Metastable structures of solid hydrogen

Abstract

The Jϭ0→2 Raman signal from solid Jϭ0 D 2 or H 2 reveals the hcp structure when deposited at a rate 0.1рR͑m/min͒р40 onto MgF 2 at T d /T tp Ͼ0.3, a mixture of hcp and fcc crystals at 0.2ϽT d /T tp Ͻ0.3 and possibly a randomly stacked close-packed structure at T d /T tp Ͻ0.2, where T tp is the triple point temperature. Non-hcp crystals transform to hcp continuously and irreversibly with increasing T. Finally, the crystal size decreases with decreasing T d and increasing R, from ϳ1 mm at T d ϳ0.8T tp and Rϳ2 m/min to ϳ1 m at 0.25 T tp and Rϳ40 m/min.

Rapid Vapor Deposition of Millimeters Thick Optically Transparent Solid Parahydrogen Samples for Matrix Isolation Spectroscopy

“…Figure 3 shows Raman scattering spectra of two different as-deposited and annealed pH 2 samples. These spectra are in excellent agreement with those of Collins, Unites, Mapoles, and Bernat [17]. The "triplet" lineshape pattern, obtained by warming our samples to 4. pH 2 [17,26].…”

“…These spectra are in excellent agreement with those of Collins, Unites, Mapoles, and Bernat [17]. The "triplet" lineshape pattern, obtained by warming our samples to 4. pH 2 [17,26]. The quartet pattern observed for the asdeposited samples indicates a mixture of hep and fee, with the low-energy shifted peak of the fee doublet appearing at 350 cm' 1 [17].…”

“…Unfortunately, such vapor deposited SMf^are notorious for exhibiting strong optical scattering [16] and must be kept fairly thin (below -100 urn) for absorption studies, even in the infrared region [9]. Reverse sublimation of pH 2 gas in an enclosed cell at T « 0.5 T,,, produces optically transparent solids [4,11,17], but impurity concentrations must be kept well below 100 PPM to avoid aggregation [12,18]. Related techniques for rapidly condensing rare gas and N 2 solids in semi-enclosed cells also produce large optically transparent samples [19][20][21][22].…”

Ortho/para hydrogen converter for rapid deposition matrix isolation spectroscopy

“…37 A "triplet" lineshape pattern is obtained by warming the sample to 4.5 K, matching the known Raman spectrum of hep solid pH 2 . 27,37 ' 38 The presence of oH 2 impurities in a close packed pH 2 solid reduces the symmetry of neighboring pH 2 molecules' environments, resulting in further distortions to their charge distributions.…”

High Resolution Infrared Absorption Spectra of Methane Molecules Isolated in Solid Parahydrogen Matrices

“…Two principal methods for producing thick, transparent, doped pH 2 solids have emerged: gas condensation in an enclosed cell 2 ' 3 held at T « 8 K, and rapid vapor deposition 5,6 onto a much colder substrate in vacuum. Since these solids can be produced under highly non^equilibrium conditions they might contain a variety of metastable microscopic structures, 7 suggesting a corresponding variety of perturbations to the spectroscopy of dopant molecules. This manuscript addresses this connection between the observed rovibrational spectroscopy of dopant molecules and the microscopic structure of the pH 2 host for the specific case of methane doped parahydrogen (CH 4 /pH 2 ) solids.…”