Infrared-laser driven vibrational excitation of relaxing adsorbates: Quantum dynamical aspects

As a first step to the active manipulation of adsorbates by external, time- dependent electromagnetic fields, the infrared-laser driven selective excit ation of molecular vibrations of adsorbates at metal surfaces is investigat ed here in the framework of time-dependent open-system density matrix theor y. Special emphasis is given to the inclusion of vibrational damping, cause d by the coupling of the adsorbate vibrations to possibly electronic substr ate degrees of freedom. For the example system NH3/Cu, a non-Markovian, two -mode open-system Liouville-von Neumann model for the vibrational relaxatio n of an excited adsorbate is proposed. After studying the field-free decay of excited adsorbates, it is shown that even in rapidly relaxing environmen ts optimal IR laser pulses in the picosecond domain can be designed which l ead to temporarily high populations of selected target states of adsorbates at metal surfaces. (C) 1999 American Institute of Physics. [S0021-9606(99) 70312-0].