Vibrational-to-rotational energy transfer

Figure 10 A typical trajectory showing rotational excitation accompanying vibrational de-excitation (i.e. a vibration to rotational energy transfer) [71]. The top panel shows the evolution in the Z (molecule-surface distance) and r (molecular bond length) coordinates. In the lower panel, the motion is projected onto the r — 0 (molecular bond orientation) plane. Coupling of vibrations and rotations occurs because the molecule attempts to dissociate at an unfavourable bond angle.

E.E.Nikitin and M.Ya.Ovchinnikova, An analytical model of vibrational-to-rotational energy transfer for highly anisotropic interaction between an atom and a diatomic molecule, Khim. Fiz. 5,291 (1986)... 

E.E.Nikitin, S.Ya.Umanskii. and D.V.Shalashilin, Vibration-to-rotation energy transfer in collisions of hydrogen halide molecules with noble gas atoms. Khim. Fiz. 8. 1011 (1989). 

Collision-induced intramolecular vibration-to-rotation energy transfer appears to be inefficient. The evidence for this inference comes from the study of rotational contours in the one collision-induced transition 7 0° in glyoxal. It is found that the emission from 0° has a distribution over rotational transitions that is close to the thermal distribution. But the vibration v-j in glyoxal is a torsional motion, and the axis of torsion very nearly coincides with the smallest axis of inertia of the molecule, so if collision-induced intramolecular vibra-tion-to-rotation transfer were efficient the emission from 0 should have a nonthermal distribution in the quantum number K (which describes quantization of the motion about the smallest axis of inertia). Note, however, that the collision partner used in this experiment was... 

The photofragmentation that occurs as a consequence of absorption of a photon is frequently viewed as a "half-collision" process (16)- The photon absorption prepares the molecule in assorted rovibrational states of an excited electronic pes and is followed by the half-collision event in which translational, vibrational, and rotational energy transfer may occur. It is the prediction of the corresponding product energy distributions and their correlation to features of the excited pes that is a major goal of theoretical efforts. In this section we summarize some of the quantum dynamical approaches that have been developed for polyatomic photodissociation. For ease of presentation we limit consideration to triatomic molecules and, further, follow in part the presentation of Heather and Light (17). 

With few exceptions, the experimental techniques that have been devised to study molecular vibrational and rotational energy transfer depend on the measurement of a relaxation time for a gas that is disturbed from its equilibrium condition. In the simplest case, one need consider only a single excitation mechanism in a pure two-state gas ... 

To summarize, two important factors controlling the rates of energy transfer involving vibration and rotation are resonance and long-range interactions. The combination of lasers and microwave techniques can be used to study a wide array of vibrational and rotational energy transfer processes. 

The simple model leading to Eqs. (14.2) and (14.3) eompletely neglects the transfer of vibrational to rotational energy. This would be justified at best for collisions of homonuclear molecules (H2, Ng, Og) or heteronuclear, slightly asymmetric molecules (CO). It certainly must not be assumed that vibrational transitions in these molecules are not accompanied by rotational transitions [394]. However, the latter have a relatively small effect on the average transition... 

The effects of molecular structure on the rates of energy transfer are manifested in many ways. Additional pathways are available in collisions with molecules, such as the influence of ionic potential surfaces [35], the availability of near resonant electronic-to-vibrational and rotational energy transfer pathways [36], and the introduction of nonadiabatic transitions due to the breaking of the molecular orbital symmetry [37]. For the studies considered here, we might also add the competition between reaction and the desired energy transfer process, the possibility of energy transfer processes in the entrance or exit channels, selective changes in... 

The characteristic energy values for vibrational to vibrational as well as rotational to rotational energy transfer and their weak dependence on temperature relates to the phenomenon of resonance. Transfer becomes less effective when the difference in energies of the modes involved becomes larger. This follows from so-called surprisal analysis (a statistical method) and is due to decreasing cross sections when mode frequencies v become very different. According to surprisal analysis the rate constant for transitions between states labeled v and v depends Boltzmann-like on the energy difference between them ... 

Resonant rotational to rotational (R-R) energy transfer may have rates exceeding the Leimard-Jones collision frequency because of long-range dipole-dipole interactions in some cases. Quasiresonant vibration to rotation transfer (V-R) has recently been discussed in the framework of a simple model [57]. 

My own research efforts [4] have, for many years, involved taking into account such non-Bom-Oppen-heimer couplings, especially in cases where vibration/rotation energy transferred to electronic motions causes... 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

Energy transfer occurring in nonreactive neutral-neutral collisions is a very active field of investigation.230 Important contributions to the understanding of collisional energy-transfer processes have also resulted from various studies of nonreactive ion-neutral collisions. The modes of energy transfer that have been investigated for the latter interactions include vibrational to relative translational (V-T), vibrational to vibrational (V-V), translational to vibrational (T-V), translational to rotational (T-R), vibrational to rotational (V-R), translational to electronic (T-E), and electronic to translational (E-T). 

---