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Driving fundamental excitations via strong light fields is one of the most important issues in solid state physics, which opens up new avenues to control material properties. Two-dimensional materials are fruitful platforms for future semiconductor applications, including opto-electronic and phononic devices, yet the phonon dynamics and nonlinear phonon–phonon coupling remain under-explored. Here, we demonstrate coherent phonon excitation in thin films of the layered two-dimensional semiconductor WSe2 induced by intense and broadband ultrafast terahertz (THz) pulses. We performed THz Kerr effect spectroscopy and observed coherent phonon oscillations assigned to the E 2 g optical phonon mode. The phonon amplitude displays a quadratic THz field strength dependence, indicating a sum-frequency THz excitation process. Furthermore, pump–probe polarization and crystal orientation relationships, supported by symmetry analysis of the nonlinear susceptibility and Raman tensors, provide helpful insight into nonlinear phonon–phonon interactions and potential coherent control schemes for the manipulation of phonon polarization and material properties in WSe2. [ABSTRACT FROM AUTHOR]

M dwarfs are common host stars to exoplanets but often lack atmospheric abundance measurements. Late-M dwarfs are also good analogs to the youngest substellar companions, which share similar T eff ∼ 2300–2800 K. We present atmospheric analyses for the M7.5 companion HIP 55507 B and its K6V primary star with Keck/KPIC high-resolution (R ∼ 35,000) K -band spectroscopy. First, by including KPIC relative radial velocities between the primary and secondary in the orbit fit, we improve the dynamical mass precision by 60% and find M B = 88.0 − 3.2 + 3.4 M Jup , putting HIP 55507 B above the stellar–substellar boundary. We also find that HIP 55507 B orbits its K6V primary star with a = 38 − 3 + 4 au and e = 0.40 ± 0.04. From atmospheric retrievals of HIP 55507 B, we measure [C/H] = 0.24 ± 0.13, [O/H] = 0.15 ± 0.13, and C/O = 0.67 ± 0.04. Moreover, we strongly detect 13CO (7.8 σ significance) and tentatively detect H 2 18 O (3.7 σ significance) in the companion's atmosphere and measure 12 CO / 13 CO = 98 − 22 + 28 and H 2 16 O / H 2 18 O = 240 − 80 + 145 after accounting for systematic errors. From a simplified retrieval analysis of HIP 55507 A, we measure 12 CO / 13 CO = 79 − 16 + 21 and C 16 O / C 18 O = 288 − 70 + 125 for the primary star. These results demonstrate that HIP 55507 A and B have consistent 12C/13C and 16O/18O to the <1 σ level, as expected for a chemically homogeneous binary system. Given the similar flux ratios and separations between HIP 55507 AB and systems with young substellar companions, our results open the door to systematically measuring 13CO and H 2 18 O abundances in the atmospheres of substellar or even planetary-mass companions with similar spectral types. [ABSTRACT FROM AUTHOR]

Two-dimensional Raman and hybrid terahertz-Raman spectroscopic techniques provide invaluable insight into molecular structures and dynamics of condensed-phase systems. However, corroborating experimental results with theory is difficult due to the high computational cost of incorporating quantum-mechanical effects in the simulations. Here, we present the equilibrium–nonequilibrium ring-polymer molecular dynamics (RPMD), a practical computational method that can account for nuclear quantum effects on the two-time response function of nonlinear optical spectroscopy. Unlike a recently developed approach based on the double Kubo transformed (DKT) correlation function, our method is exact in the classical limit, where it reduces to the established equilibrium-nonequilibrium classical molecular dynamics method. Using benchmark model calculations, we demonstrate the advantages of the equilibrium–nonequilibrium RPMD over classical and DKT-based approaches. Importantly, its derivation, which is based on the nonequilibrium RPMD, obviates the need for identifying an appropriate Kubo transformed correlation function and paves the way for applying real-time path-integral techniques to multidimensional spectroscopy. [ABSTRACT FROM AUTHOR]

Much of what is known about chemistry in star-forming regions comes from observations of nearby (d < 500 pc) low-mass protostars. For chemistry in high-mass star-forming regions, several more distant (d ∼ 2–8 kpc), exceptionally bright molecular sources have also been the subjects of repeated observations but with concomitantly poorer linear spatial resolution. Facilities such as ALMA and JWST, however, now provide the means for observing distant sources at dramatically higher spatial resolution and sensitivity. We used the modest resolving power of the Atacama Compact Array, a dedicated subset of ALMA antennas, to carry out a pilot survey of 11 giant molecular clouds selected from the Bolocam Galactic Plane Survey [Battisti & Heyer, Astrophys. J., 2014, 780, 173] within the so-called molecular ring between about 4 and 8 kpc from the galactic center. Within our observed sample, molecular emission regions—most of which correspond to at least one (candidate) young stellar object—exhibit a range of chemical complexity and diversity. Furthermore, nine target giant molecular clouds contain well-fit methanol emission, giving us a first look at the spatial chemical variability within the objects at relatively high (compared to past observations) resolutions of ∼5′′. This work lays the foundation for future high angular resolution studies of gas-phase chemistry with the full ALMA. [ABSTRACT FROM AUTHOR]

