Molecular Triplet States: Excitation, Detection, and Dynamics Wilton L. Virgo Kyle L. Bittinger Robert W. Field Collisional Excitation Transfer in the Xe*-N 2 System: Proxies for Hg*-acetylene,ethylene

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

Photosensitized Excitation Transfer Our Goal: use atomic photosensitization, exciting atoms via 2-photon optical pumping Hg* + C 2 H 2  Hg + C 2 H 2 * Xe* + N 2  Xe + N 2 * A Pulsed Beam Source of Metastable Molecules

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

Excite to short-lived 3 D 2 state via two-photon transition at 252nm Decays in 28 nsec to the lowest two excited states: 6 3 D 2 two-photon pump state 2 photon transition from ground state New Optical Pumping Scheme for Populating Xe ( 3 P 2 ) 33% to 3 P 1 (895nm, 10 ns) decays to ground state 67% to 3 P 2 (823nm, 150 s) metastable state

Detect N 2 * B 3  g A 3  u emission. (5,3) and (5,2) bands dominant Krumpelmann CPL 140, 142 (1987) Previous Studies of Xe* + N 2 by Ottinger Excitation Transfer Detected via Dispersed Fluorescence Excite Xe by electron impact or electrical discharge Excitation transfer via Xe beam / N 2 gas target or crossed beam

Two Methods of Detection LIF Sensitive to short-lived states < 10s Determine the number of metastables produced SEELEM (Surface Electron Ejection by Laser Excited Metastables) Sensitive to long-lived states  > 600s Time-of-flight spectra

SEELEM: Electronic De-Excitation at Metal Surface Criterion for e - emission: E el >  metal (work function) 5.1 eV (Au) Surface Electron Ejection by Laser Excited Metastables e-e- Au Surface

Co-expand a mixture of Xe and N 2 Excitation Transfer in the Molecular Beam

Possible Xe ( 3 P 2 )  N 2 Metastable Resonances And Franck-Condon Factors 0.096

Xe and N 2 LIF: Signals on two different timescales Xe ( 3 D 2  3 P nm ~30ns N 2 (B 3  g  A 3  u nm ~5s

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

TOF-SEELEM Excitation in Expansion Region 50 PSI backing pressure

TOF-SEELEM 120 PSI Backing Pressure

How Well Are We Doing? 0.01 bar, 1 mm 3  Xe atoms 2-photon 1% saturated  Xe* Observe 1x10 6 Xe*, 1x10 6 N 2 * SEELEM Counts: 2500 each Xe* & N 2 * Xe*+Xe*  Xe + Xe + + e - Penning Ionization? Associative ionization to Xe e - ?

Future Experiments LIF probe of N 2 * states 3 Photon excitation of Xe*, Kr*, etc. Ablation jet for Hg*, Cd*, Zn* Hg* on acetylene and ethylene

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

The Ultimate Goal: Hg/Acetylene & Ethylene Laser Ablation to the Rescue !  Hg Reservoir and Acetylene too ! Ablation Pulse 

Orbital Mechanism of Excitation Transfer Xe 5p -1 6s N 2  g  g * Hg6s6p HCCH u  g *

Detection of Xe and N 2 Metastables via Fluorescence

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 Laser Induced Fluorescence of Xe + N 2 : Estimating Excitation Transfer Efficiency Excitation transfer efficiency: calculate the relative number of Xe, N 2 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: V ave t charge collected Q e at 823nm, 748nm, 677nm

Rearrange equations for n e and remove constant factors Calculation based on relative band intensities observed in similar experiments Laser Induced Fluorescence of Xe + N 2 : Excitation Transfer Efficiency Calculation Xe 3 D 2 3 P nmQ e = 0.21% N 2 B A (4,2) A (4,1) 748 nm 677 nm 1 % 3 %

1% NO 2 in He 625 Torr backing pressure 90 shot averaging Speed of beam: 1800 m/s Doppler broadening limit using 3mm skimmer: cm -1 Measured Doppler broadening: cm -1 NO 2 spectra recorded with frequency-doubled CW ring laser

Franck-Condon factors for low-lying excited states of N 2 v’FC factor E, cm v’FC factor E, cm B3gB3g W3uW3u v’FC factor E, cm B’ 3  u - v’FC factor E, cm A 3  u + -X 1  + g (v’’=0) Gilmore, Laher, and Espy. J Phys Chem Ref Data 21, 1005 (1992) Lofthus and Krupenie. J Phys Chem Ref Data. 6, 113 (1977) Xe 3 P 2 energy

Previous Studies of Xe + N 2 Excitation Transfer 3 P 2 state of xenon lies 475 cm -1 below v=5 and 1114 cm -1 above v=4 of N 2 B 3  g state. Energy transfer into v=5 occurs w/absolute cross-section of 12.5A 2 at avg. collision energy of 452cm -1 a) Ottinger Chem Phys 192, 49 (1995) Krumpelmann CPL 140, 142 (1987) N 2 B 3  g Levels

Laser tuned to Xe 6 3 D 2  6 1 S 0 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 Laser Induced Fluorescence of Xe* + N 2 : Preparing Metastable Xe