Enhanced photodesorption by vibrational pre-excitation: Quantum model simulations for Cs/Cu(1 1 1)

We present quantum model simulations for the laser-induced desorption of Cs from Cu(1 1 1) induced by a charge transfer from the Cu surface state to an unoccupied resonance state of the Cs-adsorbate. Based on empirical two-state model potentials along the Cs–Cu distance quantum nuclear dynamics were performed including electronic relaxation to account for the ultrashort lifetime of the resonance state. In order to increase the negligible desorption yield obtained from direct UV excitation the system was vibrationally pre-excited by a THz-laser pulse. Using an “jumping” wavepacket incoherent averaging scheme we calculated the desorption probability with respect to the time evolution of a vibrational wavepacket on the electronic ground state. The desorption probability is significantly increased for short Cs–Cu distances, but it is almost negligible for the stretched Cs–Cu bond. Vibrational pre-excitation enhances the photodesorption of Cs from Cu(1 1 1), but only if the timing of the laser-induced charged transfer is chosen correctly.