Measurement of the Vibrational Population Distribution of Barium Sulfide, Seeded in an Argon Supersonic Expansion, Following Production Through the Reaction of Laser Ablated Barium with Carbonyl Sulfide Chris T. Dewberry, Garry S. Grubbs II, Kerry C. Etchison and Stephen A. Cooke, Department of Chemistry, University of North Texas, Denton, TX, USA

Chirp Pulse Techniques The recent advancement in microwave spectroscopy of Chirped Pulse Fourier Transform Microwave a,b,c (CP-FTMW) techniques have greatly broadened search regions shortening acquisition times while also allowing for relative correct intensities of spectra a. G.G. Brown, B.C. Dian, K.O. Douglass, S.M. Geyer and B.H. Pate, J. Mol. Spec., 238, 200. b. Brown et al, Rev. Sci. Instr., 79, c. G.S. Grubbs II et al, J. Mol. Spec., 251, 378.

Laser Ablation Techniques Advancements in Laser Ablation Techniques, particularly the Walker-Gerry Ablation Nozzle a (Pictured), allow for the introduction of solid, transient, and plasma chemistry to be studied in the gas phase by microwave spectrometers a. K.A. Walker and M.C.L. Gerry, J. Mol. Spec., 182, 178.

Instruments The instruments used in these experiments were a Search Accelerated, Correct Intensity Fourier Transform Microwave (SACI-FTMW) a spectrometer with Laser Ablation Source and a Balle-Flygare type FTMW spectrometer with Laser Ablation Source b SACI-FTMW (upper left) Balle-Flygare Type (above) a. G.S. Grubbs II, C.T. Dewberry, K.C. Etchison, K. Kerr, and S.A. Cooke, Rev. Sci. Instr., 78, b. K.C. Etchison, C.T. Dewberry, and S.A. Cooke, Chem. Phys., 342, 71.

SACI-FTMW with Laser Ablation Pictured above is a sampling of spectra from SACI-FTMW with Laser Ablation Source a The relative intensities of the spectra demonstrate good agreement with isotopic abundance a. G.S. Grubbs II, C.T. Dewberry, K.C. Etchison, K. Kerr, and S.A. Cooke, Rev. Sci. Instr., 78,

Barium Sulfide Given these advancements, new questions can be asked about the chemistry of the molecule in the laser ablation event These questions are more easily approached using simple diatomic species Barium Sulfide, a previously studied a closed shell molecule, seemed a good candidate for these experiments due to its large dipole moment a. D. A. Helms, M. Winnewisser, and G. Winnewisser, J. Phys. Chem., 84 (1980), 1758

Barium and Sulphur Isotopes IsotopesNatural Abundance (%) Nuclear Spin, I 130 Ba Ba Ba Ba6.5923/2 136 Ba Ba /2 138 Ba IsotopesNatural Abundance (%) Nuclear Spin, I 32 S S0.763/2 34 S S0.020

Barium Sulfide J = 2 – 1, ν = 0 transition of 138 Ba 32 S to 136 Ba 32 S was 8.94 : 1 and the natural isotopic ratio is 9.12 : 1 QUESTION: Is there a parameter we can manipulate to alter the vibrational state distribution intensities or is everything dominated by the supersonic expansion?

Parameters Studied Laser Power Backing Gas Pressure OCS Concentration OCS in Argon and Helium H 2 S in Argon and Helium Variables held constant while not being studied: 75% Laser Power,.3% OCS Concentration, Argon backing gas, 4.5 atm backing pressure

Notes BaS was seen to be present simply by a test of the Laser being on vs. off RELATIVE intensities have been used in the experiments by as ratios of one transition to another Any other transitions have been measured with the Balle-Flygare type FTMW spectrometer Experiments were performed on the 3 lowest vibrational states of the main isotopologue

Experimental Control 20 gas units of OCS in Argon at 7000 gas units (~4.5 atm) with a Laser Power of 75% maximum power (1560 gas units = 1 atm) Chirp pulse lengths are 3 μs with a span of 2 GHz being examined at a time Timings are generally the same throughout the experiment

BaS Spectra A sample BaS spectra taken from the SACI- FTMW spectrometer (96508 Averaging Cycles)

Control Scan Zoom In MHz

Control BaS RunBand RatioIntensity RatioValue Shotsν=1/ν=02.57/ J = 2-1ν=2/ν=01.81/ ν=3/ν=01.32/ ν=4/ν=01.01/ ν=5/ν=00.73/ ν=6/ν=00.56/

Laser Power Results Laser PowerBand RatioIntensity RatioValueControl 65%ν=1/ν=02.434/ ν=2/ν=01.857/ %ν=1/ν=06.851/ ν=2/ν=05.564/ %ν=1/ν=03.504/ ν=2/ν=02.864/ %ν=1/ν=01.841/ ν=2/ν=01.456/ %ν=1/ν=00.648/ ν=2/ν=00.576/

Laser Power Conclusion A trend of the ν =1/ν=0 and ν =2/ν=0 ratios show an increase toward the populations of higher vibrational states as laser power increases Tradeoff of intensity of the spectra with alteration of the distribution

