Atomic polarisation in molecular photodissociation

Velocity mapped ion imaging has been used to experimentally investigate the photodissociation of the diatomic and triatomic molecules, Cl2 and CS2, respectively. For Cl2, sets of results are presented following excitation from both the v = 0 and v = 1 vibrational levels of the electronic ground state. Dissociation of Cl2 into both the Cl(2P3/2) + Cl(2P3/2) and Cl(2P3/2) + Cl*(2P1/2) product channels was observed experimentally, with the photodissociation of vibrationally excited chlorine also investigated theoretically using a timedependent wavepacket formalism. Results for CS2 are presented following dissociation into both the singlet channel, CS(X 1Σ+) + S(1D2), and the spin-forbidden triplet channel, CS(X 1Σ+) + S(3PJ ).
Experimental results on the photodissociation of Cl2 (v = 0) have been compared to recent time dependent wavepacket calculations performed on a set of ab initio potential energy curves. The experimental results include the first determination of high order electronic polarisation of Cl(2P3/2) atoms and, in general, good agreement was found between experiment and theory. Negligible cooling of the vibrational degree of freedom in the molecular beam allowed experimental investigation of the photodissociation of vibrationally excited Cl2 (v = 1). These results are supplemented with theoretical time dependent wavepacket calculations. Although the same electronic states were found to be important as for the photodissociation of Cl2 (v = 0), significant differences were found regarding many of the observables.
The indirect photodissociation of CS2 giving rise to the singlet product channel has been investigated following excitation above and below the barrier to linearity in the 1Σ+u(1B2) state. Above the barrier the vibrational populations of the CS(X 1Σ +) products appear inverted, while below the barrier the diatomic product state distributions point to a statistical partitioning of energy. Below the barrier, modelling of the spatial anisotropy parameter, based on angular momentum conservation, indicates the importance of nonaxial recoil effects. In this region PHOFEX spectra point to an enhancement of the singlet channel for K = 1, relative to K = 0, in agreement with previous work. Surprisingly, in both regions the S(1D2) atomic products were found to be unpolarised.
For the dissociation of CS2 into the spin forbidden triplet channel, following excitation below the barrier to linearity in the 1Σ+u(1B2) state, both the polarisation of triplet Satoms and the populations of the fine-structure levels are found to depend on the vibronic band accessed. Following excitation of K = 1 levels, and like the singlet S-atoms, all S(3PJ) products were found to be unpolarised. For K = 0, however, the S(3P2) species dominate, and are characterised by equal MJ populations, while the S(3P1) species show a preference for the MJ = ±1 sub-levels. For these bands the electronic alignment is very similar to that observed in the dissociation of OCS, indicating a similar mechanism, at least in the exit channel, is responsible for the polarisation in both systems.

Keywords: molecular photodissociation
Advisor: Brouard, M