Observation of rotationally mediated focused inelastic resonances in D-2 scattering from Cu(001)

Rotationally mediated focused inelastic resonances (RMFIR's) in the angular distributions of 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 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 epsilon(n,J) = -21.5 meV, -12.4 meV, and -10.3 meV are determined. They are attributed to the lowest bound states epsilon(0) = -28.9 meV and epsilon(1) = -19.8 meV combined with the rotational excitation energy for J = 1 to be B(rot)J(J + 1) = 7.41 meV, respectively, and epsilon(3) = -10.3 meV combined with the rotational ground state (J = 0). While the epsilon(1) and epsilon(3) states appear as maxima in the angular distribution at RMFIR conditions, the 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 meV which is somewhat deeper than that found previously.