Path integral Monte Carlo simulation of the absorption spectra of an Al atom embedded in helium

Infrared-active spin-orbit transitions of halogen atom dopants in solid parahydrogen: the role of trapping site geometry

We present theoretical calculations of the (2)P(1/2) ← (2)P(3/2) spin-orbit transition of Cl dopants embedded as substitutional impurities in solid parahydrogen (pH2) matrices. In the lower-energy (2)P(3/2) spin-orbit level, the Cl atom's electron density distribution is anisotropic, and slightly distorts the geometry of the atom's trapping site. This distortion leads to a blue shift in the spin-orbit transition energy; the blue shift is enhanced when we account for the large-amplitude zero point motions of the pH2 molecules surrounding the Cl dopant. We also show that the intensity of the transition depends on the geometry of the trapping site. In the gas phase, the (2)P(1/2) ← (2)P(3/2) atomic transition is electric dipole forbidden. However, when the Cl atom resides in trapping sites that mimic the hexagonal close packed morphology of pure solid pH2, the transition becomes electric dipole allowed through interaction-induced transition dipole moments. These transition dipole moments originate in the anisotropic electron density distribution of the lower-energy (2)P(3/2) spin-orbit level.

Path-integral Monte Carlo simulation of the recombination of two Al atoms embedded in parahydrogen

We report the use of path-integral Monte Carlo (PIMC) simulations in the study of the stability against recombination of two Al atoms trapped in solid parahydrogen (pH2) at 4 K. The many-body interactions involving open-shell Al atoms are described with a pairwise additive Hamiltonian model. To estimate the lifetime against recombination, we use PIMC simulations to define an effective potential averaged over the position of the pH2 molecules, followed by a transition-state treatment. Different initial embedding sites are explored. If the initial substitution sites are within a distance of approximately 13 bohrs, the Al atoms will significantly distort the lattice structure to allow recombination, with an accompanying release of energy during the process. For substitution distances longer than approximately 14 bohrs, the dispersion of Al atoms is shown to be metastable, with lifetimes varying from approximately 30 min to several days. The electronic anisotropy is a factor that helps to stabilize the dispersion.