Floquet representation of absolute phase and pulse-shape effects on laser-driven molecular photodissociation

Abstract

Using a recent reformulation of Floquet theory [S. Gu\'erin and H. R. Jauslin, Adv. Chem. Phys. 125, 1 (2003)], we discuss the dynamical role of the absolute phase in the photofragmentation of molecules subjected to laser pulses. We show how the dependence of Floquet states on an absolute phase is related to the complexity of the dressed molecular scheme and to the multiphoton character of the molecular dynamics. The general theory is applied to the study of the photodissociation of $\mathrm{H}_{2}{}^{+}$ in a $400\text{\penalty1000-\hskip0pt}\mathrm{nm}$ periodic laser pulse, repeated with a frequency lying in the IR. The dependence of the dynamics on the phase of the pulse envelope is highlighted through an effect previously called dynamical dissociation quenching (DDQ) [F. Ch\^ateauneuf et al., J. Chem. Phys. 108, 3974 (1998)] and through photofragment kinetic energy spectra. These spectra allow us to map out the Floquet content of the dynamics---i.e., its multiphoton character both with respect to the carrier-wave frequency, which gives rise to the usual bond-softening mechanism, and with respect to the pulse modulation frequency in the IR. The synchronization of this pulse modulation with the wave packet motion governs the DDQ effect in this uv-visible pulsed excitation case.