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 Microwavea,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, 053103.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 Nozzlea (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 Sourceb

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, 096106.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 Sourcea

• 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, 096106.

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 studieda 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

Isotopes Natural Abundance (%)

Nuclear Spin, I

130Ba 0.106 0132Ba 0.101 0134Ba 2.417 0135Ba 6.592 3/2

136Ba 7.854 0137Ba 11.232 3/2138Ba 71.698 0

Isotopes Natural Abundance (%)

Nuclear Spin, I

32S 94.93 0

33S 0.76 3/2

34S 4.29 0

36S 0.02 0

Barium Sulfide• J = 2 – 1, ν = 0 transition of 138Ba32S to 136Ba32S

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• H2S in Argon and Helium

Variables held constant while not being studied: 75% LaserPower, .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 12100-12450 MHz

Control

BaS Run Band Ratio Intensity Ratio Value

96508 Shots ν=1/ν=0 2.57/15.9 0.162

J = 2-1 ν=2/ν=0 1.81/15.9 0.114

ν=3/ν=0 1.32/15.9 0.083

ν=4/ν=0 1.01/15.9 0.064

ν=5/ν=0 0.73/15.9 0.046

ν=6/ν=0 0.56/15.9 0.035

Laser Power ResultsLaser Power Band Ratio Intensity Ratio Value Control

65% ν=1/ν=0 2.434/16.782 0.145 0.162

ν=2/ν=0 1.857/16.782 0.111 0.114

75% ν=1/ν=0 6.851/32.666 0.210 0.162

ν=2/ν=0 5.564/32.666 0.170 0.114

80% ν=1/ν=0 3.504/17.022 0.206 0.162

ν=2/ν=0 2.864/17.022 0.168 0.114

85% ν=1/ν=0 1.841/7.033 0.262 0.162

ν=2/ν=0 1.456/7.033 0.207 0.114

90% ν=1/ν=0 0.648/2.351 0.276 0.162

ν=2/ν=0 0.576/2.351 0.245 0.114

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 Ratio Intensity Ratio Value Control

6718 ν=1/ν=0 4.330/21.721 0.199 0.162

ν=2/ν=0 3.583/21.721 0.165 0.114

5720 ν=1/ν=0 4.157/20.620 0.202 0.162

ν=2/ν=0 3.235/20.620 0.157 0.114

4704 ν=1/ν=0 2.375/11.642 0.204 0.162

ν=2/ν=0 1.921/11.642 0.165 0.114

3709 ν=1/ν=0 0.861/4.60 0.187 0.162

ν=2/ν=0 0.530/4.60 0.115 0.114

2692 ν=1/ν=0 0.702/3.36 0.209 0.162

ν=2/ν=0 0.5401/3.36 0.161 0.114

1707 ν=1/ν=0 0.738/4.01 0.184 0.162

ν=2/ν=0 0.611/4.01 0.152 0.114

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

Concentration Band Ratio Intensity Ratio Value Control

.3% ν=1/ν=0 0.729/4.58 0.164 0.162

ν=2/ν=0 0.560/4.58 0.122 0.114

.6% ν=1/ν=0 0.979/6.16 0.159 0.162

ν=2/ν=0 0.739/6.16 0.120 0.114

.9% ν=1/ν=0 No Signal N/A 0.162

ν=2/ν=0 No Signal N/A 0.114

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 Gas Band Ratio Intensity Ratio Value Control

Argon ν=1/ν=0 3.085/14.518 0.212 0.162

ν=2/ν=0 2.131/14.518 0.147 0.114

Helium ν=1/ν=0 0.935/6.267 0.149 0.162

ν=2/ν=0 0.742/6.267 0.118 0.114

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, 1758.

Ba + OCS BaS + ___ Laser Ablation

Ba + Scontaining gas BaS + ___ Parameter

H2S Gas Mixture ResultsBacking Gas Band Ratio Intensity Ratio Value Control

Argon ν=1/ν=0 0.788/3.805 0.207 0.162

ν=2/ν=0 0.397/3.805 0.104 0.114

Helium ν=1/ν=0 No Signal/.771 N/A 0.162

ν=2/ν=0 No Signal/.771 N/A 0.114

Backing Gas Band Ratio Intensity Ratio Value Control

Argon ν=1/ν=0 3.085/14.518 0.212 0.162

ν=2/ν=0 2.131/14.518 0.147 0.114

Helium ν=1/ν=0 0.935/6.267 0.149 0.162

ν=2/ν=0 0.742/6.267 0.118 0.114

OCS Gas Mixture Results

H2S 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)

• H2S 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

• H2S 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 TransitionsIsotopomer ν J’ - J’’

Frequency (MHz)

138Ba 32S 0 1 - 0 6185.108 2 - 1 12370.194 3 - 2 18555.236 4 - 3 24740.207 1 1 - 0 6166.163 2 - 1 12332.301 3 - 2 18498.397 2 2 - 1 12294.305 3 - 2 18441.404 3 2 - 1 12256.203 3 - 2 18384.248 4 2 - 1 12217.991 3 - 2 18384.248 5 2 - 1 12179.666 6 2 - 1 12141.227 7 2 - 1 12102.669

137Ba 32S 0 2 - 1 12388.006 12384.321 12387.169

3-2 18581.17018578.77618575.922

Isotopomer ν J’ - J’’Frequency

(MHz)136Ba 32S 0 1 - 0 6202.231

2 - 1 12404.438 3 - 2 18606.602 1 2 - 1 12366.387 3 - 2 18549.526 2 2 - 1 12328.235 3 2 - 1 12289.972

135Ba 32S 0 2 - 1 12422.457 12416.185 12426.926 3 - 2 18633.120 18631.563 1 2 - 1 12377.540 12378.359

134Ba 32S 0 2 - 1 12439.697 3 - 2 18659.488 1 2 - 1 12401.487

138Ba 34S 0 2 - 1 11780.667 3 - 2 17670.952 1 2 - 1 11745.456

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 135Ba and 137Ba species

Constants Frequency (MHz)

Y01 3097.28318(674)

Y02 -0.000918966(198)

Y03 0.000000000033(32)

Y11 -9.448321(323)

Y21 -0.012240(113)

Y31 -0.0001314(105)

Y12 -0.000001017(234)

Y22 -0.0000001956(873)

Δ01 -3.257(559)

Δ01 -5.1045(780)

Ba

S

Our Work for 138Ba32S Constants Frequency (MHz)

Y01 3097.28216(26)

Y02 -0.000918568(63)

Y03 NOT REPORTED

Y11 -9.44620(33)

Y21 -0.013323(66)

Y31 NOT REPORTED

Y12 -0.000001554(73)

Y22 NOT REPORTED

Δ01 NOT REPORTED

Δ01 NOT REPORTEDS

G. Winnewissera for 138Ba32S

a. D. A. Helms, M. Winnewisser, and G. Winnewisser, J. Phys. Chem., 84, 1758

Ba

Overall Conclusions

• Laser Power and H2S 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 (CS2)

Balle-Flygare Spectrometer Techniques

• Balle-Flygare Fourier Transform Microwave1 (FTMW) spectrometer advancements of coaxial orientation of the sample nozzle2 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