Mechanism of the coherent control of the photoionization and photodissociation of HI and DI

Two experiments were performed to determine the mechanism responsible for the phase lag between the HI+ and I+ signals observed by Zhu et al. (Science, 270 (1995) 77) in the one- versus three-photon coherent control of the decay of excited HI and DI. In the first experiment a pulse of 266 nn radiation was introduced before the UV and VUV control pulses, It was observed in this case that the modulation amplitudes of the I+ signal decreased slightly, whereas the modulation depths decreased by a factor of two. This experiment rules out the possibility that modulation of I+ is caused by coherent control of the ionization of I atoms generated by two-photon photodissociation of the parent molecule. In the second experiment a fluorescence excitation spectrum produced by a single VUV photon was recorded and is attributed to emission from the atomic iodine product. Both experiments support our original interpretation of the phase lag arising from coherent control of the branching ratio for autoionization versus predissociation of the excited parent molecule. (C) 1998 Elsevier Science B.V. All rights reserved.