1. LASER-INDUCED FLUORESCENCE (LIF) SPECTROSCOPY
HIGH-RESOLUTION
LASER-INDUCED FLUORESCENCE (LIF) SPECTROSCOPY
OF THE DEUTERATED ISOTOPOMERS
OF THE METHOXY RADICAL
JINJUN LIU, JOHN T. YI AND TERRY A. MILLER
Laser Spectroscopy Facility
Department of Chemistry
The Ohio State University
06/20/06

2. Outline Talk I (TJ04): Talk II (TJ05): Motivation and goals Experiment
Experimental apparatus
Calibration method
Error analysis and experimental accuracy
CH3O, control and example
Simulation and fitting
Prediction of microwave transitions
Summary
Talk II (TJ05):
CH2DO and CHD2O
Experiment
Effective Hamiltonian
Transition intensities
CD3O
Global fitting (LIF and microwave)
Summary and future work
Acknowledgment

3. General hydrocarbon oxidation scheme in the atmosphere.
Motivation
Alkoxy radicals (RO·) are key components in the oxidation of hydrocarbons both in combustion and in the atmosphere.
Methoxy (CH3O), the simplest alkoxy radical is an interesting molecule for dynamic studies.
CH3OCH2O+H
Methoxy has also very important theoretical interest due to its Jahn-Teller effect, coupled to spin-orbit interaction.
The partial deuteration of the methoxy radical
(CHD2O & CH2DO).
Breaks the molecular symmetry.
Removes the degeneracy in the electronic ground state.
Turns Jahn-Teller effect to pseudo-Jahn-Teller effect.
Introduces new terms into the rovibronic Hamiltonian of CH3O.
Extra vibrational bands.
Different rotational structures.
CH3O(X 2E) ~
Energy/103cm-1 30 20 10
CH3+O(3P)
HCO+H2
CH2OH
CH3O(A 2A1)
H2CO+H
General hydrocarbon oxidation scheme in the atmosphere.
* J. J. Orlando, G. S. Tyndall, T. J. Wallington, Chem. Rev. 103, 4657 (2003)
* A. Geers, J. Kappert, F. Temps, and J. W. Wiebrecht, J. Chem. Phys. 101, 3618(1994)
* J. Han, Y. G. Utkin, H. Chen, L. A. Burns, and R. F. Curl, J. Chem. Phys. 117, 6538 (2002).

4. Chronicle Microwave LIF 1984 Endo et al, microwave spectrum of CH3O.
2004
Melnik et al,
microwave spectrum of CH2DO and CHD2O
Microwave
LIF
1988
Foster et al,
moderate-resolution LIF of CH3O.
2006,
Liu et al,
high-resolution LIF of CH3O, CH2DO, CHD2O and CD3O, and global fitting with mw.
2001,
Kalinovski,
low-resolution LIF of CH2DO and CHD2O
Both IR and WM have inconsistency with LIF. IR and MW don’t overlap much.
1990
Liu et al,
high-resolution LIF of CH3O (A state spin-rotation splittings only). ~

5. Experimental Apparatus
PMT
Doubling
Crystal
~5-10 mJ
XeF
Photolysis
Laser
Etalon
Chopper
(2KHz)
λ/2 Plate
PBS PD
Calibration
System I2
50cm
~100mW
1-3 mW
Pulse Amplifier
Excimer
Laser
Vacuum
Chamber
(XeCl)
CHxD3-xONO
+first-run Ne
(75% Ne+25%He)
T~3K
~0.5mJ
Box-Car
CW Ring
Dye Laser
Ar+ Laser 20W
Lock-in
Computer
a. H. Kato et. al., “Doppler-Free High Resolution Spectral Atlas of Iodine Molecule”, Japan Society of the Promotion of Science, (2002) (Experimental)
b. B. Bodermann, H. Knöckel, E. Tiemann, IodineSpec4 (2002) (Computational)

6. Experimental Apparatus
PMT
Doubling
Crystal
~5-10 mJ
XeF
Photolysis
Laser
Etalon
Chopper
(2KHz)
λ/2 Plate
PBS PD
Calibration
System I2
50cm
~100mW
1-3 mW
Pulse Amplifier
Excimer
Laser
Vacuum
Chamber
(XeCl)
~0.5mJ
Box-Car
CHxD3-xONO
+first-run Ne
(75% Ne+
25%He)
Photolysis
Excitation
Lower temperature than He
Narrower linewidth than He
Less noise due to scattering of photolysis laser
Cheaper than pure Ne 3K
CW Ring
Dye Laser
Ar+ Laser 20W
Lock-in
Computer
* H. Kato et. al., “Doppler-Free High Resolution Spectral Atlas of Iodine Molecule”, Japan Society of the Promotion of Science, (2002)

7. Experimental Conditions
Ours
Kato’s
Laser Power (Pump ~100 mW 50 mW
/Probe) 1-3 mW mW
Beam size ~1mm ~1mm
Step Size 5MHz 1MHz
Time constant ms 1s
Modulation Freq 2 kHz 50 kHz
(chopper) (EOM)
I2 cell temp room temp oC
I2 cell length 50 cm 50 cm

8. LIF Spectrum of CH3O
John-Teller active mode. Allowed due to 2E symmetry of the X state and the Jahn-Teller distortion. ~
* S.C.Foster, X.P.Misra, T.D.Lin, C.P.Damo, C.C.Carter, and T.A.Miller, J. Phys. Chem. 92, 5914 (1988)

9. Rotationally Resolved LIF Spectrum of Band
Moderate-resolution*
(FWHM~6GHz)
High-resolution
(FWHM~300MHz)
* D. E. Powers, M. B. Pushkarsky, and T. A. Miller, J. Chem. Phys. 106, 6863 (1997)

10. b a
Frequency/cm-1
a. Broader than the other three isotopomers (~250MHz).
b. B. Bodermann, H. Knöckel, E. Tiemann, IodineSpec4, Toptica Photonics, Munich, Germany, (2002)

11. Error Analysis and Calibration Method
Computational iodine Atlas (σ~1.5MHz)
Density of iodine lines (~1/(10GHz), i.e. separation~20FSRs)
Instability of FSR of the etalon (σ~0.05MHz)
Mechanical instability (~-1.5KHz/μm)
Drift of the index of refraction of the air (thermal: ~0.5KHz/oC, flow…)
Thermal expansion of the Invar frame of the etalon (~-0.1KHz/oC)
Incident angle
Nonlinearity of cw ring laser (~1%, <2.5MHz(=1/2×500MHz×1%))
Uncertainty of picking up the peaks
LIF peaks (~50MHz, dominant)
Iodine peaks (~1.5MHz)
Etalon fringes (~1.5MHz)
Frequency chirping of the pulsed dye amplifier (~10MHz, affects only T00)*
Iodine peaks for absolute calibration
Etalon fringes for relative calibration
Whole spectrum calibrated using cubic spline interpolation
* I. Reinhard, M. Gabrysch, B. F. Weikersthal, K. Jungmann, and G. Putlitz, Appl. Phys. B., 63, 476 (1996)

12. Frequency Chirping of the Pulsed Dye Amplifier
The phase perturbations depend mostly on the deposition of energy in the active volume by the pump pulse and the resulting thermal expansion of the dye solvents.
Influence of the concentrations of different dyes in different solvents on the frequency chirping is significant only for very low dye concentrations (below 0.1 g/l). For Higher concentrations the chirping effect appears to be independent of these parameter. Ours are “high”: ~0.6g/l, ~0.45g/l, and ~0.2g/l for the 1st, 2nd and 3rd stage, respectively;
There is a strong correlation between the thermal expansivity of the solvents and the magnitude of the chirp frequency.
The chirp frequency depends linearly on the pump laser intensity in the range between 50 and 500mJ. Ours: mJ;
No direct correlation of the chirp frequency with other parameters like the heat conductivity or the heat capacity of the dye solutions.
For XeCl pumped red dyes (redder than 308nm) in methanol, the frequency is blueshifted by 10-12MHz.
The resolution achieved by dye amplifiers was limited by phase fluctuations of the pulsed amplified light caused by rapid changes of the refractive index in the dye solution within the duration of the pulse. The average frequency of the pulsed laser light was systematically blueshifted with respect to the cw seed radiation by some 10 MHz; the effect is known as frequency chirping. the phase perturbations depend mostly on the deposition of energy in the active volume by the pump pulse and the resulting thermal expansion of the dye solvents.
Measurement and compensation of frequency chirping
in pulsed dye laser amplifiers, I. Reinhard, M. Gabrysch, B. Fischer von Weikersthal, K. Jungmann, G. zu Putlitz, Appl. Phys. B., 63, 476 (1996)

13. Theoretical Prediction of Errors_1

15. Theoretical Prediction of Errors_2

17. Experimental Accuracy and Proof
Based on the propagation of all experimental errors, an estimation of accuracy σ~50MHz
Calibration of iodine peaks comparing with atlas (σ<10MHz)
Reproducibility of different calibrated scans (σ<50MHz)
σ depends on the frequency separation between the calibrated peaks to the closest iodine peaks and/or etalon fringes
Prediction of microwave transitions based on combination differences of LIF spectra
a. Corrected for hyperfine splittings.
b. Y. Endo, S. Saito, and E. Hirota, J. Chem. Phys. 81, 122, (1984)

18. Experimental
Simulation
CH3O, 320 Band

19. HEFF = HROT + HCOR + HSO + HSR 11 Constants
73 transitions
HEFF = HROT + HCOR + HSO + HSR
11 Constants
Ground state
(2e, Hund’s case a)
A”, B”, Aζt”, aζed”, ε aa”, εbc”
Excited state
(2a, Hund’s case a)
A’, B’, εaa’, εbc’ Te
M_|_:M||=1:0
T=3K
Standard deviation
= 41MHz
Rotational
Coriolis interaction
Spin-orbit interaction
Spin-rotation interaction

20. Standard deviation = 74MHz
Experimental
Simulation
CH3O, 610 Band
90 transitions
13 Constants
Ground state (2e, Hund’s case a):
A”, B”, Aζt”, aζed”, ε aa”, εbc”
Excited state (2e, Hund’s case a):
A’, B’, Aζt’, εaa’, εbc’, ε1’ Te
M_|_:M||=1:2
Standard deviation = 74MHz
A zeta t: Coriolis interaction
Eps 1: (2,2) interaction, sigh can’t be determined

21. Molecular Constants

22. Prediction of Microwave Transitions
a. Corrected for hyperfine splittings.
b. Y. Endo, S. Saito, and E. Hirota, J. Chem. Phys. 81, 122, (1984)

23. Summary and Future Work (TJ05)
Hi-resolution LIF apparatus improved by the new calibration system (σ~50MHz).
Hi-resolution LIF spectra of CH3O ( and bands).
Successful simulation and fitting.
Ground electronic state constants from the global fitting (LIF and microwave).
Hi-resolution LIF spectra of all other isotopomers.