Two-dimensional spectroscopic techniques combining terahertz (THz), infrared (IR), and visible pulses offer a wealth of information about coupling among vibrational modes in molecular liquids, thus providing a promising probe of their local structure. However, the capabilities of these spectroscopies are still largely unexplored due to experimental limitations and inherently weak nonlinear signals. Here, through a combination of equilibrium-nonequilibrium molecular dynamics (MD) and a tailored spectrum decomposition scheme, we identify a relationship between the tetrahedral order of liquid water and its two-dimensional IR-IR-Raman (IIR) spectrum. The structure-spectrum relationship can explain the temperature dependence of the spectral features corresponding to the anharmonic coupling between low-frequency intermolecular and high-frequency intramolecular vibrational modes of water. In light of these results, we propose new experiments and discuss the implications for the study of tetrahedrality of liquid water. Direct spectroscopic probes of the impact of structure on dynamical processes in liquids remain scarce. Here, the authors use molecular dynamics simulations to show that the correlation between vibrational coupling and the local tetrahedral structure of liquid water can be studied via hybrid terahertz- and infrared-Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Direct imaging studies have mainly used low-resolution spectroscopy (R ∼ 20â€"100) to study the atmospheres of giant exoplanets and brown dwarf companions, but the presence of clouds has often led to degeneracies in the retrieved atmospheric abundances (e.g., carbon-to-oxygen ratio, metallicity). This precludes clear insights into the formation mechanisms of these companions. The Keck Planet Imager and Characterizer (KPIC) uses adaptive optics and single-mode fibers to transport light into NIRSPEC (R ∼ 35,000 in the K band), and aims to address these challenges with high-resolution spectroscopy. Using an atmospheric retrieval framework based on petitRADTRANS, we analyze the KPIC high-resolution spectrum (2.29â€"2.49 ÎĽ m) and the archival low-resolution spectrum (1â€"2.2 ÎĽ m) of the benchmark brown dwarf HD 4747 B (m = 67.2 ± 1.8 M Jup, a = 10.0 ± 0.2 au, T eff ≈ 1400 K). We find that our measured C/O and metallicity for the companion from the KPIC high-resolution spectrum agree with those of its host star within 1 σ â€"2 σ. The retrieved parameters from the K -band high-resolution spectrum are also independent of our choice of cloud model. In contrast, the retrieved parameters from the low-resolution spectrum are highly sensitive to our chosen cloud model. Finally, we detect CO, H2O, and CH4 (volume-mixing ratio of log(CH4) = âˆ'4.82 ± 0.23) in this L/T transition companion with the KPIC data. The relative molecular abundances allow us to constrain the degree of chemical disequilibrium in the atmosphere of HD 4747 B, and infer a vertical diffusion coefficient that is at the upper limit predicted from mixing length theory. [ABSTRACT FROM AUTHOR]

The Orion Kleinmann-Low nebula (Orion KL) is notoriously complex and exhibits a range of physical and chemical components. We conducted high-angular-resolution (subarcsecond) observations of 13CH3OH ν = 0 (∼0.″3 and ∼0.″7) and CH3CN ν 8 = 1 (∼0.″2 and ∼0.″9) line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate Orion KL’s structure on small spatial scales (≤350 au). Gas kinematics, excitation temperatures, and column densities were derived from the molecular emission via a pixel-by-pixel spectral line fitting of the image cubes, enabling us to examine the small-scale variation of these parameters. Subregions of the Hot Core have a higher excitation temperature in a 0.″2 beam than in a 0.″9 beam, indicative of possible internal sources of heating. Furthermore, the velocity field includes a bipolar ∼7â€"8 km sâˆ'1 feature with a southeastâ€"northwest orientation against the surrounding ∼4â€"5 km sâˆ'1 velocity field, which may be due to an outflow. We also find evidence of a possible source of internal heating toward the Northwest Clump, since the excitation temperature there is higher in a smaller beam versus a larger beam. Finally, the region southwest of the Hot Core (Hot Core-SW) presents itself as a particularly heterogeneous region bridging the Hot Core and Compact Ridge. Additional studies to identify the (hidden) sources of luminosity and heating within Orion KL are necessary to better understand the nebula and its chemistry. [ABSTRACT FROM AUTHOR]

