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Action spectroscopies use a readout created by the action of light on the molecules or material rather than optical absorption. Ultrafast 2D photocurrent and 2D fluorescence spectroscopies are two such action spectroscopies. Despite their utility, multidimensional action spectroscopies suffer from a background created by incoherent population mixing. These backgrounds appear when the action of one molecule impacts that of another, creating a signal that mimics a fourth-order population response but is really just the convolution of two linear responses. The background created by incoherent mixing is often much larger than the desired foreground signals. In this paper, we describe the physical mechanisms that give rise to the incoherent signals, drawing Feynman paths for each. There are three variations of incoherent signals, differing by their pulse ordering. They all have the same phase dependence as the desired fourth-order population signals and so cannot be removed by standard phase cycling, but they do differ in their polarization responses and dephasing times. We propose, and implement, a spectrometer design that eliminates the background signals for isotropically oriented samples, leaving only the desired fourth-order 2D action spectra. Our spectrometer utilizes a TWINS interferometer and a pulse shaper interferometer, each driven with a different white-light source so that the pulse pairs within each interferometer are phase stable, but not between the two. The lack of phase stability between the two interferometers eliminates two of the three incoherent responses. The third incoherent response is eliminated with the polarization scheme ⟨0, π/2, π/4, π/4⟩. Our spectrometer also enables both 2D photocurrent and 2D white-light spectra to be collected simultaneously, thereby enabling a direct comparison between action and optical detection under identical conditions and at the exact same position on the sample. Using this spectrometer and photovoltaic devices made from thin films of semiconducting carbon nanotubes, we demonstrate 2D photocurrent spectra free of incoherent background. [ABSTRACT FROM AUTHOR]

Understanding water dynamics at charged interfaces is of great importance in various fields, such as catalysis, biomedical processes, and solar cell materials. In this study, we implemented molecular dynamics simulations of a system of pure water interfaced with Au electrodes, on one side of which 4-mercaptobenzonitrile (4-MBN) molecules are adsorbed. We calculated time correlation functions of various dynamic quantities, such as the hydrogen bond status of the N atom of the adsorbed 4-MBN molecules, the rotational motion of the water OH bond, hydrogen bonds between 4-MBN and water, and hydrogen bonds between water molecules in the interface region. Using the Luzar–Chandler model, we analyzed the hydrogen bond dynamics between a 4-MBN and a water molecule. The dynamic quantities we calculated can be divided into two categories: those related to the collective behavior of interfacial water molecules and the H-bond interaction between a water molecule and the CN group of 4-MBN. We found that these two categories of dynamic quantities exhibit opposite trends in response to applied potentials on the Au electrode. We anticipate that the present work will help improve our understanding of the interfacial dynamics of water in various electrolyte systems. [ABSTRACT FROM AUTHOR]

We demonstrate the first phase stable measurement of a third-order 2Q spectrum using a pulse shaper in the pump–probe geometry. This measurement was achieved by permuting the time-ordering of the pump pulses, thus rearranging the signal pathways that are emitted in the probe direction. The third-order 2Q spectrum is self-heterodyned by the probe pulse. Using this method, one can interconvert between a 1Q experiment and a 2Q experiment by simply reprogramming a pulse shaper or delay stage. We also measure a fifth-order absorptive 2Q spectrum in the pump–probe geometry, which contains similar information as a third-order experiment but does not suffer from dispersive line shapes. To do so, we introduce methods to minimize saturation-induced artifacts of the pulse shaper, improving fifth-order signals. These techniques add new capabilities for 2D spectrometers that use pulse shapers in the pump–probe beam geometry. [ABSTRACT FROM AUTHOR]

