Dipole Active Vibrational Motion in the Physisorption Well

We demonstrate by electron energy-loss measurements that the vibrational motion in the physisorption well is dipole active. This is a general phenomenon and data for adsorbed ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD reveal a strong nonlinear dependence of the induced dipole moments on displacement. We find that a good estimate of the dipole moment function can be obtained from a van der Waals model.