Knowledge Bank

Rotational structure in the spectrum of the H-C1 stretching bands of the HC1 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 $(T\sim 130K)$ of HC1 at low pressures (3-4 Torr) and long pathlengths (64-80 m). Rotational assignments have been made for $K\prime \prime =0,1$, and 2 subbands of a perpendicular-type band centered near $2880cm-1$ and a parallel-type band near $2857cm-1$ sharing a common vibrational ground state. For the ground state of the $H35C1-H35C1$ dimer, the $K\prime \prime =0$ rotational constant, $B¯=1944.29(7)MHz$, and the $K\prime \prime =1$ asymmetry splitting, $B-C=19.5(3)MHz$, yield a zero-point center-of-mass separation of the two monomer constitutents of $3.797(1)\backslash AA$. Another difference band for the $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}C1$ isotope has also been observed as well as a similar set of bands for the mixed $H35C1-H37C1$ species. The spectra suggest an extremely rapid tunneling motion between equivalent forms of nearly orthogonally oriented monomer units.