Population transfer to theE Σ1gexcited electronic state ofLi2by femtosecond laser pulses with all possible interstate radiative couplings

Abstract

Population transfer to the excited electronic state $E\text{ }{^{1}\ensuremath{\Sigma}}_{\text{g}}$ of ${\text{Li}}_{2}$ by transform limited delayed femtosecond pulses has been investigated. The validity of the usual neglect of some field-molecule coupling terms in such transfers, accomplished using counterintuitively ordered pulses through ``adiabatic passage on light-induced potentials,'' has been reexamined. It has been found that such an approximation cannot be justified in the case of the ladder system $X\text{ }^{1}\ensuremath{\Sigma}^{+}_{\text{g}}\text{\ensuremath{-}}A\text{ }^{1}\ensuremath{\Sigma}^{+}_{\text{u}}\text{\ensuremath{-}}E\text{ }^{1}\ensuremath{\Sigma}^{+}_{\text{g}}$ of ${\text{Li}}_{2}$. However, the transfer can be successfully accomplished over a range of pulse parameters, which is more restricted than those obtained with the usual approximation. In this scheme, when two pulses with a small frequency difference between them are used, the lower-frequency pulse must precede the higher-frequency pulse for a successful transfer. We introduce an adiabatic representation, appropriate to this full coupling case, for examining the mechanism of transfer. In our representation the initial wave packet may be prepared on more than one adiabatic state, and at an early stage it is transferred to a single adiabatic surface after which the evolution mostly proceeds adiabatically. The vibrational analysis of the final wave packet shows that for short pulses high vibrational states are generally excited, while for pulses with larger width low-lying vibrational states are preferred in cases where successful transfer occurs. These results can be understood in terms of the motion of the wave packet on the time-dependent adiabatic potentials.