We report the first coherent excitation of intramolecular vibrational modes via the nonlinear interaction of a TeraHertz (THz) light field with molecular liquids. A terahertz-terahertz-Raman pulse sequence prepares the coherences with a broadband, high-energy, (sub)picosecond terahertz pulse, that are then measured in a terahertz Kerr effect spectrometer via phase-sensitive, heterodyne detection with an optical pulse. The spectrometer reported here has broader terahertz frequency coverage, and an increased sensitivity relative to previously reported terahertz Kerr effect experiments. Vibrational coherences are observed in liquid diiodomethane at 3.66 THz (122 cm-1), and in carbon tetrachloride at 6.50 THz (217 cm-1), in exact agreement with literature values of those intramolecular modes. This work opens the door to 2D spectroscopies, nonlinear in terahertz field, that can study the dynamics of condensed-phase molecular systems, as well as coherent control at terahertz frequencies. [ABSTRACT FROM AUTHOR]

Connecting the composition of planet-forming disks with that of gas giant exoplanet atmospheres, in particular through C/O ratios, is one of the key goals of disk chemistry. Small hydrocarbons like C2H and C3H2 have been identified as tracers of C/O, as they form abundantly under high C/O conditions. We present resolved c–C3H2 observations from the TW Hya Rosetta Stone Project, a program designed to map the chemistry of common molecules at 15–20 au resolution in the TW Hya disk. Augmented by archival data, these observations comprise the most extensive multi-line set for disks of both ortho and para spin isomers spanning a wide range of energies, Eu = 29–97 K. We find the ortho-to-para ratio of c–C3H2 is consistent with 3 throughout extent of the emission, and the total abundance of both c–C3H2 isomers is (7.5–10) × 10−11 per H atom, or 1%–10% of the previously published C2H abundance in the same source. We find c–C3H2 comes from a layer near the surface that extends no deeper than z/r = 0.25. Our observations are consistent with substantial radial variation in gas-phase C/O in TW Hya, with a sharp increase outside ∼30 au. Even if we are not directly tracing the midplane, if planets accrete from the surface via, e.g., meridional flows, then such a change should be imprinted on forming planets. Perhaps interestingly, the HR 8799 planetary system also shows an increasing gradient in its giant planets' atmospheric C/O ratios. While these stars are quite different, hydrocarbon rings in disks are common, and therefore our results are consistent with the young planets of HR 8799 still bearing the imprint of their parent disk's volatile chemistry. [ABSTRACT FROM AUTHOR]

Infrared observations probe the warm gas in the inner regions of planet-forming disks around young Sun-like T Tauri stars. In these systems, H2O, OH, CO, CO2, C2H2, and HCN have been widely observed. However, the potentially abundant carbon carrier CH4 remains largely unconstrained. The James Webb Space Telescope (JWST) will be able to characterize mid-infrared fluxes of CH4 along with several other carriers of carbon and oxygen. In anticipation of the JWST mission, we model the physical and chemical structure of a T Tauri disk to predict the abundances and mid-infrared fluxes of observable molecules. A range of compositional scenarios are explored involving the destruction of refractory carbon materials and alterations to the total elemental (volatile and refractory) C/O ratio. Photon-driven chemistry in the inner disk surface layers largely destroys the initial carbon and oxygen carriers. This causes models with the same physical structure and C/O ratio to have similar steady-state surface compositions, regardless of the initial chemical abundances. Initial disk compositions are better preserved in the shielded inner disk midplane. The degree of similarity between the surface and midplane compositions in the inner disk will depend on the characteristics of vertical mixing at these radii. Our modeled fluxes of observable molecules respond sensitively to changes in the disk gas temperature, inner radius, and total elemental C/O ratio. As a result, mid-infrared observations of disks will be useful probes of these fundamental disk parameters, including the C/O ratio, which can be compared to values determined for planetary atmospheres. [ABSTRACT FROM AUTHOR]

The thermal structure of protoplanetary disks is a fundamental characteristic of the system that has wide-reaching effects on disk evolution and planet formation. In this study, we constrain the 2D thermal structure of the protoplanetary disk TW Hya structure utilizing images of seven CO lines. This includes new ALMA observations of 12CO J = 2–1 and C18O J = 2–1 as well as archival ALMA observations of 12CO J = 3–2, 13CO J = 3–2 and 6–5, and C18O J = 3–2 and 6–5. Additionally, we reproduce a Herschel observation of the HD J = 1–0 line flux and the spectral energy distribution and utilize a recent quantification of CO radial depletion in TW Hya. These observations were modeled using the thermochemical code RAC2D, and our best-fit model reproduces all spatially resolved CO surface brightness profiles. The resulting thermal profile finds a disk mass of 0.025 M⊙ and a thin upper layer of gas depleted of small dust with a thickness of ∼1.2% of the corresponding radius. Using our final thermal structure, we find that CO alone is not a viable mass tracer, as its abundance is degenerate with the total H2 surface density. Different mass models can readily match the spatially resolved CO line profiles with disparate abundance assumptions. Mass determination requires additional knowledge, and, in this work, HD provides the additional constraint to derive the gas mass and support the inference of CO depletion in the TW Hya disk. Our final thermal structure confirms the use of HD as a powerful probe of protoplanetary disk mass. Additionally, the method laid out in this paper is an employable strategy for extraction of disk temperatures and masses in the future. [ABSTRACT FROM AUTHOR]

