Quantitative Absorption Spectroscopy of Aluminum Atoms in Cryogenic Parahydrogen Solids

Abstract

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 2P1/2 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.

Catastrophic Recombination/Reaction of Aluminum Atoms in Cryogenic Parahydrogen Solids

“…A detailed description of our experimental apparatus and sample preparation protocol can be found in ref . Briefly, we produce our Al/pH 2 samples by codeposition of thermally evaporated Al atoms with precooled pH 2 gas onto a T B ≈ 2 K transparent substrate-in-vacuum (either CsI or BaF 2 ).…”

“…As reported in ref , the RVD approach works very well at producing optically transparent millimeters-thick Al/pH 2 samples with Al atom concentrations [Al] ≲ 200 parts-per-million (ppm). This success allowed us to map out and model the effects of varying the sample deposition parameters (e.g., effusive Al atom source temperature, pH 2 flow rate, and deposition times) on the final Al/pH 2 sample properties (i.e., Al atom concentration and sample thickness).…”

“…This article is the third in a series describing the trapping of Al atoms and Al x H y molecules in cryogenic parahydrogen (pH 2 ) solids. The immediately preceding companion manuscript, ref , details our experimental sample preparation and characterization techniques and refined data analysis methods for performing quantitative ultraviolet (UV) and infrared (IR) absorption spectroscopy. Reference follows several years after the first of the series: a more contemporaneous report of a collaborative study with the Andrews group on the IR spectroscopy of Al x H y species produced by Al atom reactions in hydrogen solids …”

“…In the course of this study, we performed a total of fourteen thermal Al/pH 2 depositions. The nine experiments yielding stable transparent solids were described in ref . The three depositions immediately producing highly scattering solids and the two producing initially transparent solids that subsequently underwent catastrophic recombination/reaction are the subjects of this manuscript.…”

“…Historically, interest in free radical chemical energy storage has been cyclical. We continue here with the detailed “nuts and bolts” presentation style adopted in ref , with the goal of providing the best possible starting point for future investigations into these phenomena. We conclude with a brief summary and suggestions for possible improvements in future experiments.…”