolet-visible spectra were also compared to time-dependent Density Functional theory (TD-DFT) to ascertain the orbital origins of long wavelength electronic absorption features. The spectroscopic evidence supports the conclusions that only a propyl-bridge flip occurs in low-temperature matrix, while early-time CO-ejection leads to formation of solvated isomeric species on the 25 ps timescale in room temperature solution.

Below bandgap two-photon excitation of bulk silicon and gallium arsenide samples is used to evenly generate low density carriers (

Efficient doping of 2D materials, including carrier type, concentration and mobility, is challenging but essential for enabling their future electronic and photonic applications. We are developing substitutional n- and p- doping of InSe semiconductor by introducing Sn and Zn, respectively, in the Bridgman bulk crystal growth. Electrical transport properties of undoped vs. n- and p- doped InSe crystals are compared by conducting Hall measurements on bulk crystals and FET transport measurements on exfoliated thin layers. Undoped InSe is intrinsically n-type in both bulk and thin-film forms, with [n]~3.5E14 cm-3 and mu values of up to 1,400 cm2 V-1 s-1 for thick layers at 300K. Carrier concentration in Sn-doped thick layers increases approximately two-fold, while the corresponding mobility reduces ~2 times at 300 K. Zn-doped InSe shows p- behavior for bulk InSe with [p]~7.9E13 cm-3 and mu~43 cm2 V-1 s-1 at 300 K, which reverts to ambipolar/n- type behavior for thin layers in FET devices.

One of the most commonly used nonsteroidal anti-inflammatory active pharmaceutical ingredient called Meloxicam has been characterized spectroscopically both by Terahertz (THz) time domain spectroscopy (THz-TDS) and by Fourier Transform Infrared (FTIR) spectroscopy in far-IR regions of electromagnetic spectrum; 0.2 THz–20 THz. While many relatively sharp features are observed in the far-IR range between 2 THz–20 THz as expected for being an organic substance, very distinct and relatively strong absorption bands are also observed at 1.00, 1.66, 2.07 and 2.57 THz in the THz range. These well separated, defined, and fairly strong spectral features can be used for discrimination and quantification of Meloxicam in drug analysis. Frequency dependent refractive index of the drug was determined in a range of 0.2 THz–2.7 THz, where an almost constant index was observed with an average index of 1.75. Powder XRD, and solid-state Density Functional Theory (SS-DFT) calculations were utilized to determine the crystalline form of the Meloxicam sample in its enolic crystalline form. Single molecule DFT calculations were also performed in all four possible structures of Meloxicam. In addition, the capability of THz waves transmission through common packaging materials is demonstrated for possibility of future on-site analysis. The results suggest that drug analysis will be possible to perform not only at every stage of manufacturing without destruction but also directly at the shelf of a market after development of portable THz technologies.

[FeFe]-hydrogenases are efficient enzymes that produce hydrogen gas under mild conditions. Synthetic model compounds containing all CO or mixed CO/PMe(3) ligands were previously studied by us and others with ultrafast ultraviolet (UV) or visible pump-infrared (IR) probe spectroscopy (TRIR) in an effort to better understand the function and interactions of the active site with light. Studies of anionic species containing cyano groups, which more closely match the biological active site, have been elusive. In this work, two model compounds dissolved in room temperature acetonitrile solution were examined: [Fe(2)(μ-S(2)C(3)H(6))(CO)(4)(CN)(2)](2−) (1) and [Fe(2)(μ-S(2)C(2)H(4))(CO)(4)(CN)(2)](2−) (2). These species exhibit long-lived transient signals consistent with loss of one CO ligand with potential isomerization of newly formed ground electronic state photoproducts, as previously observed with all-CO and CO/PMe(3) containing models. We find no evidence for fast (ca. 150 ps) relaxation seen in the all-CO and CO/PMe(3) compounds because of the absence of the metal-to-metal charge transfer band in the cyano-functionalized models. These results indicate that incorporation of cyano ligands may significantly alter the electronic properties and photoproducts produced immediately after photoexcitation, which may influence the catalytic activity of model compounds when attached to photosensitizers.

Though there have been many studies on photosensitizers coupled to model complexes of the [FeFe]-hydrogenases, few have looked at how the models react upon exposure to light. To extract photoreaction information, ultrafast time-resolved UV/visible pump, IR probe spectroscopy was performed on Fe(2)(μ-S(2)C(2)H(4))(CO)(4)(PMe(3))(2) (2b) dissolved in heptane and acetonitrile and the photochemical dynamics were determined. Excitation with 532 and 355 nm light produces bleaches and new absorptions that decay to half their original intensity with time constants of 300 ± 120 ps and 380 ± 210 ps in heptane and acetonitrile, respectively. These features persist to the microsecond timescale. The dynamics of 2b are assigned to formation of an initial set of photoproducts, which were a mixture of excited-state tricarbonyl isomers. These isomers decay into another set of long-lived photoproducts in which approximately half the excited-state tricarbonyl isomers recombine with CO to form another complex mixture of tricarbonyl and tetracarbonyl isomers.

