Far-infrared spectra of hydrogen dimers: Comparisons of experiment and theory for (H2)2 and (D2)2 at 20 K

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Abstract
Far‐infrared spectra of weakly bound complexes of molecular hydrogen have been studied using an infrared Fourier transform spectrometer and a long absorption path (98 m) through equilibrium gas at low temperature. The dimer transitions accompany pure rotational transitions of H2 or D2 monomers. Para‐H2 was studied in the S0(0) region (350 cm−1 ), normal H2 in the S0(1) region (590 cm−1 ), and ortho‐D2 in the S0(0) region (180 cm−1 ). The extensive and well resolved (13 sharp lines) spectrum observed for (D2)2 was of special interest. A new empirical (rigid rotor) fit potential of the H2–H2 system has been used for calculating eigenvalues and numerical eigenfunctions of the dimers in the close coupled formalism. Dipole moment transition probabilities were calculated by using the previously tested induced dipole moment surface of Meyer. In order to compare with experiment, bound–bound transition frequencies have been calculated for the three cases, along with the full collision‐induced spectrum for the para‐H2 S0(0) case. It was found that the dimer binding energies give quite good agreement with experiment, and that the line and continuum intensities were also in satisfying agreement with the measurements. However, there is evidence that the rigid rotor potential function is not completely adequate, especially for dimers containing rotationally excited monomers. The next step will be to utilize a nonrigid rotor potential including the effects of the individual hydrogen monomer bond lengths.