Formaldehyde (H2CO) is an important precursor to organics like methanol (CH3OH). It is important to understand the conditions that produce H2CO and prebiotic molecules during star and planet formation. H2CO possesses both gas-phase and solid-state formation pathways, involving either UV-produced radical precursors or CO ice and cold (≲20 K) dust grains. To understand which pathway dominates, gaseous H2CO's ortho-to-para ratio (OPR) has been used as a probe, with a value of 3 indicating "warm" conditions and <3 linked to cold formation in the solid state. We present spatially resolved Atacama Large Millimeter/submillimeter Array observations of multiple ortho- and para-H2CO transitions in the TW Hya protoplanetary disk to test H2CO formation theories during planet formation. We find disk-averaged rotational temperatures and column densities of 33 ± 2 K, (1.1 ± 0.1) × 1012 cm−2 and 25 ± 2 K, (4.4 ± 0.3) × 1011 cm−2 for ortho- and para-H2CO, respectively, and an OPR of 2.49 ± 0.23. A radially resolved analysis shows that the observed H2CO emits mostly at rotational temperatures of 30–40 K, corresponding to a layer with z/R ≥ 0.25. The OPR is consistent with 3 within 60 au, the extent of the pebble disk, and decreases beyond 60 au to 2.0 ± 0.5. The latter corresponds to a spin temperature of 12 K, well below the rotational temperature. The combination of relatively uniform emitting conditions, a radial gradient in the OPR, and recent laboratory experiments and theory on OPR ratios after sublimation, led us to speculate that gas-phase formation is responsible for the observed H2CO across the TW Hya disk. [ABSTRACT FROM AUTHOR]

We analyze high-resolution (Δv ≤ 10 km s−1) optical and infrared spectra covering the [O i ] λ6300 and [Ne ii ] 12.81 μm lines from a sample of 31 disks in different evolutionary stages. Following work at optical wavelengths, we use Gaussian profiles to fit the [Ne ii ] lines and classify them into high-velocity component (HVC) or low-velocity component (LVC) if the line centroid is more or less blueshifted than 30 km s−1 with respect to the stellar radial velocity, respectively. Unlike for the [O i ], where an HVC is often accompanied by an LVC, all 17 sources with an [Ne ii ] detection have either an HVC or an LVC. [Ne ii ] HVCs are preferentially detected toward high accretors (M⊙ yr−1), while LVCs are found in sources with low , low [O i ] luminosity, and large infrared spectral index (n13–31). Interestingly, the [Ne ii ] and [O i ] LVC luminosities display an opposite behavior with n13–31: as the inner dust disk depletes (higher n13–31), the [Ne ii ] luminosity increases while the [O i ] weakens. The [Ne ii ] and [O i ] HVC profiles are generally similar, with centroids and FWHMs showing the expected behavior from shocked gas in microjets. In contrast, the [Ne ii ] LVC profiles are typically more blueshifted and narrower than the [O i ] profiles. The FWHM and centroid versus disk inclination suggest that the [Ne ii ] LVC predominantly traces unbound gas from a slow, wide-angle wind that has not lost completely the Keplerian signature from its launching region. We sketch an evolutionary scenario that could explain the combined [O i ] and [Ne ii ] results and includes screening of hard (∼1 keV) X-rays in inner, mostly molecular, MHD winds. [ABSTRACT FROM AUTHOR]

The volatile composition of a planet is determined by the inventory of gas and ice in the parent disk. The volatile chemistry in the disk is expected to evolve over time, though this evolution is poorly constrained observationally. We present Atacama Large Millimeter/submillimeter Array observations of C18O, C2H, and the isotopologues H13CN, HC15N, and DCN toward five Class 0/I disk candidates. Combined with a sample of 14 Class II disks presented in Bergner et al., this data set offers a view of volatile chemical evolution over the disk lifetime. Our estimates of C18O abundances are consistent with a rapid depletion of CO in the first ∼0.5–1 Myr of the disk lifetime. We do not see evidence that C2H and HCN formation are enhanced by CO depletion, possibly because the gas is already quite under-abundant in CO. Further CO depletion may actually hinder their production by limiting the gas-phase carbon supply. The embedded sources show several chemical differences compared to the Class II stage, which seem to arise from shielding of radiation by the envelope (impacting C2H formation and HC15N fractionation) and sublimation of ices from infalling material (impacting HCN and C18O abundances). Such chemical differences between Class 0/I and Class II sources may affect the volatile composition of planet-forming material at different stages in the disk lifetime. [ABSTRACT FROM AUTHOR]

