Observation of the infrared spectrum of the helium hydride molecular ion

This dissertation describes the first high-precision observation of the infrared spectrum of the helium hydride molecular ion HeH+. The frequencies of five vibrational-rotational transitions in the range 1700 to 1900 cm-1 in the X1Σ+ ground electronic state of 4HeH+ have been measured to +-0.002 cm-1(+-1 ppM). The Doppler tuned ion beam laser spectroscopic method was used in making the measurements: In a region of constant electrostatic potential, an HeH+ ion beam of several keV energy is intercepted at a small angle by a beam from a carbon monoxide infrared gas laser. The energy of the ion beam is adjusted to Doppler-shift an ion transition into resonance with a nearby laser line. On resonance the laser light stimulates transitions to take place. If the resonating states differ in population, the laser-induced transitions produce a net population transfer. The occurrence of population transfer is detected by monitoring the transmission of the ion beam through a gas target downstream from the laser beam interaction region. The transmission through the target is dependent upon the ion beam vibrational-state population distribution and therefore is sensitive to changes in the population distribution, because the cross-section for charge-exchange neutralization of an incident ion is dependent upon the vibrational state of the ion.The current interest in molecular ions in general, and in HeH+ in particular, is explained. The existing theory of the structure of HeH+ is summarized and a comprehensive listing of theoretical treatments of the structure of HeH+ is given. The meager previous experimental work on HeH+ is reviewed. The principles of the Doppler tuned ion beam laser resonance method are discussed and the experimental apparatus used is described in detail