1. INVESTIGATION OF O 2 (C 3 , v=2) BY NOVEL LASER PHOTOIONIZATION TECHNIQUE IN AIR AT ATMOSPHERIC PRESSURE Jonathan D. Umbel, Dr. Steven F. Adams, Dr. Charles A. DeJoseph, Jr. Air Force Research Laboratory Wright Patterson AFB, OH 18 Jun 07

2. Laser Diagnostics Facility for Plasma Studies AFRL Propulsion Directorate, Wright Patterson AFB, OH Building 450, WPAFB, OH

3. Introduction Resonant-Enhanced Multiphoton Ionization (REMPI) in dry atmospheric pressure air studied for possible low-jitter laser triggering of air spark gap switch Interesting REMPI phenomena observed in dry air at 1 atm – Strong REMPI signal with O 2 (C 3  ) Rydberg resonance – Strong N 2 + fluorescence at all REMPI transitions In this work, we characterize the O 2 (C 3 , v=2) state using both fluorescence and traditional REMPI spectra.

4. Previous work: O 2 REMPI with ultraviolet laser Laser Photons O 2 (X 3  g - ) O 2 + (X 2  g ) O 2 (C 3  g ) e-e- Ionization O( 1 D)+O( 3 P) 3g3g REMPI Band Corresponds to O 2 (C 3  g,v=2 → X 3  g - ) Two-Photon Resonant Intermediate R D Johnson, G R Long, and J W Hudgens. J. Chem. Phys., 87 (1987). Previous REMPI studies with O 2 (C 3  g ) intermediate found bands with very diffuse structure due to predissociation. Only the O 2 (C 3  g,v=2) state was rotationally resolved.

5. Previous Characterization of O 2 (C 3  g,v=2) State Lewis et. al (1999) analyzed O 2 REMPI spectra from Sur et. al (1986) and Ogorzalek-Loo (1989, unpublished) and derived spectroscopic constants for the F1, F2, and F3 sublevels of O 2 (C 3  g,v=2). Total term energy = o + B eff J(J+1) – D v J 2 (J+1) 2 (For weak spin uncoupling) cm -1 F 1 (  =0)F 2 (  =1)F 3 (  =2) o 693666944569550 B eff 1.61.651.68 DvDv -1.6X10 -5 1.3X10 -5 B. R. Lewis, S. T. Gibson, J. S. Morrill, M. L. Ginter J. Chem. Phys., Vol. 111, No. 1, 1 July 1999. A Sur, C V Ramana, W A Chupka, S D Colson. J. Chem Phys., 84, 1, (1986).

6. This Work: Laser REMPI / Fluorescence Experimental Setup YAG Pumped Dye Laser Laser 280-290 nm Laser 560-580 nm SHG Dry Air In Resonant Enhanced Multi-Photon Ionization (REMPI) Computer Al Electrodes Voltage Source R Digital O-scope ICCD Camera/Spectrometer 391 nm Fluorescence Computer Post REMPI N 2 + Fluorescence

7. N 2 + Fluorescence Signal in Dry Air at Atmospheric Pressure Dry Air at Atmospheric Pressure Strong N 2 + Fluorescence Band Corresponding to O 2 (C,v=2) Resonant Intermediate N 2 + (B 2  u + → X 2  u - ) Laser Excitation Spectrum Typical Spectrum of N 2 + Fluorescence Coinciding with REMPI Transitions in Dry Air at Atmospheric Pressure

8. Proposed Mechanism for N 2 + Fluorescence with O 2 (C 3  g ) 2 Photon Resonance N 2 (X ) N 2 + (X) Laser Photons O 2 (X ) O 2 + (X) O 2 (C) N 2 + (B) Fluorescence N 2 (a’) Collisional Energy Transfer Laser Photons

9. Experiment: – Compare N 2 + fluorescence spectrum in air with traditional O 2 REMPI. – Analyze both spectra for O 2 (C 3  g ) spectroscopic constants.

10. Experimental Spectra N 2 + Fluorescence Spectrum 760 Torr Dry Air 4 mJ Laser Pulse Energy Traditional REMPI Spectrum 25 Torr Pure O 2 0.5 mJ Laser Pulse Energy Note: Fluorescence spectrum is more diffuse than REMPI spectrum

11. Term Energy Equations Applied to Fit Our Spectral Data Term Energy Calculations Ground State O 2 (X 3 , v=0) F 1 = B v J(J+1) – D v J 2 (J+1) 2 + (2J+3)B v –  – √((2J+3) 2 B v 2 +  2 -2  B v ) +  (J+1) F 2 = B v J(J+1) – D v J 2 (J+1) 2 F 3 = B v J(J+1) – D v J 2 (J+1) 2 + (2J-1)B v –  + √((2J-1) 2 B v 2 +  2 -2  B v ) +  J *Constants for O 2 (X 3 , v=0) taken from: R R Laher, F R Gilmore, J. Chem. Phys. Ref. Data, 20, 4, (1991). *Note: For N = 1, J = 0, the sign in front of the square root was inverted Upper State O 2 (C 3 , v=2) S 1 (  =0) = o1 + B eff1 J(J+1) – D v1 J 2 (J+1) 2 Term energy formula for weak spin uncoupling S 2 (  =1) = o2 + B eff2 J(J+1) – D v2 J 2 (J+1) 2 S 3 (  =2) = o3 + B eff3 J(J+1) – D v3 J 2 (J+1) 2 Constants o, B eff, and D v for each sublevel, F 1 (  =0), F 2 (  =1), and F 3 (  =2) will be fit to the spectral data

