Focused inelastic resonances in the scattering of He atoms from NaCl(001)

Abstract

High-resolution inelastic He-atom surface scattering experiments have been carried out on an in situ cleaved NaCl͑001͒ crystal surface for the incident wave-vector range of 4.29-6.07 Å Ϫ1 in the ͗110͘ high-symmetry direction to detect the recently predicted effect, the focused inelastic resonance ͑FIR͒. The scattering angle for the resonance in the ͗110͘ direction is uniquely predicted based on the reciprocal lattice vector exchanged in the process for the bound state ͉⑀ 1 ͉ϭ3.4Ϯ0.1 meV. This value is slightly lower than the 3.7Ϯ0.4 meV value found in the literature. The convincing evidence for the FIR effect comes from a systematic analysis of the angular distributions in combination with an energy analysis based on time-of-flight spectra.

Selective adsorption resonances: Quantum and stochastic approaches

“…Fano-type profiles in scattering processes appear when there is an interference between the resonance and background contributions. Since this interference behavior is very common in many physical processes, we are going to give a general discussion about it and will extend this study to three more interference mechanisms: double resonances, dipole resonances [199], and the so-called critical profiles recently observed in atom-surface [180] and diatom-surface scattering [86,172,181]; the originally predicted resonance mechanisms leading to such singular behaviors were the CKE [115,173] and the FIR [153,179], already studied in previous sections. We will show that the critical temperature for the topological transition should be very close to the surface Debye temperature.…”

“…In Fig. 19, the angular distribution for NaCl(001) 110 taken at 200 K and with an incidence wave vector of 4.73Å −1 clearly shows the FIR and KFE features together with some additional peaks, some of them assigned to SARs [180]. The resonant channel is B = (1, 0) (in units of 2 /a), the FIR incidence angle is 38 • and corresponds to the bound state 3.7 meV.…”

“…Since the resonance enhancement of inelastic processes extends over a finite portion of the SC under FIR conditions, a signature of this effect can also be found in time-of-flight spectra [180]. The locus of tangency points in the surface phonon dispersion space is given by…”

“…As has been seen in previous sections, several of such mechanisms have been predicted and observed. Here we are going to analyze the so-called focussed inelastic resonance (FIR) [179,180], the inelastic counterpart of the CKE (ICKE) [86,181], and some combinations of the IF singularity together with FIR and ICKE [106]. Because the origin of most inelastic features can be understood from kinematics, the following kinematic equations can be taken as the starting point of our theoretical development.…”

“…The FIR mechanism [173,180] takes place when the following two conditions in the dispersion ( E, K) space meet: (i) the SC and RC are tangent to each other for a particular bound state of the system, and (ii) this tangency point belongs to the RWDC or any other surface or bulk phonon dispersion curve. The resulting kinematic equation,…”