Hyperfine-resolved rovibrational spectrum of the X 2Π state of HI+

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Abstract
A high resolution spectroscopic study of HI+ has been carried out for the first time. The absorption spectrum in the inverted 2Π ground electronic state has been measured with a linewidth of 0.004 cm−1 between 1995 and 2245 cm−1. A total of 117 vibration–rotation transitions were observed with a tunable diode laser spectrometer coupled to an ac glow discharge cell employing velocity modulation. Lines were measured in the vibrational fundamental of the 2Π1/2 spin substate and in the three lowest (v+1←v) bands of the 2Π3/2 spin substate. A good fit to the data was obtained using a standard vibration–rotation, fine structure Hamiltonian. Equilibrium values were determined for 16 molecular parameters including the harmonic vibrational frequency ωe, the rotational constant Be, and the Λ‐doubling constants pe and qe. A review of the ground state properties of the hydrogen halide ions HX+ (X=F, Cl, Br, I) shows that the harmonic force constant is, to excellent approximation, a linear function of the internuclear spacing. In ten different vibration–rotation transitions of the two spin substates of HI+, hyperfine splittings were observed. A total of 58 relative splittings of hyperfine components were analyzed to determine the iodine quadrupole coupling constant eQq0 as well as the Frosch–Foley magnetic hyperfine constants a, (b+c), and d. The results have been used to investigate the electronic properties of the ion.