Michael Porambo, Jessica Pearson, Craig Riccardo, Benjamin J. McCall
International Symposium on Molecular Spectroscopy, The Ohio State University
June 20, 2013
Expansion Discharge Source for Ion Beam Spectroscopy of
Cold Molecular Ions
Outline• Motivation• Review of Ion Beam Instrument• Ion Source Work• Summary
Molecular Ions
NASA Picture of the Day, Expedition 13 Crew, International Space Station, NASA
Atmospheric Reactions
Interstellar ChemistryCH5
+
C6H7+
H2CO+
Combustion Processes
Fundamental Chemical Physics
Oka Ion Factory, http://fermi.uchicago.edu/research/ch5plus/
http://www.navy.mil/view_single.asp?id=40674
Resolved, high precision spectroscopy to study these
fields
Challenges• Low number density• Small fraction of ions compared to
neutrals• High rotational temperature
Ion Production Methods
Low rotational temperature
No ion-neutral discrimination
Positive Column Velocity Modulation
Hollow Cathode Discharge Cell
No discrimination between ions and neutral molecules
Ion-neutral discrimination
High rotational temperature
Supersonic Expansion Discharge
Ion Production MethodsWay to bring low rotational temperature and
ion-neutral discrimination together?
1Coe et al. J. Chem. Phys. 1989, 90, 3893.
Ion Beam Spectroscopy- last attempted in 1980s–1990s1
- advances in technology open new opportunities- can obtain both ion-neutral discrimination and low rotational temperatures
Sensitive, Cooled, Resolved Ion BEam Spectroscopy - SCRIBES
TOF massspectrometer
Sourcechamber
Overlapregion
Electrostatic Bender2
Low ion density106 cm-3
Make up for this with cavity-enhanced spectroscopy
2Kreckel et al. Rev. Sci. Instrum. 2010, 81, 063304.
Sensitive, Cooled, Resolved Ion BEam Spectroscopy – SCRIBES
Spectroscopic Detection
Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy
Cavity enhancement for longer pathlength (× Finesse/π)
Spectroscopic DetectionEOM
NICE-OHMS Signal
Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy
Heterodyne/Frequency Modulation Detection for Lower Noise
Spectroscopic DetectionEOM
Lock-In Amplifier
NICE-OHMS Signal
Noise ImmuneCavityEnhanced-OpticalHeterodyneMolecularSpectroscopy
Also velocity modulate the ion beam and demodulate at this signal.
Spectroscopic DetectionEOM
Lock-In Amplifier
NICE-OHMS Signal
Frequency calibrated with optical frequency comb
Line centers to ~1 MHz!
AccomplishmentsOperational mass spectrometer
Comb calibrated hot ion beam spectra in near-infrared (N2
+)
From Mills et al. J. Chem. Phys. 2011, 135, 224201.
Mid-infrared optical system finished and optimized on CH4
First broadly tunable NICE-OHMS spectrometer in mid-IRFrom Porambo et al., Opt. Lett., 2012, 37, 4422.
Next: Supersonic Expansion Discharge Source
Produces ions with low rotational temperature
Designed and characterized with H3
+
cavity ring-down spectroscopy3
H3+ rotational
temperature of 80–110 K3
3Crabtree et al., Rev. Sci. Instrum., 2010, 81, 086103.
Supersonic expansion discharge source in ion beam source chamber producing a nitrogenic expansion
Instrument RearrangementIon source chamber moved to connect to Roots blower system
Leybold RUTA 13000 system
10” bellows connected to Roots blower
system
Unistrut brace to prevent
chamber from rising
Beam skimmer used to extract ions from expansion
Source Chamber
Beam SkimmersMachined in-house from EDM graphite4
Two designs:
4Jordan et al., Rev. Sci. Instrum., 1999, 70, 1640.5Keim, E. PhD Thesis, UC Berkeley, 1992.
1 2
Length
(in.)
Orifice Diameter
(mm)1 0.5 32 1.024 0.8
Difference Frequency Generation
Preliminary Studies: Characterization of HN2
+ Expansion
InSb
Double-pass setup to perform wavelength-modulation optical heterodyne spectroscopy
Rovibrational lines of HN2
+ will be used to estimate a rotational temperature
Instrument Rearrangement
Difference Frequency Generation
setup
Determine HN2+
rotational temperature by NICE-OHMS in ion beam
Summary• Molecular ion spectroscopy is challenging but
useful for studying many phenomena• Ion beam spectrometer development with ion-
neutral discrimination, NICE-OHMS, and cold ions
• Successful first signal and mid-infrared arrangement; now integrating source
• Source works well in setup, attempting to finish characterization of rotational temperature
• Preparing to extract ion beam from expansion
AcknowledgmentsMcCall Research Group Former Members
Brian SillerAndrew MillsManori PereraHolger KreckelKyle CrabtreeCarrie Kauffman
NASA Earth and Space Science Fellowship Program
Springborn Endowment