FE06 : Collisional Cross-Sections at Low Temperatures

David L. GraffT. J. RonningenF. C. De Lucia

The Ohio State University

Low Energy Molecular Interactions

Typical Cross-Section as a Function of Temperature

• Doppler Width– Std Dev < 1.5%

• Temperature– Std Dev < 3%

Molecular Thermometry

Low Temperature Molecules

Analysis: Extract from each Lineshape

Line ShapeParameter

PressureDependence

TemperatureDependence

Line Origin Independent Independent

Doppler Width Independent Square root

Line Center Linear shift Not analytical

Lorentzian Width

Linear increase Not analytical

HC14N - He : Broadening / Shifting• Experimental errors were reduced to 3%• Data are compared to three PES• Ronningen et al. JCP 122 184319 (2005)

8

6

4

2

0-5

0

5

Translation Stage

ΔP = (P2P1+1)ΔP1

P2

P1

Adjust Mirror Until Absorption “Fits”0 Steps-2 Steps 2 Steps

-15

-10

-5

0

5

10

15

6543210

0-221101201012010101012010101

Lorentzian Width Deviation From Linear Fit

Number Indicate Mirror Position

2.0

1.5

1.0

0.5

0.0

172.110172.108172.106GHz

-100-50

050

100

2.0

1.5

1.0

0.5

0.0

172.110172.108172.106GHz

-100-50

050

100

2.0

1.5

1.0

0.5

0.0

172.110172.108172.106GHz

-100-50

050

100

-15

-10

-5

0

5

10

15

6543210Pressure /mTorr

0

-2

21

1012

01

012

010

101

012

01

0101

Pressure Shift as Affected by Mirror Position

± 1.5 MHz/Torr-15

-10

-5

0

5

10

15

6543210

201

10121101

Select Lineshapes "Best Fit"

± 1.5 MHz/Torr

Fit Each Lineshape for Dispersion

• Functional Form is Voigt Absorption plus Lorentzian Dispersion term– Each term has the same line-center and

Lorentzian linewidth

2.0

1.5

1.0

0.5

0.0

172.110172.109172.108172.107172.106GHz

172.110172.109172.108172.107172.106GHz

-1000

100

Lineshape Fit With and Without Dispersion

Include Dispersion in the Fit

-15-10

-505

1015

6543210Pressure /mTorr

-15-10

-505

1015

0

-2

21

1012

01

012

0101

01012

010101

0-221101201012010101012010101

Fit For Dispersion

Fit Without Dispersion

± 1.5 MHz/TorrNumber = Mirror Position

Comparison of Methods

6543210-15-10

-505

10

15

-15-10

-5

05

1015

0-221101201012010101012010101

201

10121101

Fit Includes Dispersion

Select Lineshapes for "Best Fit"

Fit: -0.164 ± 0.269 MHz/Torr

Fit: -0.458 ± 0.051 MHz/Torr

The two analyses do not yield the same answer

Compare Cross-Sections

Temp (K) Broadening (A2) Shifting (A2) Broadening (A2) Shifting (A2)

2.01 17.1(0.5) -0.8(1.0) 17.0(0.4) 0.5(1.9)

2.47 18.8(0.6) -1.1(1.3) 19.4(0.4) 0.7(2.0)

3.14 20.7(0.6) -0.5(1.5) 21.6(0.4) 0.3(2.2)

4.21 23.0(0.7) -0.8(1.7) 22.9(0.5) 0.0(2.6)

HC15N HC14NJ =2 ← 1,F=3← 2J =2 ← 1

Conclusions

• Low temperature pressure broadening measurements continue to disagree with theoretical predictions

• Careful consideration of our experimental variables have sharpened this difference

• We have improved our method of measuring the pressure shift parameter to gain greater precision

Acknowledgements• Co-Authors

– T.J. Ronningen– Frank De Lucia

• De Lucia Group– Brenda, Manfred– Ivan, Atsuko– Sarko, Kerra and Corey

• Funding– ARO and NSF

CaH Buffer Gas Trapping

• CaH -He(3) PES was calcuated by Balakrishnan et. Al.

• Diffusion rate was measured to be 2-8 times less than predicted values

Doppler Width as Thermometer

• Standard deviation of retrieved Doppler width from thermometer measurement is < 1.5%

• This corresponds to < 3% standard deviation in Temperature measurement

• Linewidths retrieved from data simulated with incorrect transpiration corrections were not linear with pressure

CO - He : Broadening / Shifting

• Measurements extended from 1 - 600K• Thachuk et al. compared data with four PES• Agreement was very good down to 20K

Thachuk et al. JCP 105 4005

CO - He : Broadening / Shifting

• Measurements extended from 1 - 600K• Thachuk et al. compared data with four PES• Agreement was very good down to 20K

Thachuk et al. JCP 105 4005

• Measured both Pressure Broadening / Shifting and Inelastic Depopulation Rates

• Substantial Disagreement Below 10K

J =10,1 ← 11,0 J =21,1 ← 22,0

Ball, et al. JCP 111 8893

H2S - He : Broadening / Shifting

HCN - He Pressure Broadening