Vibrational state distributions

Hamilton C E, Bierbaum V M and Leone S R 1985 Product vibrational state distributions of thermal energy charge transfer reactions determined by laser-induced fluorescence in a flowing afterglow Ar" + CC -> CC (v= 0-6) + Ar J. Chem. Rhys. 83 2284-92... 

This lineshape analysis also implies tliat electron-transfer rates should be vibrational-state dependent, which has been observed experimentally [44]- Spin-orbit relaxation has also been identified as an important factor in controlling tire identity of botli electron and vibrational-state distributions in radiationless ET reactions. 

Differential cross-sections for particular final rotational states (f) of a particular vibrational state (v ) are usually smoothened by the moment expansion (M) in cosine functions mentioned in Eq, (38). Rotational state distributions for the final vibrational state v = 0 and 1 are presented in [88]. In each case, with or without GP results are shown. The peak position of the rotational state distribution for v = 0 is slightly left shifted due to the GP effect, on the contrary for v = 1, these peaks are at the same position. But both these figures clearly indicate that the absolute numbers in each case (with or without GP) are different. 

For a useful separation of pathways, the variation in final state distributions within each pathway must be at least somewhat smaller than the variation between pathways. The aforementioned dissociation of H2CO provides a perfect example of this technique, in which the H2 produced through the three-center ehmination leads to extensive rotational excitation of CO, with only moderate vibrational excitation of H2. By contrast, the competing pathway involving roaming of one H atom leaves much less energy in CO rotation, with very significant vibrational excitation of H2 [8]. 

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

The conclusion that highly vibrationally excited H2 correlated with low-7 CO represents a new mechanistic pathway, and the elucidation of that pathway, is greatly facilitated by comparison with quasiclassical trajectory calculations of Bowman and co-workers [8, 53] performed on a PES fit to high level electronic structure calculations [54]. The correlated H2 / CO state distributions from these trajectories, shown as the dashed lines in Fig. 8, show reasonably good agreement with the data. Analysis of the trajectories confirms that the H2(v = 0—4) population represents dissociation over the skewed transition state, as expected. 

Figure 12. Potential energy contour plots for He + I Cl(B,v = 3) and the corresponding probability densities for the n = 0, 2, and 4 intermolecular vibrational levels, (a), (c), and (e) plotted as a function of intermolecular angle, 0 and distance, R. Modified with permission from Ref. 40. The I Cl(B,v = 2/) rotational product state distributions measured following excitation to n = 0, 2, and 4 within the He + I Cl(B,v = 3) potential are plotted as black squares in (b), (d), and (f), respectively. The populations are normalized so that their sum is unity. The l Cl(B,v = 2/) rotational product state distributions calculated by Gray and Wozny [101] for the vibrational predissociation of He I Cl(B,v = 3,n = 0,/ = 0) complexes are shown as open circles in panel (b). Modified with permission from Ref. [51].

Gray and Wozny [101, 102] later disclosed the role of quantum interference in the vibrational predissociation of He Cl2(B, v, n = 0) and Ne Cl2(B, v, = 0) using three-dimensional wave packet calculations. Their results revealed that the high / tail for the VP product distribution of Ne Cl2(B, v ) was consistent with the final-state interactions during predissociation of the complex, while the node at in the He Cl2(B, v )Av = — 1 rotational distribution could only be accounted for through interference effects. They also implemented this model in calculations of the VP from the T-shaped He I C1(B, v = 3, n = 0) intermolecular level forming He+ I C1(B, v = 2) products [101]. The calculated I C1(B, v = 2,/) product state distribution remarkably resembles the distribution obtained by our group, open circles in Fig. 12(b). 

The time-of-flight spectrum of the H-atom product from the H20 photodissociation at 157 nm was measured using the HRTOF technique described above. The experimental TOF spectrum is then converted into the total product translational distribution of the photodissociation products. Figure 5 shows the total product translational energy spectrum of H20 photodissociation at 157.6 nm in the molecular beam condition (with rotational temperature 10 K or less). Five vibrational features have been observed in each of this spectrum, which can be easily assigned to the vibrationally excited OH (v = 0 to 4) products from the photodissociation of H20 at 157.6 nm. In the experiment under the molecular beam condition, rotational structures with larger N quantum numbers are partially resolved. By integrating the whole area of each vibrational manifold, the OH vibrational state distribution from the H2O sample at 10 K can be obtained. In... 

In order to see the effect of the rotational excitation of the parent H2O molecules on the OH vibrational state distribution, the experimental TOF spectrum of the H atom from photodissociation of a room temperature vapor H2O sample has also been measured with longer flight distance y 78 cm). By integrating each individual peak in the translational energy spectrum, the OH product vibrational distribution from H2O photodissociation at room temperature can be obtained. 

The OH product vibrational state distributions obtained from the above experimental studies are listed in Table 1. From the results obtained under the two extreme conditions (molecular beam and room temperature vapor)... 

Rotational state distributions of the OH(A) product for v = 0 to 3 have also been determined. Highly rotationally excited OH(A,v = 0,1) products are dominant as in the ground state, indicating that the angular anisotropy of the potential is also very important to the production of these product states on the H2O B lA state surface. The vibrational distribution... 

Fig. 25. The total OH product vibrational and rovibrational state distributions for the...

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