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Fine and hyperfine structure in the 2II ground electronic state of HBr+ and HI+ Chanda, Alak


The vibration-rotation spectrum of HBr⁺ in the ²H[sub 1/2] and ²H[sub 2/3] spin substates of the ground electronic state has been investigated between 1975 cm⁻¹ and 2360 cm⁻¹ using a tunable diode laser spectrometer coupled to an a.c. glow discharge cell. Both fine and hyperfine studies have been carried out. In the former, about 300 vibration-rotation tran sitions were measured for each of the isotopomers H⁷⁹Br+ andH⁸¹Br+. These belonged to the five bands (v’—v”)=(1—0) to (5—4). The observed linewidth was ‘--‘0.006 cm⁻¹. In this inverted 2 state, the difference ((A[sub e] — ω[sub e]) is small (‘∼‘200 cm⁻¹) compared to We (2440 cm⁻¹). Here A[sub e] and ω[sub e] are the equilibrium values of the spin-orbit constant and the harmonic vibrational frequency, respectively. As a result, the energy levels oc cur in neighbouring, but non-resonant, pairs with (v, ²π[sub 3/2]) coupled to ( v — 1, ²π[sub 1/2]). The one exception is the ²π[sub 3/2] state with v=0, which is isolated. Centrifugal distor tion matrix elements between partner states have been shown to effect significantly the A-doubling. A model has been developed in which these distortion matrix elements are treated by a vibrational Van Vleck transformation carried to third order. A good fit has been obtained without introducing any new fitting parameters to characterize the (Δv ≠ 0; ΔΩ = ±1) effects. Equilibrium values were determined for the principal pa rameters which characterize the individual vibrational levels. In the hyperfine study, a combined total of 57 hyperfine splittings were observed in the two spin substates of H⁷⁹Br, the transitions in the ²π[sub 1/2] spin substate being ob served for the first time. An equal number were measured for H⁸¹Br+. These transitions were distributed over the P, Q, and R branches of the four lowest vibrational bands. The matrix elements for the magnetic dipole and electric quadrupole interactions have been written in the e/f symmetrized scheme more commonly used in vibration-rotation prob lems. Values have been obtained for the Frosch and Foley magnetic hyperfine constants a, c, and d by using the value of b determined by Lubic et al., J. Mol. Spectrosc. 131, 21-31 (1989). These results have been used to investigate the electronic properties of the ion. The analysis supports a model in which the electron distribution is close to that of a bromine atom perturbed by a proton. A similar study of the HI⁺ molecular ions has also been carried out. Prior to the current investigation, the spectroscopic information on HI⁺ was limited to that obtained from low resolution. A total of more than 100 vibration-rotation transitions belonging to the (v’ — v”)=(1—0), (2—1) and the (3—2) vibrational bands of the ²π[sub 3/2], spin substate and to the (1—0) vibrational band of the ²π[sub 1/2] spin substate have been recorded in the frequency range from 1995 cm⁻¹ to 2245 cm⁻¹. The observed linewidth was ∼0.004 cm⁻¹ Equilibrium values were determined for all the principal parameters characterizing the individual vibrational levels. The precision of the vibrational constants, ω[sub e] and ω[sub e]x[sub e], were improved by a factor of ∼10⁶ over the values determined recently by Böwering et aL, Chem. Phys. Lett. 189, 467 (1992) and by Zietkiewicz et al., J. Chem. Phys. 101, 86 (1994), using photoelectron techniques. Large hyperfine splittings arising from magnetic dipole and electric quadrupole in teractions were observed for low J transitions belonging to both of the Ω substates of HI⁺. The Frosch and Foley magnetic hyperfine constants a, b+c, and d, and the electric quadrupole constant eQq₀ were determined for the first time in this molecular ion.

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