Solid Parahydrogen Thickness Revisited

Electron thermalization length in solid para-hydrogen at low-temperature

We report the first ever measurements of the thermalization length of low-energy electrons injected into solid para-hydrogen at a temperature T ≈ 2.8 K. The use of the pulsed Townsend photoinjection technique has allowed us to investigate the behavior of quasi-free electrons rather than of massive, slow negative charges, as reported in all previous literature. We have found an average thermalization length ⟨z0⟩ = 26.1 nm, which is three to five times longer than that in liquid helium at the same temperature.

Infrared Spectroscopic Studies of Oxygen Atom Quantum Diffusion in Solid Parahydrogen

The thermally induced diffusion of atomic species in noble gas matrices was studied extensively in the 1990s to investigate low-temperature solid-state reactions and to synthesize reactive intermediates. In contrast, much less is known about the diffusion of atomic species in quantum solids such as solid parahydrogen (p-H₂). While hydrogen atoms were shown to diffuse in normal-hydrogen solids at 4.2 K as early as 1989, the diffusion of other atomic species in solid p-H₂ has not been reported in the literature. The in situ photogeneration of atomic oxygen, by ArF laser irradiation of an O₂-doped p-H₂ solid at 193 nm, is studied here to investigate the diffusion of O(³P) atoms in a quantum solid. The O(³P) atom mobility is detected by measuring the kinetics of the O(³P) + O₂ → O₃ reaction after photolysis via infrared spectroscopy of the O₃ reaction product. This reaction is barrierless and is thus assumed to be diffusion-controlled under these conditions such that the reaction rate constant can be used to estimate the oxygen atom diffusion coefficient. The O₃ growth curves are well fit by single exponential expressions allowing the pseudo-first-order rate constant for the O(³P) + O₂ → O₃ reaction to be extracted. The reaction rates are affected strongly by the p-H₂ crystal morphology and display a non-Arrhenius-type temperature dependence consistent with quantum diffusion of the O(³P) atom. The experimental results are compared to H(²S) atom reaction studies in p-H₂, analogous studies in noble gas matrices, and laboratory studies of atomic diffusion in astronomical ices and surfaces.

Infrared spectra of isoquinolinium (iso-C9H7NH+) and isoquinolinyl radicals (iso-C9H7NH and 1-, 3-, 4-, 5-, 6-, 7- and 8-iso-HC9H7N) isolated in solid para-hydrogen

Protonated polycyclic aromatic nitrogen heterocycles (H⁺PANH) are prospective candidates that may contribute to interstellar unidentified infrared (UIR) emission bands because protonation enhances the relative intensities of the bands near 6.2, 7.7 and 8.6 μm, and the presence of the N atom induces a blue shift of the ring-stretching modes so that the spectra of H⁺PANH match better with the 6.2 μm feature in class-A UIR spectra. We report the infrared (IR) spectra of protonated isoquinoline (the 2-isoquinolinium cation, iso-C₉H₇NH⁺), its neutral counterpart (the 2-isoquinolinyl radical, iso-C₉H₇NH), and another mono-hydrogenated product (the 6-isoquinolinyl radical, 6-iso-HC₉H₇N), produced on the electron-bombardment of a mixture of isoquinoline (iso-C₉H₇N) with excess para-hydrogen (p-H₂) during matrix deposition at 3.2 K. To generate additional isomers of hydrogenated isoquinoline, we irradiated iso-C₉H₇N/Cl₂/p-H₂ matrices at 365 nm to generate Cl atoms, followed by IR irradiation to generate H atoms via Cl + H₂ (v = 1) → HCl + H; the H atoms thus generated reacted with iso-C₉H₇N. In addition to iso-C₉H₇NH and 6-iso-HC₉H₇N observed in the electron-bombardment experiments, we identified six additional hydrogenated isoquinoline species, 1-, 3-, 4-, 5-, 7- and 8-iso-HC₉H₇N, via their IR spectra; hydrogenation on the N atom and all available carbon atoms except for the two sharing carbon atoms on the fused ring was observed. Spectral groupings were achieved according to their behaviors after maintenance of the matrix in darkness and on secondary photolysis at various wavelengths. The assignments were supported via comparison of the experimental results with the vibrational wavenumbers and IR intensities of possible isomers predicted using the B3LYP/6-311++G(d,p) method. The implications in the identification of the UIR band are discussed.

High-purity solid parahydrogen

Alkali atoms trapped in solid hydrogen matrices have demonstrated ultralong electron spin coherence times and are promising as quantum sensors. Their spin coherence is limited by magnetic noise from naturally occurring orthohydrogen molecules in the parahydrogen matrix. In the gas phase, the orthohydrogen component of hydrogen can be converted to parahydrogen by flowing it over a catalyst held at cryogenic temperatures, with lower temperatures giving a lower orthohydrogen fraction. In this work, we use a single cryostat to reduce the orthohydrogen fraction of hydrogen gas and grow a solid matrix from the resulting high-purity parahydrogen. We demonstrate the operation of the catalyst down to a temperature of 8 K, and we spectroscopically verify that orthohydrogen impurities in the resulting solid are at a level <10^-6. We also find that, at sufficiently low temperatures, the cryogenic catalyst provides isotopic purification, reducing the HD fraction.

