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. 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 ²P_1/2 transition, as previously discussed for boron-atom-doped pH₂ solids. 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 sample thickness and to establish a constant pH₂ deposition efficiency independent of the flow rate. Using all of these absorption features in concert, we show that the Al-atom flux delivered by the effusive source is well described by the Knudsen-Langmuir equation, calculate an absolute Al-atom deposition yield per mass of aluminum evaporated, and demonstrate both constant Al-atom deposition and isolation efficiencies for [Al] ≲200 ppm. We discuss this unexpected constant Al-atom isolation efficiency in detail and speculate that it indicates nonuniform Al-atom recombination/reaction on the surface of the accreting sample, perhaps dominated by processes occurring near pH₂ crystallite grain boundaries. We demonstrate "control" over the deposition process, which we define as the ability to set, achieve, and verify "targets" for the final Al-atom concentrations and pH₂ solid thicknesses. This ability is key to sorting out the very different phenomena observed in samples targeting [Al] ≳300 ppm, which are described in the immediately following companion manuscript.