Vibrational energy relaxation of azulene studied by the transient grating method. II. Liquid solvents

The vibrational energy dissipation process of the ground-state azulene in various liquids has been studied by the transient grating spectroscopy. The acoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored. The temperature rise-time constant of the solvent has been determined both by the fitting of the acoustic signal to a theoretical model equation and by the analysis of the acoustic peak shift. We found that the temperature rise-time constants determined by the transient grating method in various solvents are larger than the vibrational energy relaxation time constants determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)]. The difference is explained by different energy dissipation pathways from azulene to solvent; vibrational-vibrational (V-V) energy transfer and vibrational-translational (V-T) energy transfer. The contribution of the V-V energy transfer is estimated in various liquid solvents from the difference between the temperature rise time and vibrational energy relaxation time, and the solvent V-T relaxation time.     

Nonequilibrium vibrational populations of diatomic species in electrical discharges: Effects on the dissociation rates

Dissociation rates of N₂ in N₂-H₂ mixtures and of H₂ in Ar-H₂ mixtures have been calculated for conditions typical of gas discharges. The results have been obtained by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The results show that these rates are much smaller than the corresponding ones for pure gases, as a consequence of the increased importance of V-T processes. A simplified model, which takes into account the main features of the distribution of vibrational levels in an electrical discharge, has been built up to rationalize the present results as well as those derived in our previous work.     

Vibrational energy relaxation of azulene studied by the transient grating method. I. Supercritical fluids

The vibrational energy dissipation process of the ground-state azulene in supercritical xenon, carbon dioxide, and ethane has been studied by the transient grating spectroscopy. In this method, azulene in these fluids was photoexcited by two counterpropagating subpicosecond laser pulses at 570 nm, which created a sinusoidal pattern of vibrationally hot ground-state azulene inside the fluids. The photoacoustic signal produced by the temperature rise of the solvent due to the vibrational energy relaxation of azulene was monitored by the diffraction of a probe pulse. The temperature-rise time constants of the solvents were determined at 383 and 298 K from 0.7 to 2.4 in rho(r), where rho(r) is the reduced density by the critical density of the fluids, by the fitting of the acoustic signal based on a theoretical model equation. In xenon, the temperature-rise time constant was almost similar to the vibrational energy-relaxation time constant of the photoexcited solute determined by the transient absorption measurement [D. Schwarzer, J. Troe, M. Votsmeier, and M. Zerezke, J. Chem. Phys. 105, 3121 (1996)] at the same reduced density irrespective of the solvent temperature. On the other hand, the temperature-rise time constants in ethane were larger than the vibrational energy-relaxation time constants by a factor of about 2. In carbon dioxide, the difference was small. From these results, the larger time constants of the solvent temperature rise than those of the vibrational energy relaxation in ethane and carbon dioxide were interpreted in terms of the vibrational-vibrational (V-V) energy transfer between azulene and solvent molecules and the vibrational-translational (V-T) energy transfer between solvent molecules. The contribution of the V-V energy transfer process against the V-T energy transfer process has been discussed.     

Progress in the non-equilibrium vibrational kinetics of hydrogen in magnetic multicusp H− ion sources

The non-equilibrium vibrational kinetics of H₂ in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H₂(v) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H⁻) shows a satisfactory agreement, thus indicating the basic correctness of our model.     

Nonequilibrium vibrational populations and dissociation rates of oxygen in electrical discharges

Nonequilibrium vibrational distributions and dissociation rates of molecular oxygen in both electrical and thermal conditions have been calculated by solving a system of master equations including V-V (vibration-vibration), V-T (vibration-translation) and e-V (electron-vibration) energy exchanges. The dissociation constant under thermal conditions (i.e. without electrons) follows an Arrhenius law with an activation energy of 120 kcal/mole, while the corresponding rates under electrical conditions (5000 ? T_e ? 15000 K, 300 ? T_g ? 1000 K, 10¹¹ ? n_e ? 10¹² cm^?3,5 ? p ? 20 torr) increase with decreasing gas (T_g) and electron (T_e) temperatures and pressure (p) and with increasing electron density (n_e). These results are explained on the basis of the different interplay of V-V and V-T energy exchanges and are rationalized by means of simplified models proposed in the literature. The accuracy of the present results is discussed paying particular attention to the dependence of V-V and V-T rate coefficients on the vibrational quantum number. A comparison of the calculated dissociation rates with the corresponding ones obtained by the direct electron impact mechanism shows that the present mechanism prevails at low electron and gas temperatures. Finally a comparison is shown between theoretical and experimental dissociation rates under electrical and thermal conditions.     

Low temperature vibrational relaxation of nH2 gas,liquid and solid

The vibrational relaxation time of the ⁿH₂ molecule has been measured as a function of density and temperature between 25 and 40 K in the gas and liquid phase, and at fixed density in the solid and liquid near the fusion point.     

