Photodissociation of state selected BrCl+ cations: branching ratios and angular anisotropies of the Br+ product forming channels

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X(2)Pi(Omega), v = 0) cations following excitation in the wavelength range 370-420 nm. The parent ions are formed by 2 + 1 resonance enhanced multiphoton ionization (REMPI) via the [(2)Pi(1/2)]5s sigma, v = 0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(P-3(J)) + Cl(P-2(J)) product channels. Ground state Br+(P-3(2)) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin-orbit excited Cl(P-2(1/2)) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A(2)Pi <- X(2)Pi (Delta Omega = 0) excitation in the parent cation, but the best-fit description of the fragment angular distributions requires use of higher terms in the Legendre expansion than is traditionally the case for a one-photon dissociation process. The additional angular modulation is rationalised by recognizing that the REMPI preparation laser induces unwanted ( but unavoidable) parent ion fragmentation, leaving a spatially aligned distribution of ions for the intended photolysis study. Interpretation of the measured product branching ratios and recoil anisotropies is guided by complementary ab initio calculations of spin-orbit free potential energy curves for many of the lower lying electronic states of BrCl+ and subsequent semi-empirical inclusion of spin-orbit effects. The adiabatic potentials so derived exhibit a plethora of avoided crossings. Preliminary wavepacket propagations following excitation to the A (2)Pi state show rapid flux redistribution to adiabatic potentials that correlate with the lower dissociation asymptotes, but are unable to reproduce the details of the experimentally determined product branching ratios.