Molecular alignment and photofragment spectroscopy

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Abstract
By illumination with polarized light, molecules with a dissociative absorption can be aligned by the selective photodissociation of different M states at different rates, as was first shown by Dehmelt and co‐workers for H+2. We have extended this technique to neutral molecules, demonstrating it for a diatomic, IBr pumped by the second harmonic of a neodymium laser at 18 780 cm−1, by observing the recoiling Br fragments with photofragment spectroscopy. Classical and quantum calculations of the selective photodissociation effect give equivalent predictions except in the limit in which only a few M states remain populated. Several methods are discussed for experimentally monitoring the degree of alignment as a function of light flux: (i) the change in fragment flux, (ii) the change in relative strength of two transitions of different symmetry as reflected in their joint angular distributions, and (iii) the change in the shape of the angular distribution of the recoiling atomic fragments for a single transition. The first two methods are demonstrated for IBr molecules. In addition, experiments using two sequential light pulses are suggested, the second pulse monitoring the alignment produced by the first either through the photofragment angular distribution or through absorption. Our results show that at achievable light fluxes, molecules with dissociative transitions of moderate strength can be strongly aligned. Molecular populations can be prepared which contain only a few M states for each J, or even only M=±J. This technique can potentially provide a wide variety of aligned molecules for molecular beam or bulb studies of relaxation as well as elastic, energy transfer, and reactive scattering, and can provide further directionality for photon and fragment angular recoil studies.