Ultrafast Time-Domain and Time-Resolved Terahertz (TDS/TRTS) spectroscopic methods are utilized to explore photoconductive properties of five conjugated donor–acceptor co-polymers (PCDTPT, CDTBTZ, IDTBT, PBDTTPD and N2200) dispersed in room temperature toluene, a nonpolar dielectric solvent. TRTS provides an effective non-contact handle to interrogate localized relative conductivity of dispersed co-polymers. Conductivity dynamics are contrasted among disparate polymer structures and between their photoconductive peak and thermalized time-delayed signatures. Photoconductivity kinetics, relative levels of real/imaginary conductivity, and carrier properties are summarized with higher energy photoexcitation increasing the real photoconductivity by up to three-fold. Molecular orbital pictures and donor–acceptor twisting mode frequencies ascertained through ab initio calculations (via Density Functional Theory) support descriptions of the relative efficacies of charge transfer from the perspective of the localized THz method.

Solution-deposited nanoscale films of RuO2 (nanoskins) are effective transparent conductors once calcined to 200 °C. Upon heating to higher temperatures, the nanoskins show increased transmission at 550 nm, indicating changes in the optical behavior of the films. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO2 nanoparticles. The mechanism for the spectral evolution is clearly observed with ultrafast optical measurements. Following excitation at 400 nm, nanoskins calcined at higher temperatures show increased transmission above 650 nm, consistent with the photobleaching of a surface plasmon resonance (SPR) band. Calculations based on the optical constants of RuO2 substantiate the presence of SPR absorption. Sheet resistance and transient terahertz photoconductivity measurements establish that the nanoskins electrically de-wire into separated particles. The plasmonic behavior of the nanoskins has implications for their use in a range of optical and electrochemical applications.

Tin(II) monosulfide (SnS) is a semiconductor material with an intermediate band gap, high absorption coefficient in the visible range, and earth abundant, non-toxic constituent elements. For these reasons, SnS has generated much interest for incorporation into optoelectronic devices, but little is known concerning the charge carrier dynamics, especially as measured by optical techniques. Here, as opposed to prior studies of vapor deposited films, phase-pure colloidal SnS was synthesized by solution chemistry in three size regimes, ranging from nanometer- to micron-scale (SnS small nanoparticles, SnS medium 2D nanosheets, and SnS large 2D μm-sheets), and evaluated by time-resolved terahertz spectroscopy (TRTS); an optical, non-contact probe of the photoconductivity. Dropcast films of the SnS colloids were studied by TRTS and compared to both thermally annealed films and dispersed suspensions of the same colloids. TRTS results revealed that the micron-scale SnS crystals and all of the annealed films undergo decay mechanisms during the first 200 ps following photoexcitation at 800 nm assigned to hot carrier cooling and carrier trapping. The charge carrier mobility of both the dropcast and annealed samples depends strongly on the size of the constituent colloids. The mobility of the SnS colloidal films, following the completion of the initial decays, ranged from 0.14 cm2/V·s for the smallest SnS crystals to 20.3 cm2/V·s for the largest. Annealing the colloidal films resulted in a ~ 20 % improvement in mobility for the large SnS 2D μm-sheets and a ~ 5-fold increase for the small nanoparticles and medium nanosheets.

Low density charge mobility from below bandgap, two-photon photoexcitation of bulk silicon (Si) is interrogated using time-resolved terahertz spectroscopy (TRTS). Total charge mobility is measured as a function of excitation frequency and fluence (charge carrier density), cut angle, and innate doping levels. Frequency dependent complex photoconductivities are extracted using the Drude model to obtain average and DC-limit mobility and carrier scattering times. These dynamic parameters are compared to values from contact-based Hall, above bandgap photoexcitation, and comparable gallium arsenide (GaAs) measurements. Mobilities are shown to increase beyond Hall values at low carrier densities and are modestly higher with increasing dopant density. The former occurs in part from below bandgap photoexcitation exhibiting abnormally small (faster) scattering times, while both reflect unique conduction characteristics at lowest (> 2x1012 cm−3) carrier densities achieved through photodoping.

We perform contactless bulk mobility measurements for ZnSe, ZnTe, GaP, CdS, and GaSe in an optical pump THz probe experiment. As opposed to above-gap excitation or contact methods, two-photon absorption excites the entire sample thickness producing measurable signals with 10(13) carriers/cm(3) and higher density. For ZnTe and GaSe samples, the measured mobility using two-photon excitation is higher than that measured with one-photon excitation.

