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