The volatile contents of protoplanetary disks both set the potential for planetary chemistry and provide valuable probes of defining disk system characteristics such as stellar mass, gas mass, ionization, and temperature structure. Current disk molecular inventories are fragmented, however, giving an incomplete picture: unbiased spectral line surveys are needed to assess the volatile content. We present here an overview of such a survey of the protoplanetary disks around the Herbig Ae star MWC 480 and the T Tauri star LkCa 15 in ALMA Band 7, spanning ∼36 GHz from 275 to 317 GHz and representing an order of magnitude increase in sensitivity over previous single-dish surveys. We detect 14 molecular species (including isotopologues), with five species (C34S, 13CS, H2CS, DNC, and C2D) detected for the first time in protoplanetary disks. Significant differences are observed in the molecular inventories of MWC 480 and LkCa 15, and we discuss how these results may be interpreted in light of the different physical conditions of these two disk systems. [ABSTRACT FROM AUTHOR]

Context. CO rovibrational lines are efficient probes of warm molecular gas and can give unique insights into the inner 10 AU of proto-planetary disks, effectively complementing ALMA observations. Recent studies find a relation between the ratio of lines originating from the second and first vibrationally excited state, denoted as v2∕v1, and the Keplerian velocity or emitting radius of CO. Counterintuitively, in disks around Herbig Ae stars the vibrational excitation is low when CO lines come from close to the star, and high when lines only probe gas at large radii (more than 5 AU). The v2∕v1 ratio is also counterintuitively anti-correlated with the near-infrared (NIR) excess, which probes hot and warm dust in the inner disk. Aims. We aim to find explanations for the observed trends between CO vibrational ratio, emitting radii and NIR excess, and to identify their implications in terms of the physical and chemical structure of inner disks around Herbig stars. Methods. First, slab model explorations in local thermal equilibrium (LTE) and non-LTE are used to identify the essential parameter space regions that can produce the observed CO emission. Second, we explore a grid of thermo-chemical models using the DALI code, varying gas-to-dust ratio and inner disk radius. Line flux, line ratios, and emitting radii are extracted from the simulated lines in the same way as the observations and directly compared to the data. Results. Broad CO lines with low vibrational ratios are best explained by a warm (400–1300 K) inner disk surface with gas-to-dust ratios below 1000 (NCO < 1018 cm−2); no CO is detected within or at the inner dust rim, due to dissociation at high temperatures. In contrast, explaining the narrow lines with high vibrational ratios requires an inner cavity of a least 5 AU in both dust and gas, followed by a cool (100–300 K) molecular gas reservoir with gas-to-dust ratios greater than 10 000 (NCO > 1018 cm−2) at the cavity wall. In all cases, the CO gas must be close to thermalization with the dust (Tgas ~ Tdust). Conclusions. The high gas-to-dust ratios needed to explain high v2∕v1 in narrow CO lines for a subset of group I disks can be naturally interpreted as due to the dust traps that are proposed to explain millimeter dust cavities. The dust trap and the low gas surface density inside the cavity are consistent with the presence of one or more massive planets. The difference between group I disks with low and high NIR excess can be explained by gap opening mechanisms that do or do not create an efficient dust trap, respectively. The broad lines seen in most group II objects indicate a very flat disk in addition to inner disk substructures within 10 AU that can be related to the substructures recently observed with ALMA. We provide simulated ELT-METIS images to directly test these scenarios in the future. [ABSTRACT FROM AUTHOR]

The mechanism for the formation of hexamethylenetetraamine predicts the formation of aminomethanol from the addition of ammonia to formaldehyde. This molecule subsequently undergoes unimolecular decomposition to form methanimine and water. Aminomethanol is the predicted precursor to interstellar glycine, and is therefore of great interest for laboratory spectroscopic study, which would serve as the basis for observational searches. The height of the water loss barrier is therefore useful in the determination of an appropriate experimental approach for spectroscopic characterization of aminomethanol. We have determined the height of this barrier to be 55 kcal/mol at ambient temperatures. In addition, we have determined the infinite-pressure Rice–Ramsperger–Kassel–Marcus unimolecular decomposition rate to be <10-25 s-1 at 300 K, indicating gas-phase kinetic stability for typical laboratory and hot core temperatures. Therefore, spectroscopic characterization of and observational searches for this molecule should be straightforward provided an efficient formation mechanism can be found. [ABSTRACT FROM AUTHOR]

