Infrared spectra of hydrogen dimers

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Abstract
Weakly bound complexes of molecular hydrogen have been studied using an infrared Fourier transform spectrometer and a long absorption path (112 m) through equilibrium gas mixtures at temperatures ∼20 K. Spectra of the dimers (H2)2, (HD)2, and (D2)2 were recorded, along with those of the mixed species H2–D2 and HD–D2. They lie in the 2900–8700 cm−1 (1.15–3.35 μm) spectral region corresponding to the monomer fundamental and first overtone bands. Since the rotation and vibration of the monomers remain largely unperturbed in a dimer, each dimer band is centered around a monomer rotation–vibration transition. The structure within these bands is associated with motions of the dimer as a whole, and particularly its end‐over‐end rotation, denoted by l. The spectra are quite detailed and well resolved, especially for the heavier isotopes HD–D2, and (D2)2, and in many cases, their rotational structure can be fully assigned. Although a detailed analysis is beyond the scope of the present paper, it should ultimately be possible to obtain precise multidimensional pair potential and induced dipole moment surfaces for molecular hydrogen directly from this experiment.