Dipole active vibrations and dipole moments of N2 and O2 physisorbed on a metal surface

We have, in infrared reflection absorption measurements, observed narrow dipole active absorption lines associated with the fundamental internal vibrational transitions of N(2) and O(2) physisorbed at 30 K on the chemically inert Pt(111)(1 x 1)H surface. Such transitions are forbidden for free homonuclear molecules and become dipole active at a metal surface due to polarization induced surface dipole moments. The measurements show that the internal stretch vibration frequencies are lowered by 7-8 cm(-1) relative to the gas phase values. The measured static and dynamic dipole moments are in the ranges of 0.06-0.07 and 0.001-0.002 D, respectively. We find that good estimates of the induced dynamic as well as the static dipole moments can in general be obtained from a van der Waals model but that the ratios of the measured static and dynamic moments indicates a need for a refinement of the dipole moment function.