Transcript Andrew A. Mills - McCall Research Group

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
Precision Cavity Enhanced Velocity Modulation
532 nm pump
laser
Lock
Box
HP
FAST
VCO
QWP
RF
Frequency
Comb
l-meter
FPI
AOM
PD
Ti:Sapph
925 nm
Reference
Cavity
EOM
FI
Optical Isolator
PZT
SLOW
`
N2
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.
Pump
5 Ohm
APD
Lock-in
Amplifier
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
D Beat
Frequency
Ti:Sapph Laser
FOffset
FREP
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
•
Rep Rate
Beat Frequency
• 0.5 - 1 MHz laser linewidth ~ Accuracy of frequency
• New frequency reference @ 22.25 MHz
Dips and Accuracy
• Second molecular ion lamb dip
• General saturation technique
• “High Resolution Scan”
with wavemeter.
Havenith, Schneider, Bohle, Urban,
Molecular Physics 72 (1991) 1149–
1158.
• “Ultra-High Resolution
Scan” with frequency
comb.
Blue, Previously reported DopplerBroadened line center
Q22(14.5) N2+
Ultra-High Resolution N2+ 2Pu-2Sg+ Q22 1-0
Ferguson, Rao, Martin,
Guelachvili. J. Mol. Spec.
153 (1992) 599-609.
10879.209 cm-1
Previously blended lines
-0.003
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?
Ion Drift Velocity
• Ion velocity is modulated by AC
voltage.
•Ion drift velocity passes through
bandwidth of laser
• ~5 ns in laser Bandwidth
• ~ 30 MHz linewidth
Laser Bandwidth
Rotational Temperature
 I DIP 
ln 

 gS ij 
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
• 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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