CH3D Near Infrared
Cavity Ring-down
Spectrum Reanalysis and
IR-IR Double Resonance
S. Luna Yang
George Y. Schwartz
Kevin K. Lehmann
University of Virginia
06/2015
Outline
Motivation for studying methane isotopomers
Experimental setup of cavity ring-down
spectroscopy (CRDS)
Spectrum analysis of CH3D
- spectrum simulation
- combination differences
- temperature dependence
- double resonance
Future plan and acknowledgement
Credit: NASA, 2009
Scientific American 296, 42 - 51 (2007)
Methane isotopes
CH3D
Applied Optics, Vol. 33, Issue 33, pp. 7704-7716 (1994)
Outline
Motivation for studying methane isotopomers
Experimental setup of cavity ring-down
spectroscopy (CRDS)
Spectrum analysis of CH3D
- spectrum simulation
- combination differences
- temperature dependence
- double resonance
Future plan and acknowledgement
Experimental setup for CRDS methane detection
DFB diode Lasers
#1
TECs
#2
MOS
#3
Current
Sources
PC
#4
70MHz
Beam
Splitter
EOM
Single Pass Cell
Detector #2
Pulse Generator
SOA
Mixer
Detector #1
M
SM1
HeNe
SM2
Cavity
M
PZT
OPM
Typical CRDS spectrum of CH3D at near infra-red region
Cavity ring-down spectroscopy of ~177 ppm CH3D in ~ 8.3 Torr N2 buffer gas, in
comparison with FTIR spectrum (76.7 Torr pressure, 105 m absorption path length ).
FTIR spectrum : K. Deng et al, Molec. Phys., VOL. 97, NO. 6 (1999).
Outline
Motivation for studying methane isotopomers
Experimental setup of cavity ring-down
spectroscopy (CRDS)
Spectrum analysis of CH3D
- spectrum simulation
- combination differences
- temperature dependence
- double resonance
Future plan and acknowledgement
1. Spectrum simulation - comparison
Comparison of CH3D CRDS spectrum with software simulation and FTIR spectrum
FITR: Mol. Phys. 1999
2. Combination differences - theory
If two transitions are from different ground states but both go to the same
overtone band state, J , K   J , K  and J , K   J   1,K  , their frequency
difference is:
H (J   1,K )  H (J ,K )  2B(J  1)  4D J (J  1)3  2D JK (J  1)K 2
 4H JK (J  1)3 K 2  2H KJ (J  1)K 4 +…
The relative intensities of combination differences can also be predicted.
3. Temperature dependence - theory
Line intensity of a certain transition for spherical top molecules
have the following dependence on temperature (when neglecting
vibrational energies and perturbations):
I  C(2J  1)e  E / KT T 3 / 2
Therefore, if we have the line intensity under two different temperatures,
their ratio would be:
 E  1 1   T2
I1
 exp      
I2
 K  T1 T2   T1
 I  3 T 
ln  1   ln  2 
I
2  T1 
E  2
1
1

T2 T1



3
2
The ground state energy, mostly rotational energy, can be estimated:
E=BJ(J+1)+(A-B)K2
3. Temperature dependence - test
CH4 Q-branch
Methane spectrum line intensity temperature dependence (CH4 Q branch)
3. Temperature dependence - test
CH4 Q-branch
Methane CH4 ground state rotational energy comparison between
approximation and temperature dependence experimental result
4. Double resonance -setup
C-H stretching bands of CH3D
…
  1  0 : Fundamental band, near 3000 cm-1, well studied
  2  0 : First overtone band, near 6000 cm-1, much more
  3
complicated and not well studied
Increase absorption
  2
probe
laser
 1
pump
laser
  0
Decrease
absorption
Pump laser:
CW Optical parametric oscillation (OPO)
output wavenumber ~3000cm-1
output power > 1W. ∆ν͌1MHz
Probe laser:
DFB diode laser
output wavenumber ~6000cm-1
output power ~10mW. ∆ν͌10MHz
4. Double resonance - result
Pump laser transition RQ0(8)
Pump laser transition RQ0(5)
• Found about 100 sharp positive&negative peaks from double resonances
for different transitions.
• Improved simulations to be more accurate – the simulation and calibrated
wavenumber of assigned peaks are within 0.004 cm-1 difference.
• Found transitions from other bands that are not yet simulated.
4. Double resonance - four features
Cell: ~300 mtorr pure CH3D
Sharp negative peaks:
The pump laser excites CH3D RR1(1) ν=1←0
transition at 3034.688 cm-1 .
Broad negative peaks:
Correspond to that the probe laser excites
molecules ν=2←0.
Cell: ~1torr pure CH4.
Sharp positive peaks:
The pump laser excites CH4 R(0) ν=1←0
transition at 3028.726 cm-1, and the probe
laser excites molecules ν=3←1.
Broad positive peaks:
Vibrational heating effect of pump laser
beam
Vibration-rotational parameters for simulating 2 4 ( A1 ) and 2 4 ( E ) bands (unit cm-1)
Parameter
Ground state
2 4 ( A1 ) 2 4 ( A1 )
2 4 ( E ) 2 4 ( E )
ν
-
5980.404(8)
6022.203(4)
A
5.250821
5.242(1)
5.2134(7)
B
3.880195
3.862(1)
3.8506(3)
DJ/10-5
5.2614
-1.5(4) ×10
4.8(7)
DJK/10-4
1.26287
-1.6(2)×10
2.6(2)
DK/10-4
-0.78844
3.4(2)×10
-2.3(3)
HJ/10-9
1.394
-
1.0(5)×102
HJK/10-8
1.146
-
3.9(2)×102
HKJ/10-9
-6.58
-
3.9(4)×102
HK/10-9
-1.05
-
-2.9(4)×103
Aζ
-
-
-0.1021(1)
ηJ/10-3
-
-
1.26(2)
ηK/10-3
-
-
1.20(5)
qeff/10-2
-
-
-1.59(2)
qeffK/10-3
-
-
-
-
-
-
/10-3
Outline
Motivation for studying methane isotopomers
Experimental setup of cavity ring-down
spectroscopy (CRDS)
Spectrum analysis of CH3D
- spectrum simulation
- combination differences
- temperature dependence
- double resonance
Future plan and acknowledgement
Future plans
• CH3D spectrum at an intermedia temperature
• ‘Ladder’ double resonance assignment
• Liquid nitrogen cooled double resonance
• Double resonance in CRDS
Acknowledgements
•NSF
•NASA
•University of Virginia
•Yongxin Tang