Absence of recombination of neighboring H atoms in highly purified solid parahydrogen: Electron spin resonance, electron-nuclear double resonance, and electron spin echo studies

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.

Purely Spatial Quantum Diffusion of H Atoms in Solid H_{2} at Temperatures below 1 K

We report on a direct measurement of the quantum diffusion of H atoms in solid molecular hydrogen films at T=0.7  K. We obtained a rate of pure spatial diffusion of H atoms in the H_{2} films, D^{d}=5(2)×10^{-17}  cm^{2} s^{-1}, which was 2 orders of magnitude faster than that obtained from H atom recombination, the quantity used in all previous work to characterize the mobility of H atoms in solid H_{2}. We also observed that the H-atom diffusion was significantly enhanced by injection of phonons. Our results provide the first measurement of the pure spatial diffusion rate for H atoms in solid H_{2}, the only solid state system beside ^{3}He-^{4}He mixtures, where atomic diffusion does not vanish even at temperatures below 1 K.

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.

Diffusion of Water Molecules in Quantum Crystals

With the use of solid parahydrogen in matrix isolation spectroscopy becoming more commonplace over the past few decades, it is increasingly important to understand the behavior of molecules isolated in this solid. The mobility of molecules in solid parahydrogen can play an important role in the dynamics of the system. Water molecules embedded in solid parahydrogen as deposited were found to be mobile at 4.0 K on the time scale of a few days. The diffusion at this temperature must be due to quantum tunneling in solid parahydrogen. The diffusion dynamics were analyzed based on the theory of nucleation. The concentration dependence on the diffusion rate indicates that there might be correlated motion of water molecules, a signature of quantum diffusion. We find that both water monomers and water dimers migrate in solid parahydrogen and provide insight into the behavior of molecules embedded in this quantum crystal.

Signatures of a quantum diffusion limited hydrogen atom tunneling reaction

We are studying the details of hydrogen atom (H atom) quantum diffusion in parahydrogen quantum solids in an effort to better understand H atom transport and reactivity under these conditions.

Tunneling chemical exchange reaction D + HD → D2 + H in solid HD and D2 at temperatures below 1 K

We report on a study of the exchange tunneling reaction D + HD → D₂ + H in a pure solid HD matrix and in a D₂ matrix with a 0.23% HD admixture at temperatures between 130 mK and 1.5 K. We found that the exchange reaction rates, k_exHD ∼ 3 × 10^-27 cm³ s^-1 in the pure HD matrix, and k_exD2 = 9(4) × 10^-28 cm³ s^-1 in the D₂ matrix, are nearly independent of temperature within this range. This confirms the quantum tunnelling nature of these reactions, and their ability to proceed at temperatures down to absolute zero. Based on these observations we concluded that exchange tunneling reaction H + H₂ → H₂ + H should also proceed in a H₂ matrix at the lowest temperatures. On the other hand, the recombination of H atoms in solid H₂ and D atoms in solid D₂ is substantially suppressed at the lowest temperatures as a result of a decreased probability of resonant tunneling of atoms when they approach each other.

Quantum Diffusion-Controlled Chemistry: Reactions of Atomic Hydrogen with Nitric Oxide in Solid Parahydrogen

Our group has been working to develop parahydrogen (pH2) matrix isolation spectroscopy as a method to study low-temperature condensed-phase reactions of atomic hydrogen with various reaction partners. Guided by the well-defined studies of cold atom chemistry in rare-gas solids, the special properties of quantum hosts such as solid pH2 afford new opportunities to study the analogous chemical reactions under quantum diffusion conditions in hopes of discovering new types of chemical reaction mechanisms. In this study, we present Fourier transform infrared spectroscopic studies of the 193 nm photoinduced chemistry of nitric oxide (NO) isolated in solid pH2 over the 1.8 to 4.3 K temperature range. Upon short-term in situ irradiation the NO readily undergoes photolysis to yield HNO, NOH, NH, NH3, H2O, and H atoms. We map the postphotolysis reactions of mobile H atoms with NO and document first-order growth in HNO and NOH reaction products for up to 5 h after photolysis. We perform three experiments at 4.3 K and one at 1.8 K to permit the temperature dependence of the reaction kinetics to be quantified. We observe Arrhenius-type behavior with a pre-exponential factor of A = 0.036(2) min(-1) and Ea = 2.39(1) cm(-1). This is in sharp contrast to previous H atom reactions we have studied in solid pH2 that display definitively non-Arrhenius behavior. The contrasting temperature dependence measured for the H + NO reaction is likely related to the details of H atom quantum diffusion in solid pH2 and deserves further study.