Colloidal-based solution syntheses offer a scalable and cost-efficient means of producing 2D nanomaterials in high yield. While much progress has been made towards the controlled and tailorable synthesis of semiconductor nanocrystals in solution, it remains a substantial challenge to fully characterize the products’ inherent electronic transport properties. This is often due to their irregular morphology or small dimensions, which demand the formation of colloidal assemblies or films as a prerequisite to performing electrical measurements. Here, we report the synthesis of nearly monodisperse 2D colloidal nanocrystals of semiconductor SnS and a thorough investigation of the intrinsic electronic transport properties of single crystals. We utilize a combination of multi-point contact probe measurements and ultrafast terahertz spectroscopy to determine the carrier concentration, carrier mobility, conductivity/resistivity, and majority carrier type of individual colloidal semiconductor nanocrystals. Employing this metrological approach, we compare the electronic properties extracted for distinct morphologies of 2D SnS and relate them to literature values. Our results indicate that the electronic transport of colloidal semiconductors may be tuned through prudent selection of the synthetic conditions. We find that these properties compare favorably to SnS grown using vapor deposition techniques, illustrating that colloidal solution synthesis is a promising route to scalable production of nanoscale 2D materials.

Ultrafast laser systems are becoming more widespread throughout the research and industrial communities yet eye protection for these high power, bright pulsed sources still require scrupulous characterization and testing before use. Femtosecond lasers, with pulses naturally possessing broad-bandwidth and high average power with variable repetition rate, can exhibit spectral side-bands and subtly changing center wavelengths, which may unknowingly affect eyewear safety protection. Pulse spectral characterization and power diagnostics are presented for a 80 MHz, Ti+3:Sapphire, ≈ 800 nm, ≈40 femtosecond oscillator system. Power and spectral transmission for 22 test samples are measured to determine whether they fall within manufacturer specifications.

Ultrafast lasers have become increasingly important as research tools in laboratories and commercial enterprises suggesting laser safety, personal protection, and awareness become ever more important. Laser safety eyewear are typically rated by their optical densities (OD) over various spectral ranges, but these measurements are usually made using low power, large beam size, and continuous beam conditions. These measurement scenarios are vastly different than the high power, small beam size, and pulsed laser beam conditions where ultrafast lasers have extremely high peak powers and broad spectra due to the short pulse durations. Many solid-state lasers are also tunable over a broad wavelength range, further complicating the selection of adequate laser safety eyewear. Eighteen laser eyewear filter samples were tested under real-world conditions using a Ti:Sapphire regenerative amplifier with output pulses centered at 800 nm running from 2 Hz to 1 kHz repetition rate. The typical maximum peak laser irradiance employed was ca. 3 TW/cm2 (800 nm wavelength, 450 μJ/pulse with 80 fs full width at half maximum pulse duration) or less when damage occurred, depending on the sample. While many samples maintained their integrity under these test conditions, many plastic samples showed signs of failure which reduced their OD, in some cases transmitting 4–5 orders of magnitude higher than expected. In general, glass filters performed significantly better than plastic filters, exhibiting less physical damage to the substrate and less absorber degradation.Ultrafast lasers have become increasingly important as research tools in laboratories and commercial enterprises suggesting laser safety, personal protection, and awareness become ever more important. Laser safety eyewear are typically rated by their optical densities (OD) over various spectral ranges, but these measurements are usually made using low power, large beam size, and continuous beam conditions. These measurement scenarios are vastly different than the high power, small beam size, and pulsed laser beam conditions where ultrafast lasers have extremely high peak powers and broad spectra due to the short pulse durations. Many solid-state lasers are also tunable over a broad wavelength range, further complicating the selection of adequate laser safety eyewear. Eighteen laser eyewear filter samples were tested under real-world conditions using a Ti:Sapphire regenerative amplifier with output pulses centered at 800 nm running from 2 Hz to 1 kHz repetition rate. The typical maximum peak laser irradian...

The extent of the photoinitiated linkage isomerization of dicarbonyl(3-cyanomethylpyridine-κN)(η(5)-methylcyclopentadienyl)manganese (4) to dicarbonyl(3-cyano-κN-methylpyridine)(η(5)-methylcyclopentadienyl)manganese (5) was examined by time-resolved infrared spectroscopy on picosecond to microsecond time scales in room temperature isooctane to determine the extent the isomerization occurs as a geminate cage rearrangement. We previously reported that a substantial part of the conversion between 4 and 5 must be a bimolecular reaction between a solvent coordinated dicarbonyl(η(5)-methylcyclopentadienyl)manganese (3) and uncoordinated 3-cyanomethylpyridine. For the purpose of designing a molecular device, it would be desirable for the photoisomerization to occur in a geminate cage reaction, because the faster the isomerization, the less opportunity for side reactions to occur. In this study, assignments of transients are identified by comparison with transients observed for model reactions. Within 100 μs after photolysis of 4 in isooctane, no 5 is observed. Instead, the solvent coordinated 3 is observed within 25 ps after irradiation. The formation of 5 is observed only in the presence of 9 mM 3-cyanomethylpyridine but not until 10-50 μs after irradiation of 4. Within the limits of detection, these results indicate the conversion of 4 to 5 occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated 3 and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl(η(5)-methylcyclopentadienyl)manganese fragment.

