Vibration-rotation spectroscopy of the HD+ion near the dissociation limit

Abstract

A sequential two-photon photodissociation experiment employing infrared lasers which interact with a fast ion beam has been used to detect vibration-rotation transitions between levels of the HD⁺ ion near the dissociation limit. Measurements of five P-branch transitions in the 17-15 band and six rotational components of the 20-17 band are reported. The proton nuclear hyperfine splittings for transitions in the 20-17 band are much larger than for the previously measured 17-14 and 18-16 bands. The resolved structure and lineshapes are shown to provide conclusive experimental evidence of an asymmetric electron (and therefore charge) distribution, in the sense H⁺ + D, as the dissociation limit is approached. The effects of this dramatic breakdown of the Born-Oppenheimer approximation are explored in theoretical calculations of the hyperfine splittings.