Hydrogen atom catalyzed ortho-to-para conversion in solid molecular hydrogen

Abstract

Infrared spectroscopy is used to investigate the process of molecular hydrogen ortho-to-para (o/p) conversion in solid hydrogen samples doped with small concentrations (10–50 ppm) of hydrogen atoms (H-atoms) as an impurity. The H-atoms are generated using the in situ 193 nm photolysis of N2O dopant molecules. For hydrogen crystals with relatively low initial ortho-H2 fractions (Xo ≤ 0.03), the o/p conversion kinetics at temperatures of 1.8 and 4.0 K follow kinetic equations developed previously for H-atom catalyzed o/p conversion. The measured atom catalyzed o/p conversion kinetics indicates the H-atoms are mobile under these conditions in agreement with previous ESR measurements. It has been proposed that the H-atoms diffuse by a quantum tunneling mechanism that is described as chemical diffusion. Detailed fits of the measured o/p conversion kinetic data allow the initial H-atom concentration after photolysis to be extracted assuming literature values for the H-atom recombination rate constant (H + H → H2). The measured o/p conversion kinetics show the observed o/p conversion is much less than expected based on the previously measured H-atom recombination rate constant and thus suggest that the H-atoms do not diffuse randomly through the crystal but rather diffuse preferentially in regions of high para-hydrogen content. The estimated H-atom concentrations from this study are consistent with previous ESR measurements but in conflict with kinetic studies of H-atom reactions with various dopants such as N2O.

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

“…The experimental apparatus for pH 2 matrix isolation spectroscopy in the Wyoming laboratory has been discussed previously, 14,29 so only details relevant to this study are presented below. Propyne-doped pH 2 solids are grown via the rapid-vapor-deposition (RVD) technique developed by Fajardo and Tam.…”

“…Room-temperature propyne (Aldrich, ≥99% as-received) was introduced into the cryostat through a separate needle valve set to achieve acceptable working concentrations that favor isolated monomers and disfavor propyne clusters. Exclusively asdeposited samples (hexagonal close packed (hcp) fractions between ∼50 and 80%, estimated using the U 1 (0) feature 29 ) were studied in this work; annealing was conducted post-NSC to check for reversible and irreversible changes in measured IR spectra to confirm rovibrational assignments. Sample temperatures were measured using a pair of Si diodes (attached to the OFHC Cu optical mount making thermal contact with the BaF 2 window) connected to a digital temperature controller.…”

“…The experimental apparatus for p H 2 matrix isolation spectroscopy in the Wyoming laboratory has been discussed previously, , so only details relevant to this study are presented below. Propyne-doped p H 2 solids are grown via the rapid-vapor-deposition (RVD) technique developed by Fajardo and Tam. , This procedure involves the codeposition of independent p H 2 and propyne gas streams onto a precooled BaF 2 optical substrate held near ∼2.5 K within a sample-in-vacuum l He cryostat, producing millimeters-thick propyne-doped p H 2 -enriched samples in ∼30 min.…”

“…Total propyne monomer concentrations are measured via Beer’s law, stated as where A i ( ν̃ ) = log 10 [ I 0 ( ν̃ )/ I ( ν̃ )] is the decadic absorbance of the i th vibrational mode of propyne integrated over the entire band area ( ν̃ to ν̃ + d ν̃ ), ε i p H 2 = 1.06 ε i gas is the gas-phase integrated absorption coefficient corrected for the refractive index of solid p H 2 , d is the thickness of the solid p H 2 slab, and V 0 = 23.16(6) cm 3 mol –1 is the molar volume of solid p H 2 at l He temperatures . Concentrations and p H 2 sample thicknesses are reported as weighted averages weighted by uncertainties.…”

Formation of Para-H2O by Vacuum-UV Photolysis of O2 in Solid Hydrogen: Implication for Astrochemistry

“…Other paramagnetic species produced as intermediates upon VUV irradiation, such as O or H atoms, might affect the kinetics of NSC in the early stage. 40 However, because these species have no IR signatures, their survival and lifetime were unclear in this work. Studies using UV−vis spectroscopy to monitor the quantities of these species will be performed in the future.…”