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 ultraviolet (uv) light-induced desorption:of different isotopomeric neutral adsorbates from metal surfaces is studied with the help of nuclear density matrix theory. As a concrete example the system NH3/ND3/Cu(111) is chosen. In a first step of the "vibrationally mediated chemistry" advocated here, based on computed two-mode dipole functions and model potentials, optimal infrared laser pulses are designed to selectively 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 electronic transitions (DIET). It is argued that despite strong dissipation, the proper vibrational preparation not only increases desorption yields substantially, but also allows for an almost complete separation of both isotopomers. (C) 1997 American Institute of Physics.