high precision, sensitive, near-ir spectroscopy in a fast ion beam michael porambo, holger kreckel,...
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High Precision, Sensitive, Near-IR Spectroscopy in a Fast Ion Beam
Michael Porambo, Holger Kreckel, Andrew Mills, Manori Perera, Brian Siller, Benjamin J. McCall
MWAM 2011University of Illinois at Urbana-Champaign
22 October 2011
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Outline
• Introduction
• Description of Instrument
• Results
• Summary
• Future Work
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Molecular Ions in Astrochemistry
H2+
H3+
CH+
CH2+
CH3+
CH5+
CH4
C2H3+
C2H2
C3H+
C3H3+
H2
H2
H2
H2
H2
C
e
C+
e
C+
OH+
H2O+
H3O+
H2O
OHe
O
H2
H2
HCO+
CO
HCN
CH3NH2
CH3CN
C2H5CN
N, e
NH3, e
HCN, eCH3
CN, e
e
CO, e
H2O, e
CH3OH, e
CH
CH2CO
CH3OH
CH3OCH3
CH3+
C2H5+e
C2H4
e
C3H2
e
C3H
e
C2H
Experimental laboratory spectra needed to aid in theoretical, observational work.
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Ion Production Techniques
Oka, Saykally, McCall Maier, Nesbitt
Ion-neutral discrimination
Low rotational temperature
Narrow linewidth
Compatible with cavity-enhanced spectroscopy
Positive Column
Supersonic
ExpansionHollow
Cathode
Hirota, Amano
High ion column density
We want…
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Sensitive, Cooled, Resolved Ion BEam Spectrometer – SCRIBES
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Ion Production Techniques
Oka, Saykally, McCall Maier, Nesbitt
Ion-neutral discrimination
Low rotational temperature
Narrow linewidth
Compatible with cavity-enhanced spectroscopy
Mass spectrometry of laser-probed ions
Spectral identification of ion mass
Velocity
Modulation
Supersonic
ExpansionSCRIBES
Hollow
Cathode
Hirota, Amano
High ion column density
McCall
So we want…
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First Generation Ion Beam Instrument
Coe, J. V. et al. J. Chem. Phys. 1989, 90, 3893–3902.
Direct Laser Absorption Spectroscopy in Fast Ion Beams –DLASFIB.
Pioneered by Saykally group in late 1980s– early 1990s.1,2,3
Studied HF+, HN2+,
HCO+, H3O+, NH4+ in the
mid-infrared, with no supersonic expansion.
Lacked sensitivity to see larger or more complex ions, especially at high temperature.
1Coe et al., J. Chem. Phys. 1989, 90, 3893–3902.2Owrutsky et al., J. Phys. Chem. 1989, 93, 5960–5963.3Keim et al., J. Chem. Phys. 1990, 93, 3111–3119.
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Ion Beam Setup
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Ion Beam Setup
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Ion Beam Setup
Source Chamber
Cold Cathode Ion SourceNote: No rotational
cooling
Beam Deflector7
4Kreckel, H. et al. Rev. Sci. Instrum. 2010, 81, 063304.
Time-of-Flight Region
Mass Spec Detector
Cavity Mirror
Detector
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Spectroscopy: Heterodyne Detection
Single Carrier Frequency
Carrier + Other Frequencies
Electro-optic modulator
EOMAnalyte
113 MHz 113 MHz
Relative Frequency (MHz)
Relative Frequency (MHz)
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Heterodyne DetectionAbsorption
Dispersion
+-
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Cavity EnhancementNoiseImmuneCavityEnhancedOpticalHeterodyneMolecularSpectroscopy (NICE-OHMS)5,6
Analyte
Optical cavity increases pathlength by factor of ~100
EOM
5Ye, Ph.D. Dissertation, University of Colorado Department of Physics, 1997.6Foltynowicz et al., Appl. Phys. B 2008, 92, 313–326.
Relative Frequency (MHz)
Cavity Modes
Laser Frequencies
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Spectroscopy LayoutPZT
EOM
~113 MHz
Lock-In Amplifier
Lock-In Amplifier
Lock-In Amplifier
Lock-In Amplifier
Vel. Mod.~ 4 kHz
Dispersion
AbsorptionDetector
DispersionAbsorption
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Ion Beam
Doppler Splitting
0red blue
Amount of shifts depend on the mass of the ion
Relative Frequency
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First Spectroscopic Target• Obtain rovibronic spectral transitions of
A 2u – X 2g+ 1–0 Meinel band of N2
+
• Near-infrared transitions probed with commercial tunable titanium–sapphire laser (700–980 nm)
• N2+ formed in cold cathode ion source; no rotational
cooling
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Experimental N2+ Signal
A 2u – X 2g+, qQ22(14.5) line
Frequency (cm−1)
Fra
ctio
nal A
bsor
ptio
n (×
10−
7 )
No absorption observed!
Absorption Lock-In Amplifier Output
Dispersion Lock-In Amplifier Output
NICE-OHMS absorption signal strongly affected by saturation; saturation of the ions decrease the absorption to below the
noise
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Spectral Signals
• FWHM ≈ 120 MHz (at 4 kV)• Noise equivalent absorption ~ 4 × 10−11 cm−1 Hz−1/2 (50× lower than last ion beam instrument)• Within ~1.5 times the shot noise limit
A 2u – X 2g+, qQ22(14.5) line,
red- and blue-shifted
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Ultra-High Resolution Spectroscopy• Rough calibration with Bristol wavelength
meter (~70 MHz precision)
• Precisely calibrate with MenloSystems optical frequency comb (<1 MHz accuracy)
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Frequency Comb Calibrated SpectraA 2u – X 2g
+, qQ22(14.5) line,
red- and blue-shifted
Only ~8 MHz from linecenter obtained in N2+ positive column work.6
Confident in improvements in the mid-IR.6Siller, B. M. et al. Opt. Express 2011, Accepted.
Average the line centers
Average the line centers
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Summary and Conclusions• Fast ion beam spectroscopy can be very
effective for general molecular ion spectroscopy.
• Integrated NICE-OHMS and velocity modulation spectroscopy for performing sensitive measurements of ion beam.
• Operational spectroscopy on rovibronic transitions of the Meinel band of N2
+ – first direct spectroscopy of electronic transition in fast ion beam.
• Performed precisely calibrated measurements with optical frequency comb to get line centers to an accuracy of ~8 MHz.
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Present and Future WorkVibrational spectroscopy in the mid-IR
• Finished construction of mid-IR DFG laser at 3.0 µm.
• Produced HN2+ in
the ion beamSupersonic Expansion Discharge Source7
• Enable rotational cooling
H3+
HN2+
Increasing N2
Time-of-flight mass spectra of hydrogenic ion beam with increasing amounts of N2
• 750 K with cold cathode;<100 K with supersonic source?
• Vibrational spectroscopy of rotationally cooled molecular ions (CH5
+, C3H3+, etc.)
Supersonic expansion discharge source7Crabtree, K. N. et al. Rev. Sci. Instrum. 2010, 81, 086103.
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AcknowledgmentsMcCall Research Group
Machine Shop
Electronics Shop
Jim Coe
Rich Saykally
Sources of Funding– Air Force – NASA– Dreyfus– Packard– NSF
– U of Illinois– Springborn Endowment