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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

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
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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.
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