Quantum dynamical aspects of rotationally and vibrationally mediated photochemistry in matrices and at surfaces HCl/DCl in Ar and NH3/ND3 at Cu(111)

Abstract

We investigate two extensions of the concept of vibrationally mediated chemistry, from small molecules in the gas phase to small molecules in matrices and at surfaces using, as examples, the systems HCl/DCl in Ar and NH ₃ /ND ₃ at Cu(111). The transition from isolated systems to the condensed phase calls for new quantum dynamical techniques and allows us to predict new phenomena. For matrix isolation, we propagate three-dimensional wavepackets representing photo-dissociated H or D atoms which penetrate from the initial cage into the lattice provided by the matrix. The cage-exit probabilities are found to depend not only on the initial vibrational, but also on the rotational states, owing to the environmental (O _h ) symmetry provided by the (fcc) lattice of the matrix. As a consequence, we suggest the extension from vibrationally to rotationally, or rovibrationally mediated chemistry for matrix-isolated molecules. For molecules at surfaces, we adopt Gadzuk's jumping wavepacket plus incoherent averaging scheme, applied to an extended two-dimensional Antoniewicz-type model for the surface–molecule bond plus the vibrational coordinate which lends itself to preferential vibrational excitation (here the umbrella mode of ammonia). The desorption depends selectively on the initial vibrational state. As a consequence, we suggest the extension of traditional desorption induced by electronic transitions (DIET) to a vibrationally mediated IR–UV DIET scheme which may be used e.g. for enrichment of specific isotopomers at surfaces.