Solution-deposited nanoscale films of RuO2 (“nanoskins”) are effective transparent conductors once calcined to 200 °C. Upon heating the nanoskins to higher temperature the nanoskins show increased transmission at 550 nm. Electronic microscopy and X-ray diffraction show that the changes in the optical spectrum are accompanied by the formation of rutile RuO2 nanoparticles. The mechanism for the spectral evolution is clearly observed with ultrafast optical measurements. Following excitation at 400 nm, nanoskins calcined at higher temperatures show increased transmission above 650 nm, consistent with the photobleaching of a surface-plasmon resonance (SPR) band. Calculations based on the optical constants of RuO2 substantiate the presence of SPR absorption. Sheet resistance and transient terahertz photoconductivity measurements establish that the nanoskins electrically de-wire into separated particles. The plasmonic behavior of the nanoskins has implications their use in a range of optical and electrochemical applications.

Thin-film ruthenium dioxide (RuO2) is a promising alternative material as a conductive electrode in electronic applications because its rutile crystalline form is metallic and highly conductive. Herein, a solution-deposition multi-layer technique is employed to fabricate ca. 70 ± 20 nm thick films (nanoskins) and terahertz spectroscopy is used to determine their photoconductive properties. Upon calcining at temperatures ranging from 373 K to 773 K, nanoskins undergo a transformation from insulating (localized charge transport) behavior to metallic behavior. Terahertz time-domain spectroscopy (THz-TDS) indicates that nanoskins attain maximum static conductivity when calcined at 673 K (σ = 1030 ± 330 S·cm−1). Picosecond time-resolved Terahertz spectroscopy (TRTS) using 400 nm and 800 nm excitation reveals a transition to metallic behavior when calcined at 523 K. For calcine temperatures less than 523 K, the conductivity increases following photoexcitation (ΔE < 0) while higher calcine temperatures yield films composed of crystalline, rutile RuO2 and the conductivity decreases (ΔE > 0) following photoexcitation.

Model compounds that structurally mimic the hydrogen-producing active site of [FeFe]-hydrogenases have been studied to explore potential ground-state electronic structure effects on reaction mechanisms compared to hexacarbonyl derivatives. The time-dependent behavior of Fe2(μ-S2C3H6)(CO)4(PMe)2 (A) in room temperature n-heptane and acetonitrile solutions was examined using various ultrafast UV and visible excitation pulses with broadband IR-probe spectroscopy of the carbonyl (CO) stretching region. Ground- and excited-state electronic and CO-stretching mode vibrational properties of the possible isomers of A were also examined using density functional theory (DFT) computations. In n-heptane, 355 and 532 nm excitation resulted in short-lived (135 ± 74 ps) bands assigned to excited-state, CO-loss photoproducts. These bands decay away, forming new long-lived absorptions that are likely a mixture of isomers of both three-CO and four-CO ground-state isomers. These new bands grow in with a time scale of 214 ± 119 ps and persist for more than 100 ns. In acetonitrile, similar results are seen with a 532 nm pump, but the 355 nm data lack evidence of the longer-lived bands. In either solvent, the 266 nm pump data seem to also lack longer-lived bands, but the intensities are significantly lower in this data, making firm conclusions more difficult. We suggest that these wavelength-dependent excitation dynamics significantly alter potential mechanisms and efficiencies for light-driven catalysis.

Static and transient infrared spectroscopy were used to investigate cis -Ru(bpy) 2 (N 3 ) 2 (bpy = 2,2′-bipyridine), cis -Ru(bpy) 2 (NCS) 2 , and cis -Ru(bpy) 2 (CN) 2 in solution. The NC stretching IR band for cis -Ru(bpy) 2 (NCS) 2 appears at higher frequency (∼2106 cm −1 in DMSO) than for the free NCS − anion while the IR bands for the azide and cyanide complexes are closer to those of the respective free anions. The vibrational energy relaxation (VER) lifetime for the azide complex is found to be much shorter (∼5 ps) than for either the NCS or CN species (both ∼70 ps in DMSO) and the lifetimes resemble those for each corresponding free anion in solution. However, for cis -Ru(bpy) 2 (N 3 ) 2 , it is determined that the transition frequency depends more on the solvent than the VER lifetime implying that intramolecular vibrational relaxation is predominant over solvent energy-extracting interactions. These results are compared to the behavior of other related metal complexes in solution.