Electron spin resonance spectroscopy of molecules in large precessional motion: a case of H6(+) and H4D2(+) in solid parahydrogen

We have measured electron spin resonance (ESR) spectra of H6+ and H4D2(+) ions produced in gamma-ray irradiated solid parahydrogen. Anisotropic hyperfine-coupling constants for H6(+) and H4D2(+) determined by the analysis of ESR lines at 4.2K were -0.06 and -0.12 mT, respectively, which were opposite in sign to and much smaller than theoretical results of 1.17-1.25 mT. Although no change was observed in H6(+), the constant for H4D2(+) increased to be 1.17 mT at 1.7 K, which is very close to the theoretical value. We concluded that H6+ both at 4.2 and 1.7 K and H4D2(+) at 4.2K should be in a large precessional motion with the angle of 57-59 degrees, but the precession of H4D2(+) is stopped at 1.7 K.

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid D2-H2 and HD-H2 mixtures: an electron-spin-resonance study

Tunneling chemical reactions D + H2 --> DH + H and D + DH --> D2 + H in solid HD-H2 and D2-H2 mixtures were studied in the temperature range between 4 and 8 K. These reactions were initiated by UV photolysis of DI molecules doped in these solids for 30 s and followed by measuring the time course of electron-spin-resonance (ESR) intensities of D and H atoms. ESR intensity of D atoms produced by the photolysis decreases but that of H atoms increases with time. Time course of the D and H intensities has the fast and slow processes. The fast process, which finishes within approximately 300 s after the photolysis, is assigned to the reaction of D atom with one of its nearest-neighboring H2 molecules, D(H2)n(HD)(12-n) --> H(H2)(n-1)(HD)(13-n) or D(H2)n(D2)(12-n) --> H(HD)(H2)(n-1)(D2)(12-n) for 12 > or = n > or = 1. Rate constant for the D + H2 reaction between neighboring D atom-H2 molecule pair is determined to be (7.5 +/- 0.7) x 10(-3) s(-1) in solid HD-H2 and (1.3+/-0.3) x 10(-2) s(-1) in D2-H2 at 4.1 K, which is very close to that calculated based on the theory of chemical reaction in gas phase by Hancock et al. [J. Chem. Phys. 91, 3492 (1989)] and Takayanagi and Sato [J. Chem. Phys. 92, 2862 (1990)]. This rate constant was found to be independent of temperature up to 7 K within experimental error of +/-30%. The slow process is assigned to the reaction of D atom produced in a cage fully surrounded by HD or D2 molecules, D(HD)12 or D(D2)12. This D atom undergoes the D + DH reaction with one of its nearest-neighboring HD molecules in solid HD-H2 or diffuses to the neighbor of H2 molecules to allow the D + H2 reaction in solid HD-H2 and D2-H2. The former is the main channel in solid HD-H2 below 6 K where D atoms diffuse very slowly, whereas the latter dominates over the former above 6 K. Rate for the reactions in the slow process is independent of temperature below 6 K but increases with the increase in temperature above 6 K. We found that the increase is due to the increase in hopping rate of D atoms to the neighbor of H2 molecules. Rate constant for the D + DH reaction was found to be independent of temperature up to 7 K as well.

Hydrogen atoms in impurity-helium solids

Electron spin resonance (ESR) is employed to study atomic impurities (H and D) stabilized in impurity-helium (Im-He) solids at 1.35-1.5 K. The kinetics of the low temperature tunneling exchange reactions (D+H2-->H+HD, D+HD-->H+D2) are investigated in Im-He samples containing several different mixtures of hydrogen and deuterium impurities. The ESR line structures help determine the local environment of atoms trapped in Im-He solids. High concentrations of atomic hydrogen stored in Im-He solids may ultimately find applications in energy storage, matrix-isolation spectroscopy, and studies of different quantum statistical effects.