Transcript Ohiomeeting-cunshun.ppt

Novel Approaches
to Doppler-Free Ion Imaging
I.
Two-Color Reduced-Doppler Imaging
II.
Doppler-Free/Doppler-Sliced Imaging
Cunshun Huang, Wen Li, Sridhar A. Lahankar, Myung Hwa Kim,
Bailin Zhang, and Arthur G. Suits
Department of Chemistry, Wayne State University, Detroit, MI, 48202
Doppler Broadening:
Two-photon Doppler Expression:
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
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
H atom images from HCO photodissociation
J. Riedel et al. Chem. Phys. Lett., 414, 473 (2005)
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
H+ + eH (2s)
266 nm
224 nm
H (1s)
I. Two-Color Reduced-Doppler Ion Imaging
224 nm
(0.05 cm-1)
Field free TOF tube MCP/phosphor/camera
Megapixel
acquisition
Molecular beam
PC
266 nm Ion optics
(~ 2 cm-1)
Two-photon
resonance
DBr
224 nm Doppler
width ~7 cm-1
Two-photon resonance condition:
{
condition:
H+ + eH (2s)
266 nm
266 nm
224 nm
224 nm
H (1s)
1. TCRD Detection of D Atom
Advantages:
Higher sensitivity
Lower background
Simplifies the optimization of the
experimental conditions
Convenient, ‘Doppler free’
Allows simple
“1-laser” probe
Not just for imaging:
H atom phofex spectroscopy
Two-color reduced-Doppler
image of D atom from DBr
photodissociation at 224 nm
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.
Scanning
probe scans
through
Doppler profile
along flight
axis
Circular probe
eliminates
1-laser signal
linear probe
circular probe
Novel Approaches to Doppler-Free Imaging
Conclusion
TCRD Advantages:
Allows “1-laser” dissociation/detection
Convenient
High Sensitivity/High Resolution
Reduce background
Move probe wavelength
Doppler-free/Doppler sliced:
Very efficient slicing
Convenient
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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