Translational excitation

Actually, collisions in which tlie batli becomes vibrationally excited are relatively rare, occurring witli a typical probability of 1% per gas-kinetic collision [6, 8, H and 13]. More common are processes tliat produce rotational and translational excitation in tlie batli acceptor while leaving tlie molecule in its ground (vibrationless) OO O state. 

Now let us consider tire implications of tliese results for energy transfer. First we recognize tliat tliere is no directed energy transfer of tire fonn considered in the incoherent case. Molecules in tire dimer cannot be recognized as well defined separate entities tliat can capture and translate excitation from one to anotlier. The captured excitation belongs to tire dimer, in otlier words, it is shared by botli molecules. The only counteriDart to energy migration... 

Figure 3.24. The rotational quadrupole alignment AJ asa function of translational energy /i ralls = Ef of D2 desorbing from Cu(lll) in the D2(v = 0, J = 11) quantum state (points connected by solid line) and in the D2(v = 1, J = 6) quantum state (points connected by dashed lines). The inset shows the same results plotted against Ef — E0(v, J), where E0(v, J) is the center of the S -shaped state-resolved translational excitation function obtained from unpolarized Df(Ef, v, J, Ts) measurements. From Ref. [421].

Comparison of the collision numbers given above for vibration-vibration transfer with those for vibration-translation, given in Section 4, shows that in many cases vibration-vibration transfer between two resonant or near-resonant modes is much more efficient than vibration-translation transfer from either. This applies equally to homomolecular and heteromolecular collisions, and carries the interesting consequence that the quickest route for vibrational excitation of upper levels from the ground level by homomolecular collisions is an initial vibration-translation excitation to the v = 1 level, followed by successive vibration-vibration transfers to higher levels. Because of the selection rule, Av = 1,... 

The study of the reactions of excited species is becoming an increasingly important area of research in kinetics [49, 50]. The excitation may take the form of enhanced translational, rotational, vibrational or electronic energy. Reactions with translational excitation are most commonly studied under molecular beam conditions using seeded nozzle beams or other types of sources to provide the enhanced energy [51, 52]. Translationally hot atoms may also be generated by nuclear recoil [53] or photodissociation [ 54 ]. 

Fig. 8. Three contour plots of the detailed rate constants k(V, R, T) for H + Cl2. The left-hand plot is for the room temperature reaction, the upper right-hand plot is for translationally excited reagents and the lower right-hand plot is for vibrationally excited reagents, H + Cl2 (u => 1). (Reproduced from ref. 196 by permission of the authors and the Royal Society of Chemistry.)...

Experiments have also been performed to study the effect of reagent translation excitation on the reactivity of K + HC1 [284] and the effect of reagent vibrational and rotational energy for Na + HF, HC1 [65, 285] and K + HC1 [286, 287]. These experiments have not probed the effect on the energy disposal of the increase in reagent energy. The polarisation of the MX product angular momentum has been studied [288] for K, Cs + HBr, HI. 

The experiments discussed at the end of the previous section provided information about the translational excitation of the products of unimolecular fragmentation of energized species formed in association reactions. The distributions of vibrational energy in the products of some reactions of this, and related, types have been determined by chemical laser measurements and by observations of infrared chemiluminescence. Some of these studies were referred to in Section III.C, other reactions have been studied more recently [388-392], In all of these investigations, the product which has been observed is HF or HC1 formed in what is frequently termed a snap-out reaction. These processes require that, almost simultaneously, two bonds break, the HX bond forms, and the order of a bond in the other product is increased. The reverse reaction, a four-centre (bimolecular) one, has a high activation barrier, so in the snap-out process a considerable proportion of the total energy is released after the system passes through the activated state. Thus reaction (120)... 

Wc have seen from consideration of the hard sphere model that translational energy is very quickly degraded. Consequently we should not expect to find that translationally excited species will persist long enough to be of importance in a kinetic scheme except for v(ny light mass( s among extremely heavy masses (i.e., very small 6). The reason for this exception (low 6) can be found in Eq. (Xlll.16.3), where it is seen that, as d becomes very small, the fractional energy transfer becomes small also"" and at a rate faster than 6. 

J or one of the lower-lying singlet states. He further suggests that at 1849 A the sequence is Hg -f O2— 02 PS ] 4- Hg, followed by predissociation of the excited O2 into two translationally excited pP] O atoms. The evidence is again very indirect. The latter proposal would be in conflict with the chain decomposition proposed above. 

The tandem-in-time instruments are mostly ion-trapping devices, including ion trap and FT-ICR. They operate in a time sequence in the scan function to yield MS/MS data, mostly product ion spectra. No additional mass analyzer is required. In the case of an ion trap, the scan function begins with the isolation of ions of interest with ejection of all other ions from the ion trap, followed by (a) translational excitation of ions by applying a supplementary RF voltage to the trap and (b) mass analysis of the product ions using resonant ejection. 

A common goal in reaction dynamics is to investigate the effects of specific reagent excitation on the rate or dynamics of a reaction. Translational excitation, especially of hydrogen atoms, can be produced by laser... 

OH(X H) internal and translational excitations under bulk and complexed conditions... 

Thus, both OH rotational and translational excitations are much lower with N2O-HX complexes than under bulk conditions, while the NH channel shows no such striking change. Why is there such a difference After all, like HO-X, the HN-X interaction is strongly attractive. However, for the case of HN-NO fragmentation, X-NO is also attractive, which is more favorable geometrically than HN-X once the H atom has shifted to the terminal nitrogen. Also, the HNNO lifetime may lead to sufficient HNNO -X distance at the point of fragmentation that HN-X exit channel interactions are lessened. No such unimolecular decomposition lifetime is anticipated for the OH + N2 channel. 

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