Fast multiquantum vibrational relaxation of highly vibrationally excited O2

Experimental study of rotational relaxation for D2(1,12) in collisions with N2

The rotational relaxation behavior of D2(1,12) in a D2-N2 mixture was investigated using coherent anti-Stokes Raman scattering (CARS) technique. The rovibrational level v = 1 and J = 12 of D2 was selectively excited through stimulated Raman pumping while monitoring the temporal evolution of population for D2(1, J ≤ 12) molecules using time-resolved CARS spectroscopy. The results demonstrate that the rotational relaxation processes of D2(1,12) encompass both multi-quantum relaxation and continuous single-quantum relaxation. When α, the molar ratio of N2, is less than 0.5, D2(1,12) predominantly undergoes a single quantum relaxation process transition. However, when α ≥ 0.5, the multi-quantum relaxation mechanism gradually predominates. The total rotational relaxation rate coefficients of D2(1,12) collisions with N2 and D2 at 295 K were determined to be 3.974 × 10-14 and 1.179 × 10-14 cm3 s-1, respectively. The temperature dependence of rotational relaxation rate of D2(1,12) was investigated within the temperature range of 295-453 K. With increasing temperature, the dominant relaxation process exhibited an accelerated behavior, while the minor relaxation process remained largely unaffected. The rotational temperature of the D2 molecule at various N2 molar ratios was determined through the utilization of Boltzmann plots. The rotational temperature undergoes a rapid decline within 2 μs, corresponding to the near-resonant rotation-vibration relaxation process of D2(1,12) collisions with N2. The system reaches a quasi-equilibrium state when the delay time is 3 μs. The findings of this study can serve as a valuable empirical basis for further validation of the kinetic theory and simulation.

Energy exchange rate coefficients from vibrational inelastic O2(Σg-3) + O2(Σg-3) collisions on a new spin-averaged potential energy surface

A new spin-averaged potential energy surface (PES) for non-reactive O₂(Σg-3) + O₂(Σg-3) collisions is presented. The potential is formulated analytically according to the nature of the principal interaction components, with the main van der Waals contribution described through the improved Lennard-Jones model. All the parameters involved in the formulation, having a physical meaning, have been modulated in restricted variation ranges, exploiting a combined analysis of experimental and ab initio reference data. The new PES is shown to be able to reproduce a wealth of different physical properties, ranging from the second virial coefficients to transport properties (shear viscosity and thermal conductivity) and rate coefficients for inelastic scattering collisions. Rate coefficients for the vibrational inelastic processes of O₂, including both vibration-to-vibration (V-V) and vibration-to-translation/rotation (V-T/R) energy exchanges, were then calculated on this PES using a mixed quantum-classical method. The effective formulation of the potential and its combination with an efficient, yet accurate, nuclear dynamics treatment allowed for the determination of a large database of V-V and V-T/R energy transfer rate coefficients in a wide temperature range.

Quantum state-resolved, bulk gas energetics: Comparison of theory and experiment

Until very recently, the computational model of state-to-state energy transfer in large gas mixtures, introduced by the author and co-workers, has had little experimental data with which to assess the accuracy of its predictions. In a novel experiment, Alghazi et al. [Chem. Phys. 448, 76 (2015)] followed the equilibration of highly vibrationally excited CsH(D) in baths of H2(D2) with simultaneous time- and quantum state-resolution. Modal temperatures of vibration, rotation, and translation for CsH(D) were obtained and presented as a function of pump-probe delay time. Here the data from this study are used as a test of the accuracy of the computational method, and in addition, the consequent changes in bath gas modal temperatures, not obtainable in the experiment, are predicted. Despite large discrepancies between initial CsH(D) vibrational states in the experiment and those available using the computational model, the quality of agreement is sufficient to conclude that the model's predictions constitute at least a very good representation of the overall equilibration that, for some measurements, is very accurate.

