High resolution spectrum of the HCl dimer

Rotational structure in the spectrum of the H–Cl stretching bands of the HCl dimer has been fully resolved using a tunable difference-frequency laser. The spectrum of a natural isotopic sample was recorded under thermal equilibrium conditions near the condensation point (T∼130 K) of HCl at low pressures (3–4 Torr) and long path lengths (64–80 m). Rotational assignments have been made for K″=0, 1, and 2 subbands of a perpendicular-type band centered near 2880 cm−1 and a parallel-type band near 2857 cm−1 sharing a common vibrational ground state. For the ground state of the H 35Cl–H 35Cl dimer, the K″=0 rotational constant B̄=1944.29(7) MHz and the K″=1 asymmetry splitting B−C=19.5(3) MHz yield a zero-point center-of-mass separation of the two monomer constituents of 3.797(1) Å. Another difference band for the 35Cl isotope has also been observed as well as a similar set of bands for the mixed H 35Cl–H 37Cl species. The spectra suggest an extremely rapid tunneling motion between equivalent forms of nearly orthogonally oriented monomer units.