Methanediol, or methylene glycol, is a product of the liquid phase reaction of water and formaldehyde and is a predicted interstellar grain surface species. Detection of this molecule in a hot core environment would advance the understanding of complex organic chemistry in the interstellar medium, but its laboratory spectroscopic characterization is a prerequisite for such observational searches. This theoretical study investigates the unimolecular decomposition of methanediol, specifically the thermodynamic and kinetic stability of the molecule under typical laboratory and interstellar conditions. Methanediol was found to be thermodynamically stable at temperatures of <100 K, which is the characteristic temperature range for interstellar grain mantles. The infinite-pressure RRKM unimolecular decomposition rate was found to be <10[sup -18] s[sup -1] at 300 K, indicating gas phase kinetic stability for typical laboratory and hot core temperatures. Therefore, both laboratory studies of and observational searches for this molecule should be feasible. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the design and performance of an echelon-based single shot visible/near-infrared spectrometer with adequate sensitivity to measure the nonlinear optical and terahertz Kerr effects in neat molecular liquids at room temperature. Useful molecular information spanning tens of picoseconds can be measured in just a few milliseconds, and the signal-to-noise performance scales favorably with respect to the standard stage scan technique. These results demonstrate the viability of stage-free nonlinear Kerr effect measurements and provide a route for improvements to the speed of future multidimensional Kerr effect studies. [ABSTRACT FROM AUTHOR]

Discusses the microwave rotation-tunneling spectroscopy of the water-methanol dimer. Transitions arising from the methyl torsional A state; Measured components of the dipole moment for the parent isotopomer.

Microwave and far-infrared spectra of the H3N–HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a-type pure rotational microwave data extend the previous m=0, K=0 A symmetry manifold measurements of Herbine and Dyke [J. Chem. Phys. 83, 3768 (1980)] to higher frequency and also provide an additional set of microwave transitions in the mK=+1 E symmetry manifold. Two sets of five b-type rotation–tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V3=10.5±5.0 cm-1 for the barrier height to internal rotation for the NH3 monomer. A nonlinear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle cos-1[〈λz>] determined from 〈λz>, the projection of the ammonia’s angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of ∼700 cm-1 is derived for the water tunneling motion about its c axis. [ABSTRACT FROM AUTHOR]

Fifty-six transitions from the K=1 lower→K=2 lower tunneling–rotation band of water dimer have been measured and assigned at 22 cm-1 by direct absorption spectroscopy in a cw planar supersonic jet expansion using a tunable far infrared laser spectrometer. Two different models were used to fit the data and several spectroscopic constants were determined for the upper and lower states. This work supports the local IAM model recently proposed by Coudert and Hougen for the hydrogen bond tunneling dynamics of the water dimer. This model includes four different tunneling motions, all of which contribute to the observed tunneling splittings. This is the most complicated hydrogen bonded system considered to be well understood at this time, at least in the lowest few K states. [ABSTRACT FROM AUTHOR]

The ground state Ka =0(u)→1(g) b-type subband of the rotation–tunneling spectrum of the symmetric 35Cl–35Cl, 37Cl–37Cl, and the mixed 35Cl–37Cl hydrogen chloride dimers have been recorded near 26.3 cm-1 with sub-Doppler resolution in a continuous two-dimensional supersonic jet with a tunable far-infrared laser spectrometer. Quadrupole hyperfine structure from the chlorine nuclei has been resolved. From the fitted rotational constants a (H35Cl)2 center-of-mass separation of 3.81 Å is derived for the Ka =1(g) levels, while the nuclear quadrupole coupling constants yield a vibrationally averaged angular structure for both tunneling states of approximately 20–25 deg for the hydrogen bonded proton and at least 70–75 deg for the external proton. This nearly orthogonal structure agrees well with that predicted by ab initio theoretical calculations, but the observed splittings and intensity alterations of the lines indicate that the chlorine nuclei are made equivalent by a large amplitude tunneling motion of the HCl monomers. A similar geared internal rotation tunneling motion has been found for the HF dimer, but here the effect is much greater. The ground state tunneling splittings are estimated to lie between 15–18 cm-1, and the selection rules observed indicate that the trans tunneling path dominates the large amplitude motion, as expected, provided the dimer remains planar. From the observed hyperfine constants, we judge the dimer and its associated tunneling motion to be planar to within 10°. [ABSTRACT FROM AUTHOR]

The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm-1, and to vb1 =1+ (b-type) at 25 cm-1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm-1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm-1 above the ground state of the complex. The experimentally determined center-of-mass separation (Rc.m. =3.75 Å) and harmonic stretching force constant (ks =0.0134 mdyn/Å) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure. [ABSTRACT FROM AUTHOR]