State-resolved collisional relaxation of highly vibrationally excited CsH by CO2

Quenching of highly vibrationally excited CsH(X(1)Σ(+), v=15-23) by collisions with CO2 was investigated. A significant fraction of the initial population of highly vibrationally excited CsH(v=22) was relaxed to a low vibrational level (Δv=-5). The near-resonant 5-1 vibration-to-vibration (V-V) energy was efficiently exchanged. The rate constants for the rotational levels of CO2(00(0)0) [J=36-60] and CO2(00(0)1) [J=5-31] from the collisions with excited CsH were determined. The experiments revealed that the collisions resulting in CO2(00(0)0) were accompanied by substantial excitation in rotation and translation. The vibrationally excited CO2(00(0)1) state exhibited rotational and translational energy distributions near those of the initial state. The total quenching rates relative to the probed state of excited CsH were determined for both CO2 states. The corresponding data indicated that the gains in the rotational and translational energies in CO2 were sensitive to the collisional depletion of excited CsH.

Energy-dependence of vibrational relaxation between highly vibrationally excited KH (X1Σ+, ν"=14-23) and H2, and N2

Vibrational state total relaxation rate coefficients, k(ν") (M), for KH (ν"=14-23) by M=H(2) and N(2) have been investigated in an overtone pump-probe configuration. At ν"=14, 15, 16 and 17, the rate coefficients k(ν)(″) (M) increase linearly with vibrational quantum number. The region (ν"=18, 19, 20 and 21) where the dependence is much stronger than linear has significant contribution from multiquantum (Δν≥2) relaxation. For ν"=18, 19, 20 and 21, 0.25, 0.31, 0.38 and 0.31 of the initially prepared population undergo two-quantum (Δν=2) vibrational relaxation in KH (ν")+H(2) collisions. In KH (ν")+N(2), the time profile of ν"=14(15) after preparation of ν"=19(20) was measured. A clear bimodal distribution is observed. The time scale of the first peak is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential single-quantum relaxation mechanism can be explicitly ruled out. Relaxation of KD with D(2) has been also investigated. The relaxation rate coefficients exhibit distinct maxima for both isotopes (KH and KD). We discuss possible explanation of the experimental results including mass effect, V-R energy transfer and V-V energy transfer.

Mechanically induced generation of highly reactive excited-state oxygen molecules in cluster scattering

Molecular electronic excitation in (O(2))(n) clusters induced by mechanical collisions via the "chemistry with a hammer" is investigated by a combination of molecular dynamics simulations and quantum chemistry calculations. Complete active space self-consistent field augmented with triple-zeta polarizable basis set quantum chemistry calculations of a compressed (O(2))(2) cluster model in various configurations reveal the emergence of possible pathways for the generation of electronically excited singlet O(2) molecules upon cluster compression and vibrational excitation, due to electronic curve-crossing and spin-orbit coupling. Extrapolation of the model (O(2))(2) results to larger clusters suggests a dramatic increase in the population of electronically excited O(2) products, and may account for the recently observed cluster-catalyzed oxidation of silicon surfaces, via singlet oxygen generation induced by cluster impact, followed by surface reaction of highly reactive singlet O(2) molecules. Extensive molecular dynamics simulations of (O(2))(n) clusters colliding onto a hot surface indeed reveal that cluster compression is sufficient under typical experimental conditions for nonadiabatic transitions to occur. This work highlights the importance of nonadiabatic effects in the "chemistry with a hammer."

