The spin-forbidden a ⁴Σ^--X ²Π_1/2 transition of GeH detected in absorption using intracavity laser spectroscopy

The a ⁴Σ^--X ²Π_1/2 transition of GeH has been recorded in absorption for the first time using Intracavity Laser Spectroscopy (ILS). The GeH molecules were produced in a 0.40-0.60 A DC plasma discharge inside an aluminum hollow cathode, using 500 mTorr of Ar, 100 mTorr of H₂, and 200 mTorr of GeH₄. This cathode is located within the resonator cavity of a Coherent Verdi™ V-10 pumped dye laser. Effective path lengths for this series of measurements using the ILS method ranged from 2 to 7 km. Spectra were calibrated using the absorption spectrum of I₂ collected from an extracavity cell, the I₂ transmission spectrum from Salami and Ross, J. Mol. Spectrosc. 223(1), 157 (2005) and PGOPHER's [C. M. Western, J. Quant. Spectrosc. Radiat. Transfer 186, 221-242 (2016)] calibration feature. Differences in peak positions between calibrated experimental spectra and the reference data were on average less than ±0.002 cm^-1. All eight branches expected to have appreciable intensity for the transition have been identified, and isotopologue splitting was observed in features of 5 of the 8 identified rotational branches. Molecular constants have been obtained for the a ⁴Σ^- states of ⁷⁰GeH (20.84% abundant), ⁷²GeH (27.54% abundant), and ⁷⁴GeH (36.28% abundant). The transitions were fit using PGOPHER, holding the ground state constants fixed to the values reported by Towle and Brown [Mol. Phys. 78(2), 249 (1993)]. The constants for the a ⁴Σ^- state of ⁷⁴GeH determined by the fit are T₀ = 16 751.5524(13) cm^-1, B₀ = 6.764 912(33) cm^-1, D₀ = 0.459 60(17) × 10^-3 cm^-1, λ_SS = 9.7453(12) cm^-1, λ_D = 0.468(14) × 10^-3 cm^-1, γ = 0.077 878(84), and γ_S = -0.361(77) × 10^-3 cm^-1.