High-resolution infrared spectroscopy of theJ=1H2pair in parahydrogen crystals

Abstract

We have conducted high-resolution laser spectroscopic studies of the ortho-${\mathrm{H}}_{2}$ impurity-pair ${Q}_{1}(1)$ ($v=1\ensuremath{\leftarrow}0,$ $J=1\ensuremath{\leftarrow}1$) transitions in solid para-${\mathrm{H}}_{2}$ with the ortho-${\mathrm{H}}_{2}$ concentration of $l~0.2%.$ Several hundred lines were observed in the frequency region between 4142.9 and $4150.3{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ with the linewidth of 7 to 200 MHz half width at half maximum. Except for the single-molecule ${Q}_{1}(1)$ transition located at $4146.5621{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ all transitions are due to the pairs and higher-order clusters of ortho-${\mathrm{H}}_{2}.$ Using calculated relative intensities for spectral lines of nearest-neighbor (NN) pairs and next-nearest-neighbor (NNN) pair, and the accurate energy levels of the ground states by microwave spectroscopy [B. W. Statt, W. N. Hardy, and R. Jochemsen, Can. J. Phys. 58, 1326 (1980)], we have assigned 180 spectral lines that are due to NN and NNN pairs, including both symmetric and antisymmetric excited states. The ground-state frequency-combination differences agree to within $0.001{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ the accuracy of the measurement. The agreement of the relative intensities and polarization dependences between the observed spectrum and the calculated spectrum is also satisfactory. The energy levels in the excited states obtained from the assignment demonstrate that the pair splitting due to the first- and second-order electric quadrupole-quadrupole (EQQ) interaction and the crystal-field interaction remains similar between the ground and excited states. Small and similar deviations from the ground state have been noted for the excited states of NN in-plane (IP), NN out-of-plane (OP), and NNN pairs. The widths of the observed spectral lines differ by more than an order of magnitude depending on the levels. We noticed that only the $F=2,$ $M=0$ level, which has the highest energy among all the $F,M$ components due to the EQQ interaction, has significant homogeneous broadening on the order of 100 MHz due to relaxation to the lower $F,M$ levels. The inhomogeneous broadening due to the randomly distributed ortho-${\mathrm{H}}_{2}$ is approximately proportional to the difference in EQQ energy between the ground and excited $F,M$ levels. These observations were useful in conducting spectral assignments. Complete energy level patterns for the six excited states: NN IP $(s,a),$ NN OP $(s,a),$ and NNN $(s,a)$ pairs have been determined and discussed. A comprehensive analysis using the Hamiltonian is left for a future work.