Measurement of bipolar moments for photofragment angular correlations in ion imaging experiments

A general numerical method is given to extract angular correlations from ph otodissociation experiments with ion imaging detection. The angular correla tions among the transition dipole moment of the parent molecule, mu, the ph otoproduct recoil velocity, v, and its angular momentum, j, are parametrize d analytically using the semiclassical bipolar moment scheme due to Dixon. The method is a forward-convolution scheme which allows quantitative extrac tion of all measurable bipolar moments and can be applied in experiments wi th both linearly and circularly polarized probe light. It avoids the cylind rical symmetry limitations of the inverse Abel transform method, traditiona lly used for extracting photoproduct recoil anisotropy and speed distributi on from imaging data. The method presented here also takes into account the possibility of multiple photodissociation channels. The features of the me thod are illustrated in a two-color 1+1' REMPI-ion imaging study of the NO photoproduct trajectories resulting from the 650 nm photodissociation of 2- chloro-2-nitrosopropane (CNP). A comparison between experimental and synthe tic images is presented for selected experimental geometries. The experimen tal images for CNP and the results from their fit confirm earlier TOF studi es showing that the recoil speed distribution is bimodal with the low and h igh speed components having average values of approximately 500 and 910 m/s . These components have been previously assigned to dissociation from the S -0 and T-1 electronic states of the parent molecule, respectively. The expe rimental results from the current study also confirm that for the high-spee d component the product NO velocity vector, v, is preferentially perpendicu lar to its angular momentum, j [beta (0)(0)(22)=-0.21], and that there is n o significant correlation between v and the transition dipole moment mu of the CNP molecule [beta (2)(0)(20)=-0.02]. (C) 2001 American Institute of Ph ysics.