Reanalysis of the a 4 - PowerPoint Presentation

Slide1

Reanalysis of the a 4Σ- - X 2Πr Transition of GeHUsing 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 Slide2

Why Investigate GeH?Germanium hydrides, GeHn, have been studied because of their role in the production of germanium thin films from the dissociation of GeH4There has been some evidence that coordination of Ge to metal complexes alters reactivity significantlyAs Group IIIA element, Ge containing species are carbon analogs and investigation of the electronic structure of these species helps to determine periodic trends2Slide3

Previous Studies of GeHFirst observed spectroscopically in 1953Kleman and Werhagen identified the A 2Δ – X 2Π transition in the near UV and the a 4Σ- - X

2

Π transition in the visible in the emission

Barrow, Drummond, and

Garton

identified the B 2Σ+ - X 2Π transition in the UV in absorptionKlynning observed the A 2Δ – X 2Π transition in absorption in 1966 and performed a more complete rotational analysisBetween 1985 and 1993, several studies were performed investigating the X 2Πr ground state of GeH and GeDTechniques included CO, CO2, and Faraday Laser Magnetic Resonance & diode laser spectroscopyComprehensive fit of all data for the ground state from all isotopologues was performed by Towle and Brown in 1993Several computational investigations have also been performedThe most recent was published in 2015 by Li et al.X 2Πr ground state 95% from the 8σ29σ24π1 configurationa 4Σ- is the first excited state, 94% from the 8σ29σ14π2 configuration

3Slide4

PECs for GeH from Li et al.4Slide5

The a 4Σ- - X 2Πr TransitionThis spin forbidden transition was observed weakly in emission by Kleman and WerhagenSpectra were not included in publication because “their weakness and the overlapping continuum do not permit an acceptable reproduction.”Observed 4 branches for the a 4

Σ

-

-

X

2Π1/2 transitionBudo and Kovacs predicted 8 branches to have reasonable intensityObserved 5 branches for the a 4Σ- - X 2Π3/2 transitionPredicted to have 10 branches with reasonable intensityNo 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 dataLi 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 calculationsIn 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

5Slide6

Experimental MethodsGeH molecules were produced in the plasma discharge of an Aluminum hollow cathode500 mtorr of Ar used as the sputter gas100 mtorr of H2 and 200 mtorr of GeH4 as reagent gassesDC power supply to cathode: 0.40 – 0.60 AThe hollow cathode was located within the laser cavity of a dye laserDCM 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-1.6Slide7

Instrument Schematic7Ar, H2, & GeH4Aluminum HollowCathodeSlide8

Intracavity Dye Laser8Slide9

ILS Spectrum9Slide10

Isotopologue splitting of GeH Transitions10Q1(8.5)P21(9.5)

70

GeH: 20.84%

72

GeH: 27.54%

74

GeH: 36.28%76GeH: 7.61%Slide11

4Σ- -Hund’s Case (a) 2Π1/2 Transitions11Slide12

Rotational AnalysisTo date, the absorption spectrum of GeH has only been measured for the a 4Σ- - X 2Π1/2 transitionThe spin-orbit splitting of the X 2Πr state is 893 cm

-1

The highest J”-line observed in this set of data is the R

1

(16.5), which is 1800 cm

-1

above the ground stateLines from the a 4Σ- - X 2Π3/2 transition are expected to be weak in absorption and have not yet been observed8 Rotational branches have been identifiedThese 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 branchesFeatures 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 region12Slide13

ILS Spectrum13Slide14

Ground State Constants for GeH from Towle and Brown14Slide15

Determined Molecular Constants for thea 4Σ- State of GeH15Slide16

ConclusionsThe a 4Σ- - X 2Π1/2 transition of GeH has been recorded in absorption for the first timeAll 8 branches expected to have non-zero intensity have been identified in the experimental spectrumRotational analysis has been performed for 3 of the 5 major isotopologues of GeH, and molecular constants have been obtained for the a 4

Σ

-

state of

70

GeH,

72GeH, and 74GeHAttempts to identify the a 4Σ- - X 2Π3/2 transition of GeH are ongoing16Slide17

AcknowledgementsNational Science FoundationUniversity of Missouri-St. Louis Department of Chemistry and Biochemistry & Center for Nanoscience17