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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? - PowerPoint PPT Presentation
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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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