Fajardo ME. Quantitative Absorption Spectroscopy of Aluminum Atoms in Cryogenic Parahydrogen Solids.
J Phys Chem A 2019;
123:10893-10905. [PMID:
31769687 DOI:
10.1021/acs.jpca.9b09318]
[Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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.
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