Transcript Lineshapes and Sensitivity of Spectroscopic Signals of N2

Initial Development of High
Precision, High Resolution Ion
Beam Spectrometer in the NearInfrared
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
Outline
• Introduction: Why a Fast Ion Beam?
• Ion Beam Description
• NIR Spectra
• Summary and Future Work
Molecular Ions
Important in many areas of nature and science
Atmospheric science
Astrochemistry
C3H2
C3H
C3H3+
e
e
H2
C3H+
C+
C2H2
C2H
e
C2H4
e
C2H3+
C2H5+
e
C+
CH3+
CH4
e
NASA Picture of the Day, Expedition 13 Crew, International Space
Station, NASA
CH3OCH3
Fundamental physics and chemistry
CH5+
CH3OH
C2H5CN
H2
CH3CN
CH3+
CH2CO
e
OH
CH2+
CH
H2O
CH3NH2
H2
HCN
H2
H3O+
H2
CH+
H2O+
C
+
Challenge: How to produce ionsH inOH
the
laboratory effectively to study them?
2
O
H3
HCO+
+
CO
H2
CH5+
H2+
From B. J. McCall, Ph.D. Thesis, Univ. of Chicago, 2001.
From White et al. Science, 1999, 284, 135–137.
Ion
Production
Methods
Hollow Cathode
No ion-neutral
discrimination
Way to bring low rotational temperature and
ion-neutral discrimination together?
Supersonic Expansion
Low rotational
temperature
No ion-neutral
discrimination
Positive Column
Ion-neutral discrimination
with velocity modulation
No low rotational
temperature
Ion Beam Spectroscopy
-last attempted in 1980s–1990s1
-advances in technology open new
opportunities
1Coe
et al. J. Chem. Phys. 1989, 90, 3893.
Sensitive, Cooled, Resolved Ion
BEam Spectroscopy – SCRIBES
Electrostatic Bender2
Source
chamber
Overlap
region
Laser in
cavity
TOF mass
spectrometer
2Kreckel
et al. Rev. Sci. Instrum. 2010, 81, 063304.
Rigorous ionneutral
discrimination
Can perform low
temperature
spectroscopy with
a supersonic
discharge source
Low ion density
Make up for this
with cavityenhanced
spectroscopy
Sensitive, Cooled, Resolved Ion
BEam Spectroscopy – SCRIBES
Spectroscopic Detection
Cavity
enhancement for
longer pathlength
(× Finesse/π)
Noise
Immune
Cavity
Enhanced
Optical
Heterodyne
Molecular
Spectroscopy
Spectroscopic Detection
EOM
NICEOHMS
Signal
Noise
Immune
Cavity
Enhanced
Optical
Heterodyne
Molecular
Spectroscopy
Heterodyne/Frequency
Modulation Detection for
Lower Noise
Spectroscopic Detection
EOM
Lock-In
Amplifier
NICE-OHMS
Signal
Noise
Immune
Cavity
Enhanced
Optical
Heterodyne
Molecular
Spectroscopy
Also velocity modulate
the ion beam and
demodulate at this
signal.
Doppler Splitting
Ion Beam
nblue
nred
Mass information
encoded in the optical
spectrum!
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
Experimental
+
N2
Signal
Fractional Absorption (× 10−7)
Absorption
Dispersion
No absorption observed!
Frequency (cm−1)
• 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.
Spectral Signals
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
• Obtain line centers, linewidths, and amplitudes from fits
• FWHM ≈ 120 MHz (at 4 kV)
TOF MS
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
Mass spectrum of nitrogenic ion beam. Energy spread in inset
corresponds to an expected linewidth of 120 MHz.
Spectral Signals
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
• 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!
1Coe
et al. J. Chem. Phys. 1989, 90, 3893.
Ultra-High Resolution Spectroscopy
• Rough calibration
with Bristol
wavelength meter
(~70 MHz precision)
• Precisely calibrate with MenloSystems
optical frequency comb (<1 MHz accuracy)
Frequency Comb Calibrated Spectra
Average
the line
centers
Average
the line
centers
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.
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.
Present and Future Work
• Ro-vibrational spectroscopy in the midIR
• Integration of supersonic cooling
Stay tuned to MG05 for more
information!
Acknowledgments
McCall Research Group
Machine Shop
Electronics Shop
Jim Coe
Rich Saykally
Sources of Funding
–
–
–
–
–
Air Force
NASA
Dreyfus
Packard
NSF
– Sloan
–Research Corp.
– Springborn
Endowment