Femtosecond photoassociation: Coherence and implications for control in bimolecular reactions

Abstract

Communication: Feshbach resonances in the water molecule revealed by state-selective spectroscopy A theoretical analysis of the recent femtosecond photoassociation spectroscopy ͑FPAS͒ experiment on mercury ͓U. Marvet and M. Dantus, Chem. Phys. Lett. 245, 393 ͑1995͔͒ is presented. It is shown that when a thermal distribution of diatom collision pairs is excited from a free to a bound electronic state on a time scale shorter than molecular vibration, an ensemble of coherent wave packets is produced. The dynamics of these wave packets created by the photoassociation pulse can be observed by firing a second probe pulse at variable time delays, and the depletion of the first excited bound state by the probe pulse is detected via fluorescence of the remaining population. Simulations of the FPAS experiment, using both wave packet propagation techniques and perturbation theory, clearly show the vibrational dynamics of the photoassociated transients. It is also demonstrated how the FPAS technique may be used as a tool for controlling the energy, impact parameter, and orientation in bimolecular reactions.

COHERENTNONLINEARSPECTROSCOPY: From Femtosecond Dynamics to Control

“…A method that has emerged as a new possibility in the study of bimolecular reactions is femtosecond photoassociation spectroscopy (99,(124)(125)(126). The photoassociation process, involving cooperative absorption of a photon by a pair of unbound atoms or molecules, causes bond formation between them in a free-tobound photonic transition.…”

“…Although photoassociation has been known at least since 1937 (127), the chemical implications of this process were described conceptually much later by Dubov et al (128). Photoassociation has been used for the spectroscopic study of excimer and exciplex molecules that have a repulsive ground state (129)(130)(131)(132)(133)(134)(135)(136)(137)(138) and more recently has gained interest because of its role in the generation of ultracold molecules (139)(140)(141)(142)(143)(144)(145)(146)(147)(148)(149)(150)(151)(152)(153), the real time observation of bimolecular reactions (124,154), and control of bimolecular encounters (99,126,155).…”

“…The yield of a bimolecular reaction is determined by the energy of the collision, relative orientation of the reactants, and impact parameter of the encounter. The photoassociation process has been demonstrated to achieve control of these three key parameters (99,126). Short-pulse photoassociation provides a well-determined initiation time for the reaction as well as an alignment with respect to the laboratory frame.…”

Training Schrödinger’s cat: quantum optimal control

“…In the context of coherent control of a chemical reaction, the bimolecular process of bond formation using femtosecond lasers remained more elusive [214][215][216][217][218][219][220][221] than bond breaking. Its coherent control was demonstrated only very recently [222].…”

Femtosecond two-photon photoassociation of hot magnesium atoms: A quantum dynamical study using thermal random phase wavefunctions

“…[5][6][7][8][9] However, a critical examination of the achievements reveals that successful control has been demonstrated almost exclusively for unimolecular processes such as ionization, dissociation and fragmentation. It is natural to ask why the reverse process of controlling binary reactions [10][11][12][13][14][15][16][17][18] is so much more difficult.…”