Rotational and nuclear-spin level dependent photodissociation dynamics of H2S.

The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. But even in a small molecule like hydrogen sulphide (H2S), the influence of non-adiabatic transitions is not yet well understood. Here we report high resolution translational spectroscopy measurements of the H and S(1D) photoproducts formed following excitation of H2S to selected quantum levels of a Rydberg state with 1B1 electronic symmetry at wavelengths λ ~ 139.1 nm, revealing rich photofragmentation dynamics. Analysis reveals formation of SH(X), SH(A), S(3P) and H2 co-fragments, and in the diatomic products, inverted internal state population distributions. These nuclear dynamics are rationalised in terms of vibronic and rotational dependent predissociations, with relative probabilities depending on the parent quantum level. The study suggests likely formation routes for the S atoms attributed to solar photolysis of H2S in the coma of comets like C/1995 O1 and C/2014 Q2.

Water Photolysis and Its Contributions to the Hydroxyl Dayglow Emissions in the Atmospheres of Earth and Mars.

Airglow is a well-known phenomenon in the Earth's upper atmosphere, which arises from the emissions of energetic atoms and molecules. The Meinel band emission from high vibrationally excited OH(X) radicals is one of the more important contributors to the airglow from the mesosphere/lower thermosphere. The H + O3 reaction has long been regarded as the dominant source of these OH(X, high v) radicals. Here we demonstrate that vacuum ultraviolet (VUV) photolysis of water vapor at λ ∼ 112.8 nm represents another source of exceptionally highly vibrationally excited OH(X) radicals, with a nascent vibrational state population distribution that maximizes at v = 9 and extends to at least the v = 15 level. Atmospheric chemistry modeling indicates that OH(X, high v) radicals from H2O photolysis might be detectable in the OH Meinel band dayglow in the upper atmosphere of Earth and should dominate the corresponding emission from the Martian atmosphere. VUV photolysis of H2O also produces electronically excited OH(A) radicals, and simultaneous detection of emissions from OH(X, high v) and OH(A) is shown to offer a route to identifying high-oxygen exoplanetary atmospheres.

Kinetic Study of the Reactions PO + O2 and PO2 + O3 and Spectroscopy of the PO Radical.

The kinetics of the reactions of PO with O2 and PO2 with O3 were studied at temperatures ranging from ∼190 to 340 K, using a pulsed laser photolysis-laser induced fluorescence technique. For the reaction of PO + O2, there is evidence of both a two- and three-body exit channel, producing PO2 + O and PO3, respectively. Potential energy surfaces of both the PO + O2 and PO2 + O3 systems were calculated using electronic structure theory and combined with RRKM calculations to explain the observed pressure and temperature dependences. For PO + O2, at pressures typical of a planetary upper atmosphere where meteoric ablation of P will occur, the reaction is effectively pressure independent with a yield of PO2 + O of >99%; the rate coefficient can be expressed by log10(k, 120-500 K, cm3 molecule-1 s-1) = -13.915 + 2.470 log10(T) - 0.5020(log10(T))2, with an uncertainty of ±10% over the experimental temperature range (191-339 K). With increasing pressure, the yield of PO3 increases, reaching ∼90% at a pressure of 1 atm and T = 300 K. For PO2 + O3, k(188-339 K) = 3.7 × 10-11 exp(-1131/T) cm3 molecule-1 s-1, with an uncertainty of ±26% over the stated temperature range. Laser-induced fluorescence spectra of PO over the wavelength range 245-248 nm were collected and simulated using pgopher to obtain new spectroscopic constants for the ground and v = 1 vibrational levels of the X2Π and A2Σ+ states of PO.

Ultraviolet photolysis of H2S and its implications for SH radical production in the interstellar medium.

