Shape resonances in molecular clusters: the 2t_2gshape resonances in S 2p-excited sulfur hexafluoride clusters

Abstract

Sulfur hexafluoride clusters are investigated in the S 2p excitation regime. For the first time special emphasis is put on high-energy resolution spectroscopy of shape resonances in free clusters. We have investigated the 2t(2g)-shape resonance occurring above the S 2p threshold as one typical example to study size effects that are related to shape resonances. A small redshift of this resonance of 30 +/- 5 meV occurs in clusters relative to the free molecule and changes in line shape are observed. A double-barrier optical potential model is applied for the analysis of intra- and intermolecular effects occurring in SF6 clusters, which is suitable for rationalizing the experimentally observed spectral changes. The experimental and theoretical results are briefly discussed in comparison to previous work on core-excited van der Waals clusters containing diatomic molecules.

Gas-to-cluster effects in S 2p-excited SF6

“…Clusters of SF 6 are of specific interest, since they also show intense core-to-valence-transitions, which occur in the S 2p core ionization continua. 8,9 Striking similarities in spectral shape have been observed for the shape resonances in free and clustered SF 6 , 8,9 which is in full agreement with earlier low resolution spectroscopic work on the isolated molecule and the solid. 12,13 S 2p-excited SF 6 has been investigated before due to its unique cage structure and its double-well potential.…”

“…1,2 Specifically, free variable size molecular Van-der-Waals clusters have been investigated in the soft X-ray regime. [3][4][5][6][7][8][9] These are regarded as suitable model systems for studying size effects of elementary processes in weakly bound quantum systems as well as cage effects in composite and biological systems. In addition to significant changes in core-to-Rydberg transitions studied by high resolution X-ray spectroscopy, significantly smaller spectral shifts in core-to-valence transitions in molecular clusters have been observed.…”

“…Small and distinct redshifts of 1s → π * -transitions have been identified in N 2 , CO, and benzene clusters including the gas-to-solid shift, which depends sensitively on the molecular system and are of the order of 2-60 meV. [7][8][9] Even larger spectral shifts have been observed for aromatic molecules with a substantial dipole moment, whereby for C 1s → π * -excited pyridine a spectral redshift of 90 meV is observed, whereas a spectral blueshift of the same magnitude is observed for the N 1s → π * -excitation. 11 The gas-to-cluster redshifts have been on the one hand attributed to the freezing of molecular rotations and the dynamic stabilization of the core-excited clustered molecules.…”

“…These resonances are known to be associated with temporary trapping of the photoelectron ejected from the sulfur core within the octahedral cage of electronegative fluorine atoms as well as with subsequent tunneling of the photoelectron through the cage into the S 2p-ionization continua. 8,9,15,16 Another assignment links the resonances with electronic transitions from the deeply bound 2t 1u -orbital to the unoccupied virtual 2t 2g -and 4e gorbitals, which are embedded in the S 2p-continua (see, e.g., Refs. 8,9,16,and 17).…”

Free clusters studied by core-level spectroscopies

“…Such small, but distinct, spectral shifts, line broadening and changes in intensity between monomers and clusters, are expected to occur generally in molecular clusters and in the corresponding condensed phase. These can be understood in terms of variations in motion of the core-excited molecule within the molecular clusters using a simple model [94], and has further been explored for molecules such as SF 6 [95].…”