Kinetic study of vibrational energy transfer from a wide range of vibrational levels of O2(X(3)Sigma(g)-, v = 6-12) to CF4

A wide range of vibrational levels of O2(X(3)Sigma(g)(-), v = 6-13) generated in the ultraviolet photolysis of O3 was selectively detected by the laser-induced fluorescence (LIF) technique. The time-resolved LIF-excited B(3)Sigma(u)(-)-X(3)Sigma(g)(-) system in the presence of CF4 has been recorded and analyzed by the integrated profiles method (IPM). The IPM permitted us to determine the rate coefficients k(v)(CF4) for vibrational relaxation of O2(X(3)Sigma(g)(-), v = 6-12) by collisions with CF4. Energy transfer from O2 (v = 6-12) to CF4 is surprisingly efficient compared to that of other polyatomic relaxation partners studied so far. The k(v)(CF4) increases with vibrational quantum number v from [1.5 +/- 0.2(2sigma)] x 10(-12) for v = 6 to [7.3 +/- 1.5(2sigma)] x 10(-11) for v = 12, indicating that the infrared-active nu3 vibrational mode of CF4 mainly governs the energy transfer with O2(X(3)Sigma(g)(-), v = 6-12). The correlation between the rate coefficients and fundamental infrared intensities has been discussed based on a comparison of the efficiency of energy transfer by several collision partners.

Spin-orbit coupling in O2(v)+O2 collisions. II. Quantum scattering calculations on dimer states involving the XΣg−3, aΔg1, and bΣg+1 states of O2

The dynamics of collisional deactivation of O(2)(X (3)Sigma(g) (-),v=20-32) by O(2)(X (3)Sigma(g) (-),v(')=0) is investigated in detail by means of quantum-mechanical calculations. The theoretical approach involves ab initio potential energy surfaces correlating to the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) states of O(2) and their corresponding spin-orbit couplings [F. Dayou, M. I. Hernandez, J. Campos-Martinez, and R. Hernandez-Lamoneda, J. Chem. Phys. 123, 074311 (2005)]. Accurate Rydberg-Klein-Rees potentials are included in order to improve the description of the vibrational structure of the fragments. The calculated Boltzmann-averaged depletion probabilities display a dependence with v in good agreement with experimental measurements. The onset of the vibrational-to-electronic (V-E) depletion mechanism is noticeable for v>/=26, and it is due to energy transfer to both a (1)Delta(g) and b (1)Sigma(g) (+) states of the diatom. For O(2)(X (3)Sigma(g) (-),v=28), a further and sharp increase in the removal probabilities is caused by a near degeneracy with the O(2)(b (1)Sigma(g) (+),v=19) vibrational state. Analysis of the temperature dependence of the Boltzmann-averaged probabilities indicates a transition from the vibrational-to-translational to the V-E energy transfer regime, which can be traced back to the behavior of the inelastic probabilities as functions of kinetic energy. Furthermore, branching ratios for outcomes through the three different electronic states show a strong propensity towards populating a unique vibrational level within each electronic state. These results provide supported evidence that spin-orbit couplings account for a large portion of the "dark channel" reported in total depletion measurements. New insight for further experimental and theoretical investigations is also given.

Vibrational relaxation of O2(X 3 Σ–g, v = 9–13) by collisions with O2

Vibrationally excited O(2)(X(3) Sigmag(-)) was generated in the UV laser flash photolysis of O(3) and single vibrational level was detected via laser-induced fluorescence (LIF) in the B(3) Sigmau(-)-X(3) Sigmag(-) system. The time-resolved LIF of adjacent vibrational levels has been analyzed by the integrated-profiles method and the rate coefficients for single-quantum relaxation, O(2)(X(3)Sigmag(-), v = 9-13)+ O(2)(v = 0)--> O(2)(X(3)Sigmag(-), v - 1)+ O(2)(v = 1), have been determined. To the best of our knowledge, the rate coefficients for v = 12 and 13 are measured for the first time in the present study. The efficiency of relaxation is higher at lower vibrational levels, indicating that a small energy mismatch is suitable for the energy transfer. The vibrational level dependence of all the rate coefficients for the relaxation measured in the present study and previously reported by several groups can be rationalized by the energy gap law.