Mode-to-mode vibrational energy transfer in D2CO: Evidence of coriolis-enhanced rotational selectivity

Infrared—ultraviolet double resonance spectroscopy is used to demonstrate rapid collision-induced V-V transfer between the v₆ and v₄ vibrational manifolds of D₂CO. The rate of transfer is at least gas-kinetic and is explained in terms of Coriolis coupling and rotationally specific, quasi-resonant relaxation channels     

A joint vibro-electronic mechanism in the dissociation of molecular hydrogen in nonequilibrium plasmas

A new mechanism Of H₂ dissociation in electrical discharges (10¹¹ ? n_e ? 10¹² cm^?3, 2.10^?16 ? E/N ? 3.10^?16 V cm², 300 ? T_g ? 1000 K, 3 ? p ? 30 torr) is presented and discussed. In this mechanism, called joint vibro-electronic mechanism (JVE), the electrons of the discharge create a strong vibrational disequilibrium with respect to the gas temperature (T_g) and promote electronic transitions from all vibrational levels of ¹Σ_g H₂ state to the repulsive ³Σ_u one. Moreover the V-V (vibration-vibration) and V-T (vibration-translation) energy exchanges are considered for building up the vibrational distribution of ¹Σ_g state. A complete set of e - D cross sections (e + H₂(¹Σ_g,ν) → e + H₂ (³Σ_u) → + 2H, ν = 0,14) is calculated by using an extension of the semiclassical Gryzinski theory in combination with the Franck-Condon principle. Dissociation rates calculated according to JVE are larger either than those obtained by the pure vibrational mechanism (PVM) discussed in our previous work or than those from the direct electronic impact mechanism (DEM) from the ground vibrational level. The behaviour of JVE rates as a function of gas temperature (T_g), of E/N, of electron density (n_e) and of pressure is then reported. The results show strong differences as compared, with the corresponding values obtained, with PVM. Finally the influence of the atoms as well as their recombination on the dissociation rates is discussed. The results have been obtained by solving a system of vibrational master equations.     

Vibrational Distributions of N2 (A3∑+u) in the nitrogen afterglow

Absolute populations of the lowest six vibrational levels of N₂ (A³∑⁺_u) have been measured under various conditions in the Lewis-Rayleigh afterglow of nitrogen for the first time. These vibrational distributions are interpreted in terms of a simple kinetic model which permits the evaluation of rate constants for vibrational relaxation of the A-state levels by V-V energy transfer to ground state nitrogen molecules.     

Temperature dependence of the vibrational deactivation of NO(ν = 1) and NO(ν = 2) by NO

The temperature dependence of the vibrational relaxation of NO(ν = 1) and NO(ν = 2) by NO has been investigated over the range 220–470K. The vibrationally excited NO was produced by the pulse radiolysis of dilute NO-Ar mixtures and temporal dependence of the NO(ν = 1) and NO(ν = 2) followed by UV absorption spectrophotometry. The results for the self-relaxation of NO(ν = 1) are in good agreement with previous measurements exhibiting a minimum in the relaxation rate constant near 300 K; however, the results for the V-V exchange between NO(ν = 2) and NO(ν = 0) are approximately a factor of two smaller that the results calculated from the relaxation rate constant of the reverse 1+1 process and detailed balance at 300 K. The temperature dependence of the V-V exchange rate constant is in good agreement with calculated estimates based on Rapp's SSH theory for V-V transfer. The long-range dipole-dipole formulation must be extended to other than ¹Σ state systems in order to explicitly take into account the influence of spin-orbit coupling on the state-to-state rate constants for vibrational relaxation.     

光学双共振研究苯的单振动能级弛豫

用红外-紫外激光双共振技术首次测定了苯的ν_(10), ν_(11), 3ν_(16), ν_4等8个单振动能级的弛豫速率。弛豫过程量现双指数衰减特性, 它们相当于V-V和V-T弛豫过程, 例如16_110_1能级的T-V和V-T弛豫速率常数分别为0.59×10~5和0.023×10~6 s~(-1)Torr~(-1)。     

SF6-UF6体系中六氟化铀的振动弛豫研究

本文以脉冲CO_2激光激发SF_6, 经碰撞再使UF_6分子激发, 通过测定振动受激UF_6的紫外吸收变化, 研究了六氟化铀分子的振动弛豫过程。对于2.0. Torr SF_6+2.0 Torr UF_6体系, 在220-320 nm范围内测定了UF_6的紫外吸收信号随时间的变化, 测得UF_6三类不同振动受激态分子间的V-V能量弛豫时间分别为6.8, 9.0和26 μs, 相应的V-T能量弛豫时间为42, 8和1 ms。此外, 还讨论了SF_6-UF_6体系中振动能量弛豫的机理。     

Laser isotope separation in SF6

Laser induced isotope separation in SF₆ and SF₆ mixtures has been investigated in a collisionally dominated pressure regime. Experimental results with SF₆/rare gas mixtures point out the importance of collision induced dissociation following the initial collisionless dissociation. Rotational relaxation induced by rare gases and H₂ is shown to play a signficant role in the dissociation of both isotopic species. Total vibrational relaxation (V-T/R) induced by H₂ as a collision partner is shown to dominate the dissociation efficiency of SF₆/H₂ mixtures.     