Non-contact, optical time-resolved terahertz spectroscopy (TRTS) has been used to study the transient photoconductivity of nanometer-scale metallic films deposited on fused quartz substrates. Samples of 8 nm thick gold or titanium show an instrument-limited (ca. 0.5 ps) decrease in conductivity following photoexcitation due to electron-phonon coupling and subsequent increased lattice temperatures which increases charge carrier scattering. In contrast, for samples of 8 nm gold with a 4 nm adhesion layer of titanium or chromium, a ca. 70 ps rise time for the lattice temperature increase is observed. These results establish the increased transient terahertz transmission sign change of metallic compared to semiconductor materials. The results also suggest nanoscale gold films that utilize an adhesion material do not consist of distinct layers.

Time-resolved terahertz spectroscopy (TRTS) was used to explore charge generation, transfer, and the role of hot carriers in organic solar cell materials. Two model molecular photovoltaic systems were investigated: with zinc phthalocyanine (ZnPc) or alpha-sexathiophene (α-6T) as the electron donors and buckminsterfullerene (C60) as the electron acceptor. TRTS provides charge carrier conductivity dynamics comprised of changes in both population and mobility. By using time-resolved optical spectroscopy in conjunction with TRTS, these two contributions can be disentangled. The sub-picosecond photo-induced conductivity decay dynamics of C60 were revealed to be caused by auto-ionization: the intrinsic process by which charge is generated in molecular solids. In donor-acceptor blends, the long-lived photo-induced conductivity is used for weight fraction optimization of the constituents. In nanoscale multilayer films, the photo-induced conductivity identifies optimal layer thicknesses. In films of ZnPc/C60, electron transfer from ZnPc yields hot charges that localize and become less mobile as they thermalize. Excitation of high-lying Franck Condon states in C60 followed by hole-transfer to ZnPc similarly produces hot charge carriers that self-localize; charge transfer clearly precedes carrier cooling. This picture is contrasted to charge transfer in α-6T/C60, where hole transfer takes place from a thermalized state and produces equilibrium carriers that do not show characteristic signs of cooling and self-localization. These results illustrate the value of terahertz spectroscopic methods for probing charge transfer reactions.

Model compounds have been found to structurally mimic the catalytic hydrogen-producing active site of Fe-Fe hydrogenases and are being explored as functional models. The time-dependent behavior of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) and Fe(2)(μ-S(2)C(2)H(4))(CO)(6) is reviewed and new ultrafast UV- and visible-excitation/IR-probe measurements of the carbonyl stretching region are presented. Ground-state and excited-state electronic and vibrational properties of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) were studied with density functional theory (DFT) calculations. For Fe(2)(μ-S(2)C(3)H(6))(CO)(6) excited with 266 nm, long-lived signals (τ = 3.7 ± 0.26 μs) are assigned to loss of a CO ligand. For 355 and 532 nm excitation, short-lived (τ = 150 ± 17 ps) bands are observed in addition to CO-loss product. Short-lived transient absorption intensities are smaller for 355 nm and much larger for 532 nm excitation and are assigned to a short-lived photoproduct resulting from excited electronic state structural reorganization of the Fe-Fe bond. Because these molecules are tethered by bridging disulfur ligands, this extended di-iron bond relaxes during the excited state decay. Interestingly, and perhaps fortuitously, the time-dependent DFT-optimized exited-state geometry of Fe(2)(μ-S(2)C(3)H(6))(CO)(6) with a semibridging CO is reminiscent of the geometry of the Fe(2)S(2) subcluster of the active site observed in Fe-Fe hydrogenase X-ray crystal structures. We suggest these wavelength-dependent excitation dynamics could significantly alter potential mechanisms for light-driven catalysis.

The chelation following photodissociation of CO for cyclopentadienyl manganese tricarbonyl derivatives with a bifunctional side chain has been investigated. Previous studies show that steady-state irradiation of 1 (Mn{η5-C5H4CH2COR}(CO)3, R = 2-pyridyl) leads to CO dissociation and formation of O-chelate 2 with smaller amounts of N-chelate 3. Subsequently, 2 rearranges thermally to 3. A new preparation for 1 is reported, while analogues 4 (R = phenyl) and 5 (R = 4-pyridyl) are prepared for the first time. Steady-state UV–vis, FTIR, and NMR studies of 4 and 5 in heptane demonstrate that O-chelates 6 and 7, respectively, are formed with the side-chain oxygen but decay on the minute time scale. The linkage isomerization of O-chelate 2 to 3 is faster than the decay observed for the O-chelate 6 (R = 2-pyridyl versus Ph), even in the presence of 0.1 M pyridine for the latter. Following irradiation of 4 during time-resolved infrared studies in heptane, ultrafast O-chelation is observed but not ultrafast solvent ...

