Jeanette A. Fiss

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

Abstract 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 nm 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.

The role of molecular and resonance phases in the coherent control of chemical reactions

Coherent control of the photoionization and photodissociation of HI and the photoionization of H2S were obtained in the region of the 5d(π, δ) resonance of HI. Interference between one- and three-photon excitation paths caused modulation of the HI+, I+, and H2S+ signals. Phase lags between the different modulated signals, measured as a function of excitation energy, revealed the roles played by molecular and resonance phases. A theory of the phase lag arising from a set of overlapping rotational resonances was developed and used to interpret the observations.

Coherent Phase Control of Photoionization and Photodissociation

A fundamental principle of quantum mechanics is that if a process can occur by more than one independent path, then the probability of that process occurring can be calculated by adding the probability amplitudes for each path and then squaring the sum. A well known example is the photoionization of an atom or a molecule. One route connecting the ground state g > with the continuum 39-1 > is direct ionization, with a probability amplitude that is proportional to 39-1