Vibration-vibration energy exchange between carbon monoxide and oxygen

The rates of V-V energy transfer for CO-O₂ in the temperature range 133 to 323°K were studied using a steady-state vibrational quenching technique. This work clears the discrepancy between previous available room temperature measurements, and demonstrates a linear dependence of log V-V exchange probability with log temperature.     

Features in the vibrational relaxation of laser excited Freon-22 molecules

The vibration-translation (V-T) of laser excited Freon-22 (CF₂HC1) molecules has been studied. An interferometric technique allows simultaneous measurement of the initial energy 〈E〉 stored in the molecules, and of the V-T relaxation time. Consequently, the V-T relaxation time and the energy released per collision can be determined as a function of the energy absorbed from the laser field. Three distinct regions have been observed for these dependences. This behaviour observed and reported by us for Freon-22 confirms the role played in the relaxation process by the initial distribution of the vibrational energy, and agrees qualitatively with the experimental results for other polyatomic molecules.     

Vibrational deactivation of DF

Rate constants for deactivation of DF in vibrational states n = 1 to 7 in collisions with DF (0) are calculated semiclassically in the temperature range from 300 K to 3000 K. The variation with the temperature agrees quite well with experiments for n = 1, although the theoretical rates are 30% lower than the experimental values for temperatures above 1200 K and 75% lower at 300 K. This difference is discussed. Single quantum transitions dominate. At 300 K the mechanism is predominantly a V—V transfer for n = 2 and a V-T/R transfer for n = 7, while both mechanisms contribute for n = 3–6. Collision complexes are important for both V—V and V-T/R energy transfer. Rotational relaxation times are calculated for HF and DF.     

The relaxation of HCN(011) by V-T,R and V-V energy transfer

Tuned output from an optical parametric oscillator has been used to excite HCN directly to its (011) level. By careful use of a “cold-gas filter”, it has proved possible to distinguish between the time-resolved fluorescence from HCN(011) and that from HCN(001) formed during collisional relaxation. Rate constants for relaxation from both levels have been obtained for the partners: (i) He, Ne, Ar and Kr, and (ii) HCN, CO₂, N₂O, OCS, CS₂, C₂H₂ and C₂D₂. With the rare gases, HCN(011) is relaxed to (001) by V-T,R energy transfer, with rate constants (cm³ molecule^?1 s^?1) at 298 ± 4 K of: k_He⁰¹¹ = (7.9 ± 1.05) × 10^?13; k_Ne⁰¹¹ = (1.56 ± 0.12) × 10^?13; k_Ar⁰¹¹ = (1.20 ± 0.17) × 10^?13; k_Kr⁰¹¹ = (6.7 ± 0.65) × 10^?14. The molecular collision partners also transfer HCN(011) to (001). The rates are much greater and clearly near-resonant V-V energy exchange is important. The results are compared to first-order Sharma-Brau theory, with fair agreement where near-resonant channels exist.     

Energy transfer pathways in gaseous systems containing CO2 elucidated by thermal lens studies

Pulsed source thermal lens measurement on CO₂ and CO₂/Ar, CO₂/N₂, CO₂/CO gaseous mixtures are reported. A theory relating the thermal lens signal to the V-V and V-T energy transfer processes in the mixtures has been developed and applied to interpret the measurements. The occurrence of convergent and divergent thermal lens signals related to endothermic and exothermic effects in the gas have been quantitatively explained.     

The vibrational relaxation of I2 by H2 in a supersonic jet

The vibrational relaxation of I₂ by H₂ has been studied in a supersonic free jet. It was observed that the addition of 5% H₂ to the helium carrier gas greatly reduces the concentration of X ¹Σ⁺_g(ν″ = 1) I₂ in the jet as compared to the concentration in a pure helium carrier. From this observation we have determined that the average vibrational relaxation cross sections of H₂ is 7.1 times as large as that of helium. Since the average vibrational relaxation cross section of deuterium is at least as large as that of hydrogen, the mechanism responsible for this phenomenon appears not to be dominated by mass effects.     

Semiclassical three-dimensional model for vibrational energy transfer in diatomic molecules

A semiclassical model for calculation of rate constants for vibrational excitation in diatomic gases at low temperatures (below 1000 K) is suggested. The model has been tested by its ability to predict the relaxation times of hydrogen (τ_H1 in the temperature region 40–1000 K. The agreement with experimental values is excellent. The isotopic ratio τ_D2/τ_H2 as a function of temperature is predicted.