New results are reported suggesting that with appropriate levels of Fe doping Mg can rapidly and reversibly absorb up to 7 mass fraction (%) hydrogen at moderate temperatures and pressures useful for hydrogen storage applications. Hydrogenation kinetics and thermodynamics of Mg–4Fe at.% (+/− 1 at.%) thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods: in situ infrared imaging, volumetric pressure-composition isotherm (PCI) measurements, and ex situ X-ray diffraction and transmission electron microscopy. The hydride growth rate was determined by utilizing wedge-shaped films and infrared imaging; assuming formation of a continuous hydride layer, the growth rate was found to range from ≈3.8 nm/s at lower temperature to ≈36.7 nm/s at higher temperature. The apparent activation energy of the thermally activated hydrogenation kinetics was measured to be 56 kJ/mol; this value suggests that at low temperatures hydrogen diffusion along grain boundaries of MgH 2 is the mechanism controlling the hydride layer growth. Reproducible PCI measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy and entropy were estimated as 66.9 kJ/mol and 0.102 kJ/(mol∗K), respectively, which are both slightly less than values for pure magnesium (as either films or bulk). The extremely rapid and cyclable kinetics of Mg-4 at.% Fe films suggest that properly grown Mg–Fe powders of 1–2 μm size can be fully charged with hydrogen within 1 min at temperature near 150 °C (423 K), with possible practical hydrogen storage applications.

Using Raman, terahertz (THz), and mid-infrared (IR) spectroscopies, the vibrational spectra of two chromophore models of hydrogen-producing [FeFe]-hydrogenase, Fe 2 (μ-S 2 C 3 H 6 )(CO) 6 and Fe 2 (μ-S 2 C 2 H 4 )(CO) 6 , have been assigned. The combination of absorption and scattering techniques, along with DFT calculations, allows for assignments to be made without traditional isotopic substitution methods.

A vibrational analysis of polycrystalline l -serine is provided using experimental terahertz, FTIR, Raman and inelastic neutron scattering (INS) spectra, calculated INS spectra – and Born–Oppenheimer molecular dynamics (BOMD) simulations from which the power spectra for the electronegative elements are compared to the THz spectra. Corrections are made to density functional theory (DFT) calculations for van der Waals interactions. Assignments and potential energy distributions are included for all 3 N = 336 normal modes of an eight molecule supercell, including those for 48 non-bonded whole molecule translating and rotating vibrations, of which three are acoustic modes, usually not considered. Calculated and observed frequencies differ by an average 3 cm −1 ( s = 4). The INS spectrum of these modes below 100 cm −1 , calculated from energy second derivatives, show a remarkable similarity to the experimental 10 K spectra. The calculated low frequency modes are insensitive to small changes in cell parameters and geometry. THz intensities are represented by power spectra and not calculated explicitly. Nevertheless, power spectra of 13 ps BOMD trajectories at classical temperatures of 20 K, 400 K, and 500 K are markedly similar to the experimental terahertz spectra at 77 K and 298 K. Calculations on a serine crystal supercell 2 × 2 × 2 molecules deep appear to include, in a crude but fortuitously accurate way, enough of the principle out of phase dispersion to yield a match with experimental frequencies and intensities.

The direct observation of a sequence of phase transitions during hydrogenation of Y thin films has been realized through the use of in situ isothermal infrared emissivity measurements. The formation of different phases, α-Y(H), YH2 and YH3, has been identified based on the observation of changes in the slope of the normalized IR emissivity vs. time curve during hydrogen loading. The presence of α-Y(H), YH2 and YH3 was confirmed by ex situ X-ray diffraction, transmission electron microscopy, and prompt gamma activation analysis. Transmission electron microscopy further demonstrated epitaxial orientation relationships between the Al2O3 substrate, Ti buffer layer, the as-deposited Y film, as well as its hydrides. These results clearly demonstrate the power of IR emissivity imaging to monitor, in real time, the formation of hydride phases of both metallic and insulating character near the surface of a thin-film sample.

In the paper we report on a new methodology, which allows measuring in-situ heterogeneous growth rates of hydride phase in films during metal-hydride phase transformation. This optical method is based on infrared imaging of a wedge-shaped thin film during hydrogen loading. In the paper the method is demonstrated for Mg 98.4 Ti 1.6 wedge-shaped thin film and main conclusions are supported by results of transmission electron microscopy. The methodology combined with the structural characterizations verified fast formation of MgH 2 layer on top followed by drastically slower growth of the MgH 2 phase. The initial averaged growth rate of the MgH 2 phase was estimated as ∼1.3 nm/s, and as ∼0.03 nm/s subsequently.

Multilayer and 1:1 blended films of zinc phthalocyanine (ZnPc) and buckminsterfullerene (C60) were investigated as model active layers for solar cells by time-resolved terahertz spectroscopy (TRTS)...

