Classical dynamics simulations are performed to study rotational excitation of ${\mathrm{H}}_2$ scattered from Pd(111), taking into account energy exchange with surface phonons through a 3D surface oscillator model. We show that dynamic trapping, identified recently in the study of dissociation dynamics, plays a prominent role. The corresponding long interaction time due to several recollisions allows an efficient energy exchange between ${\mathrm{H}}_2$ molecules and surface phonons. This microscopic mechanism explains the puzzling experimental finding on the role of surface temperature in ${\mathrm{H}}_2(\nu =0,J=1\mathrm{}\to 3)$ excitation.

• Received 5 December 2000

DOI:https://doi.org/10.1103/PhysRevLett.87.127601