Spectroscopic techniques that are capable of measuring surfaces and interfaces must overcome two technical challenges: one, the low coverage of molecules at the surface, and two, discerning between signals from the bulk and surface. We present surface enhanced attenuated reflection 2D infrared (SEAR 2D IR) spectroscopy, a method that combines localized surface plasmons with a reflection pump-probe geometry to achieve monolayer sensitivity. The method is demonstrated at 6 µm with the amide I band of a model peptide, a cysteine terminated α-helical peptide tethered to a gold surface. Using SEAR 2D IR spectroscopy, the signal from this sample is enhanced 20 000-times over a monolayer on a dielectric surface. Like attenuated total reflection IR spectroscopy, SEAR 2D IR spectroscopy can be applied to strongly absorbing solvents. We demonstrated this capability by solvating a peptide monolayer with H2O, which cannot normally be used when measuring the amide I band. SEAR 2D IR spectroscopy will be advantageous for studying chemical reactions at electrochemical surfaces, interfacial charge transfer in photovoltaics, and structural changes of transmembrane proteins in lipid membranes. [ABSTRACT FROM AUTHOR]

Determining the number of molecules in a given assembly, such as the number of proteins in a toxic aggregate, is often critical to understanding chemistry and function. Herein, we report a variation of a limitless method for counting photoactivatable fluorescent dyes in which single dye molecules are photoswitched to a fluorescent state, counted, and then irreversibly photobleached. We use this method to count the number of CAGE 552 covalently bound to the surface of 500 nm polystyrene beads. Activation of CAGE 552 was achieved with a 405 nm laser pulse. Once activated, the dye was excited with 532 nm light, and the fluorescence emission was collected with a CCD camera. The results from the fluorescence experiments were then compared to bulk fluorescence measurements to assess the error in counting. There are other ways of counting molecules, such as photobleaching and statistical analysis of reversible switchable chromophores. The method reported here provides a lower bound to the number of chromophores, with no upper limit to the number of molecules that can be quantified. [ABSTRACT FROM AUTHOR]

We report that a model dye, Re(CO)3(bypy)CO2H, aggregates into clusters on TiO2 nanoparticles regardless of our preparation conditions. Using two-dimensional infrared (2D IR) spectroscopy, we have identified characteristic frequencies of monomers, dimers, and trimers. A comparison of 2D IR spectra in solution versus those deposited on TiO2 shows that the propensity to dimerize in solution leads to higher dimer formation on TiO2, but that dimers are formed even if there are only monomers in solution. Aggregates cannot be washed off with standard protocols and are present even at submonolayer coverages. We observe cross peaks between aggregates of different sizes, primarily dimers and trimers, indicating that clusters consist of microdomains in close proximity. 2D IR spectroscopy is used to draw these conclusions from measurements of vibrational couplings, but if molecules are close enough to be vibrationally coupled, then they are also likely to be electronically coupled, which could alter charge transfer. [ABSTRACT FROM AUTHOR]

This paper presents the implementation of 3D IR spectroscopy by adding a second pump beam to a two-beam 2D IR spectrometer. An independent mid-IR pulse shaper is used for each pump beam, which can be programmed to collect its corresponding dimension in either the frequency or time-domains. Due to the phase matching geometry employed here, absorptive 3D IR spectra are automatically obtained, since all four of the rephasing and non-rephasing signals necessary to generate absorptive spectra are collected simultaneously. Phase cycling is used to isolate the fifth-order from the third-order signals. The method is demonstrated on tungsten hexacarbonyl (W(CO)6) and dicarbonylacetylacetonato rhodium (I), for which the eigenstates are extracted up to the third excited state. Pulse shaping affords a high degree of control over 3D IR experiments by making possible mixed time- and frequency-domain experiments, fast data acquisition and straightforward implementation. [ABSTRACT FROM AUTHOR]

