initial development of high precision, high resolution ion beam spectrometer in the near-infrared
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Initial Development of High Precision, High Resolution Ion Beam Spectrometer in the Near-Infrared. Michael Porambo , Brian Siller, Andrew Mills, Manori Perera, Holger Kreckel, Benjamin J. McCall International Symposium on Molecular Spectroscopy The Ohio State University 18 June 2012. - PowerPoint PPT PresentationTRANSCRIPT
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Initial Development of High Precision, High Resolution Ion
Beam Spectrometer in the Near-Infrared
Michael Porambo, Brian Siller, Andrew Mills, Manori Perera, Holger Kreckel, Benjamin J. McCall
International Symposium on Molecular SpectroscopyThe Ohio State University
18 June 2012
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Outline
• Introduction: Why a Fast Ion Beam?• Ion Beam Description• NIR Spectra• Summary and Future Work
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Molecular IonsImportant in many areas of nature and science
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
HCNCH3NH2
CH3CN
C2H5CN
N, e
NH3, e
HCN, eCH3CN, e
eCO, e
H2O, e
CH3OH, e
CHCH2CO
CH3OH
CH3OCH3
CH3+
C2H5+e
C2H4
e
C3H2
e
C3H
e
C2H
AstrochemistryAtmospheric science
Fundamental physics and chemistry
CH5+
From White et al. Science, 1999, 284, 135–137.From B. J. McCall, Ph.D. Thesis, Univ. of Chicago, 2001.
NASA Picture of the Day, Expedition 13 Crew, International Space Station, NASA
Challenge: How to produce ions in the laboratory effectively to study them?
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Ion Production MethodsHollow Cathode
Supersonic Expansion
Positive Column
Way to bring low rotational temperature and ion-neutral discrimination together?
No ion-neutral discrimination
Low rotational temperature
No ion-neutral discrimination
Ion-neutral discrimination with velocity modulation
No low rotational temperature
Ion Beam Spectroscopy-last attempted in 1980s–1990s1
-advances in technology open newopportunities
1Coe et al. J. Chem. Phys. 1989, 90, 3893.
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Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES
TOF massspectrometer
Sourcechamber
Overlapregion
Laser incavity
Electrostatic Bender2
Rigorous ion-neutral discrimination
Can perform low temperature spectroscopy with a supersonic discharge source
Low ion densityMake up for this with cavity-enhanced spectroscopy2Kreckel et al. Rev. Sci. Instrum. 2010, 81, 063304.
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Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES
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Spectroscopic Detection
Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy
Cavity enhancement for longer pathlength (× Finesse/π)
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Spectroscopic Detection
Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy
Heterodyne/Frequency Modulation Detection for Lower Noise
EOM
NICE-OHMS Signal
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Spectroscopic DetectionEOM
Lock-In Amplifier
NICE-OHMS Signal
Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy
Also velocity modulate the ion beam and demodulate at this signal.
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Ion Beam
Doppler Splitting
nred nblue
Mass information encoded in the optical
spectrum!
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First Spectroscopic Target• Obtain rovibronic spectral transitions of 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
Frequency (cm−1)
Frac
tiona
l Abs
orpt
ion
(× 1
0−7)
No absorption observed!
Absorption
Dispersion
• Absorption signal strongly attenuated by saturation.3 Not observable!• Saturation parameters: 30,000 carrier, 6300 sidebands.• Dispersion signal attenuated by a factor of 2 due to saturation.3Ma et al. J. Opt. Soc. Am. B 2008, 25, 1144–1155.
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Spectral Signals
• Obtain line centers, linewidths, and amplitudes from fits• FWHM ≈ 120 MHz (at 4 kV)
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
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TOF MS
Mass spectrum of nitrogenic ion beam. Energy spread in inset corresponds to an expected linewidth of 120 MHz.
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
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Spectral Signals
• Obtain line centers, linewidths, and amplitudes from fits• FWHM ≈ 120 MHz (at 4 kV)• Noise equivalent absorption ~ 2 × 10−11 cm−1 Hz−1/2 (50× lower than last ion
beam instrument)1
• Within ~1.5 times the shot noise limit!
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
1Coe et al. J. Chem. Phys. 1989, 90, 3893.
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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 Spectra
Only ~8 MHz from line center obtained in N2+ positive column work.4
Confident in improvements in the mid-IR.4Siller, B. M. et al. Opt. Express 2011, 19, 24822.
Average the line centers
Average the line centers
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Summary and Conclusions• Ion Beam Spectroscopy – effective in studying
molecular ions.• High sensitivity spectroscopy used to study ion
beam – high S/N, Doppler splitting.
• Spectroscopy on rovibronic transitions of N2+ –
first direct spectroscopy of electronic transition in fast ion beam.
• Accurate frequency calibration with optical frequency comb.
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Present and Future Work• Ro-vibrational spectroscopy in the mid-
IR• Integration of supersonic cooling
Stay tuned to MG05 for more information!
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AcknowledgmentsMcCall Research Group Machine ShopElectronics ShopJim CoeRich SaykallySources of Funding
– Air Force – NASA– Dreyfus– Packard– NSF
– Sloan–Research Corp.– Springborn Endowment