Transcript Carroll_OSU2012_FC04.pptx

Development of a submillimeter
multipass spectrometer for the
study of molecular ions
Molecular Ions in Space
• Molecules of
Astrochemical
interest
• THz spectra to
support astronomical
observations
McCall, B. J. “Spectroscopy of H3+ in laboratory and astrophysical
plasmas." Ph. D. Thesis, University of Chicago, 2001.
Molecular Ions in Lab
• Spectrometer with sufficient sensitivity
• Ion source that produces cold gas-phase ions
with sufficient concentration
Submillimeter Spectroscopy
Spectrometer
α min
Characteristics
Long Pathlength Discharge
Cell + Direct Absorption
10-5 cm-1
High abundance of ions, Long path
length, Warm sample
Long Pathlength Discharge
Cell + Multipass Direct
Absorption
10-7 cm-1
High abundance of ions, Longer path
length, Warm sample
Supersonic Expansion
Discharge Source +
Direct Absorption
10-3 cm-1
Low abundance of ions, Short path
length, Cold sample
Supersonic Expansion
Discharge Source +
Cavity Ringdown
10-10 cm-1
Low abundance of ions, Very long path
length, Cold sample
Beer-Lambert Law: I=I0e-αd
Multipass Spectrometer
• Based on design by David Perry and coworkers
(Kaur et al. 1990)
• Uses two nearly concentric spherical mirrors
Multipass In Vacuum
• Small Chamber: 7 - 9 passes
Barrel Shock
Zone of Silence
Mach Disk
THz multipass beams
Multipass In Vacuum
• Large Chamber: 9 - 11 passes
Zone of Silence
THz multipass beams
Mach Disk
Barrel Shock
Methanol Spectroscopy
• 1 scan using double-modulation with two Lock-In amplifiers to
detect cold sample
Signal/Noise
Single Pass
7 Passes
319.05
Methanol Spectroscopy
• 1 scan using double-modulation with two lock-in amplifiers to
detect cold sample
Signal/Noise
Single Pass
319.05
7 Passes
449.51
SNR Enhancement:
1.41
Standing Wave Challenge
• Free spectral range in large chamber changes
with number of passes
Ion Sources
• Inspired by designs by McCarthy and Thaddeus
(McCarthy et al. 2000)
Teflon Spacer
Expansion
- HV
Grounded
Valve Face Plate
Teflon Spacers
Expansion
- HV
Valve
Face Plate
Grounded
Electrode
Picture Credit: Brett McGuire
Ion Sources
• Inspired by hollow cathode designs of Amano and
coworkers
Tube Design
Expansion
Grounded
Copper Tube
Port for - HV
Valve Face Plate
Picture Credit: Brett McGuire
Sphere Design
Valve Face
Plate
Grounded
Sphere
- HV
Expansion
Methoxy in Discharge
• CH3OH
Teflon Spacers
CH3O + H
Expansion
+ High Voltage
Valve
Face Plate
Grounded
Electrode
Methoxy in Discharge
• CH3OH
CH3O + H
• Single Pass in Small Chamber
Future Work
• Improve multipass in large chamber
• Further compare effect of valve orientation on
spectral quality
• Test ion sources with N2H+
• Use optimized source to study other
protonated species
Acknowledgments
The Widicus Weaver Group, Emory University:
Bridget DePrince, Brian Hays, Jay Kroll, Jake Laas, Mary
Radhuber, Blithe Rocher, Jim Sanders, Jessica Walsh
O’Sullivan, Shiya Wang
Former group member: Brett McGuire
Mike McCarthy (Harvard-Smithsonian CfA)
NASA APRA Grant NNX11AI07G
NSF CAREER Award CHE-1150492
• Schematic for multipass is big chamber