A tunable far infrared laser system has been used to measure the vibration–rotation spectrum of the lowest Σ bending state of ArHCl near 24 cm-1 in a cw planar jet operating with a terminal jet temperature near 3 K. Over 60 transitions have been observed for both 35Cl and 37Cl isotopes with resolution of the quadrupole hyperfine structure. An improved set of molecular parameters was determined, including B, D, H, and eqQ for both upper and lower states. Very narrow linewidths (approximately 300 kHz) resulting in high resolution and sensitivity make this technique a powerful new method for the detailed investigation of intermolecular forces. [ABSTRACT FROM AUTHOR]

Over 70 transitions among the lowest six vibrational states of C35Cl+ and C37Cl+ have been measured between 1070–1210 cm-1. The spectrum has been fitted to a sixth order Dunham expansion to yield an accurate mapping of the Born–Oppenheimer potential function of CCl+. The spectroscopic constants obtained are ωe =1177.7196(8) cm-1, ωexe =6.6475(3) cm-1, and Be =0.797 940(3) cm-1. The rotational constants for both CCl+ isotopes reported here show the results of the previous electronic emission studies to be incorrect. A fit of the data to a Morse function yields a dissociation energy D of 52 828(50) cm-1. The rotational temperature has been determined as 540 K±30%. The increase in the effective vibrational temperature with vibrational excitation indicates that CCl+ is formed with high internal energy. [ABSTRACT FROM AUTHOR]

Focuses on the tunability of generated far-infrared radiation. Techniques developed which generate tunable coherent radiation at submillimeter and far-infrared wavelengths; Limited tuning range of the individual infrared transitions; Interferometer path difference; Measurement of submillimeter rotational absorption lines.

Core-accretion theory predicts that the formation of giant planets predominantly occurs at the dense mid-plane of the inner ∼50 AU of protoplanetary disks. However, due to observational limitation, this critical region remains to be the least charted area in protoplanetary disks. With its great sensitivity, ALMA recently started to image optically thin line emissions arisen from the mid-plane of the inner 50AU in nearby disks, which unlocks an exciting new path to directly constrain the physical properties of the giant planet formation zone through gas tracers. Here we present the first spatially resolved observations of the 13C18O J =3-2 line emission in the TW Hya disk. We show that this emission is optically thin even inside the CO mid-plane snowline. Combining it with the C18O J =3-2 images and the previously detected HD J=1-0 flux, we directly constrain the mid-plane temperature and optical depths of the CO gas and dust. We report a mid-plane CO snowline at 20.5 ± 1.3 AU, a mid-plane temperature distribution of 27+4−3×(R/20.5AU)-0.47+0.06−0.07 K, and a gas mass distribution of 13+8−5×(R/20.5AU)-0.9+0.4−0.3 g cm−2 between 5-20.5 AU in the TW Hya protoplanetary disk. We find a total gas/mm-sized dust mass ratio of 140 ± 40 in this region, suggesting that ∼2.4 earth mass of dust aggregates have grown to > cm sizes (and perhaps much larger). [ABSTRACT FROM AUTHOR]

The molecular complex between iso-propanol and water has been investigated by Fourier transform microwave spectroscopy. Two distinct rotational spectra have been assigned, corresponding to two different isomers of the adduct. In both cases the water molecule acts as a proton donor to the alcoholic oxygen atom of iso-propanol in its gauche arrangement. The isomer in which the water molecule is oriented along the symmetry plane of the iso-propanol molecule (inner) is more stable than the second isomer, where the water is positioned outside the iso-propanol symmetry plane (outer). The rotational transitions of the inner isomer display a doubling, due to the two equivalent minima related to the internal rotation of the hydroxyl group (concerted with a rearrangement of the water unit). The tunneling splitting has been determined to be 25.16(8) GHz, corresponding to a B2 barrier of ∼440 cm−1. [ABSTRACT FROM AUTHOR]

Previous theoretical work on the ethanol–methanol dimer has been inconclusive in predicting the preferred hydrogen bond donor/acceptor configuration. Here, we report the microwave spectrum of the dimer using a chirped pulse Fourier transform microwave spectrometer from 8–18 GHz. In an argon-backed expansion, 50 transitions have been assigned to a trans-ethanol-acceptor/methanol-donor structure that is likely stabilized by a secondary weak C–H…O hydrogen bond. A higher energy conformer was observed in a helium-backed expansion and tentatively assigned to a gauche-ethanol-acceptor/methanol-donor structure. No ethanol-donor/methanol-acceptor dimers have been found, suggesting such interactions are energetically disfavored. A preliminary analysis of the A–E splitting due to the internal rotation of the methanol methyl group in the ground state species is also presented. We find evidence of the Ubbelohde effect in the measured A–E splittings of three deuterated isotopologues and the normal species of this conformer. [ABSTRACT FROM AUTHOR]

