Path-integral Monte Carlo study of a lithium impurity in para-hydrogen: Clusters and the bulk liquid

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Abstract
Simulation studies using the path‐integral formulation of quantum statistical mechanics are reported for single atomic lithium impurities in bulk liquid para‐hydrogen and in clusters, Li(p‐H2) n , with n=12, 13, 32, 33, and 34. Over the range of temperatures studied in the clusters (T=2.5–6.0 K), the lithium impurity is found to reside outside or at the surface of the clusters. Nevertheless, perturbations of the structure are observed in comparison to neat para‐hydrogen clusters. The solvation energy of the lithium in the bulk liquid and subcritical gas (T=14–25 K) is found to be slightly positive. In both the clusters and the liquid, the inhomogeneously broadened dipole spectrum of the lithium atom was calculated using the radial fast Fourier transform Lanczos method. In the clusters, the spectra exhibit a main absorption band near the unperturbed atomic Li value and a second, asymmetric band shifted to the blue. The latter can be identified as the p orbital oriented radially towards the cluster, while the main band is composed of two p orbitals oriented parallel to the cluster surface. The spectrum in the liquid, while broader, has a more symmetric line shape. The ionization spectrum of the lithium atom in the clusters was also obtained. These spectra have finite width and a band center that progressively red shifts from the atomic value as the number of para‐hydrogen molecules increases. The predicted trends should be amenable to testing using molecular beam experiments.