Backing Gas Pressure (with OCS) Results Pressure (Arb. Units) Band RatioIntensity RatioValueControl 6718ν=1/ν=04.330/ ν=2/ν=03.583/ ν=1/ν=04.157/ ν=2/ν=03.235/ ν=1/ν=02.375/ ν=2/ν=01.921/ ν=1/ν=00.861/ ν=2/ν=00.530/ ν=1/ν=00.702/ ν=2/ν= / ν=1/ν=00.738/ ν=2/ν=00.611/

Backing Gas Pressure (with OCS) Conclusion Non-conclusive results due to inconsistent trends in the data Higher backing pressures intensify the bands (as expected with the chemistry in the jet)

Concentration of OCS Results ConcentrationBand RatioIntensity RatioValueControl.3%ν=1/ν=00.729/ ν=2/ν=00.560/ %ν=1/ν=00.979/ ν=2/ν=00.739/ %ν=1/ν=0No SignalN/A0.162 ν=2/ν=0No SignalN/A0.114

Concentration of OCS Conclusion No observed concentration dependence for the vibrational state distribution There is a point at which the mixture becomes too rich causing BaS signal elimination

OCS in Argon and Helium Result Backing GasBand RatioIntensity RatioValueControl Argonν=1/ν=03.085/ ν=2/ν=02.131/ Heliumν=1/ν=00.935/ ν=2/ν=00.742/

OCS in Argon and Helium Conclusion As expected, the Helium lowered the intensity of the ν = 0 transition due to a warmer rotational temperature No real change to the vibrational state populations due to Argon or Helium with OCS

Barium Sulfide Synthesis Ba + OCS BaS + CO Winnewisser* Done in an Oven *D. A. Helms, M. Winnewisser, and G. Winnewisser, J. Phys. Chem., 84, Ba + OCS BaS + ___ Laser Ablation Ba + S containing gas BaS + ___ Parameter

H 2 S Gas Mixture Results Backing GasBand RatioIntensity RatioValueControl Argonν=1/ν=00.788/ ν=2/ν=00.397/ Heliumν=1/ν=0No Signal/.771N/A0.162 ν=2/ν=0No Signal/.771N/A0.114 Backing GasBand RatioIntensity RatioValueControl Argonν=1/ν=03.085/ ν=2/ν=02.131/ Heliumν=1/ν=00.935/ ν=2/ν=00.742/ OCS Gas Mixture Results

H 2 S Gas Mixture Conclusion There is a change of the population distribution not seen in the previous experiments (ν=1/ν=0 ratio goes up while the ν=2/ν=0 ratio relatively remains the same) H 2 S mixed with Helium is not strong enough to be detected currently

Vibrational State Testing Laser power had a trend of increasing the ratios of ν=1/ν=0 and ν=2/ν=0, but with a signal intensity tradeoff Backing pressures were non-conclusive Mixtures of OCS in Argon and Helium only provided knowledge currently understood by the supersonic expansion H 2 S mixtures in Argon yielded an increase in one vibrational state ratio, ν=1/ν=0, while leaving the ν=2/ν=0 ratio relatively unchanged while signal issues prevented the Helium mixed analog to be studied further

Measured Transitions IsotopomerνJ’ - J’’ Frequency (MHz) 138 Ba 32 S Ba 32 S IsotopomerνJ’ - J’’ Frequency (MHz) 136 Ba 32 S Ba 32 S Ba 32 S Ba 34 S This listing is all measured transitions to date including transitions only observed by the cavity experiment. High resolution of the transitions are due to the cavity experiment.

Rotational Constants Hyperfine structure observed for 135 Ba and 137 Ba species ConstantsFrequency (MHz) Y (674) Y (198) Y (32) Y (323) Y (113) Y (105) Y (234) Y (873) Δ (559) Δ (780) Ba S Our Work for 138 Ba 32 S ConstantsFrequency (MHz) Y (26) Y (63) Y 03 NOT REPORTED Y (33) Y (66) Y 31 NOT REPORTED Y (73) Y 22 NOT REPORTED Δ 01 NOT REPORTED Δ 01 NOT REPORTED S G. Winnewisser a for 138 Ba 32 S a. D. A. Helms, M. Winnewisser, and G. Winnewisser, J. Phys. Chem., 84, 1758 Ba

Overall Conclusions Laser Power and H 2 S gas mixtures shifted the distribution of the vibrational band ratio for populations suggesting different chemistry is happening in the ablation process for the molecule Many vibrational state transitions have been studied for isotopologues of the BaS molecule and rotational constants as well as Born- Oppenheimer Breakdown terms have been determined and reported

Acknowledgements Funding from NSF The Cooke Group

Future Work Nozzle Design Other Backing Gases (Ne, He/Ne, Xe, etc.) Other Gases containing Sulphur (CS 2 )

Balle-Flygare Spectrometer Techniques Balle-Flygare Fourier Transform Microwave 1 (FTMW) spectrometer advancements of coaxial orientation of the sample nozzle 2 allows for increased resolution of measured spectra ( ~7 kHz linewidths).

Overview Techniques in Microwave Spectroscopy Dynamics of the Ablation Process Experiments Measured Transitions Rotational Constants Overall Conclusions