OH rotational distribution

Witting and coworkers (1988) used laser (193 nm) to photo dissociate HBr in a C02-HBr van der Waal complex and monitored the OH rotational distribution arising out of the reaction... 

Figure 3-7. Hypothetical situation in which two distinct mechanisms yield hot and cold OH rotational distributions (open and filled bars, respectively). Though the open and filled bars represent 75% and 25%, respectively, of the total OH, the filled bar dominates at N = 1. Thus, state specific detection of N = 1 senses the minor channel in preference to the major channel. This might occur when two pathways yield hot and cold OH, for example, Br + HOCO1 versus HO(Br)CO+, respectively.

The OH rotational distribution peaks at N = 14. The reaction 0(3P) + CH2 produces CO in vibrational levels up to v = 18 with a rotational temperature of 104 K [429]. This suggests that both reaction channels, giving CO + 2 H and CO + H2, occur simultaneously. 

The case of higher-than-binary complexes is harder to deal with. Such complexes are almost certainly present to some extent under the experimental conditions used in our work, but it is hard to quantify their contributions. For example, they could contribute to the relatively cold components of the OH rotational distributions observed in our measurements, This point remains to be settled quantitatively. 

HX) are created in 10% of the collisions for the HQ case and 20% for the HBr case [86]. For the purely singlet reactions, producing OH(2]T) + H(2s), energy partitioning measurements [87-89] indicate that the internal excitation of the product molecule is very large. The OH rotational distribution in the lowest vibrational levels (the only ones which have been measured to date) is inverted. This may not carry any dynamical information, however, as it is possibly the result of simple angular momentum constraints. (See Ref, 63 and Refs. 90-93 for a discussion of this in the case of the H2 reaction). 

In this initial attempt at understanding the HBr C02 reaction dynamics we have found that both direct and complex reaction mechamsms are possible in the van der Waals cluster and that the relative amounts of these mechanisms and the product state distributions obtained from them can be different fiom that in the bulk reaction. At this point we are not able to quantitatively reproduce the observed change in the OH rotational distribution in going from bulk to van der Waals complex, but reasons for this discrepancy are understandable based on known errors in the potential surface and due to quantum effects. 

Dixon et al. [75] use a simple quantum mechanical model to predict the rotational quantum state distribution of OH. As discussed by Clary [78], the component of the molecular wave function that describes dissociation to a particular OH rotational state N is approximated as... 

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... 

The dynamics of the O Dj) + H2S -> OH(t/) + HS reaction have recently been investigated. Time-resolved spectra at 0.4 cm-1 resolution were recorded at 40-/rs intervals, beginning at 20 fj.s and continuing until 540 fis after the laser pulse. The time-dependent OH vibrational populations recorded in this experiment are shown in Figure 15. The rotational distributions in all vibrational levels at all observation times could be fitted by near room-temperature Boltzmann distributions. The vibrational distribution obtained at the earliest time (corresponding to approximately two gas-kinetic collisions after the reaction) was strongly inverted [45]. The LIF measurements... 

The flash photolysis of water in this region has produced OH(X2n), which according to Welge and Stuhl (1033), is rotationally excited only up to N" = 5 and no vibrational excitation is found. The rotational distribution of OH is practically equal to that at room temperature, suggesting that the excess energy, the difference between hv and D0(H—OH), is distributed between translational energies of H and OH [also see Masanet et al. (241, 665)]. The excited state of water responsible for dissociation in this region is considered to be the unstable A( B,) state [Horsley and Fink (485), Miller et al. (704)]. 

Jacobs, A., Wahl, M., Weller, R., and Wolfram, J. (1987). Rotational distribution of nascent OH radicals after H2O2 photolysis at 193 nm, Appl. Phys. B 42, 173-179. 

Schinke, R., Engel, V., Andresen, P., Hausler, D., and Balint-Kurti, G.G. (1985). Photodissociation of single H2O quantum states in the first absorption band Complete characterization of OH rotational and A-doublet state distributions, Phys. Rev. Lett. 55, 1180-1183. 

The OH radical has been selected for preliminary saturated molecular fluorescence studies. A NdrYAG pumped dye laser is used to excite an isolated rotational transition, and the resulting fluorescence signal is analyzed both spectrally and temporally in order to study the development of the excited state rotational distribution. It is found that steady state is not established throughout the upper rotational levels, although the directly excited upper rotational level remains approximately in steady state during the laser pulse. The fluorescence signal from the directly excited upper level exhibits considerable saturation. 

The following channels have been observed [484, 485] from the reaction 0 ( D) + NH3, giving OH(X) + NH2(X), OH(X) + NH (A) and NH (a) + HzO. No evidence was found for the reaction channel forming HNO + H2 [485]. A bimodal rotational distribution for the OH(z = 0) is suggested to result from the electronic branching associated with the formation of NH2 in either its ground state (2J51) or its excited state (2At). The lack of a bimodal rotational distribution for OH formed... 

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