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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