Rovibrational Energy Transfer

Bhattachaijee and W. Forst, XVth Interaational Symposium on Combustion, The Combustion Institute, 1975, p. 681. 

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Fig. 3. Potential energy curves of I2 showing a typical state preparation and detection scheme for studies of rovibrational energy transfer.

Fig. 4. Spectra showing rovibrational level state preparation and collisional energy transfer. The upper trace demonstrates clean preparation of the l2(.X ), v = 23, J = 57 level under collision-free conditions. The middle and lower traces show the effects of rovibrational energy transfer induced by collisions with H2O (middle trace) and Ar (lower trace). The peaks marked with asterisks originate from levels populated by vibrational energy transfer.

Rovibrational Energy Transfer. Little work has been done as yet on the particular influence of combined rotational and vibrational energy transfer. The importance of this effect in the rovibrational relaxation of a diatomic molecule has been clearly shown for Hi, where the bottleneck of the flux is found to be near to the (v = 0, / = 10) -> (v = 1, / = 8) step. Similar importance may be expected in dissociation when the effective dissociation energies vary with angular momentum quantum number J. A factorization of the transition probability into a vibrational and rotational factor together with an exponential model for FG/i) has been employed in ref. 37. Experimental evidence shows that both contributions are in reality coupled. 

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). 

The vibrational heating efficiency of LiH molecules in collisions with He atoms was the subject of further study [34], The excitation and relaxation rates over a broad range of temperatures were reported, together with the average energy transfer indices. It was found that in spite of the weak nature of the van der Waals interaction, the strong anisotropy of the surface leads to rovibrational excitation rates which are larger, for example, than those exhibited by the He-CO [35] or He-N2 [36] systems. 

The quantitative description of ultracold molecule-molecule collisions is another challenging topic. The recent progress on the H2-H2 system will be difficult to implement for heavier systems due to the large number of rovibrational levels of the molecules. The study of H2-H2 collisions has shown that, for certain combinations of rovibrational levels, the energy transfer may occur to specific final rovibrational states. In such cases, the calculations can use a much smaller basis set without compromising the accuracy. 

A CO2 laser operates on the emission bands between vibrational combination states generating emission on discrete rovibrational transitions in the i>i and 2 2 3 bands, centred around 10.6 and 9.6 pm, respectively. Population inversion is achieved by collisional energy transfer from plasma-excited N2 to CO2, usually in a mixture with He. A particular rovibrational emission line can be selected using a rotatable diffraction grating incorporated in the laser cavity. CO2 lasers can achieve very high continuous-wave (cw) power levels of up to 100 W from commercially available systems. In addition, CO2 lasers are robust, narrow-bandwidth and low-cost systems well able to induce IRMPD, but a disadvantage is clearly its limited tunability. It should be noted that fixed-frequency CO2 lasers are used routinely in commercial MS platforms to induce dissociation as an alternative to CID. 

In fact, the / value usually does change during a rovibrational or rovibronic transition, even though the rotational state has relatively little impact on the overall transition energy. In electric-dipole transitions, the photon transfers one unit of angular momentum to the molecule, and this must be absorbed by the... 

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