Vibrational and electronic transition dipole strengths are often good probes of molecular structures, especially in excitonically coupled systems of chromophores. One cannot determine transition dipole strengths using linear spectroscopy unless the concentration is known, which in many cases it is not. In this paper, we report a simple method for measuring transition dipole moments from linear absorption and 2D IR spectra that does not require knowledge of concentrations. Our method is tested on several model compounds and applied to the amide I′ band of a polypeptide in its random coil and α-helical conformation as modulated by the solution temperature. It is often difficult to confidently assign polypeptide and protein secondary structures to random coil or α-helix by linear spectroscopy alone, because they absorb in the same frequency range. We find that the transition dipole strength of the random coil state is 0.12 ± 0.013 D2, which is similar to a single peptide unit, indicating that the vibrational mode of random coil is localized on a single peptide unit. In an α-helix, the lower bound of transition dipole strength is 0.26 ± 0.03 D2. When taking into account the angle of the amide I′ transition dipole vector with respect to the helix axis, our measurements indicate that the amide I′ vibrational mode is delocalized across a minimum of 3.5 residues in an α-helix. Thus, one can confidently assign secondary structure based on exciton delocalization through its effect on the transition dipole strength. Our method will be especially useful for kinetically evolving systems, systems with overlapping molecular conformations, and other situations in which concentrations are difficult to determine. [ABSTRACT FROM AUTHOR]

A heterodyned fifth-order two-dimensional (2D) IR spectrum of a model coupled oscillator system, Ir(CO)2(C5H7O2), is reported. The spectrum is generated by a pulse sequence that probes the eigenstate energies up to the second overtone and combination bands, providing a more rigorous potential-energy surface of the coupled carbonyl local modes than can be obtained with third-order spectroscopy. Furthermore, the pulse sequence is designed to generate and then rephase a two-quantum coherence so that the spectrum is line narrowed and the resolution improved for inhomogeneously broadened systems. Features arising from coherence transfer processes are identified, which are more pronounced than in third-order 2D IR spectroscopy because the transition dipoles of the second overtone and combination states are not rigorously orthogonal, relaxing the polarization constraints on the signal intensity for these features. The spectrum provides a stringent test of cascading signals caused by third-order emitted fields and no cascading is observed. In the Appendix, formulas for calculating the signal intensities for resonant fifth-order spectroscopies with arbitrarily polarized pulses and transition dipoles are reported. These relationships are useful for interpreting and designing polarization conditions to enhance specific spectral features. [ABSTRACT FROM AUTHOR]

A heterodyned fifth-order infrared pulse sequence has been used to measure a two-dimensional infrared (2D-IR) spectrum of azide in the ionic glass 3KNO3:2Ca(NO3)2. By rephasing a two-quantum coherence, a process not possible with third-order spectroscopy, the 2D-IR spectra are line narrowed, allowing the frequencies, anharmonicities, and their correlations to be measured for the first four (ν=0-3) antisymmetric stretch vibrational levels. In this glass, the vibrational levels are extremely inhomogeneously broadened. Furthermore, the glass shifts the energy of the ν=3 state more than the others, causing an inhomogeneous distribution in the anharmonic constants that are partially correlated to the inhomogeneous distribution of the fundamental frequency. These effects are discussed in light of the strong interactions that exist between the charged solute and solvent. Since this is the first example of a heterodyned fifth-order infrared pulse sequence, possible cascaded contributions to the signal are investigated. No evidence of cascaded signals is found. Compared to third-order spectroscopies, fifth-order pulse sequences provide advanced control over vibrational coherence and population times that promise to extend the capabilities of ultrafast infrared spectroscopy. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Heterodyned two-dimensional infrared (2D IR) spectroscopy has been used to study the amide I vibrational dynamics of a 27-residue peptide in lipid vesicles that encompasses the transmembrane domain of the T-cell receptor CD3ζ. Using 1–13C==18O isotope labeling, the amide I mode of the 49-Leucine residue was spectroscopically isolated and the homogeneous and inhomogeneous linewidths of this mode were measured by fitting the 2D IR spectrum collected with a photon echo pulse sequence. The pure dephasing and inhomogeneous linewidths are 2 and 32 cm-1, respectively. The population relaxation time of the amide I band was measured with a transient grating, and it contributes 9 cm-1 to the linewidth. Comparison of the 49-Leucine amide I mode and the amide I band of the entire CD3ζ peptide reveals that the vibrational dynamics are not uniform along the length of the peptide. Possible origins for the large amount of inhomogeneity present at the 49-Leucine site are discussed. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have experimentally demonstrated a pulse sequence for the acquisition of heterodyned two-dimensional infrared (2D IR) spectra that correlates the overtone and combination bands to the fundamental frequencies. The spectra are generated by Fourier transforming the time domain signal that is allowed to evolve during one- and two-quantum coherence times. In this manner, the overtone and combination bands appear along the two-quantum axis, resulting in a direct determination of the diagonal and off-diagonal anharmonicities. To demonstrate this pulse sequence, we have collected two-quantum 2D IR spectra of a ruthenium dicarbonyl complex, extracted the diagonal and off-diagonal anharmonicities, and simulated the spectra using an exciton model. Several polarization conditions are presented that suppress the diagonal or cross peaks and we have used them to improve the accuracy of the measurement. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the femtosecond photoelectron spectroscopy of iodine clusters. Determination of the initial solvent configuration from measuring the number of solvent atoms around iodine; Measurement of the time-resolved number of solvent atoms; Vibrational relaxation in caged photofragments.

