Dipole active rotations of physisorbed H2 and D

Journal article, 2005

We have, in electron-energy-loss measurements, observed dipole activity associated with the j=02 rotational transitions for H2 and D2 physisorbed on a Cu(100) surface and at Au adatoms on this surface. Such dipole transitions, which are forbidden for the free molecules, provide for example, via rotation-translation conversion, channels for infrared photodesorption. Using two simple dipole moment functions, we have, for the bare Cu(100) surface, calculated dipole matrix elements for the rotational transition and its combination mode with the =01 vibrational transition in the physisorption well. Our calculations reveal that the anisotropy of the molecular electronic polarizability induces a significant dynamic dipole coupled to the rotational and rotational-vibrational motion, but are unable to reproduce the measured relative intensities of the corresponding transitions.