OBSERVATION OF THE INFRARED SPECTRUM OF THE HELIUM-HYDRIDE MOLECULAR ION

Abstract

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 1700-1900 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. The acquisition and analysis of the data is described and the results are compared with the best existing theoretical predictions of the transition frequencies. The present experimental values (given by D. E. Tolliver, G. A. Kyrala, and W. H. Wing, Phys. Rev. Lett. 43, 1719) for the measured transitions are (with the corresponding values calculated by D. L. Bishop and L. M. Cheung, J. Mol. Spectrosc. 75, 462, given in parentheses): (v,J)=(1,11)↔(0,12), 1855.905 cm⁻¹ (1856.152 cm⁻¹); (1,12)↔(0,13), 1751.971 cm⁻¹ (1752.198 cm⁻¹); (2,8)↔(1,9), 1896.992 cm⁻¹ (1897.139 cm⁻¹); (2,9)↔(1,10), 1802.349 cm⁻¹ (1802.492 cm⁻¹); and (2,10)↔(1,11), 1705.543 cm⁻¹ (1705.684 cm⁻¹). It is seen that the present experimental values deviate from the theory by typically 0.2 cm⁻¹, and are two orders of magnitude more precise than the theoretical values.