VUV photochemistry and nuclear spin conversion of water and water-orthohydrogen complexes in parahydrogen crystals at 4 K

Samples of H2O, HDO, and D2O were isolated in solid parahydrogen (pH2) matrices and irradiated by vacuum ultraviolet (VUV) radiation at 147 nm. Fourier-Transform Infrared (FTIR) spectra showed a clear depletion of D2O and an enrichment of both HDO and H2O by 147 nm irradiation. These irradiation-dependent changes are attributed to the production of OH and/or OD radicals through photodissociations of H2O, HDO, and D2O. The radicals subsequently react with the hydrogen matrix, leading to the observed enrichment of H2O. No trace of isolated OH or OD was detected in the FTIR spectra, indicating that the OH/OD radicals react with the surrounding matrix hydrogen molecules via quantum tunneling within our experimental timescale. The observed temporal changes in concentrations, especially the increase of HDO concentration during VUV irradiation, can be interpreted by a model with a rapid conversion from orthohydrogen (oH2) to pH2 in water-oH2 complexes upon VUV photodissociation, indicating either the acceleration of the nuclear spin conversion (NSC) of H2 due to the magnetic moment of the intermediate OH/OD radical, or the preferential reaction of the OH/OD radical with a nearby oH2 molecule over other pH2 molecules. We have also identified and quantified an anomalously slow NSC of H2O and D2O complexed with oH2 in solid pH2.

Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N₂O, which induces a two-step tunneling reaction, H + N₂O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N₂O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N₂O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.

Infrared Spectra of Monohydrogenated Aniline, ortho- and para-HC6H5NH2, Generated in Solid para-Hydrogen

The isomers of monohydrogenated aniline (HC₆H₅NH₂) are regarded as important intermediates in reduction reactions of aniline, but their spectral identification has been limited to electron paramagnetic resonance in an adamantane matrix. We report here infrared (IR) spectra of two least-energy isomers of HC₆H₅NH₂, produced on electron bombardment during the deposition of a matrix of aniline and para-hydrogen at 3.2 K. The intensities of IR lines of HC₆H₅NH₂ increased during maintenance of the electron-bombarded matrix in darkness for a prolonged period because of the neutralization of protonated aniline, H⁺C₆H₅NH₂, by trapped electrons and further reactions between aniline and the unreacted hydrogen atoms that were produced during electron bombardment. The observed lines were grouped according to their behaviors on secondary photolysis with light at 520, 465, and 375 nm. On comparison of experimental spectra with quantum chemically predicted spectra for four possible isomers of HC₆H₅NH₂, lines in one group were assigned to the most stable ortho-HC₆H₅NH₂ and those in the other group were assigned to the secondmost stable para-HC₆H₅NH₂. Their photolytic behaviors at varied wavelengths are consistent with predicted ultraviolet absorption bands. The mechanisms of formation of these isomers are discussed according to semiquantitative analysis.

Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen

Parahydrogen (pH₂) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CH₃CCH) isolated in solid pH₂ is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm^-1 region. Detailed analyses of a variety of parallel (ΔK = 0) and perpendicular (ΔK=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C₃ symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH₂ such that the energies of the K rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual ortho- and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.

Catastrophic Recombination/Reaction of Aluminum Atoms in Cryogenic Parahydrogen Solids

We report the results from experiments testing the limits of chemical energy storage in Al-atom-doped cryogenic parahydrogen (pH₂) solids produced by codeposition of Al vapor and pH₂ gas. These results map out three regimes of behavior. As described in the immediately preceding companion manuscript, for target Al atom concentrations, [Al]_target ≲ 200 parts-per-million (ppm), the Al/pH₂ solids are optically transparent and primarily contain isolated Al atoms, with a small admixture of Al_xH_y molecules formed by Al atom recombination/reaction. For [Al]_target ≳ 500 ppm, the depositions fail immediately, producing highly optically scattering solids with evidence of extensive Al atom recombination/reaction. For intermediate [Al]_target concentrations, near-threshold metastable transparent solids are formed which suffer catastrophic recombination/reaction as they grow past some critical thickness. Analysis of these catastrophic events using a minimal two parameter model [Jackson. J. Chem. Phys. 1959, 31, 722-729] yields a critical Al atom concentration near [Al] ≈ 280 ppm and an Al atom diffusion length ≈ 100 μm. While this diffusion length appears unphysically large, it is roughly compatible with even larger 300-900 μm wavelengths of spatial inhomogeneities visible in images of a remnant postrecombination sample. This spatial pattern is evocative of Turing structures or perhaps the structures sometimes formed upon phase separation; however, the actual mechanism for morphogenesis remains undetermined. The largest achieved Al atom concentrations of [Al] ≈ 300 ppm are roughly two orders-of-magnitude lower than those required for advanced chemical propulsion applications.

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.