Design of optimal infrared femtosecond laser pulses for the overtone excitation in acetylene

Optimal laser pulses in the femtosecond region are designed for a state sel ective excitation of infrared overtone vibrations in acetylene, for which a n experiment is actually set up in our institute. To attack the preparation problem the molecule's Hamiltonian is reduced to the IR-active modes, whic h are the twice degenerated cis-bending and the asymmetric CH-stretching mo de. For distinct populations we compute the corresponding two-dimensional p otential energy surface, dipole moments and eigenfunctions and then use opt imal control theory strategies to calculate applicable laser pulses. These optimized pulses are analysed in the time and frequency domain. Ladder clim bing is found to be the preferred mechanism for very efficient and highly s elective overtone excitation. This mechanism is discussed in detail and a m odel for the observed transition frequency shift is presented. (C) 2001 Els evier Science B.V. All rights reserved.