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

Abstract

High resolution velocity map ion imaging methods have been used to explore the photofragmentation dynamics of state-selected BrCl+(X2ΠΩ, 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 [2Π1/2]5sσ, v=0 Rydberg level. Experimental measurables include the wavelength dependent branching ratios and recoil anisotropies of the various Br+(3PJ) + Cl(2PJ) product channels. Ground state Br+(3P2) fragments dominate the fragment ion yield throughout the photolysis wavelength range investigated, with spin-orbit excited Cl(2P1/2) atoms the preferred co-fragment whenever energetically allowed. All fragmentation channels show a predominantly parallel recoil anisotropy, consistent with an initial A2Π←X2Π (ΔΩ=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 A2Π 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.