PRECISION CAVITY ENHANCED VELOCITY MODULATION SPECTROSCOPY
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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?

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PRECISION CAVITY ENHANCED VELOCITY MODULATION SPECTROSCOPY

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Precision cavity enhanced velocity modulation spectroscopy

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

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

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


Precision cavity enhanced velocity modulation spectroscopy

Precision Cavity Enhanced Velocity Modulation

FI

532 nm pump laser

FAST

Lock

Box

VCO

Frequency Comb

Ti:Sapph

925 nm

QWP

HP

RF

l-meter

AOM

FPI

PD

Reference Cavity

EOM

Optical Isolator

SLOW

`

PZT

APD

Pump

N2

5 Ohm

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.

Lock-in Amplifier


Absolute frequency reference

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

Original Comb

Doubled Comb

Ti:Sapph Laser

D Beat Frequency

FOffset

FREP

Frequency


Calibrating with comb

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 Rate

Beat Frequency


Dips and accuracy

Dips and Accuracy

  • Second molecular ion lamb dip

  • General saturation technique

Q22(14.5) N2+

  • “High Resolution Scan” with wavemeter.

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

Havenith, Schneider, Bohle, Urban, Molecular Physics 72 (1991) 1149–1158.

Blue, Previously reported Doppler-Broadened line center


Ultra high resolution n 2 2 p u 2 s g q 22 1 0

Ultra-High Resolution N2+ 2Pu-2Sg+ Q22 1-0

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

10879.209 cm-1

-0.003

Previously blended lines


Lamb dip linewidth

Lamb Dip Linewidth

  • 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.

  • No laser power broadening observed.


Velocity modulation broadening

Velocity Modulation Broadening?

  • Ion velocity is modulated by AC voltage.

  • Ion drift velocity passes through bandwidth of laser

Ion Drift Velocity

  • ~5 ns in laser Bandwidth

  • ~ 30 MHz linewidth

Laser Bandwidth


Rotational temperature

Rotational Temperature

702 ± 67K

R2=0.95

Boltzmann plot from the Lamb dip depth can be used to determine the rotational temperature of ions in positive column.


Conclusions

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.


Precision cavity enhanced velocity modulation spectroscopy

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