Infrared Observations of E-V and T-V Energy Transfer

1. Prof. Giles Henderson Retirement Symposium Eastern Illinois University 24 March 2001 Infrared Observations of E-V and T-V Energy Transfer David A. Dolson Department of Chemistry Wright State University Dayton, OH

2. Thanks to . . . • Giles Henderson • Richard Keiter • EIU Faculty • Diana West, Craig Bartell, Bryan Berger, Justin Henderson, JaCinta Batson • AFOSR & WSU RIP • Paul Davies (Cambridge, UK) • Glen Perram (AFIT)

3. Occurrence of collisional E-V transfer • Quenching of electronically excited states • Excitation mechanisms of infrared lasers CO2 HCN NO N2O I* Br* Cl* • Semiconductor processing plasmas • Atmospheric reactions • Combustion reactions

4. Halogen atoms have doublet ns2 np5 ground states (spin-orbit splitting increases with Z) • I* (7603 cm-1) and Br* (3685 cm-1) exhibit efficient E-V transfer to small molecules. • E-V probability is greater for small DE and for small Dv. • Cl* (882 cm-1) E-V excitation of SO2, CH4, N2O and SF6 has been demonstrated, rate coefficients measured and E-V efficiency measurements are in progress.

5. Kinetic Studies via PulsedLaser Photolysis / IR Fluorescence • Selective production of reactive or energetic photofragment at t=0 . . . . “instantaneous” • Control of concentrations • Sensitive (1012 cm-3) real-time (1 ms) IR detection of reactants and/or products • Narrow band spectral interference filters provide vibrational state selectivity • IR diode lasers offer options

6. Experimental Apparatus

7. Cl* + SO2 E-V Excitation of Stretching Modes is Endoergic

8. Cl* + SO2 Kinetic Scheme SO2(n3) E-V Siebert & Flynn V-V Cl* SO2(n1) 2n2 n2

9. Success with Cl* + SO2 prompted othertests of the theoretical vib. dependence • Environmental significance of Cl* + CH4 ? • Symmetry forbids E-V excitation of “dark” states: CH4, CD4, CF4, SF6, CO2 • Stretch vs. bend for CF4, N2O

10. Probable Kinetic Mechanism for Cl* + SF6 Initiation at t=0: ICl + hn (532 nm)  I + Cl* (1) Cl* + SF6  Cl + SF6(n3, v, or 0) (2) Cl*  losses with ICl & Ar/He (3) SF6(n3) + M  SF6(v, 0) + M Under pseudo 1st-order conditions: Cl*  SF6(n3)  SF6(v, 0) Expect exponential rise & fall of SF6(n3) fluorescence at 10.5 mm

11. Energy Level Kinetic Scheme SF6(n3) E-V Cl* V-V SF6(v) other 1st-order Cl* losses 10.5 mm V-T,R

13. Cl* + SF6 E-V Conclusion • Cl* quenching by SF6 is nearly gas kinetic, in keeping with small DE (62 cm-1) and strong n3 absorption band (A = 41 s-1) • Fractional excitation of n3 is 1-2% from preliminary intensity measurements

14. P-Chem Laboratory Experiments • Green HeNe Laser @ 543 nm • Raman Spectroscopy • I2 Fluorescence • Ruby Luminescence • Velocity Modulation Spectroscopy (N2+)

15. Green HeNe Laser (543.365 nm) • Commercially available since about 1977, but largely unused in teaching laboratories • Turn-key cw operation, like red HeNe • Shorter wavelength is advantageous for fluorescence & Raman excitation

16. Experimental Apparatus for Emission Spectroscopy with Gre-Ne Excitation

17. I2 Fluorescence Excitation on26-0 R(12) and 28-0 R(106)

18. Birge-Sponer plot yields vibrational constants

19. Rotationally Resolved Structure

20. Rotational Constants for X state

21. Raman Spectroscopy with Gre-Ne Laser • Modest n4 wavelength advantage vs. 633nm • High PMT response for neat liquids • CCl4, CHCl3, CH2Cl2, CH3I, CS2 & others • Sufficient sensitivity for ions in conc. sol’n • NO3-, NH4+, SO42-, BO2-, N3-, NO2+ • Plastic polarizer sheets provide adequate polarization with 25% throughput

22. CCl4 Polarized Raman Spectrum

23. 10 M NH4NO3

24. Plasma Lines Interference

25. Velocity Modulation Spectroscopy of Ions • Developed about 15 years ago by Sakally and others for high resolution IR diode laser spectroscopy of small molecular ions • All spectral lines of gas phase ions created in an AC discharge exhibit Doppler shifts, which offers phase-sensitive detection at high resolution. (suppresses neutrals) • Derivative lineshapes

26. . . . In the P-Chem Teaching Laboratory • Demonstrated for N2+ in emission (UK) • Inexpensive addition to existing emission spectroscopy experiments • Exposes students to phase-sensitive detection schemes • Intensity alternation of even-odd rotational states due to nuclear spin statistics

27. N2 BX Emission Spectrum remember this ? N2+ is here also

28. Experimental Method • 1 kHz sine wave generator • 50 W audio amplifier (kit) • Step-up transformer = auto ignition coil • Glass discharge cell* with flowing N2/He mix (1:5) at 0.5 - 5 Torr •  5 W discharge • Home built lock-in amplifier

29. Velocity Modulation Cell

30. Home Built Lock-In Amplifier

31. N2+ BX 391 nm 0-0 Band

32. n = n0 + (B'-B")(K"+1) + (B'-B")(K"+1)2 n0 B' B" 25564.5 2.11 1.96 25566.0 2.062 1.922

33. Velocity Modulation Suppresses Interference from Neutral Species

34. A Tribute Prof. Giles L. Henderson involved me in undergraduate research at Eastern Illinois University and strongly encouraged me to pursue a Ph.D. degree. My fascination with spectroscopy originated in his P-Chem lectures and laboratory experiments. My interest in developing P-Chem laboratory experiments may be traced to my experiences in Giles' laboratory courses.