PRECISION CAVITY ENHANCED VELOCITY MODULATION SPECTROSCOPY Andrew A. Mills, Brian M. Siller, Benjamin J. McCall University of Illinois, Department of Chemistry June 25, 2009

Ion Spectroscopy Why study ions spectroscopically? –Ion spectroscopy yields fundamental insight in studying combustion chemistry, physical chemistry, and astrochemistry Challenges to studying ions: –Ions are transient species Made in dilute manner wrt neutrals Requires discriminating against neutrals –Doppler-broadened spectra can yield unresolved lines –Rotationally excited yields complex spectra with reduced intensities Cavity Enhanced Velocity Modulation: –Cavity and modulation use increases sensitivity –Modulation yields ion/neutral discrimination –Lamb dip gives more precise line center B. M. Siller, A. A. Mills, & B. J. McCall, Optics Letters, 35, 1266 (2010)

Doppler Free Lamb Dips Lamb dips –Intense electric field saturates molecular transition. –Near zero longitudinal velocity is probed by both directions of the laser beam. –Much narrower linewidth. Narrower feature begs for higher accuracy spectroscopy – Frequency comb to increase accuracy

Ti:Sapph 925 nm 532 nm pump laser Reference Cavity Optical Isolator Precision Cavity Enhanced Velocity Modulation ` 5 Ohm RF Lock-in Amplifier FPI Pump N2N2 APD PD HP Frequency Comb VCO Lock Box EOM PZT AOM FI QWP -meter FAST SLOW R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, H. Ward, Applied Physics B-31 (1983) 97–105.

Absolute Frequency Reference Mode-locked fs laser equally spaced lines in frequency (F REP ) Phase offset can be locked providing absolute calibration of comb lines. The comb can now be used as an absolute frequency reference. Unknown laser comb  beat frequency Beat frequency, repetition frequency, offset frequency and comb mode contribution to get laser frequency  Beat Frequency F REP F Offset Original Comb Doubled Comb Ti:Sapph Laser Frequency

Calibrating with Comb Beat conditioning requires filters before the counter.  limits where the beat note can be located. –18.7 – 25.8 MHz –Frequency Reference 20 MHz Unable to lock laser to comb Slew comb frequency instead MHz laser linewidth ~ Accuracy of frequency New frequency MHz Rep Rate Beat Frequency

Dips and Accuracy “High Resolution Scan” with wavemeter. “Ultra-High Resolution Scan” with frequency comb. Q 22 (14.5) N 2 + Blue, Previously reported Doppler- Broadened line center Second molecular ion lamb dip General saturation technique Havenith, Schneider, Bohle, Urban, Molecular Physics 72 (1991) 1149– 1158.

Ultra-High Resolution N  u - 2  g + Q Ferguson, Rao, Martin, Guelachvili. J. Mol. Spec. 153 (1992) Previously blended lines cm

Lamb Dip Linewidth No laser power broadening observed. Pressure broadening expected –Slope higher than expected –Intercept not zero J. C. Pearson, L. C. Oesterling, E. Herbst, F. C. De Lucia, Physical Review Letters 75 (1995) 2940–2943.

Velocity Modulation Broadening? ~5 ns in laser Bandwidth ~ 30 MHz linewidth Laser Bandwidth Ion Drift Velocity Ion velocity is modulated by AC voltage. Ion drift velocity passes through bandwidth of laser

Rotational Temperature Boltzmann plot from the Lamb dip depth can be used to determine the rotational temperature of ions in positive column. 702 ± 67K R 2 =0.95

Conclusions Cavity Enhanced Velocity Modulation can help to revitalize positive column ion spectroscopy. This measurement may be only the second molecular ion with Lamb dip detection. Cavity Enhanced Velocity Modulation is the first generalized technique able to observe Lamb dips of molecular ions. The resulting Lamb dip can help to resolve previously blended lines. This measurement may be the first of a molecular ion with a frequency comb. Several line center positions for N 2 + have been determined to higher accuracy by 1.5 Orders of Magnitude. Lamb dip linewidth is dependent on pressure & modulation.

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