CONTROLLED SURFACE PHOTOCHEMISTRY - BOND-SELECTIVE AND ISOTOPE-SELECTIVE PHOTODESORPTION OF NEUTRALS BY ADSORBATE VIBRATIONAL PREPARATION WITH INFRARED-LASER PULSES

The possibility of controlling surface photochemistry by the selective vibrational preparation of adsorbates with infrared (ir) laser pulses is investigated theoretically. In particular, the selective ir plus u ltraviolet (uv) light-induced desorption:of different isotopomeric neu tral adsorbates from metal surfaces is studied with the help of nuclea r density matrix theory. As a concrete example the system NH3/ND3/Cu(1 11) is chosen. In a first step of the ''vibrationally mediated chemist ry'' advocated here, based on computed two-mode dipole functions and m odel potentials, optimal infrared laser pulses are designed to selecti vely excite the umbrella mode nu(2) of either adsorbed NH3 or ND3. In a second step, an uv/visible photon enforces an electronic transition, leading, after ultrafast quenching, to desorption induced by electron ic transitions (DIET). It is argued that despite strong dissipation, t he proper vibrational preparation not only increases desorption yields substantially, but also allows for an almost complete separation of b oth isotopomers. (C) 1997 American Institute of Physics.