Quantitative Absorption Spectroscopy of Aluminum Atoms in Cryogenic Parahydrogen Solids

We report results of quantitative ultraviolet (UV) and infrared (IR) absorption spectroscopy experiments on Al-atom-doped cryogenic parahydrogen (pH2) solids produced by codeposition of Al vapor and pH2 gas. For Al atom concentrations [Al] ≲ 200 parts-per-million (ppm) the Al/pH2 solids are optically transparent and primarily contain isolated Al atoms – with a small admixture of AlH, Al2H2 and Al2H4 molecules formed by UV irradiation and Al atom recombination/reaction [Wang, et al. J. Am. Chem. Soc. 2003, 125, 9218]. We assign the Al/pH2 UV absorption spectrum by invoking a large (≈ 0.6 eV) gas-to-matrix blue shift to accompany the increase in principal quantum number in the 4s 2S ← 3p 2P1/2 transition, as previously discussed for boron-atom-doped pH2 solids [Tam, et al. J. Chem. Phys. 2000, 113, 9067]. We assign a series of sharp features observed in the 4140 – 4155 cm-1 IR region to Al-atom-induced Q1(0) and Q1(1) absorptions of the pH2 solid. We use the solid pH2 Q1(0)+S0(0) absorption to determine the...