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Novel Approaches to Doppler-Free Ion Imaging
I. Two-Color Reduced-Doppler
Imaging
II. Doppler-Free/Doppler-Sliced
ImagingCunshun Huang, Wen Li, Sridhar A. Lahankar, Myung Hwa Kim,Bailin Zhang, and Arthur G. Suits
Department of Chemistry, Wayne State University, Detroit, MI, 48202
Two-photon Doppler Expression:
Doppler Broadening:
Doppler-Free Techniques
Doppler free multi-photon spectroscopy was developed theoretically and experimentally in the 1970s
DF absorption spectroscopy: Improve the resolution Pioneer: Arthur L. Schawlow, one of the recipients of the 1981 Nobel prize in physics
DF Multi-photon Ionization: Pioneers: Y. T. Lee’s group (1993) R. N. Zare’s group (1996)
Ion Imaging by DF 2+1 REMPI Probe
H atom images from HCO photodissociation
J. Riedel et al. Chem. Phys. Lett., 414, 473 (2005)
a. Doppler scanning
b. DF with π polarized probe beams
c. DF with π polarized probe beams with intra-cavity etalon
d. DF with ++ polarized
probe beams
I. Two-Color Reduced-Doppler Ion Imaging
Two-Color Reduced-Doppler (TCRD) probe for ion imaging offers advantages over conventional 2+1 resonance-enhanced multiphoton ionization detection.
For counterpropagating beams, Doppler width is reduced by a factor
266 nm
224 nm
H (1s)
H (2s)
H+ + e-
I. Two-Color Reduced-Doppler Ion Imaging
Two-photon resonance condition:
Ion optics
PCMolecular beam
Field free TOF tubeMCP/phosphor/camera
Megapixel acquisition
224 nm(0.05 cm-1)
266 nm(~ 2 cm-1)
266 nm
224 nm
H (1s)
H (2s)
H+ + e- Two-photon resonance condition:
224 nm266 nm
{
DBr 224 nm Doppler width ~7 cm-1
Two-color reduced-Doppler image of D atom from DBr photodissociation at 224 nm
Advantages:Higher sensitivityLower backgroundSimplifies the optimization of
the experimental conditions
Convenient, ‘Doppler free’Allows simple “1-laser” probe
Not just for imaging:H atom phofex
spectroscopy
1. TCRD Detection of D Atom
C. Huang, W. Li, A. G. Suits, J. Chem. Phys. 125 121101 (2006)
2. TCRD REMPI Spectroscopy
Second application: Two-color approach is applied to HCCO photodissociation to give very high resolution REMPI spectra.
HCCO (ketenyl radical) a key intermediate in combustion.
Prepared by reaction O + C2H2 at the nozzle of a pulsed beam.
2.TCRD REMPI Spectroscopy: CO from HCCO
One-color Doppler-free has “pedestal” that limits resolution
(cm-1)
2.TCRD REMPI Spectroscopy: CO from HCCO
One-color Doppler-free has “pedestal” that limits resolution
(cm-1)
II. Doppler-Free/Doppler-Sliced Ion Imaging
Hybrid Doppler-free/Doppler-sliced ion imaging approach that is well-suited for sliced detection of H or D atoms. The method relies on 2+1 resonant ionization with identical, nearly counterpropagating beams that are coplanar but directed at a small angle relative to the detector face.
C. Huang, S. A. Lahankar, M. Y. Kim, B. Zhang, A. G. Suits, Phys. Chem. Chem. Phys., 8, 4652 (2006)
Slicing methods
M. N. R. Ashfold, N. H. Nahler, A. J. Orr-Ewing et al., Phys. Chem. Chem. Phys., 8, 26 (2006)
Doppler-Free/Doppler-Sliced Ion Imaging
Doppler-free/Doppler-sliced imaging of H atom from HBr photodissociation at 243 nm, linearly polarized probe laser. Two ellipses come from the contribution from ionization by each laser beam alone. They will overlap and the center slicing is obtained when the laser is tuned to the center of resonance.
linear probe
circular probe
Circular probeeliminates 1-laser signal
Scanning probe scans through Doppler profile along flight axis
Novel Approaches to Doppler-Free Imaging
ConclusionTCRD Advantages:
Allows “1-laser” dissociation/detectionConvenientHigh Sensitivity/High ResolutionReduce backgroundMove probe wavelength
Doppler-free/Doppler sliced:
Very efficient slicingConvenient
Acknowledgments
Advisor: Prof. Arthur G. Suits
Co-operators: Dr. Wen Li
Dr. Myung Hwa Kim
Dr. Bailin Zhang
Mr. Sridhar A. Lahankar
Dr. Suk Kyoung Lee
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