Hydrogen sulfide radicals in the ground state, SH(X), and hydrogen disulfide molecules, H2S, are both detected in the interstellar medium, but the returned SH(X)/H2S abundance ratios imply a depletion of the former relative to that predicted by current models (which assume that photon absorption by H2S at energies below the ionization limit results in H + SH photoproducts). Here we report that translational spectroscopy measurements of the H atoms and S(1D) atoms formed by photolysis of jet-cooled H2S molecules at many wavelengths in the range 122 ≤ λ ≤155 nm offer a rationale for this apparent depletion; the quantum yield for forming SH(X) products, Γ, decreases from unity (at the longest excitation wavelengths) to zero at short wavelengths. Convoluting the wavelength dependences of Γ, the H2S parent absorption and the interstellar radiation field implies that only ~26% of photoexcitation events result in SH(X) products. The findings suggest a need to revise the relevant astrochemical models.

Precise Measurements of 12CH2D2 by Tunable Infrared Laser Direct Absorption Spectroscopy.

We present precise measurements of doubly deuterated methane (12CH2D2) in natural methane samples using tunable infrared laser direct absorption spectroscopy (TILDAS). Using a 413 m optical path length astigmatic Herriott cell and two quantum cascade lasers (QCLs) scanning the spectral regions of 1090.46 ± 0.1 and 1200.23 ± 0.1 cm-1, the instrument simultaneously measures the five main isotopologues of methane. The ratios 13CH3D/12CH4 and 12CH2D2/12CH4 are measured at 0.01‰ and 0.5‰ (1σ) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of δ13C and δD values but with the abundances of all isotopologues driven to thermal equilibrium at 250 °C. The estimated accuracy of Δ12CH2D2 is 1‰ (1σ) on the basis of the results of the heated methane samples. This new TILDAS instrument provides a simple and rapid technique to explore the sources of methane in the environment.

Automatic and semi-automatic assignment and fitting of spectra with PGOPHER.

Two new tools for computer assisted assignment of rotational spectra with the PGOPHER program are presented, aimed particularly at spectra where many individual lines are resolved. The first tool tries many different assignments, presenting a small number for possible refinement and a preliminary version of this has already been presented. The second tool, the nearest lines plot (a new style of residual plot) provides a clear indication as to whether a trial calculation is plausible, and allows rapid assignment of sets of related lines. It gives good results even for dense spectra with no obvious structure and in the presence of multiple interfering absorptions. The effectiveness of these tools is demonstrated by the analysis of high resolution IR spectra of 8 bands of cis- and trans-1,2-dichloroethene where, including hot bands and isotopologues, 31 vibrational transitions and 158 316 individual lines have been analysed, including perturbations for the higher energy states. Walkthroughs are presented to show this process is rapid.

Automatic assignment and fitting of spectra with pgopher.

An initial implementation of a tool for automatic assignment of spectra within the pgopher program is presented, together with its application to rotational analysis of the ν11 band of cis-1,2-dichloroethene. It is based on the autofit algorithm presented by N. A. Seifert et al. (J. Mol. Spectrosc., 2015, 312, 13) but implemented in a more efficient and general way, allowing it to be applied to a much wider variety of spectra.

Note: Improved line strengths of rovibrational and rotational transitions within the X(3)Σ(-) ground state of NH.

Recently, a line list including positions and transition strengths was published for the NH X(3)Σ(-) rovibrational and rotational transitions. The calculation of the transition strengths requires a conversion of transition matrix elements from Hund's case (b) to (a). The method of this conversion has recently been improved during other work on the OH X(2)Π rovibrational transitions, by removing an approximation that was present previously. The adjusted method has been applied to the NH line list, resulting in more accurate transition strengths. An updated line list is presented that contains all possible transitions with v' and v″ up to 6, and J up to between 25 and 44, depending on the band.

The Jahn-Teller effect in the presence of partial isotopic substitution: the B̃(1)E'' state of NH2D and NHD2.

Rotationally resolved resonance enhanced multiphoton ionisation spectra of the B̃(1)E'' state of NH2D are presented and analysed. The analysis indicates a small (34.9 cm(-1)) lifting of the vibronic degeneracy of the zero point level, approximately equal in sign but opposite in magnitude to the splitting observed in NHD2 in previous work. This observation is consistent with previous measurements on systems with partial isotopic substitution subject to a mild Jahn-Teller effect. A model is developed to calculate the splitting induced by asymmetric isotopic substitution of a degenerate electronic state, based on a harmonic force field with linear and quadratic Jahn-Teller terms added. The force field is developed in internal co-ordinates to allow the same parameters to be used to calculate the pattern of vibronic levels for all four isotopologues. The lifting of the degeneracy of the zero point level on asymmetric substitution comes from the quadratic Jahn-Teller effect; the linear term does not lift the degeneracy.