The icy mantles of interstellar dust grains are the birthplaces of the primordial prebiotic molecular inventory that may eventually seed nascent solar systems and the planets and planetesimals that form therein. Here, we present a study of two of the most abundant species in these ices after water: carbon dioxide (CO2) and methanol (CH3OH), using TeraHertz (THz) time-domain spectroscopy and mid-infrared spectroscopy. We study pure and mixed-ices of these species, and demonstrate the power of the THz region of the spectrum to elucidate the long-range structure (i.e. crystalline versus amorphous) of the ice, the degree of segregation of these species within the ice, and the thermal history of the species within the ice. Finally, we comment on the utility of the THz transitions arising from these ices for use in astronomical observations of interstellar ices. [ABSTRACT FROM AUTHOR]

We present 2D terahertz-terahertz-Raman (2D TTR) spectroscopy, the first technique, to our knowledge, to interrogate a liquid with multiple pulses of terahertz (THz) light. This hybrid approach isolates nonlinear signatures in isotropic media, and is sensitive to the coupling and anharmonicity of thermally activated THz modes that play a central role in liquid-phase chemistry. Specifically, by varying the timing between two intense THz pulses, we control the orientational alignment of molecules in a liquid, and nonlinearly excite vibrational coherences. A comparison of experimental and simulated 2D TTR spectra of bromoform (CHBr3), carbon tetrachloride (CCl4), and dibromodichloromethane (CBr2Cl2) shows previously unobserved off-diagonal anharmonic coupling between thermally populated vibrational modes. [ABSTRACT FROM AUTHOR]

We report the first rotational spectrum of the ground state of the isolated ethanol–water dimer using chirped-pulse Fourier transform microwave spectroscopy between 8–18 GHz. With the aid of isotopic substitutions, and ab initio calculations, we identify the measured conformer as a water-donor/ethanol-acceptor structure. Ethanol is found to be in the gauche conformation, while the monomer distances and orientations likely reflect a cooperation between the strong (O–H…O) and weak (C–H…O) hydrogen bonds that stabilizes the measured conformer. No other conformers were assigned in an argon expansion, confirming that this is the ground-state structure. This result is consistent with previous vibrationally-resolved Raman and infrared work, but sheds additional light on the structure, due to the specificity of rotational spectroscopy. [ABSTRACT FROM AUTHOR]

We present the design and capabilities of a high-resolution, decade-spanning A Synchronous OPtical Sampling (ASOPS)-based TeraHertz Time-Domain Spectroscopy (THz-TDS) instrument. Our system employs dual mode-locked femtosecond Ti:Sapphire oscillators with repetition rates offset locked at 100 Hz via a Phase-Locked Loop (PLL) operating at the 60th harmonic of the ~80 MHz oscillator repetition rates. The respective time delays of the individual laser pulses are scanned across a 12.5 ns window in a laboratory scan time of 10 ms, supporting a time delay resolution as fine as 15.6 fs. The repetition rate of the pump oscillator is synchronized to a Rb frequency standard via a PLL operating at the 12th harmonic of the oscillator repetition rate, achieving milliHertz (mHz) stability. We characterize the timing jitter of the system using an air-spaced etalon, an optical cross correlator, and the phase noise spectrum of the PLL. Spectroscopic applications of ASOPS-THz-TDS are demonstrated by measuring water vapor absorption lines from 0.55 to 3.35 THz and acetonitrile absorption lines from 0.13 to 1.39 THz in a short pathlength gas cell. With 70 min of data acquisition, a 50 dB signal-to-noise ratio is achieved. The achieved root-mean-square deviation is 14.6 MHz, with a mean deviation of 11.6 MHz, for the measured water line center frequencies as compared to the JPL molecular spectroscopy database. Further, with the same instrument and data acquisition hardware, we use the ability to control the repetition rate of the pump oscillator to enable THz frequency comb spectroscopy (THz-FCS). Here, a frequency comb with a tooth width of 5 MHz is generated and used to fully resolve the pure rotational spectrum of acetonitrile with Doppler-limited precision. The oscillator repetition rate stability achieved by our PLL lock circuits enables sub-MHz tooth width generation, if desired. This instrument provides unprecedented decade-spanning, tunable resolution, from 80 MHz down to sub-MHz, and heralds a new generation of gas-phase spectroscopic tools in the THz region. [ABSTRACT FROM AUTHOR]

We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites, and terrestrial planets; we include an updated estimate for the bulk silicate Earth (C/N = 49.0 ± 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macromolecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of ~1-12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N bulk silicate Earth ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable. [ABSTRACT FROM AUTHOR]