12. Line Intensities Applied to Generate Simulated Spectrum Relative Line Intensities for 2-Photon Transitions R G Bray, R M Hochstrasser. Mol. Phys. 31, (1976). P branch* Q branch R branch S branch *Note: The P branch equation was modified through a change in sign to correspond with symmetry seen in the models. However, there is no current published acclimation of this error. O branch

13. Theoretical Linewidth Effect Due to O 2 (C 3 , v=2) Pre-Dissociation Laser Photons O 2 (X 3  g - ) O 2 + (X 2  g ) O 2 (C 3  g ) O( 1 D)+O( 3 P) 3g3g N 2 + (X) Fluorescence N 2 (a’) Laser Photons Li et. al (1996) calculated O 2 (C 3 , v=2) linewidths for various J values due to pre-dissociation Y. Li, D. Petsalakis, H-P Liebermann, G. Hirsch, R. Buenker, J.Chem. Phys. 106 (3), 15 January 1997. We used a function fits to these values to determine the widths for each F(J) for our simulated spectra

14. Simulated Spectrum Using Constants and Linewidths from Literature Initial Simulated Spectrum: Constants derived by Lewis et. al Linewidths calculated by Li et. al Entire simulated spectrum is red-shifted compared to both experimental spectra Linewidths are not well matched, especially in the fluorescence case REMPI in O 2 N 2 + Fluorescence in Air

15. Simulated Spectra with Adjusted Constants and Linewidths Linewidth Adjustment REMPI in O 2 N 2 + Fluorescence in Air

16. Spectroscopic Constants cm -1 F 1 (  ) F 2 (  ) F 3 (  ) o 69369(2) 69366 69449(1) 69445 69552(1) 69550 B eff 1.61(5) 1.6 1.65(1) 1.65 1.69(1) 1.68 DvDv 1.9(5)x10 -5 _ 1.6(2)x10 -5 1.6X10 -5 1.9(2)x10 -5 1.3X10 -5 Our improved fits required an increase in o term energies by 2-3 cm -1 over the published constants. REMPI data provided more precise fit than Fluorescence data Higher signal-to-noise Less line broadening * Previous constants

17. Summary of O 2 (C 3 , v=2) Investigation by Novel Laser Photoionization Fluorescence phenomenon introduced for REMPI transitions in atmospheric pressure air O 2 (C 3  g,v=2) state characterized using fluorescence and REMPI spectra Derived spectroscopic constants differ slightly from literature Fluorescence linewidths in atmospheric air are significantly broadened Likely due to pressure broadening and laser power broadening Plasma Diagnostics Research Team

18. Back-up Slide References 1) M Aldén, W Wendt. Optics Communications, 69, 1, (1988). 2) A Sur, L Nguyen, N Nikoi. J. Chem. Phys., 96, 9, (1992). 3) A Sur, C V Ramana, W A Chupka, S D Colson. J. Chem Phys., 84, 1, (1986). 4) P H Krupenie. J. Chem. Phys. Ref. Data, 1, 2, (1972). 5) G. Herzberg. Molecular Spectra and Molecular Structure, Krieger Publishing: Malabar, Florida. (1989). 6) W Demtroder. Laser Spectroscopy, Springer-Verlag: Berlin, Germany. (1982). 7) Y Li, I D Petsalakis, H Liebermann, G Hirsch, R Buenker. J. Chem. Phys., 106, 3, (1997). 8) R R Laher, F R Gilmore. J. Chem. Phys. Ref. Data, 20, 4, (1991). 9) National Institute of Standards and Technology. “Diatomic Spectral Database.” http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/Html/sec3 http://physics.nist.gov/PhysRefData/MolSpec/Diatomic/Html/sec3 10) Kwok, S., and Volk, K.: 1985, `On the Energetics of High-Velocity Molecular Flows', Astrophys.J.Lett., 299, L191. http://www.iras.ucalgary.ca/~volk/o2lev.gifhttp://www.iras.ucalgary.ca/~volk/o2lev.gif 11) R D Johnson, G R Long, and J W Hudgens. J. Chem. Phys., 87 (1987). 12) I N Levine. Quantum Chemistry, 4th Ed., Prentice Hall: Englewood Cliffs, NJ. (1991). 13) R G Bray, R M Hochstrasser. Mol. Phys. 31, (1976). 14) W Kaiser, C G Garrett. Phys. Rev. Lett., 7, 6, (1961).

19. Back-up Slide Calibration with N 2 For energy calibration, the laser was scanned over the N 2 (a 1  g → X 1  g + ) intermediate transition and the resulting fluorescence signal was fit with a simulated spectrum using the accepted N 2 constants. N 2 (a 1  Wavelength Intensity Simulated Experimental

20. Back-up Slide Collisional O 2 - N 2 Energy Exchange Term Energies of O 2 (C 3  g ) and N 2 (a’ 1  u - ) are nearly coincident N2N2 O2*O2* O 2 (C 3  g ) + N 2 (X) → O 2 (X) + N 2 (a’ 1  u - ) is energetically probable