Molecular Triplet States: Excitation, Detection, and Dynamics

1. Molecular Triplet States: Excitation, Detection, and Dynamics Wilton L. Virgo Kyle L. Bittinger Robert W. Field Collisional Excitation Transfer in the Xe*-N2 System:Proxies for Hg*-acetylene,ethylene

2. Why Triplet States ? • Reactive (E* 100 kcal/mol) • Long-lived (t > 100 ms) • Difficult to detect (No UV fluorescence) • Properties differ from ground state • Easily populated unintentionally • Unknown: Structure Excitation mechanisms Decay mechanisms

3. A Pulsed Beam Source of Metastable Molecules • Photosensitized Excitation Transfer • Our Goal: use atomic photosensitization, exciting atoms via 2-photon optical pumping • Hg* + C2H2 Hg + C2H2* • Xe* + N2 Xe + N2*

4. Metastable States of Closed-Shell Atoms • Excite an electron on a closed-shell (1S0) atom into p orbital • L=1 , S=1,0 J=2,1,0 (L+S,...,0) Terms: 1P1, 3P0, 3P1, 3P2 • Order of triplet sublevels: sign of spin-orbit constant Hg: 0,1,2 normal Xe: 2,1,0 inverted 1P1 decays to ground state 3P2 or 0metastable 3P1 mixes with 1P1 decays 3P0 or 2metastable

5. New Optical Pumping Scheme for Populating Xe (3P2) 63D2 two-photon pump state • Excite to short-lived 3D2 state via two-photon transition at 252nm • Decays in 28 nsec to the lowest two excited states: 67% to 3P2 (823nm, 150 s) metastable state 33% to 3P1 (895nm, 10 ns) decays to ground state 2 photon transition from ground state

6. Previous Studies of Xe* + N2 by OttingerExcitation Transfer Detected via Dispersed Fluorescence • Detect N2* B3Pg A3Su emission. • (5,3) and (5,2) bands dominant • Excite Xe by electron impact or electrical discharge • Excitation transfer via Xe beam / N2 gas target or crossed beam Krumpelmann CPL140, 142 (1987)

7. Two Methods of Detection • LIF • Sensitive to short-lived states t < 10 ms • Determine the number of metastables produced • SEELEM (Surface Electron Ejection by Laser Excited Metastables) • Sensitive to long-lived states t > 600 ms • Time-of-flight spectra

8. SEELEM: Electronic De-Excitation at Metal Surface e- Surface Electron Ejection by Laser Excited Metastables Au Surface Criterion for e- emission: Eel > Fmetal (work function) F = 5.1 eV (Au)

9. Excitation Transfer in the Molecular Beam Co-expand a mixture of Xe and N2

10. Possible Xe (3P2)N2 Metastable Resonances And Franck-Condon Factors 0.096 0.105 0.008 0.015 0.082 0.053 0.151 0.002

11. Xe and N2 LIF:Signals on two different timescales Xe (3D23P2) @ 823 nm t~30ns N2 (B3Pg A3Su) @748 nm t~5ms

12. Time-of-Flight SEELEM: ‘Slow Collisions’ Excitation in Post-Expansion Region .75” in front of Nozzle

13. TOF-SEELEMExcitation in Expansion Region 50 PSI backing pressure

14. TOF-SEELEM 120 PSI Backing Pressure

15. How Well Are We Doing? • 0.01 bar, 1 mm3  1014 Xe atoms • 2-photon 1% saturated  1012 Xe* • Observe 1x106 Xe*, 1x106 N2* • SEELEM Counts: 2500 each Xe* & N2* • Xe*+Xe* Xe+ Xe+ + e- Penning Ionization? • Associative ionization to Xe2+ + e- ?

16. Future Experiments • LIF probe of N2* states • 3 Photon excitation of Xe*, Kr*, etc. • Ablation jet for Hg*, Cd*, Zn* • Hg* on acetylene and ethylene

17. Acknowledgements • Prof. Robert W. Field • Kyle Bittinger • Sam Lipoff • Jessica Lam • AFOSR

18. The Ultimate Goal: Hg/Acetylene & EthyleneLaser Ablation to the Rescue ! and Acetylene too ! Ablation Pulse   Hg Reservoir

19. Orbital Mechanism of Excitation Transfer Xe 5p-1 6s N2sgpg* Hg 6s 6p HCCH pupg*

20. Detection of Xe and N2 Metastables via Fluorescence

21. Laser Induced Fluorescence of Xe + N2: Estimating Excitation Transfer Efficiency • Excitation transfer efficiency: calculate the relative number of Xe, N2 molecules observed during simultaneous measurement • Many factors are the same in both measurements: • Geometry of optics • Laser power • Gain of detector • Resistance of detector circuit • Number of molecules observed is a function of: • VaveDt charge collected • Qeat 823nm, 748nm, 677nm The total charge collected… is the number of excited species …times the efficiency of the optics …times the quantum efficiency of the detector at each fluorescence wavelength …times the gain of the detector and the electron charge

22. Laser Induced Fluorescence of Xe + N2: Excitation Transfer Efficiency Calculation • Rearrange equations for ne and remove constant factors Calculation based on relative band intensities observed in similar experiments

23. NO2 spectra recorded with frequency-doubled CW ring laser 1% NO2 in He 625 Torr backing pressure 90 shot averaging Speed of beam: 1800 m/s Doppler broadening limit using 3mm skimmer: 0.007 cm-1 Measured Doppler broadening: 0.010 cm-1

24. Franck-Condon factors for low-lying excited states of N2 A3Su+-X1S+g (v’’=0) B3Pg W3Du B’3Su- Xe 3P2 energy Gilmore, Laher, and Espy. J Phys Chem Ref Data21, 1005 (1992) Lofthus and Krupenie. J Phys Chem Ref Data. 6, 113 (1977)

25. Previous Studies of Xe + N2 Excitation Transfer N2 B3Pg Levels • 3P2 state of xenon lies 475 cm-1 below v=5 and 1114 cm-1 above v=4 of N2 B3Pg state. • Energy transfer into v=5 occurs w/absolute cross-section of 12.5A2 at avg. collision energy of 452cm-1 a) Ottinger Chem Phys192, 49 (1995) Krumpelmann CPL140, 142 (1987)

26. Laser Induced Fluorescence of Xe* + N2: Preparing Metastable Xe • Laser tuned to Xe 63D2 61S0 two-photon transition • Detect Xe* by fluorescence to metastable state at 823nm (used 610nm long-pass filter) • We have done this before in a cell, but this was the first time for us in the molecular beam • Fluorescence lifetime is comparable to detector response time (28ns) • Two-photon transition probability ~1/10 that of comparable transition in Hg