Rapid vapor deposition of millimeters thick optically transparent parahydrogen solids for matrix isolation spectroscopy

We report the rapid vapor deposition of millimeters thick parahydrogen $({\mathrm{pH}}_{\mathrm{2}})$ solids of remarkable optical clarity. Characterization of pure ${\mathrm{pH}}_{\mathrm{2}}$ samples by IR and Raman spectra show a very low orthohydrogen and vacancy content, and a mixed hexagonal-closed-packed/face-centered-cubic (hcp/fcc) polycrystalline structure for as-deposited samples, which converts to hcp upon annealing. Efficient isolation of atomic and molecular dopants is achieved by conventional matrix isolation spectroscopy (MIS) techniques. The increased optical path lengths offer significant improvements in spectroscopic data quality, and reveal novel dopant-induced IR absorptions of the ${\mathrm{pH}}_{\mathrm{2}}$ matrix host itself. Thus, while traditional MIS studies in rare gas hosts can only probe the influence of the matrix environment on the spectrum of the dopant “solute,” in ${\mathrm{pH}}_{\mathrm{2}}$ the response of the host “solvent” is directly observable as well. This complementary information may prove to be the key to identifying conclusively the microscopic structures of dopant trapping sites.