Claude Dion
Laboratoire de Photophysique Moléculaire, Bâtiment 213,
Université Paris-Sud, 91405 Orsay, France
and
Département de chimie, Université de Sherbrooke, Sherbrooke,
Québec, Canada, J1K 2R1
28 June 1999
We study by numerical simulation the rotational dynamics of HCN induced by short (1.7 ps) intense (1013 W/cm2) linearly-polarized infrared laser pulses. We consider a linear model with two internal vibrational degrees of freedom (C-H and C-N) and the polar angle between the molecular axis and the laser electric field vector. The wave packet propagation is carried out using the split-operator method and FFTs. We also develop a Floquet model using a high-frequency approximation, giving a time-independent representation of the system, to which we add an adiabatic approximation to treat the case of pulsed lasers. We find that the molecule aligns (the molecular axis colinear with the field polarisation axis) in around 0.6 ps. This alignment is nearly conserved for the entire duration of the pulse but disappears after the laser is turned off. The molecule is found to be in pendular states created by the field. We observe that the alignment dynamics is essentially governed by the molecular polarisability, the permanent dipole moment acting only when the field frequency is chosen in resonance with a vibrational transition. We show that orientation of molecules (i.e., all pointing in the same direction) is possible using a combination of two lasers, with the frequency of one double that of the other and chosen in resonance with vibrationnal transitions. No orientation is observed during the pulse, but only briefly after the laser is turned off. Changing relative intensities and phases of the two lasers affects the degree of orientation obtained.
Full text (in French) available as a gzipped PostScript file [0.9 Mb].
Part of this work has been published (see refs. [3-5] ).