PRECISION CAVITY ENHANCED VELOCITY MODULATION SPECTROSCOPY

Andrew A. Mills, Brian M. Siller, Benjamin J. McCall

University of Illinois, Department of ChemistryJune 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:Sapph925 nm

532 nm pump laser

Reference Cavity

Optical Isolator

Precision Cavity Enhanced Velocity Modulation

`

5 Ohm

RF

Lock-in Amplifier

FPI

PumpN2

APD

PD

HP

Frequency Comb

VCOLockBox

EOM

PZ

T

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 (FREP)• 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

FREP

FOffset

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

•

• 0.5 - 1 MHz laser linewidth ~ Accuracy of frequency• New frequency reference @ 22.25 MHz

Rep RateBeat Frequency

Dips and Accuracy

• “High Resolution Scan” with wavemeter.

• “Ultra-High Resolution Scan” with frequency comb.

Q22(14.5) N2+

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 N2+ 2u-2g

+ Q22 1-0

Ferguson, Rao, Martin, Guelachvili. J. Mol. Spec. 153 (1992) 599-609.

Previously blended lines10879.209 cm-1

-0.003

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 ± 67KR2=0.95

ij

DIP

gS

Iln

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 N2+ have been

determined to higher accuracy by 1.5 Orders of Magnitude.

• Lamb dip linewidth is dependent on pressure & modulation.

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