Unusual Time-Dependant Absorption of Naphthalene-Gas mixtures, after

1. UV Unusual Time-Dependant Absorption of Naphthalene-Gas mixtures, after UV flash photolysis, measured with Cavity Ring-Down Spectroscopy Kieran Lynch Supervisors: Andy Ruth Michael Mansfield 27/03/02

2. Outline • Motivation • Cavity Ring-Down Spectroscopy • Oscillating Chemical Reactions • Naphthalene • Experimental Set-up • Results • Conclusions

3. Motivation • Naphthalene Cation • Unusual results • Other results in the form of decays and very complicated behaviour • Revised Motivation  Investigate dependance on initial conditions

4. Beers’ Law • Absorption occurs in atoms and • molecules is wavelength dependant. • Standard Absorption Spectroscopy measures • change in intensity.

5. Empty Cavity Cavity Ring-Down Spectroscopy (CRD)

6. Difference in ring-down times Extinction Coefficient t0 t Absorbing Sample Cavities containing absorbing samples • Intensity Independent • High sensitivity, long effective path length

7. Oscillating Chemical Reactions BZ Reaction (~ 1950) >>>> FKN Mechanism (~ 1975) 2nd Law of Thermodynamics DSTOTAL > 0 , always

8. Naphthalene (C10H8) • White Crystaline Solid at room temperature • Vapour Pressure ~ 0.5 mbar • Melting point ~ 750C • Photostable

9. Absorption Spectrum of the cation Possible Results of UV Photolysis of Naphthalene • Excitation (Triplet State) • Ionization

10. Products Potential / eV Products Potential / eV C10H7+ + H 15.41 C8H6+ + C2H2 15.50 C6H6+ + C4H2 15.64 C7H5+ + C3H3 15.90 C6H5+ + C4H3 18.72 C3H3+ + C7H5 19.35 Azulene C5H3+ + C5H5 19.74 • Isomerisation • Fragmentation

11. Experimental Set-Up

12. P-M Tube Cylindrical Lens PM Tube Voltage Control Window Excimer Laser Control Window # of shotsRep rate Shutter ControlLaser Info Graph of (Decay Time)-1 vs. Time Software to interpret experiment results Excimer Laser CRD Resonator Dye Laser

13. P-M Tube Cylindrical Lens PM Tube Voltage Control Window Excimer Laser Control Window # of shotsRep rate Shutter ControlLaser Info Graph of (Decay Time)-1 vs. Time Software to interpret experiment results Excimer Laser CRD Resonator Dye Laser

14. Absorbing Species

15. Excimer fired 1000 shots Excimer fired 200 shots Excimer fired 50 shots Varying Shot Number

16. Argon Pressure ~ 6 mbar Argon Pressure ~ 12 mbar Argon Pressure ~ 24 mbar Varying Pressure

17. Energy Per Pulse Further Study Required

18. Buffer Gas (Helium) 80 shots with Helium pressure ~ 32 mbar 160 Shots with Helium Pressure ~ 100 mbar 100 mbar ~ 10 mbar 30 mbar ~ 8 mbar Similar results with Argon but going from

19. Varying Shot Number for Helium A + B  C [A] [B] = k [C]

20. Spectrum • Dye Laser Slewed to different • wavelengths • Measurements in Helium • Low pressure ~ 32 mbar • How accurate? • Naphthalene used up • Needs to be repeated

21. Conclusions • Pressure Dependance •  Increasing Pressure of Buffer Gas increases complexity •  Higher Pressures of Naphthalene produce more • intense absorptions • Type of buffer gas •  Oscillations begin at lower pressures for Argon than • for Helium •  Helium results were more reproducible • Number of shots (increasing) •  Argon: Decay  Oscillations  Complex •  Helium: Quadratic Dependance on shot number • Lots of work yet to be done