Observation of rotationally mediated focused inelastic resonances inD2scattering from Cu(001)

Abstract

Rotationally mediated focused inelastic resonances (RMFIR's) in the angular distributions of ${\mathrm{D}}_{2}$ scattered from Cu(001) are observed. The FIR effect involves a phonon-assisted focusing of an incident beam of arbitrary energy and direction into a final channel of one single well-defined energy and direction. Surprisingly for an incident energy ${E}_{i}=27\mathrm{meV}$ the RMFIR conditions for the scattered beam coincide with the kinematic conditions required for a further elastic selective adsorption mechanism called the rotationally mediated critical kinematic (RMCK) effect. By taking advantage of the RMFIR and elastic RMCK effects, three effective bound states of energy ${\ensuremath{\epsilon}}_{n,J}=\ensuremath{-}21.5\mathrm{meV},$ $\ensuremath{-}12.4\mathrm{meV},$ and $\ensuremath{-}10.3\mathrm{meV}$ are determined. They are attributed to the lowest bound states ${\ensuremath{\epsilon}}_{0}=\ensuremath{-}28.9\mathrm{meV}$ and ${\ensuremath{\epsilon}}_{1}=\ensuremath{-}19.8\mathrm{meV}$ combined with the rotational excitation energy for $J=1$ to be ${B}_{\mathrm{rot}}J(J+1)=7.41\mathrm{meV},$ respectively, and ${\ensuremath{\epsilon}}_{3}=\ensuremath{-}10.3\mathrm{meV}$ combined with the rotational ground state $(J=0).$ While the ${\ensuremath{\epsilon}}_{1}$ and ${\ensuremath{\epsilon}}_{3}$ states appear as maxima in the angular distribution at RMFIR conditions, the ${\ensuremath{\epsilon}}_{0}$ yields a striking minimum which represents the first evidence of what we call an anti-FIR feature. Theoretical arguments to explain the different FIR signatures observed are provided. A fit of a phenomenological interaction potential to the experimental bound-state values yields a value for the well depth $D=32.5\mathrm{meV}$ which is somewhat deeper than that found previously.