Infrared–microwave double resonance spectroscopy of molecular ions: HN+2

Abstract

Infrared–microwave double resonance spectroscopy has been used to study the simple molecular ion HN+2. This is the first application of this powerful method to a molecular ion. The HN+2 ion was produced in a hollow cathode discharge cell, which served two purposes. First, the hollow cathode discharge is capable of producing relatively large concentrations of ions at the low pressures required for saturation. Also, the hollow cathode serves as a waveguide for the propagation of the microwave radiation. A color center laser was used as the infrared source. Its high power was essential in this work for saturation of the infrared transitions. The HN+2 ion was chosen as a test case for double resonance because of the availability of detailed spectroscopic information and its ease of production. We have observed the pure rotational transition J=1←0 in both the ground and ν1 states using the ν1 infrared fundamental. The determined molecular constants are eqQ=−5.88(10) MHz and B=46 586.895(18) MHz for the ground state, and eqQ=−5.71(17) MHz and B=46 208.986(27) MHz for the ν1 state. The high sensitivity of the double resonance technique has also allowed us to study the direct l-type doubling transitions in the ν2 and ν1+ν2 states using the ν1+ν2←ν2 infrared hot band. To the best of our knowledge, this is the first centimeter wave spectrum of a molecular ion. The determined molecular constants are q=254.966(11) MHz and qJ =−2.83(17) kHz for the ν2 state, and q=258.912(32) MHz and qJ =−3.15(65) kHz for the ν1+ν2 state.