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

Abstract

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.

High-resolution infrared spectroscopy of atomic bromine in solid parahydrogen and orthodeuterium

“…We were originally confused by this increase in the integrated intensity of the Br SO transition upon annealing, but theoretical calculations on the related Cl-atom SO transition conducted by the Hinde group and presented in Paper II 6 provide a theoretical understanding for the changes observed. In this work, we will use the intensity of the U 1 (0) solid pH 2 single transition (i.e., v, j = 1, 4 ← 0, 0) as an independent measure of the hcp content in the solid pH 2 sample.…”

“…This would require theoretical treatment of not only the many-body spin-orbit interaction, but the hyperfine interactions as well. 6 Clearly the shift (+39.43 cm −1 ) in the Br SO transition produced by solvation of the Br-atom in solid pH 2 is the larger effect, but the relative intensities and positions of the hyperfine components may differ significantly due to the change in the way the Br(pH 2 ) 12 cluster interacts with light to compared to the isolated Br-atom. The Br + Q 1 (0) double transition also shows evidence of resolved fine structure.…”

“…In a similar fashion, Br-H 2 intermolecular interactions can only lead to a net induced electric dipole moment for the Br SO transition if the Br atom is solvated in a site that lacks a center of inversion. 6 However, in this case, the Br SO transition even for Br-atoms in fcc solvation sites have a non-zero absorption intensity due to the inherent magnetic dipole transition strength of the Br-atom. 10,11 In contrast, the Br + Q 1 (0) double transition does not suffer from the same morphology effects because the double excitation corresponds to a unique Br-pH 2 excitation pair within the crystal, and is not the net result of summing over all the Br-H 2 intermolecular interactions.…”

“…Typically, these small intensity differences are over looked or modeled by simple solvent continuum models (e.g., Polo-Wilson equation); 8 however, there is growing evidence from van der a) Electronic mail: danderso@uwyo.edu Waals dimer spectroscopy 9 and collision-induced phenomena that for simple atom-diatom systems ab initio calculations of the full dipole moment surface is tractable and can bring experiment and theory into quantitative agreement. In the present Br-atom experimental study we will focus on intensities, and when combined with Paper II, 6 the more accurate physical picture that emerges is to view the IR absorption as originating in a cluster consisting of the Br-atom and the first solvation shell of H 2 molecules surrounding it.…”

“…Near-infrared spectroscopic studies of halogen atoms (Cl, Br, I) trapped in low temperature solid molecular hydrogen (SMH) provide a proving ground for both experiment [1][2][3][4] and theory 5,6 in the study of how weak intermolecular forces modify the spectroscopic properties of the embedded halogen atom. One of the unique properties of SMH that is an advantage in these studies is the individual H 2 molecules retain gas phase rotational and vibrational quantum numbers in the solid.…”