Reanalysis of the a 4Σ- - X 2Πr Transition of GeH Using Intracavity Laser Spectroscopy Jack C. Harms, Leah C. O’Brien,* and James J. O’Brien University of Missouri – St. Louis Department of Chemistry and Biochemistry *Southern Illinois University Edwardsville Department of Chemistry

Why Investigate GeH? Germanium hydrides, GeHn, have been studied because of their role in the production of germanium thin films from the dissociation of GeH4 There has been some evidence that coordination of Ge to metal complexes alters reactivity significantly As Group IIIA element, Ge containing species are carbon analogs and investigation of the electronic structure of these species helps to determine periodic trends

Previous Studies of GeH First observed spectroscopically in 1953 Kleman and Werhagen identified the A 2Δ – X 2Π transition in the near UV and the a 4Σ- - X 2Π transition in the visible in the emission Barrow, Drummond, and Garton identified the B 2Σ+ - X 2Π transition in the UV in absorption Klynning observed the A 2Δ – X 2Π transition in absorption in 1966 and performed a more complete rotational analysis Between 1985 and 1993, several studies were performed investigating the X 2Πr ground state of GeH and GeD Techniques included CO, CO2, and Faraday Laser Magnetic Resonance & diode laser spectroscopy Comprehensive fit of all data for the ground state from all isotopologues was performed by Towle and Brown in 1993 Several computational investigations have also been performed The most recent was published in 2015 by Li et al. X 2Πr ground state 95% from the 8σ29σ24π1 configuration a 4Σ- is the first excited state, 94% from the 8σ29σ14π2 configuration

PECs for GeH from Li et al.

The a 4Σ- - X 2Πr Transition This spin forbidden transition was observed weakly in emission by Kleman and Werhagen Spectra were not included in publication because “their weakness and the overlapping continuum do not permit an acceptable reproduction.” Observed 4 branches for the a 4Σ- - X 2Π1/2 transition Budo and Kovacs predicted 8 branches to have reasonable intensity Observed 5 branches for the a 4Σ- - X 2Π3/2 transition Predicted to have 10 branches with reasonable intensity No isotopic resolution of peaks (70Ge: 21%, 72Ge: 28%, 74Ge: 36%, 76Ge: 8%) Klynning attempted to observe the transition in absorption, but was unsuccessful, so he reanalyzed the Kleman data Li et al. suggest that “…experimental effort should be made to decrease the gap between experiment and ab initio calculations,” based on the deviation of ~0.02 Å in Re for the a 4Σ- state between experiment and the past two high-level ab initio calculations In our work, the a 4Σ- - X 2Π1/2 transition has been recorded in absorption using Intracavity Laser Spectroscopy (ILS). All 8 of the expected branches have been identified, and 2 branches are isotopically resolved

Experimental Methods GeH molecules were produced in the plasma discharge of an Aluminum hollow cathode 500 mtorr of Ar used as the sputter gas 100 mtorr of H2 and 200 mtorr of GeH4 as reagent gasses DC power supply to cathode: 0.40 – 0.60 A The hollow cathode was located within the laser cavity of a dye laser DCM Laser Dye Verdi V10 pump laser operating at 1.50 W XYZ translation of highly refractive wedge for tuning Cathode lengths ranged from 75-150 mm and generation times for experiments ranged from 90-200 μsec Laser cavity ~1.1 m long: effective pathlengths 2-7 km A spectrum from an external I2 cell was collected after each plasma spectrum, and the recorded I2 positions were calibrated in PGOPHER using the reference data of Salami and Ross. The I2 calibrations were then applied to the corresponding plasma spectra. Average deviations in the calibrations were typically less than ±0.002 cm-1.

Instrument Schematic Ar, H2, & GeH4 Aluminum Hollow Cathode

Intracavity Dye Laser

ILS Spectrum

Isotopologue splitting of GeH Transitions Q1(8.5) P21(9.5) 70GeH: 20.84% 72GeH: 27.54% 74GeH: 36.28% 76GeH: 7.61%

4Σ- -Hund’s Case (a) 2Π1/2 Transitions

Rotational Analysis To date, the absorption spectrum of GeH has only been measured for the a 4Σ- - X 2Π1/2 transition The spin-orbit splitting of the X 2Πr state is 893 cm-1 The highest J”-line observed in this set of data is the R1(16.5), which is 1800 cm-1 above the ground state Lines from the a 4Σ- - X 2Π3/2 transition are expected to be weak in absorption and have not yet been observed 8 Rotational branches have been identified These match the branches predicted by Budo and Kovacs to be observable for a 4Σ- - Hund’s Case (a) 2Π transition Isotopologue splitting is resolved in the P21 and Q1 branches Features of the other 2 red and 4 blue branches were degraded to the red and blue, respectively, but isotopologue separation was not fully resolved. The ground state constants were held fixed to the values provided by Towle and Brown from the fit of the various infrared spectroscopic investigations The molecular constants of GeH2 from Smith et al. were used in the PGOPHER simulation to predict the spectrum of GeH2 in this region

ILS Spectrum

Ground State Constants for GeH from Towle and Brown

Determined Molecular Constants for the a 4Σ- State of GeH

Conclusions The a 4Σ- - X 2Π1/2 transition of GeH has been recorded in absorption for the first time All 8 branches expected to have non-zero intensity have been identified in the experimental spectrum Rotational analysis has been performed for 3 of the 5 major isotopologues of GeH, and molecular constants have been obtained for the a 4Σ- state of 70GeH, 72GeH, and 74GeH Attempts to identify the a 4Σ- - X 2Π3/2 transition of GeH are ongoing

Acknowledgements National Science Foundation University of Missouri-St. Louis Department of Chemistry and Biochemistry & Center for Nanoscience