Laser spectroscopical investigations of small polyatomic molecules

Campbell-Shaw, Nebil Osama (1996) Laser spectroscopical investigations of small polyatomic molecules. University of Southampton, Doctoral Thesis.

Abstract

Details of the photodissociation dynamics of the hypohalous acids HOCI and HOBr have been investigated in a molecular beam/laser pump-probe experiment. Photolysis wavelengths of 307-309nm have been used to photodissociate the molecules and their deuterated analogues DOCI and DOBr. A set of experiments have also been carried out on HOBr and DOBr at a photolysis wavelength of 266nm to investigate differences between the dynamics observed under effusive, helium-driven and argon-driven expansion conditions. Resolution of the nascent internal state distribution of the OH and OD photofragments has been achieve using the technique of laser-excited fluorescence (LEF). The photolysis wavelengths are of energies greater than those required for O-X bond fission and the surplus energy of all photodissociation cases is predominantly manifest as translational excitation of the photofragments.

Photolysis of HOBr/DOBr at 266nm indicates an appreciable degree of vibrational excitation in the OH fragment (estimated at up to 5%). These transitions have been probed in a saturation LEF experiment. Vibrational excitation is not resolved in the DOBr or HOC1/DOC1 experiments. Only lower N" rotational energy levels are populated (N"=1-11) and populations generally follow a Gaussian distribution. The OH(OD) ²II_3/2 spin-orbit and II(A') Λ-doublet states are preferentially populated. No spectrum could be recorded using a photolysis wavelength of 248nm in marked contrast to HOC1/DOC1. Varying the expansion conditions indicates a trend towards bimodality in the rotational level population distributions for colder expansions of DOBr in argon. The alignment parameter β_o⁽²⁾ is derived and found to be negative, tending towards the limit of -0.5. It is proposed that dissociation follows excitation of a non-bonding electron to a σ-antibonding orbital, which promotes rapid and direct bond fission via an upper state of A' symmetry.

A parallel study has been performed into the photolysis of HOC1/DOC1 and HOBr/DOBr at 307-309nm in a single laser saturation LEF experiment. The observations are similar to those obtained using shorter wavelengths but feature a colder rotational distribution (N" = 1-8). No vibrational excitation is evident in the OH fragment produced from HOBr and the bimodality which appears in the DOBr rotational population distribution is not observed.

The results for all cases are in accord with a theoretical model suggesting OH(D) fragment rotation originates in HOX zero-point bending motions.

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