Ortho/para hydrogen converter for rapid deposition matrix isolation spectroscopy

Abstract

We describe the construction and operation of a catalyst-bed type device for precooling and equilibrating the ortho/para composition of a hydrogen gas flow. We use this device to vapor deposit millimeters thick cryogenic parahydrogen (pH2) solids which are remarkably transparent [M. E. Fajardo and S. Tam, J. Chem. Phys. 108, 4237 (1998)]. Infrared absorption spectra of solids deposited at pH2 flow rates up to 290 mmol/h (solid thickness growth rates up to 75 μm/min indicate a residual orthohydrogen (oH2) content of less than 0.01%. Gas phase thermal conductivity measurements indicate a residual oH2 content of 0(−0+5)% for flow rates up to 1.9 mol/h. These pH2 solids can be doped readily by simple codeposition of various impurities produced by any of the numerous dopant sources developed in previous matrix isolation spectroscopy (MIS) studies. The long achievable path lengths, and the desirable properties of pH2 as a matrix host, will enable significant new fundamental and practical applications for MIS.

Infrared studies of ortho-para conversion at Cl-atom and H-atom impurity centers in cryogenic solid hydrogen

“…We use the «rapid vapor deposition» technique developed by Fajardo and Tam to produce ~2 mm thick optically transparent crystals of Cl 2 doped solid hydrogen [16,17]. The Cl 2 doped hydrogen crystals are grown by co-deposition of two separate gas streams, the H 2 host (Linde Gas, purity: 99.999%) and Cl 2 dopant (Aldrich, purity: 99.5%), onto a BaF 2 optical substrate cooled to ~2.5 K within a liquid He bath cryostat.…”

“…Note using Eq. (1) from Tam and Fajardo [17] the proportionality constant b for the I(4736) feature is predicted to be 73 cm -2 (with ν in cm -1 and l in cm).…”

“…Using this equation we calculate X o = 0.5479(2) and 0.5497(2) for two separate samples that were deposited with T o/p = 80 K corresponding to X o = 0.5137. This ana- We can independently measure the X o value using the integrated intensity of the Q 1 (0) transition from 4151 to 4154 cm -1 and the proportionality constant reported by Tam and Fajardo [17], which leads to X o = 0.0269 (5). Based on this analysis, for X o values determined using just I(4736) for crystals with X o ≤ 0.05, the reported X o value is likely good to within 10%, similar to the errors for the method of Abouaf-Marguin and Vasserot [9].…”

Infrared-active vibron bands associated with rare gas atom dopants isolated in solid parahydrogen

“…Rg atom doped solid p-H 2 crystals are prepared using rapid vapor deposition [21,23] of separate gas streams of precooled p-H 2 and room temperature Rg onto a 1 inch diameter BaF 2 substrate (T » 2.5 K) within a liquid helium bath cryostat (Janis SSVT-100). High purity p-H 2 gas (99.99%) is prepared by passing normal H 2 (n-H 2 ) gas through a low temperature catalytic converter containing granular Fe(OH) 3 (hydrous ferric oxide) just prior to deposition.…”

Matrix isolation spectroscopy of H2O, D2O, and HDO in solid parahydrogen

“…Our Rapid Vapor Deposition (RVD) technique (25,26), in which volatilized dopants are co-deposited with an independent flow of precooled pH2 gas directly onto a substrate-in-vacuum held at T Z 2 K, offers excellent dopant isolation efficiencies for concentrations up to -100 ppm --ideal for FTIR-based absorption diagnostics. However, this improved isolation efficiency comes at the price of a more complicated host morphology with higher defect concentrations, and a maximum Approved for public release; distribution unlimited.…”