Line strengths of rovibrational and rotational transitions within the X³Σ⁻ ground state of NH.

A new line list for rovibrational and rotational transitions, including fine structure, within the NH X³Σ⁻ ground state has been created. It contains line intensities in the form of Einstein A and f-values, for all possible bands up to v' = 6, and for J up to between 25 and 44. The intensities are based on a new dipole moment function (DMF), which has been calculated using the internally contracted multi-reference configuration interaction method with an aug-cc-pV6Z basis set. The programs RKR1, LEVEL, and PGOPHER were used to calculate line positions and intensities using the most recent spectroscopic line position observations and the new DMF, including the rotational dependence on the matrix elements. The Hund's case (b) matrix elements from the LEVEL output (available as Supplement 1 of the supplementary material) have been transformed to the case (a) form required by PGOPHER. New relative intensities for the (1,0) band have been measured, and the calculated and observed Herman-Wallis effects are compared, showing good agreement. The line list (see Supplement 5 of the supplementary material) will be useful for the study of NH in astronomy, cold and ultracold molecular systems, and in the nitrogen chemistry of combustion.

Resonance enhanced multiphoton ionization spectroscopy of NHD2 via the B1E''-state.

Rotational analysis of the (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectrum of the B[combining tilde](1)E'' Rydberg state of the ammonia isotopologue NHD2 is reported. While the electronic degeneracy is lifted in NHD2 the splitting is small enough that interactions between the two states must be considered, particularly to model the intensity of the transitions. A simple model is developed to account for these interactions, relating them to terms present in the symmetric isotopologues. Spectroscopic parameters for the zero point and (ν2' = 1-6) vibrational levels of the B (1)E'' state have been derived using this model and the spectra are accurately simulated for the first time using the pgopher program. The current work provides the basis for on-going velocity map imaging studies of rotational energy transfer in the mixed isotopologues of ammonia.

Exploring the plasma chemistry in microwave chemical vapor deposition of diamond from C/H/O gas mixtures.

Microwave (MW)-activated CH(4)/CO(2)/H(2) gas mixtures operating under conditions relevant to diamond chemical vapor deposition (i.e., X(C/Σ) = X(elem)(C)/(X(elem)(C) + X(elem)(O)) ≈ 0.5, H(2) mole fraction = 0.3, pressure, p = 150 Torr, and input power, P = 1 kW) have been explored in detail by a combination of spatially resolved absorption measurements (of CH, C(2)(a), and OH radicals and H(n = 2) atoms) within the hot plasma region and companion 2-dimensional modeling of the plasma. CO and H(2) are identified as the dominant species in the plasma core. The lower thermal conductivity of such a mixture (cf. the H(2)-rich plasmas used in most diamond chemical vapor deposition) accounts for the finding that CH(4)/CO(2)/H(2) plasmas can yield similar maximal gas temperatures and diamond growth rates at lower input powers than traditional CH(4)/H(2) plasmas. The plasma chemistry and composition is seen to switch upon changing from oxygen-rich (X(C/Σ) < 0.5) to carbon-rich (X(C/Σ) > 0.5) source gas mixtures and, by comparing CH(4)/CO(2)/H(2) (X(C/Σ) = 0.5) and CO/H(2) plasmas, to be sensitive to the choice of source gas (by virtue of the different prevailing gas activation mechanisms), in contrast to C/H process gas mixtures. CH(3) radicals are identified as the most abundant C(1)H(x) [x = 0-3] species near the growing diamond surface within the process window for successful diamond growth (X(C/Σ) ≈ 0.5-0.54) identified by Bachmann et al. (Diamond Relat. Mater.1991, 1, 1). This, and the findings of similar maximal gas temperatures (T(gas) ~2800-3000 K) and H atom mole fractions (X(H)~5-10%) to those found in MW-activated C/H plasmas, points to the prevalence of similar CH(3) radical based diamond growth mechanisms in both C/H and C/H/O plasmas.

