Computation of time-dependent transition probabilities in excimer molecules induced by femtosecond laser pulses Original Research Article

We have computed the time-dependent probabilities of exciting and deexciting the bound states of the excimer molecules NeH, ArH and HeF, via the interaction of these systems with femtosecond laser pulses. The method involves the expansion of the time-dependent wavefunction in terms of stationary states with time-dependent coefficients and complex energies, whose imaginary parts represent the lifetimes of the collision complex on the initial repulsive ground state and of the excited states which are coupled by the field. The resulting system of differential equations is solved by a Taylor series expansion method. We have studied the efficiency of laser-induced molecular formation (LIMP) from the ground repulsive surfaces as a function of frequency and intensity, for trapezoidal pulses. Given the shortness of the pulse and the characteristics of the spectra of these molecules, for reasons of economy the bulk of the calculations was carried out in a ‘frozen nuclei’ approximation. Additional calculations for NeH, using a wavepacket representation of the initial state on the repulsive curve produced similar results as regards the possibility of LIMF. Additional information has been obtained regarding transitions among excited states. For example, starting the photoreaction in HeF from the first excited repulsive state 1 2Π with a pulse frequency of 4 eV allows an experimentally verifiable probability of obtaining bound-continuum emission at about 1320 Å (9.4 eV). For resonance conditions, the probabilities are appreciable during the pulse and go through maxima as a function of intensity of the order of 1011 W/cm2-1014W/cm2.