Water rotational product distributions

An impressive example of large rotational excitation is the photodissociation of water in the second absorption band (the B-state), where OH rotational states are populated up to N=45. In contrast, in the first absorption band of the same molecule, very little rotational excitation is found in the OH product, indicating an extremely small anisotropy in the excited state potential surface. This demonstrates, that the rotational state distribution in the products is very sensitive to the featurels of the excited state potential surface, in this case to its anisotrbpy with respect to Y. The large difference in the rotational distributions for the same molecule demonstrates also that dynamics and not kinematical constraints are responsible for this effect. 

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... 

Careful determination of the internal state distributions of one of the photoproducts can lead to an understanding of the nature of the forces that act upon the molecule as it breaks apart. One of the nicest examples of this kind of work is water photolysis through the A state. Photolysis via the A state produces little rotational excitation as has been observed by LIF of the OH product [19,20], Ab initio studies of the A state have shown that it is purely repulsive, that is, a direct dissociation similar to methyl iodide in Figure 1(a). There is calculated to be little or no variation in the PES as a function of the HOH bending angle [21]. This result means that the OH recoils from the dissociation as if from a central force field and little angular momentum is imparted. This example of photodissociation is an example that nearly perfectly follows the simple one-dimensional model. 

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