Determining the dissociation threshold of ammonia trimers from action spectroscopy of small clusters.

Infrared-action spectroscopy of small ammonia clusters obtained by detecting ammonia fragments from vibrational predissociation provides an estimate of the dissociation energy of the trimer. The product detection uses resonance enhanced multiphoton ionization (REMPI) of individual rovibrational states of ammonia identified by simulations using a consistent set of ground-electronic-state spectroscopic constants in the PGOPHER program. Comparison of the infrared-action spectra to a less congested spectrum measured in He droplets [M. N. Slipchenko, B. G. Sartakov, A. F. Vilesov, and S. S. Xantheas, J. Phys. Chem. A 111, 7460 (2007)] identifies the contributions from the dimer and the trimer. The relative intensities of the dimer and trimer features in the infrared-action spectra depend on the amount of energy available for breaking the hydrogen bonds in the cluster, a quantity that depends on the energy content of the detected fragment. Infrared-action spectra for ammonia fragments with large amounts of internal energy have almost no trimer component because there is not enough energy available to break two bonds in the cyclic trimer. By contrast, infrared-action spectra for fragments with low amounts of internal energy have a substantial trimer component. Analyzing the trimer contribution quantitatively shows that fragmentation of the trimer into a monomer and dimer requires an energy of 1700 to 1800 cm(-1), a range that is consistent with several theoretical estimates.

A theoretical and experimental study on translational and internal energies of H2O and OH from the 157 nm irradiation of amorphous solid water at 90 K.

The photodesorption of H(2)O in its vibrational ground state, and of OH radicals in their ground and first excited vibrational states, following 157 nm photoexcitation of amorphous solid water has been studied using molecular dynamics simulations and detected experimentally by resonance-enhanced multiphoton ionization techniques. There is good agreement between the simulated and measured energy distributions. In addition, signals of H(+) and OH(+) were detected in the experiments. These are inferred to originate from vibrationally excited H(2)O molecules that are ejected from the surface by two distinct mechanisms: a direct desorption mechanism and desorption induced by secondary recombination of photoproducts at the ice surface. This is the first reported experimental evidence of photodesorption of vibrationally excited H(2)O molecules from water ice.

A desorption mechanism of water following vacuum-ultraviolet irradiation on amorphous solid water at 90 K.

Following 157 nm photoexcitation of amorphous solid water and polycrystalline water ice, photodesorbed water molecules (H(2)O and D(2)O), in the ground vibrational state, have been observed using resonance-enhanced multiphoton ionization detection methods. Time-of-flight and rotationally resolved spectra of the photodesorbed water molecules were measured, and the kinetic and internal energy distributions were obtained. The measured energy distributions are in good accord with those predicted by classical molecular dynamics calculations for the kick-out mechanism of a water molecule from the ice surface by a hot hydrogen (deuterium) atom formed by photodissociation of a neighboring water molecule. Desorption of D(2)O following 193 nm photoirradiation of a D(2)O/H(2)S mixed ice was also investigated to provide further direct evidence for the operation of a kick-out mechanism.

Translational and internal energy distributions of methyl and hydroxyl radicals produced by 157 nm photodissociation of amorphous solid methanol.

Methanol is typically observed within water-rich interstellar ices and is a source of interstellar organic species. Following the 157 nm photoexcitation of solid methanol at 90 K, desorbed CH(3)(v=0) and OH(v=0,1) radicals have been observed in situ, near the solid surface, using resonance-enhanced multiphoton ionization (REMPI) detection methods. Time-of-flight and rotationally resolved REMPI spectra of the desorbed species were measured, and the respective fragment internal energy and kinetic energy distributions were obtained. Photoproduction mechanisms for CH(3) and OH radicals from solid methanol are discussed. The formation of O((1)D and (3)P) atoms and H(2)O was investigated, but the yield of these species was found to be negligible. CH(3) products arising following the photoexcitation of water-methanol mixed ice showed similar kinetic and internal energy distributions to those from neat methanol ice.

Spectroscopic investigation of the A (1)A(")-X (1)A(') electronic transition of HSiNCO.