Studies the photodissociation dynamics of gas phase I[sup -][sub 3] after an excitation using femtosecond photoelectron spectroscopy. Coherent oscillations that correspond to vibrational excitation; Photoelectron spectrum induced by resonance impulsive stimulated Raman scattering.

Studies a semiclassical method for stimulating femtosecond pump-probe photoelectron spectroscopy and its implementation to examine excited state photodissociation dynamics of iodine anion. Initial value representation method for calculating the time dependent photodetachment spectrum as a function of kinetic energy; Radiation-chromophore interaction.

Analyzes the potential energy curve of an excited state of iodine anion constructed based on femtosecond photoelectron spectroscopy. Potential for the ground state of the anion; Comparison of the present technique with previous quantum mechanical simulation methods; Sensitivity of the spectra to three parts of the potential.

Examines the solvation effects on vibrational frequency of I[sub 2]-chromophore in size-selected I[sub 2]-(AR)[sub n] and I[sub 2]-(CO[sub 2])[sub n] clusters. Stimulation of Raman scattering process to determine frequency; Detection of wave-packet oscillations with femtosecond photoelectron spectroscopy; Effect of solvation on frequency.

Describes the characterization of the I[sup -][sub 2] anion ground state with the use of conventional and femtosecond photoelectron spectroscopy. Conventional photoelectron spectrum; Femtosecond photoelectron spectroscopy; Origin and significance of the coherences.

Presents a correction to an article titled "Heterodyned Fifth-Order 2D-IR Spectroscopy of the Azide Ion in an Ionic Glass" that was previously published in an issue of the "Journal of Chemical Physics."

The stimulated infrared photon echo of N-methylacetamide-D [NMAD; CH[sub 3](CO)ND(CH[sub 3])] was measured and used to determine the vibrational frequency correlation function. The correlation function was modeled as a single exponential plus a constant, and it was found that most of the NMAD vibrational frequency distribution is motionally narrowed with a pure dephasing time of 1.12 ps. The two-dimensional infrared (2D IR) spectrum of NMAD was also obtained by heterodyning the echo field with a weak local oscillator pulse. The real and imaginary portions of the 2D IR spectrum exhibit multiple peaks due to υ=0-1 and 1-2 coherences that are excited, which are not resolved in the absolute magnitude of the 2D IR spectrum. Using the correlation function determined from the stimulated photon echo, the 2D IR spectrum was accurately simulated. Resolution enhancement of the 2D IR spectrum was performed by manipulating the photon echo field with window functions. The enhanced experimental and simulated 2D IR spectra are dramatically narrowed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a correction to a printing error in the article 'Femtosecond Stimulated Emission Pumping: Characterization of the I[sub 2] [sup -] Ground State' which was published in the 112th volume of the periodical 'Journal of Chemical Physics.'

Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I[sub 2][sup -](X˜ [sup 2]Σ[sub u][sup +]) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I[sub 2][sup -](A˜[sup ′] [sup 2]Π[sub g,1/2]) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the X˜ [sup 2]Σ[sub u][sup +] state has been fit to a modified Morse potential with the β-parameter expanded in a Taylor series, and the bond length, well depth, and υ=0-1 fundamental frequency set equal to our previously determined Morse potential [J. Chem. Phys. 107, 7613 (1997)]. At high vibrational energies, the modified potential deviates significantly from the previously determined potential. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of I[sub 2][sup -](CO[sub 2])[sub n](n=4-16) clusters excited at 780 nm have been studied with femtosecond photoelectron spectroscopy (FPES). The range of cluster sizes spans the uncaged and fully-caged product limits for this reaction. We observe time scales for a variety of processes in these clusters, including dissociation of the I[sub 2][sup -] chromophore, solvation of the I[sup -] fragment, the onset of recombination on the ground state of I[sub 2][sup -], vibrational relaxation, and solvent evaporation. In addition, substantial trapping in a "solvent-separated" state is seen for clusters with n>=9; this state persists for at least 200 ps, the longest time delay probed here. Simulations of the spectra were performed in order to determine the time dependence of the electronic state populations, the I[sub 2][sup -] vibrational distribution, and the number of CO[sub 2] molecules in the cluster. Results are compared to previous experimental and theoretical studies of I[sub 2][sup -](CO[sub 2])[sub n] photodissociation, and to a recent FPES study of I[sub 2][sup -](Ar)[sub n] clusters. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ytterbium lasers make possible shot-to-shot data collection of two-dimensional infrared (2D IR) spectra at 100 kHz and higher repetition rates. At those rates, the power absorbed by the sample is appreciable and creates a steady state temperature rise and an accumulated thermal grating artifact in the spectra that can obscure weak or low concentration IR chromophores. We report the magnitude of the temperature rise, the pulse ordering by which it is created, and ways to mitigate it. Using a calibrant molecule, we measured a steady-state temperature up to 32.5 and 45 °C for laser light at 4 µm in H2O and 6 µm in D2O, respectively, for a typical optical density used in 2D IR experiments. The temperature reached a steady state in ∼60 s. The temperature rise scales with the integrated optical density of the sample across the laser spectrum. By cooling the sample cell, we returned the steady state temperature to room temperature within the laser focus. For samples that undergo rotation, the accumulated thermal grating artifact is removed using a perpendicular ⟨XXYY⟩ polarization because the permuted time-orderings of the thermal grating artifact has the orientational response ⟨XYXY⟩, which decays to zero during the delay between consecutive laser pulses. The procedure described in this study can be used to characterize and minimize the thermal effects in experiments where repetition rate and/or pulse energy cause an appreciable temperature rise., (© 2024 Author(s). Published under an exclusive license by AIP Publishing.)

A heterodyned fifth-order two-dimensional (2D) IR spectrum of a model coupled oscillator system, Ir(CO)(2)(C(5)H(7)O(2)), is reported. The spectrum is generated by a pulse sequence that probes the eigenstate energies up to the second overtone and combination bands, providing a more rigorous potential-energy surface of the coupled carbonyl local modes than can be obtained with third-order spectroscopy. Furthermore, the pulse sequence is designed to generate and then rephase a two-quantum coherence so that the spectrum is line narrowed and the resolution improved for inhomogeneously broadened systems. Features arising from coherence transfer processes are identified, which are more pronounced than in third-order 2D IR spectroscopy because the transition dipoles of the second overtone and combination states are not rigorously orthogonal, relaxing the polarization constraints on the signal intensity for these features. The spectrum provides a stringent test of cascading signals caused by third-order emitted fields and no cascading is observed. In the Appendix, formulas for calculating the signal intensities for resonant fifth-order spectroscopies with arbitrarily polarized pulses and transition dipoles are reported. These relationships are useful for interpreting and designing polarization conditions to enhance specific spectral features.