Nonadiabatic ladder climbing during molecular collisions

Combining classical molecular dynamics simulations with high level, multiconfigurational ab initio calculations, we demonstrate that even relatively mild collisions between ground state oxygen molecules can readily lead to the formation of highly reactive singlet oxygen molecules via a novel "ladder climbing" mechanism. We employ our findings to shed some light on two recent experiments that have remained poorly understood until now. The first one concerns the highly efficient cluster-catalyzed etching of silicon surfaces, whereas the second one involves a yet to be explained "dark channel" observed for the ozone photolysis in the stratosphere.

Efficient vibrational relaxation of O2(X 3sigma(g)-, nu = 8) by collisions with CF4

A laser flash photolysis-laser-induced fluorescence (LIF) technique has been employed to study the relaxation kinetics of vibrationally excited O2(X 3sigma(g)-. The time-resolved LIF excited B 3sigma(u)(-)-X 3sigma(g)- system has been recorded and analyzed by the integrated-profiles method. The rate coefficient for vibrational relaxation of O2(X 3sigma(g)-, nu = 8) by collisions with CF(4), [1.4 +/- 0.3(2sigma)] x 10(-11) cm3 molecule(-1) s(-1), indicates that CF4 is an efficient relaxant of O2(X 3sigma(g)- and that the propensity rule for O2 relaxation suggested by Mack et al. (J. A. Mack, K. Mikulecky and A. M. Wodtke, J. Chem. Phys., 1996, 105, 4105) has been observed experimentally.

Spin-orbit coupling in O2(υ)+O2 collisions: I. Electronic structure calculations on dimer states involving the XΣg−3, aΔg1, and bΣg+1 states of O2

The importance of vibrational-to-electronic (V-E) energy transfer mediated by spin-orbit coupling in the collisional removal of O2(X 3Sigmag-,upsilon>or=26) by O2 has been reported in a recent communication [F. Dayou, J. Campos-Martinez, M. I. Hernandez, and R. Hernandez-Lamoneda, J. Chem. Phys. 120, 10355 (2004)]. The present work provides details on the electronic properties of the dimer (O2)2 relevant to the self-relaxation of O2(X 3Sigmag-,upsilon>>0) where V-E energy transfer involving the O2(a 1Deltag) and O2(b 1Sigmag+) states is incorporated. Two-dimensional electronic structure calculations based on highly correlated ab initio methods have been carried out for the potential-energy and spin-orbit coupling surfaces associated with the ground singlet and two low-lying excited triplet states of the dimer dissociating into O2(X 3Sigmag-)+O2(X 3Sigmag-), O2(a 1Deltag)+O2(X 3Sigmag-), and O2(b 1Sigmag+)+O2(X 3Sigmag-). The resulting interaction potentials for the two excited triplet states display very similar features along the intermolecular separation, whereas differences arise with the ground singlet state for which the spin-exchange interaction produces a shorter equilibrium distance and higher binding energy. The vibrational dependence is qualitatively similar for the three studied interaction potentials. The spin-orbit coupling between the ground and second excited states is already nonzero in the O2+O2 dissociation limit and keeps its asymptotic value up to relatively short intermolecular separations, where the coupling increases for intramolecular distances close to the equilibrium of the isolated diatom. On the other hand, state mixing between the two excited triplet states leads to a noticeable collision-induced spin-orbit coupling between the ground and first excited states. The results are discussed in terms of specific features of the dimer electronic structure (including a simple four-electron model) and compared with existing theoretical and experimental data. This work gives theoretical insight into the origin of electronic energy-transfer mechanisms in O2+O2 collisions.

The dynamics of "stretched molecules": experimental studies of highly vibrationally excited molecules with stimulated emission pumping

We review stimulated emission pumping as used to study molecular dynamics. The review presents unimolecular as well as scattering studies. Topics include intramolecular vibrational redistribution, unimolecular isomerization and dissociation, van der Waals clusters, rotational energy transfer, vibrational energy transfer, gas-surface interactions, atmospheric effects resulting from nonequilibrium vibrational excitation, and vibrational promotion of electron transfer.