OH vibrational excitation

An experiment that may be simpler in its reactions, more sensitive than the detection limit of O isotope exchange need to explain the discrepancy, and as free as possible from OH vibrational excitation, would be helpful to explore the issue further. On the other hand the isotope effect is very small and the system sufficiently complicated that the theoretical calculations themselves may be the source of the discrepancy. In either case we infer that the reaction itself is mass anomalous rather than mass-independent in the strict sense of the term, both experimentally and in terms of current theoretical insight. In concluding this section we comment in physical terms on the small isotope effects, about —5 to —10 per mil, observed for and in this system at low pressures. 

The dynamics of the reactions of 0( P) with cyclohexane, cyclohexene, and cyclohexa-1,4-diene have been studied by measurement of the product OH(X II) internal state distributions in a molecular beam/LIF apparatus. The rotational state distributions were found to be similar for all three reactions and consistent with small (1—3%) partitioning of the available energy, indicating that H-abstraction occurs only when the O atom is collinear with the C-H bond under attack. Comparisons with model predictions suggested that some of the extra energy available in the more exoergic reactions between 0( P) and the unsaturated hydrocarbons is released into internal excitation of the hydrocarbon radical product, resulting in only a modest increase in OH vibrational excitation. 

OH vibrational excitation varies with precursor and photolysis wavel-... 

The major changes observed in the OH-Ar potential energy surface upon electronic excitation of OH prompted us to examine the dynamics occurring on these surfaces. The unimolecular dissociation dynamics of OH-Ar complexes were investigated by preparing the complexes with one quantum of OH vibrational excitation (vqh), which is more than sufficient energy to break the OH-Ar bond. We have found that the resultant vibrational predissociation dynamics of OH-Ar differ enormously in its ground and excited electronic states. ... 

Fig. 5. Dispersed fluorescence spectrum from OH A(v =0) products following vibrational predissociation of OH-Ar complexes with one quantum of OH vibrational excitation.

This aspect of the reaction has been discussed in detail elsewhere [52,60,63]. The experimental evidence for non-statistical behaviour is contained in the OH vibrational excitation [59-62], which is greater than that of the statistical distribution, and the OH/OD branching ratio in the 0( D) + HD reaction, which is larger than the statistical ratio. These non-statistical effects are not large, however, and the dynamical behaviour which they reflect is subtle. 

The singlet reaction with H2S creates vibrationally inverted OH [101], although in this case as weU, the added molecular complexity has an influence, since the OH vibrational distribution actually appears to be bimodal [102]. The NH3 singlet reaction, unlike the others discussed in this article, gives very little OH vibrational excitation [103,104]. The dynamics in this case are not understood. 

The model introduced in Problem G concludes that the bond that is unaffected is a spectator in such reactions. Here we follow a more chemical argument OH is isoelectronic with the F atom. The F + H2 exoergic reaction selectively populates the vibrational states of the HF product. The H + H—(OH) reaction is expected to have similar forces to the H + HF reaction and the masses are also similar. Therefore, by microscopic reversibility, H—(OH) vibrational excitation should promote the endoergic H + H—(OH) reaction. 

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

Hossenlopp J M, Anderson D T, Todd M W and Lester M I 1998 State-to-state inelastic scattering from vibrationally excited OH-Hj complexes J. Chem. Phys. 109 10 707-18... 

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 overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

Fig. 24. The angular distribution of different vibrational excited OH products for the 0(1D) + H2 —> OH + H reaction at the collision energy of 1.3kcal/mol.

Polanyi and co-workers used the technique, which involved the experiments in bulk. The reactants are put into desired vibrational and rotational states by producing them in an exothermic pre-reaction. For example, OH radical in vibrational excited states can be formed by the reactions... 

The second set of problems in dynamics are those of scattering theory where the Hamiltonian is of the form H = H0 + V and the interaction V vanishes when the colliding particles are far apart. It is usually assumed that the H0 part is already solved and that the interesting or the hard part is to account for the role of V. For realistic systems, which are anharmonic, even the role of H() can be quite significant. An example that has received much recent attention is the reaction of vibrationally excited HOD with H atoms (Sinha et al., 1991 Figure 8.2). The large difference in the OH and OD vibrational frequencies means that the stretch overtones of HOD are primarily local in character (cf. Section 4.21). It follows that one can excite HOD to overtones localized preferentially on either one of the two bonds and that an approaching H atom will abstract prefer-... 

Finlayson, B. J., J. N. Pitts, Jr., and H. Akimoto, Production of Vibrationally Excited OH in Chemiluminescent Ozone-Olefin Reactions, Chem. Phys. Lett., 12, 495-498 (1972). 

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