The role of rotational excitation in H2 diffraction from LiF(001) at grazing incidence conditions
del Cueto1, A. S. Muzas1, G. Füchsel2, F. Martín1,3,4 and C. Díaz1,3
1.Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, 28049, Spain
2.Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
3.Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
4.Instituto Madrileño de Estudios Avnzados en Nanociencia (IMDEA-Nanociencia), Cantoblanco 28049 Madrid, Spain
Grazing incidence fast atom and molecule diffraction (GIFAD and GIFMD) techniques have been extensively used as a surface analyzing tool during the last years . Diffraction at GIFAD and GIFMD conditions is observed thanks to the strong decoupling between the normal and parallel motion, so the diffraction is ruled by the slow normal motion. Molecular diffraction spectra tend to be richer than their atomic counterparts, because of the extra internal degree of freedom, and a detailed theoretical analysis may shed light on the role of rotational excitation in these diffraction processes.
To carry out this study, we have built a 6D PES describing the electronic structure of the H2/LiF(001) system, by interpolation of a set of DFT energies, which have been calculated using the Vienna ab initio simulation package (VASP) . The interpolation has been performed using a modified version of the corrugation reducing procedure (CRP) . Then, we have used a time-dependent wave-packet propagation method  to study the dynamics of the system. In this method, a discrete variable representation (DVR) is used for all degrees of freedom, the wavepacket is propagated using the split-operator (SPO) method , and then the scattered wavepacket is analyzed using a scattering amplitude formalism .
In order to validate the method used, we have performed dynamics at low incidence energy, which successfully compare with previous experimental and theoretical studies [7,8]. Then, we have extended our analysis to high energy grazing incidence conditions, for which there are previous experimental GIFMD results . These experimental diffraction results strongly depend on the incidence direction, and our results indicate that this difference may be due to the different initial rotational distribution of the molecular beam.
In summary, we have been able to accurately simulate the experimental diffraction patterns of the H2/LiF(001) system at low and high energy at different incidence directions, and our results allow us to rationalize the role of rotational excitation in the diffraction processes, granting us new insight into the experimental patterns.
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