1. 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

2. Two-photon Doppler Expression: Doppler Broadening:

3. 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)

4. 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

5. 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 -

6. I. Two-Color Reduced-Doppler Ion Imaging Two-photon resonance condition: Ion optics PC Molecular beam Field free TOF tube MCP/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 nm 266 nm { DBr 224 nm Doppler width ~7 cm -1

7. Two-color reduced-Doppler image of D atom from DBr photodissociation at 224 nm 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 1. TCRD Detection of D Atom C. Huang, W. Li, A. G. Suits, J. Chem. Phys. 125 121101 (2006)

8. 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 + C 2 H 2 at the nozzle of a pulsed beam.

9. 2.TCRD REMPI Spectroscopy: CO from HCCO One-color Doppler-free has “pedestal” that limits resolution (cm -1 )

10. 2.TCRD REMPI Spectroscopy: CO from HCCO One-color Doppler-free has “pedestal” that limits resolution (cm -1 )

11. 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)

12. Slicing methods M. N. R. Ashfold, N. H. Nahler, A. J. Orr-Ewing et al., Phys. Chem. Chem. Phys., 8, 26 (2006)

13. Doppler-Free/Doppler-Sliced Ion Imaging

14. 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.

15. linear probecircular probe Circular probe eliminates 1-laser signal Scanning probe scans through Doppler profile along flight axis

16. 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

17. 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 $$$: