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

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

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).

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). ~

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)

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)

Experimental Conditions Ours Kato’s Laser Power (Pump ~100 mW 50 mW /Probe) 1-3 mW 2.5 mW Beam size ~1mm ~1mm Step Size 5MHz 1MHz Time constant 30-100ms 1s Modulation Freq 2 kHz 50 kHz (chopper) (EOM) I2 cell temp room temp 0-77 oC I2 cell length 50 cm 50 cm

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)

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)

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)

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)

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: 100-150mJ; 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)

Theoretical Prediction of Errors_1

Theoretical Prediction of Errors_2

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)

Experimental Simulation CH3O, 320 Band

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

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

Molecular Constants

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

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.