John E. Dove

The Vibrational Relaxation of H2. I. Experimental Measurements of the Rate of Relaxation by H2, He, Ne, Ar and Kr,

Abstract The vibrational relaxation of gaseous H2 in mixtures with He, Ne, Ar, and Kr was studied by the laser Schlieren technique in incidents shock waves at 1350–3000 K. From the results of 155 experiments the following standard relaxation times for self-relaxation of H2 and relaxation of H2 by He, Ne, Ar and Kr were obtained: pτ is in atm s, and the qouted uncertainties are standard deviations. The results for H2/H2 and H2/Ar are in very good agreement with previous results of Kiefer and Lutz, and the extrapolated for H2/H2, H2/He and H2/Ar agree very well with low temperature data Ducuing. The linear mixture rule for a additivity of relaxation rates was found to hold, to within experimental accuracy, for the mixtures studied in the present work.

Direct study of energy transfer of vibrationally highly excited CS2 molecules

Collisional energy transfer at 300 K of vibrationally highly excited CS2 molecules (excitation energies 28 660 and 32 640 cm−1) is studied directly by time resolved UV absorption spectroscopy after laser excitation at 351 and 308 nm. Average energies 〈ΔE〉 transferred per collision for 27 bath gases are derived. A marked dependence of 〈ΔE〉 on the excitation energy is noted; the steepness of this dependence is not the same for all bath gases. The results are compared with energy transfer data for vibrationally highly excited large molecules.

Numerical Calculation of Vibrational Relaxation and Dissociation for a Quantum Anharmonic Oscillator

The vibrational relaxation and dissociation of gaseous H2 highly diluted in an argon heat bath have been studied numerically at temperatures between 2500 and 15 000°K. Collisional transition probabilities between the vibrational levels of H2 were calculated by the method of Schwartz, Slawsky, and Herzfeld in a form modified for a Morse oscillator. The master equation was solved by numerical integration to give the time dependence of the vibrational level populations and of dissociated molecules in a simulated shock‐wave experiment. The absolute values of the calculated rate coefficients for dissociation are in good agreement with experiment, but the predicted vibrational relaxation times are about a factor of 35 shorter than those determined experimentally. At all temperatures, the upper vibrational levels in the dissociating gas are substantially depleted below their equilibrium populations. The Arrhenius activation energy for dissociation is 102.5 kcal mole−1, slightly lower than the dissociation energy...

Excitation and dissociation of molecular hydrogen in shock waves at interstellar densities

The rate constant (kD) for the thermal dissociation of molecular parahydrogen by collisions with helium at kinetic temperatures of 2000-10,000 K is determined for the entire range of densities appropriate to interstellar molecular clouds. Included in a solution of the master equation for the dissociation reaction are collisional transitions among the rotation-vibration levels, collision induced dissociation out of all levels, and infrared quadrupole emission. The rate constant decreases strongly over the density range of 1 million-10,000/cu cm due to the competition between radiative and collisional processes. At low densities, kD approaches a constant value corresponding to direct collisional dissociation out of the (v = 0, J = 0) level. It is noted that the population distribution among the rotation-vibration levels can show large deviations from a Boltzmann distribution and that, under certain circumstances, there are even population inversions. 74 references.

Accurate abinitio potential energy computations for the H4 system: Tests of some analytic potential energy surfaces

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry, as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H4, using a multiple reference (single and) double excitation configuration interaction (MRD‐CI) program. Both systematic and ‘‘random’’ errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self‐consistent way ab initio energies calculated by Schwenke, bringing the number of H4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H3; adding ab initio energies calculated by other authors yielded a total of 772 ...

Ultraviolet spectra of vibrationally highly excited CS2 molecules

Vibrationally highly excited CS2 molecules with energies close to the dissociation energy of the electronic ground state have been prepared by laser excitation at 308 and 351 nm. The UV spectra recorded are compared with UV spectra from CS2 molecules excited thermally in shock waves at temperatures up to 4000 K. A numerical representation of the absorption coefficients as a function of the average excitation energy of the molecules is given.

A quasiclassical trajectory study of the collisional dissociation of H2 by He

Abstract Collision-induced dissociation of molecular H 2 by He was studied by the method of quasi-classical trajectories using the ab initio interaction pote

Rate of dissociation of molecular hydrogen by hydrogen atoms at very low densities

The collision-induced dissociation of H2 in the (v = 0, J = 0) internal state by H atoms was studied by three-dimensional quasi-classical trajectories on the Liu-Siegbahn-Truhlar-Horowitz ab initio potential energy surface. The dynamic threshold for dissociation was found to be essentially coincident with the energetic threshold. The dissociation cross sections were fitted to an excitation function and used to predict the rate constant for dissociation of H2 by H atoms by shock waves at very low interstellar densities. 16 references.

A quasiclassical trajectory study of molecular energy transfer in H2He collisions

Abstract Molecular energy transfer in collisions of H 2 with He was investigated by quasiclassical trajectory methods, using an ab initio interaction potential. All of the initial parameters were Monte Carlo selected except for the vibrational and rotational quantum numbers (υ, J ), the translational energy, and the initial separation of atom and molecule. A total of 150000 trajectories were calculated for 40 different (υ, J ) levels of para-H 2 , in many cases at a number of different translational energies. For low-lying initial states, rotational excitation is the predominant process. Direct vibrational excitation of non-rotating molecules has a high classical threshold energy and a relatively low cross section above threshold. However, molecular rotation strongly enhances the probability of collisional excitation of vibration. For highly excited molecules, the dominant process is collisional interconversion of rotational and vibrational energy, for which large cross sections are found. These findings indicate that the rotational degree of freedom participates in both vibrational relaxation and dissociation at high temperatures. The mechanism of these processes is discussed.

Competition between dissociation and exchange processes: Contrasting dynamical behaviors in collinear H+H2 and He+H+2 collisions

Dissociative, exchange, and nonreactive collisions of the H+H2 and He+H+2 systems in collinear geometry are examined. The behavior of the two systems is found to differ qualitatively and quantitatively. For H+H2 (v=0), quasiclassical trajectory (QCT) calculations on the Siegbahn–Liu–Truhlar–Horowitz surface show that the dynamic threshold energy (Edyth) for dissociation is twice the energetic threshold (Eeth). For v=1, the elevation of Edyth is slightly less. There is vibrational enhancement of collision induced dissociation (CID) near threshold, but slight vibrational inhibition at higher energies. At energies above that required for dissociation, a second threshold to exchange is observed and the exchange process eventually takes over from dissociation. For He+H+2 (v=0,1), QCT calculations on the McLaughlin–Thompson surface yield Edyth∼Eeth for dissociation, but also show an antithreshold, with the exchange process becoming dominant at a higher energy. There is only vibrational enhancement of the dissoc...