sub-doppler spectroscopy of molecular ions in the mid-ir
DESCRIPTION
Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR. James N. Hodges, Kyle N. Crabtree, & Benjamin J. McCall WI06 – June 20, 2012 University of Illinois at Urbana-Champaign. Outline. Motivation Spectroscopic Techniques for Ions: N 2 + Mid-IR Instrument H 3 + Spectroscopy - PowerPoint PPT PresentationTRANSCRIPT
Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR
James N. Hodges, Kyle N. Crabtree, & Benjamin J. McCall WI06 – June 20, 2012 University of Illinois at Urbana-Champaign
Outline Motivation Spectroscopic Techniques for Ions: N2
+
Mid-IR Instrument H3
+ Spectroscopy Conclusions
AstrochemistryIons reactive
intermediates in ISM
~20 ions have been observed
Many carbo-cations have transitions in mid-IR
Lab spectra help observations
H2
+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
C4H2+
C4H3+
C6H5+
C6H7+ C6H6
H2
H2
H2
H2
H2
C
e
C+
e
C+
C
H
C2H2
H2
e
OH+H2O+
H3O+H2O
OHe
OH2
H2
HCO+
CO
HCNCH3NH2
CH3CN
C2H5CN
N, eNH3, e
HCN, eCH 3CN, e
eCO, e
H2O, e
CH3OH, e
CHCH2CO
CH3OH
CH3OCH3
CH3+
C2H5+e
C2H4
eC3H2
eC3H
eC2H
B.J. McCall. Ph.D. Thesis, U. Chicago, 2001.
Indirect THz Spectroscopy
Combination differences extract energy spacings for rotational levels.
Useful for ions with transitions in the THz region - Herschel, SOFIA
Fundamental ScienceFluxional species
Spectrum remains unassigned
WI07 up next!
White et al. Science, 284, 135 (1999).
CH5+
MotivationGeneral, Sensitive, High Precision, Mid-IR Spectrometer for Molecular Ions
General – Multiple Ions of InterestSensitive – Weak Transitions & Trace DetectionHigh Precision – Reduced Uncertainty in Combination
Differences
Velocity Modulation SpectroscopyCations go to cathode
Plasma Discharge Cell
+HV -HV
S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).
Velocity Modulation SpectroscopyCations go to cathodeDoppler Shifted
Plasma Discharge Cell
+HV -HV
Plasma Discharge Cell
+HV -HV
Laser
Detector
S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).
Velocity Modulation SpectroscopyCations go to cathodeDoppler Shifted
Plasma Discharge Cell
-HV +HV
Plasma Discharge Cell
Laser
DetectorPlasma Discharge Cell
Laser
Detector
S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).
Velocity Modulation SpectroscopyCations go to cathodeDoppler Shifted AC Driven – Absorption Profile ModulatedVelocity Modulation Provides Ion-Neutral Discrimination
Plasma Discharge CellPlasma Discharge Cell
Laser
DetectorPlasma Discharge Cell
Laser
Detector
S.K. Stephenson and R. J. Saykally. Chem. Rev., 105, 3220-3234, (2005).
Velocity Modulation of N2+
Heterodyne Spectroscopy
Creates fm-triplet with spacing typically in the rfMixers demodulate rf signal Sensitive to relative sizes/phases of sidebandsAbsorption/Dispersion - 90o Phase Separation“Zero background”Operation at rf frequencies reduces 1/f noise
LaserDetector
EOM
Signal
Velocity Modulation of N2+
Velocity Modulation & Heterodyne at 1 GHz
Cavity EnhancementLaser
CavityDetector
Enhances Pathlength
Increases Intracavity Power
Allows saturation of rovibrational transitions – sub-Doppler features
Requires active locking to maintain resonance – PDH locking
Velocity Modulation in a CavityVelocity Modulation Provides Ion-Neutral Discrimination
Velocity Modulation
Ion Signal Encoded at 2x the Plasma Frequency
Velocity Modulation Provides Ion-Neutral Discrimination
Cavity Enhanced Velocity Modulation Spectroscopy of N2+
PZT Detector
Detector
EOMLaser Lock-In Amplifier2f
B. M. Siller et al., Opt. Lett., 35, 1266-1268. (2010)
NICE-OHVMS
Large Signal Small Noise
Cavity Enhancement
Heterodyne Spectroscopy
NICE-OHVMS
N oiseI mmuneC avityE nhanced-O pticalH eterodyneV elocityM odulationS pectroscopy
Velocity Modulation
Sensitivity to Ions
B. M. Siller et al., Opt. Exp., 19, 24822-24827. (2011)
NICE-OHVMSHeterodyne sidebands at the cavity FSR allows the combination of heterodyne spectroscopy with a cavity. Cavity Modes
Laser
NICE-OHVMS
Lock-In Amplifier
AbsorptionSignal
PlasmaFrequency
Detector
Lock-In Amplifier
DispersionSignal
90° PhaseShift
1 × Cavity FSR
Laser EOM
2f
Comparison of Techniques on N2+
NICE-OHVMS
Mid-IR Instrument
Optical Parametric Oscillator (OPO)
High optical powerSaturation of rovibrational transitions
Spans 3.2 – 3.9 μm range
OPO Light Generation
Yb Doped Fiber Laser OPOEOM Am
p
1064 nm
OPO Light Generation
Signal 1.5-1.6 m
Pump1064 nm
Idler 3.2-3.9 m
Periodically Poled Li:NbO3
Ion Production/Velocity Modulation
~AC HV40 kHz
L-N2 InL-N2 Out Gas In
Liquid Nitrogen Cooled Positive Column Discharge Cell- ”Black Widow”
Ion Production
Mid-IR Instrument
OPO
YDFL
EOMLock-In
Amplifier
AbsorptionSignal
Lock-In Amplifier
DispersionSignal
Wave-meter
40 kHzPlasma
Frequency
80 MHz1 × Cavity FSR
90o Phase Shift
IPS
2f
ni=np-ns
H3+ Spectra
Sensitivity = 2 x 10-9 cm-1 Hz-1/2
Shot Noise Limit = 8 x 10-11 cm-1 Hz-1/2
Sig
nal
H3+ Spectra
S/N ~ 500Precision of Line Center ~ 300 kHz
Sig
nal
Summary & Conclusions
• Constructed a general high precision mid-IR spectrometer• Demonstrated the first NICE-OHVMS spectra of H3
+
• 1.5 orders of magnitude from the shot noise limit
Acknowledgements
McCall Group with Special thanks to:Brian Siller &Joseph Kelly
NSF GRF# DGE 11-44245 FLLWSpringborn Endowment