The spin-forbidden a 4Σ−–X 2Π1/2 transition of GeH detected in absorption using intracavity laser spectroscopy

The  a   4 Σ − – X   2 Π 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 H 2 , and 200 mTorr of GeH 4 . 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 I 2  collected from an extracavity cell, the I 2  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   4 Σ −  states of  70 GeH (20.84% abundant),  72 GeH (27.54% abundant), and  74 GeH (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   4 Σ −  state of  74 GeH determined by the fit are  T 0  = 16 751.5524(13) cm −1 ,  B 0  = 6.764 912(33) cm −1 ,  D 0  = 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 .