Combinatorial thin films provide an opportunity for studying a variety of properties over a wide range of compositions and microstructures on a single substrate, allowing substantial acceleration of both the fabrication and study of materials and their properties. This paper details the use of infrared (IR) emissivity imaging for studying the in situ hydrogenation of MgxNi1−x films with hydrogen gas; the method is shown to be a powerful combinatorial screening tool for metal hydride storage materials. The 100 nm thick MgxNi1−x composition gradient films (0.4

In pulsed-laser deposition (PLD), there are many processing parameters that influence film properties such as substrate–target distance, background reactive gas pressure, laser energy, substrate temperature and composition in multi-component systems. By introducing a 12.7-mm diameter circular aperture in front of a 76.2-mm silicon wafer and rotating the substrate while changing conditions during the PLD process, these parameters may be studied in a combinatorial fashion, discretely as a function of processing conditions. We demonstrate the use of the aperture technique to systematically study the effects of oxygen partial pressure on the film stoichiometry and growth rate of VO x , using Rutherford backscattering spectrometry (RBS). In another example, we discuss the effect of growth temperature on TiO 2 films characterized by X-ray diffraction and Fourier transform far-infrared (Terahertz) absorption spectroscopy. We demonstrate that we have considerable combinatorial control of other processing variables besides composition in our combi-PLD system. These may be used to systematically study film growth and properties.

Metal-Organic Frameworks (MOFs) are three-dimensional coordination polymers that are well known for large pore surface area and their ability to adsorb molecules from both the gaseous and solution phases. In general, MOFs are electrically insulating, but promising opportunities for tuning the electronic structure exist because MOFs possess synthetic versatility; the metal and organic ligand subunits can be exchanged or dopant molecules can be introduced into the pore space. Two such MOFs with demonstrated electrical conductivity are Cu 3 (1,3,5-benzenetricarboxylate) 2 , a.k.a HKUST-1, and Cu[Ni(pyrazine-2,3-dithiolate) 2 ]. Herein, these two MOFs have been infiltrated with the redox active species 7,7,8,8-tetracyanoquinodimethane (TCNQ) and iodine under solution phase conditions and shown to produce redox products within the MOF pore space. Vibrational bands assignable to TCNQ anion and triiodide anion have been observed in the Mid-IR and Terahertz ranges using FTIR Spectroscopy. The MOF samples have been further investigated by Time-Resolved Terehertz Spectroscopy (TRTS). Using this technique, the charge mobility, separation, and recombination dynamics have been followed on the picosecond time scale following photoexcitation with visible radiation. The preliminary results show that the MOF samples have small inherent photoconductivity with charge separation lifetimes on the order of a few picoseconds. In the case of HKUST-1, the MOF can also be supported by a TiO 2 film and initial results show that charge injection into the TiO 2 layer occurs with a comparable efficiency to the dye sensitizer N3, [cis-Bis(isothiocyanato)-bis(2,2’-bipyridyl-4,4’-dicarboxylato ruthenium(II)], and therefore this MOF has potential as a new light absorbing and charge conducting material in photovoltaic devices.

Ultrafast photoinduced carrier dynamics in prototypical low band gap polymer:fullerene photovoltaic blend films PTB7:PC70BM and P3HT:PC70BM is investigated using ultrafast terahertz (THz) spectroscopy. The subpicosecond and few-picosecond decays of THz-probed photoconductivities for both compounds are observed, attributed to the rapid formation of polaron pairs by exciton-exciton annihilation and subsequent polaron pair annihilation, respectively. The transient THz photoconductivity spectra of PTB7:PC70BM are well described by the Drude-Smith (DS) model, directly yielding the important charge transport parameters such as charge carrier density, momentum scattering time, and effective localization. By comparison with P3HT:PC70BM, we find that in PTB7:PC70BM the mobile charge carrier photoconductivity is significantly enhanced by a factor of 1.8 and prevails for longer times after charge formation, due to both improved mobile charge carrier yield and lower charge localization. In PTB7:PC70BM, a strong dependency of electron momentum scattering time on electron density was found, well parametrized by the empirical Caughey-Thomas model. The difference in ultrafast photoconductivities of both P3HT:PC70BM and PTB7:PC70BM is found to correlate very well with the performance of photovoltaic devices based on those materials.

Photocurrent in an organic solar cell is generated by a charge transfer reaction between electron donors and acceptors. Charge transfer is expected to proceed from thermalized states, but this picture has been challenged by recent studies that have investigated the role of hot excitons. Here we show a direct link between excess excitation energy and photocarrier mobility. Charge transfer from excited donor molecules generates hot photocarriers with excess energy coming from the offset between the lowest unoccupied molecular orbital of the donor and that of the acceptor. Hot photocarriers manifest themselves through a short-lived spike in terahertz photoconductivity that decays on a picosecond timescale as carriers thermalize. Different dynamics are observed when exciting the acceptor at its absorption edge to a thermalized state. Charge transfer in this case generates thermalized carriers described by terahertz photoconductivity dynamics consisting of an instrument-limited rise to a long-lived signal.

