FLUORESCENCE-DEPLETION INFRARED SPECTROSCOPY OF METHOXY RADICAL JINJUN LIU, LILY ZU, WEIDONG ZHOU, VLADIMIR LOZOVSKY, PATRICK DUPRÉ, C. BRADLEY MOORE AND TERRY A. MILLER, Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University 120 W. 18th Avenue, Columbus, OH 43210. 06/20/05

Motivation and Goals Energy/103cm-1 30 20 10 Alkoxy radicals are key components in the oxidation of hydrocarbons both in combustion and in the atmosphere. Methoxy (CH3O), the simplest alkoxy radical is especially interesting to both theoretical and experimental spectroscopists due to its Jahn-Teller effect, coupled to spin-orbit interaction. Methoxy is also an interesting molecule for dynamic studies. CH3OCH2O+H CH3O(X 2E) ~ Energy/103cm-1 30 20 10 CH3+O(3P) HCO+H2 CH2OH CH3O(A 2A1) H2CO+H To obtain the fluorescence-depletion infrared (FDIR) spectra of methoxy radical with different transitions in the laser induced fluorescence (LIF) spectra pumped. To build the correlation between the FDIR spectrum, the IR absorption spectrum, the dispersed fluorescence (DF) spectrum and the stimulated emission pumping (SEP) spectrum. To help the analysis of these spectra making use of the correlation. * A. Geers, J. Kappert, F. Temps, and J. W. Wiebrecht, J. Chem. Phys. 101, 3618(1994) * J. J. Orlando, G. S. Tyndall, T. J. Wallington, Chem. Rev. 103, 4657 (2003) * J. Han, Y. G. Utkin, H. Chen, L. A. Burns, and R. F. Curl, J. Chem. Phys. 117, 6538 (2002).

Principle of FDIR Spectroscopy 1. ro-vibronic level 2. confomers

Experimental Setup I

LIF of Methoxy and Simulation T~3K 31644.1cm-1 A: N'=1, K'=0, X: J"=3/2, P"=3/2 ~

Vibrational frequencies of CH3O ν4, Q branch (P=1.50.5) (P=0.5-0.5) * J. Han, Y. G. Utkin, H. Chen, L. A. Burns, and R. F. Curl, J. Chem. Phys. 117, 6538 (2002). symmetric C-H stretch CH3 umbrella C-O stretch asymmetric C-H stretch scissors CH3 rock 1. JT &SO (2. Fermi w/ v1) 3. Dynamics 4. Prevous work

FDIR Spectrum of Methoxy I Degree of Depletion ~50% * Calibrated by optogalvanic lines of Ar and Ne.

Experimental Setup II Sirah Dye Laser Optical Parameter Oscillator

FDIR Spectrum of Methoxy II

Calibration of Spectrum The single mode OPO has a longitudinal mode gap of ~4GHz. When it is tuning, controlled by a software, the back mirror will move back and forth to fit the right cavity length in order to work at a signal mode. But due to the mechanical unstability, the back mirror sometimes cannot move to the right position, then the OPO will run at two modes or jump to another mode that is ~4 GHz away to the “right” mode. As a result, we will lose some FDIR peaks when it happens and the missing mode is coincident with the FDIR peak. On the other hand, you will see a duplicated FDIR peak if it jumps back or is running at two modes.

FDIR transitions of Methoxy In total, 22 FDIR transitions were observed in the region of 2875-2980 cm-1, with J”=3/2, P”=3/2 rotational level of the X 2E3/2, ν”=0 state of methoxy depleted. a. The values in the parentheses are the standard deviations (σ) of the transition frequencies in units of the least significant figures. b. Obtained by OPO, calibrated by methane photoacoustic cell (absolute calibration) and etalon (relative calibration). c. Obtained by OPO, calibrated by methane photoacoustic cell. d. Obtained by difference frequency mixing, calibrated by transition frequencies obtained by OPO. ~ ……

Comparison with IR Absorption Spectra I

Comparison with IR Absorption Spectra II

Comparison with DF and SEP Spectra Dispersed florescence spectrum with 3141 band in A2A state pumped. ~ FDIR spectrum. SEP spectrum through the excitation of |N=1, K=0> rotational line of 35 band in A2A state. ~

Conclusions and Future Work The fluorescence-depletion infrared (FDIR) spectra of methoxy radical were obtained, with J”=3/2, P”=3/2 rotational level of the X 2E3/2,ν=0 state of methoxy depleted. Two IR sources were used to deplete the ground-state level: difference frequency mixing and an optical parameter oscillator (OPO). FDIR spectra were compared with the IR absorption spectrum, the stimulated emission pumping (SEP) spectrum and the dispersed fluorescence (DF) spectrum. Tentative assignment to some of the transitions was made based on the correlation between these spectra. ~ Broader scan for FDIR spectra of methoxy radical, with different LIF transitions pumped. Assignment of all FDIR transitions of methoxy radical. FDIR spectroscopy of larger alkoxy radicals and conformational analysis of other molecules.