The first spectroscopic observation of the previously unknown species HSiNCO has been reported. HSiNCO was generated by the fragmentation of trimethylsilylisocyanate with a high-voltage discharge source. The 0(0)(0) band of the A (1)A(")-X (1)A(') transition has been recorded with full rotational resolution using laser-induced fluorescence spectroscopy and ground and excited state rotational and centrifugal distortion constants determined. Ten additional vibrational bands belonging to HSiNCO have also been observed in the laser-induced fluorescence spectrum and have been assigned based on predicted anharmonic vibrational frequencies. Due to the large change in geometry upon excitation, a number of axis-rotation peaks have been observed in the 0(0)(0) band and the axis-rotation angle (theta(T)) has been estimated to be 0.6 degrees +/-0.2 degrees. Dispersed fluorescence spectroscopy has been carried out and nu(8) (the N-C-O out-of-plane bending mode) and a number of overtones of nu(4) (the Si-H wagging mode) have been observed in the ground electronic state.

Photodissociation imaging of diatomic sulfur (S2).

The photodissociation of diatomic sulfur, S(2), in the region of the first dissociation limit is studied with velocity map imaging. Correlated fine structure distributions P(J1,J2) for the two S((3)P(J)) fragments are determined at selected photolysis wavelengths. Image analysis of the speed distributions of the atomic fragments following product-state-specific detection results in a revision of the bond energy to D(0) = 35636.9 +/- 2.5 cm(-1) with respect to the lowest rovibrational level. This value arises from reinterpretation of previous spectroscopic data showing onset of predissociation in the B(3)Sigma(u)(-) state, as the measurements presented here demonstrate that the long-range correlation of the excited state invoked as causing the dissociation is S((3)P(2)) + S((3)P(2)) rather than S((3)P(2)) + S((3)P(1)). The wavelength dependence of data for the S((3)P(2)) + S((3)P(2)) channel suggests involvement of photoexcitation through the optically forbidden Herzberg continuum bands in addition to dissociation initiated via the optically allowed B(3)Sigma(u)(-)-X(3)Sigma(g)(-) and B''(3)Pi(u)-X(3)Sigma(g)(-) bands. Changes in product recoil velocity angular distributions and atomic angular momentum polarization were also measured as a function of dissociation wavelength. The results are compared with predictions from an adiabatic model for dissociation, which provides a basis for interpretation but does not explain quantitatively the experimental results.

A reanalysis of the A 1A"-X 1A' transition of CFBr.

The laser induced fluorescence spectrum of the A (1)A(")-X (1)A(') transition of CFBr is presented, with selected bands recorded at sub-Doppler resolution, allowing the rotational constants to be fully determined. Analysis of dispersed fluorescence spectra and the pattern of (79)Br/(81)Br isotope splittings indicate that the origin must be shifted from previous assignments in the literature to 23 271.0 cm(-1). This implies that only the lowest four vibrational levels in the A state have significant quantum yields for fluorescence, with all other levels strongly predissociated. Comparison with photofragment measurements implies that the A state is metastable, with a barrier to dissociation of approximately 1000 cm(-1).

Sub-Doppler spectroscopy of the A 2Sigma(+)-X 2Pi and B 2Pi-X 2Pi transitions of NCO.

Sub-Doppler laser induced florescence spectra were recorded of a selection of bands within the A (2)Sigma(+)-X (2)Pi and B (2)Pi-X (2)Pi transitions of NCO in a supersonic molecular beam. The light source was a diode seeded optical parametric oscillator, which gave an effective resolution of 0.01 cm(-1) in the ultraviolet. Analysis of the A (2)Sigma(+)-X (2)Pi transition at high resolution allowed fitting of both the fine and hyperfine structures, and a set of rotational and hyperfine constants were obtained for 13 vibronic levels within the A (2)Sigma(+) state including levels of both (2)Sigma(+) and (2)Pi vibronic characters. Analysis of the 0(0) (0) and 1(0) (1) bands of the B (2)Pi-X (2)Pi transition, together with a band from the A (2)Sigma(+) state at the same energy, was also performed. These did not reveal any hyperfine structure although the resolution was the same as the work at lower total energy, and this observation is discussed. Refined rotational constants and perturbation parameters for the interaction between the A and B states were also obtained.