Dopant location in SF6He39,40

Effect of helium nanoclusters on the spectroscopic properties of embedded SF6: Ionization, excitation and vibration

Ionization and excitation energies, IR and Raman spectra of sulfur hexafluoride (SF₆), located inside helium (He) nanoclusters with different sizes (SF₆@He_n; n=20, 40, 60), were calculated. The effect of the cluster size on the spectroscopic properties of the SF₆ was investigated and found that the He_n-SF₆ interaction in the He clusters with large number of atoms is small so that the ionization and absorption energies of SF₆ are not affected while for small He nanoclusters the He_n-SF₆ interaction is more important. The effect of He_n-SF₆ interaction and deformation of the fragments on the photoelectron and absorption spectra of SF₆@He_n were separated theoretically and discussed in details. It was deduced that the effect of the cluster size on the IR and Raman vibrational frequencies of the SF₆ is negligible for the cluster size range considered in this work. Density functional theory (DFT) employing M06-2X functional and 6-31+G(df) basis set were used for optimizing the structures of SF₆@He_n. Symmetry adapted cluster-configuration interaction (SAC-CI) methodology, with the same basis set, were used to calculate the ionization and excitation energies of the SF₆@He_n structures. Using the calculated ionization and absorption energies and their intensities, the photoelectron and absorption spectra of the considered SF₆@He_n structures were simulated and compared with the experiment.

Elementary Excitations of Superfluid Helium Droplets Probed by Ion Spectroscopy

Electronic spectra of molecules in helium droplets reveal spectral features that are related to the elementary excitations of the superfluid helium environment. In order to determine to what extent the interaction strength of the molecule with the helium affects these excitations, the spectrum corresponding to the B̃(2)A″ ← X̃ (2)A″ transition of 2,5-difluorophenol cations in helium droplets has been recorded. The vibronic resonances reveal a sharp zero-phonon line whose width is largely determined by the rotational band contour, followed by a broad structureless phonon wing at higher frequencies. The splitting between the zero-phonon line and phonon wing is approximately half of that found for neutral 2,5-difluorophenol. This difference is attributed to the increased helium density around the ion due to its strong interaction with the helium.

Monte Carlo methods in electronic structures for large systems

Quantum Monte Carlo methods have recently made it possible to calculate the electronic structure of relatively large molecular systems with very high accuracy. These large systems range from positron complexes [NH(2),Ps] with approximately 10 electrons to C(20) isomers with 120 electrons, to silicon crystal structures of 250 atoms and 1000 valence electrons. The techniques for such calculations and a sampling of applications are reviewed.

SPECTROSCOPY OF ATOMS AND MOLECULES IN LIQUID HELIUM

Laser ablation of in situ metals has recently made it possible to immerse a large number of different metal atoms and ions and small clusters of metal atoms in liquid helium (He) and thus study their absorption and emission spectra in the visible region. Atoms and molecules are readily picked up by large (N > or = 103 atoms) He droplets, and their spectra are sensitively detected through the use of either beam depletion following absorption or laser-induced fluorescence. Within the past three years, a wide variety of molecules, ranging from OCS to large organic molecules such as amino acids and a number of van der Waals complexes and even large metal clusters, have been embedded in He droplets and studied either in infrared or in the visible region. These results are discussed here in detail, and the evidence for the effect of superfluidity on the spectral features is reviewed.