A remeasurement of the 2 : 4 l m spectrum of J 1⁄4 1 H 2 pairs in a parahydrogen crystal q

The solid parahydrogen crystal has proven to be an extraordinary matrix medium. The very weak intermolecular interaction with relatively high-zero-point lattice vibration in combination with the small mass of the molecule lead to a highly quantum system in which hydrogen molecules rapidly tunnel between equivalent sites. This tunnelling process is the basis for a powerful self-annealing behavior of the crystal and the exquisitely homogenous crystal lattice. Its homogeneity, weak intermolecular interactions, and large lattice constant make the parahydrogen crystal a nearly ideal matrix medium for high-resolution spectroscopy. Over the past decade, spectra of a variety of simple impurities have been studied in this material and have demonstrated line widths that can be orders of magnitude narrower than their gas phase counterparts, small vibrational matrix shifts (usually <10 cm ), and nearly free rotation. Such high-resolution investigations have included hydrogenic species (i.e., various isotopomers [1] and nuclear spin modifications of hydrogen [2]), stable neutral molecules (HCl [3], CH4 [4], H2O [5], C60 [6], etc.), free radicals (CH3 [7], CH3CH2 [8], etc.), and recently molecular ion clusters (H3 ðH2Þn and H ðH2Þn [9]). Another benefit of using parahydrogen crystals as a spectroscopic matrix, that is absent from conventional noble gas matrices, is that the electric field of the impurities induces infrared activity in the matrix itself near the natural vibrational band centers of H2 (the strongest being the v 1⁄4 1 0 at 2:4lm). These spectral features not only reveal additional information about the dopant but also provide information on the condensed phase microscopic interactions and dynamics—properties that are difficult to obtain by other means. Both broad and sharp induced infrared activity near the v 1⁄4 1 0 band from hydrogenic ion clusters and several non-hydrogenic neutral species have been reported by FTIR and laser studies [3,9–12], but an understanding of these features is still primitive. When orthohydrogen (o-H2) concentration in excess of 0.05% is present in the parahydrogen (p-H2) crystal, an intricate absorption spectrum is produced. This spectrum, which consists of the strong v 1⁄4 1 0 transition of lone o-H2 at the center of o-H2 pairs which are split into a few hundred lines due to intermolecular interaction, was analyzed in the work by Zhang et al. [2]. Technical limitations of their spectrometer, however, kept them from recording a continuous spectrum. We have recently made improvements to the spectrometer and detection techniques [13,14] and present the re-recorded pair spectrum for the v 1⁄4 1 0 band in Fig. 1 of this Note. The parahydrogen crystal was produced by depositing parahydrogen vapor into a 9 cm long, 2 cm inner diameter cylindrical copper cell which is attached to a liquid helium cryostat. The copper cell was sealed by two sapphire windows with indium gaskets. The flow rate of the gas into the cell was controlled to maintain a cell temperature of 10.8K. Once grown, the crystal was allowed to cool to the liquid helium temperature of 4.2K. The final crystal was transparent except near the center axis of the cylindrical cell where the crystal was incompletely formed. Before crystal production, the parahydrogen gas was prepared by flowing commercial-grade hydrogen into a copper vessel filled with APACHE catalyst which had Journal of Molecular Spectroscopy 218 (2003) 131–133