Control of photodissociation: vibrational mode selection and quantum interference

We present two methods for using lasers to control the photodissociation of small molecules and molecular complexes. The first method, which involves ternary aniline cluster cations consisting of aniline, water, and an aromatic molecule (benzene or pyrrole), utilizes the strong interaction between an NH stretching vibration and an adjacent hydrogen bond The infrared predissociation reactions of the cluster cations were investigated. Upon absorption of an infrared photon, the clusters dissociated and eliminated either water or the aromatic molecule. Measurements of the branching ratio for this reaction revealed that fission of the hydrogen bond was accelerated by excitation of the NH stretching vibration adjacent to the targeted hydrogen bond. The second method involves coherent control in the frequency domain. We investigated interference effects on the two-color dissociation of IBr using the fundamental (560 nm) and its second harmonic (280 nm). In the strong-excitation regime by the fundamental light (∼1.3×1010 W cm−2), the yield of iodine atoms from IBr oscillated with the relative phase of the 560 and 280 nm light, whereas no change was observed in its angular distribution. This phase-dependent behavior can be explained by interference between the one-photon transition induced by the second-harmonic light and the dipole-forbidden two-photon transition induced by the strong fundamental light.