Terahertz (THz) absorption spectra of the similarly structured amino acids l -serine and l -cysteine in the solid phase at 77 and 298 K are reported and compared to isolated molecule and solid-state infrared vibrational spectral calculations using empirical force field and density functional theory. These comparisons suggest that many higher frequency internal modes can readily be assigned but lowest frequency intermolecular and phonon modes can adequately be understood with more advanced solid-state theory using currently available potential functions.

We compare terahertz time domain (THz-TDS) and optical heterodyne-detected Raman-induced Kerr effect (OHD-RIKES) spectroscopic techniques as probes of molecular and vibrational dynamics in the frequency range of 0.1 to 6 THz (3 to 200 cm - 1 ). The spectra of molecular liquids benzene, toluene, and bromobenzene, with dipole moments in the range of 0 to 1.7 D, are analyzed within a multimode spectral decomposition to examine the relative sensitivities of the techniques to distinguishable dynamical coordinates. We emphasize the use of minimum modal bases primarily for the purpose of comparing the spectra. Significant differences are found between the THz and OHD-RIKES spectra of benzene and bromobenzene that cannot be traced to rescaled oscillator strengths within a set of fixed-frequency modes.

Femtosecond mid-infrared excitation of room temperature W(CO) 6 / n -hexane solutions resonant with the T 1u CO-stretching mode transfers vibrational population to at least v =6. The population distribution in levels v =0 to v =5 was measured by transient infrared absorption spectroscopy. Resonance frequencies, anharmonic shifts and spectral broadening of observed ladder transitions were also characterized. Populating v =6 deposits ∼12 000 cm −1 in a single mode which is close to the dissociation energy of one W–C bond. These results constitute a significant step toward initiation and ultimate control of condensed phase, mid-infrared induced ground state reactions relevant to practical synthetic transformations.

The photoinduced dynamics of two new [η5-C5H4C(O)R]Mn(CO)3 complexes 2 (R = CH(SCH3)2) and 3 (R = C(SCH3)3) have been investigated in n-heptane solution on the ps to μs time scale by UV-pump IR-probe transient absorption spectroscopy. Irradiation of 2 at 266 or 289 nm induces CO loss to yield two initial products in approximately equal abundance, assigned by their CO-stretching bands to be a heptane solvate of the unsaturated Mn fragment and a ring-formed product in which the pendant sulfide moiety is coordinated to the metal center. In direct analogy with the previously observed behavior of [η5-C5H4C(O)CH2(SCH3)]Mn(CO)3 (1), the solvate reacts through a secondary pathway to afford the S-bound product within 200 ns. Irradiation of 3 under identical conditions yields the chelated product exclusively, with no evidence of a competing solvation pathway.

Terahertz (THz) absorption and neutron vibrational spectra for solid-phase short-chain peptide sequences at 77 and 298 K are reported. We demonstrate that a high degree of spectral and structural information exists for these systems in the 1–15 THz (33–500 cm � 1 ) spectral range. The density and uniqueness of distinct spectral features for each system suggests that sequence-dependent structural information can potentially be extracted from these spectra using quantum mechanical solid-state modeling and theories. Published by Elsevier Science B.V.

Broadband femtosecond and longer timescale mid-infrared and far-infrared (THz) probing techniques being explored at NIST using multi-element, mid-infrared focal plane arrays and solid-state THz generators/detectors are presented. Ultrafast infrared pulse generation, detection and pump-probe spectroscopy schemes are first reviewed in detail. These methods are used to examine transitory vibrational, chemical and electron transfer phenomena amenable to time-resolved infrared spectroscopy. Direct monitoring of electron transfer rates (> 5×1012 s) from dye sensitizers ([Ru(dceb)(bpy)2]+2 and Ru(dcb)2(NCS)2 derivatives) anchored to semiconductor solar-cell substrates (nanoparticle TiO2, ZrO2 and SnO2 films) is accomplished by measuring increased mid-IR absorption originating from C=O groups and injected electrons in the substrate. Coherent control of CO-stretching vibrational overtone population distributions in liquid-phase metal-hexacarbonyl systems using deliberately chirped mid-IR excitation pulses has also...

Ultrafast laser systems are becoming more widespread throughout the research and industrial communities, yet eye protection for these high power, bright pulsed sources still requires scrupulous characterization and testing before use. Femtosecond lasers, with pulses naturally possessing a broad-bandwidth and high average power with a variable repetition rate, can exhibit spectral side-bands and subtly changing center wavelengths, which may unknowingly affect eyewear safety protection. Pulse spectral characterization and power diagnostics are presented for a 80 MHz, Ti+3:Sapphire, ≈800 nm, and ≈40 fs oscillator system. Power and spectral transmission for 22 test samples are measured to determine whether they fall within manufacturer specifications.