Introduction to PhD research

1. Introduction to PhD research Edward Gash 30 June 2004

2. Introduction to PhD research Investigation into the role of naphthalene cations in the interstellar medium Edward Gash 30 June 2004

3. Absorption lines observed when light from a distant star passes through an interstellar cloud. Ref: Jenniskens und Désert Original motivation Diffuse interstellar bands (DIB) Search for responsible compounds The origins are unknown. The cations of polycyclic aromatic hydrocarbons (PAHs) e.g. naphthalene, have been proposed to be responsible for some DIBs.

4. Original motivation Need a spectrum of cold, isolated, ions to compare the with the DIBs e.g. in a molecular jet. Ions may be created using multi-photon photolysis. To determine possible cation yields, the absorption at 650 nm of a naphthalene-argon mixturewas investigated following UV ionisation. Absorption Spectrum Preliminary experiment Surprising results An unexpected time-dependent absorption was observed. The original objectives of the project were revised in order to study this extraordinary phenomenon.

5. Introduction to PhD research Unusual dynamic absorption of naphthalene-buffer gas mixtures following UV photolysis Edward Gash 30 June 2004

6. Outline Introduction Experimental Results Discussion Conclusion

7. Introduction absorption: the technique used for measuring the absorption is ‘Cavity Ring-down Spectroscopy’ - CRDS In this experiment, measured is the absorption at 650 nm of a naphthalene buffer gas mixture following multi-photon UV excitation. Absorption

8. Introduction absorption: the technique used for measuring the absorption is ‘Cavity Ring-down Spectroscopy’ - CRDS • Light coupled into an optically stable cavity. • Emerging light decays exponentially, the characteristic decay time is called the ring-down time, CRD. • CRD depends on the reflectivity of the mirrors and the absorption in the cavity. Absorption

9. Introduction absorption: the technique used for measuring the absorption is ‘Cavity Ring-down Spectroscopy’ - CRDS • Highly sensitive ~10-8 cm-1. Conventional absorption experiments ~10-6 cm-1. • (Can be seen as having a long effective path length) • Independent of fluctuations in light source intensity. Absorption

10. Introduction absorption: the technique used for measuring the absorption is ‘Cavity Ring-down Spectroscopy’ - CRDS In this experiment, measured is the absorption at 650 nm of a naphthalene buffer gas mixture following multi-photon UV excitation. Absorption

11. Introduction naphthaleneis a polycyclic aromatic hydrocarbon. C10H8 • White crystalline solid. • Commonly used in mothballs. • Vapour pressure at room temperature is 0.08 mbar. In this experiment, measured is the absorption at 650 nm of a naphthalene buffer gas mixture following multi-photon UV excitation. Naphthalene

12. Introduction In this experiment, measured is the absorption at 650 nm of a naphthalene buffer gas mixture following multi-photon UV excitation.

13. Introduction 1 2 3 4 photon Mutli-photon excitation of • 1 photon absorption excites the molecule to the 180 state of S1. • ~2 % of the naphthalene molecules with undergo intersystem crossing to the triplet manifold. • The molecule can absorb more photons from the metastable T1 state.

14. Introduction 1 2 3 4 photons Multi-photon excitation of • 2 photon absorption is resonance enhanced. It leaves the molecule just below the ionisation threshold. • At 298 K, some of the naphthalene will begin in an excited vibrational level of the ground state – these may be ionised by 2 photons. • The molecule can absorb more photons from the metastable T1 state.

15. Introduction + + 1 2 3 4 photons Multi-photon excitation of • 3 photon absorption is again resonance enhanced and may lead to ionisation. • There is also a chance that the naphthalene ion may isomerise to the azulene ion.

16. Introduction 1 2 3 4 photons Mutli-photon excitation of • 4 or 5 photon absorption the ion may fragment. • H and C2H2 are the most likely fragments.

17. Experimental Photolysis Attenuators Excimer Laser (XeCl) absorption 308 nm Shutter Lens time Quartz Window Absorption Filters Dye-laser (Rhod 101) Iris PMT HR Mirror HR Mirror Oscilloscope Quartz Window Oscilloscope 650 nm WLM JM Schematic

18. Experimental Procedure

19. Experimental Buffer gas absorption time 1. Fill in naphthalene. 2. Fill in buffer gas. Procedure 3. Measure the absorption. 4. Photolyse the mixture. 5. Measure the absorption as a function of time.

20. Results absorption time Types of response 5. Measure the absorption as a function of time. Type II response Type I response The responses observed are divided into 3 types. Type I: Growth-decay responses Type II: Oscillating responses Type III: Complex responses Type III response

21. Results Type I Type II response Type I response Type I(a) response Type I(b) response The responses observed are divided into 3 types. Type I: Growth-decay responses Type II: Oscillating responses Type III: Complex responses 2 classes of Type I response Type III response

22. Results Type II Type II(a) response Type II(b) response The responses observed are divided into 3 types. Type I: Growth-decay responses Type II: Oscillating responses Type III: Complex responses Type II(c) response 3 classes of Type II response

23. Results Type III Most Type III responses are either (a) or (b). Type III(b) response The responses observed are divided into 3 types. Type I: Growth-decay responses Type II: Oscillating responses Type III: Complex responses Type III(a) response 3 classes of Type III response

24. Results Experimental variables Investigate how the responses measured depend on the experimental conditions Laser fluence Buffer gas pressure Type I Number of pulses Energy of pulses Type II Buffer gas pressure Stirring of the gas mixture

25. Results No response -Laser fluence

26. Results Type I response Type II response or 0 10 20 30 40 50 60 70 80 90 mbar + He 75 Ne 7.5 Ar 18 What determines the type of response seen? Buffer gas pressure Buffer gas pressure.

27. Results H tmax Parameters of Type I responses • H – the height of the response Type I • kd – the rate of decay • tmax – the time of the maximum kd Reproducible if naphthalene was allowed to equilibrate. How these parameters depend on the experimental variables was investigated. • In particular, • Number of pulses, Np • Energy of pulses, Ep

28. Results HNp2 kd= O + m Np kd,Np For all buffer gases and lenses. Except in helium photolysed using a spherical lens HNpa, 2 < a < 2.8 Slope is linearly proportional to P. At a fixed pressure, the variation in the Type I response parameters with Np was determined. Number of pulses, Np

29. Results Q + S A rate = k0sq A B rate = kua A + 2B 3B rate = k1 B C rate = k2b . b = kua - k2b Chemical system 1 Chemical system 1 • Can be solved analytically. • S is present in excess.

30. Results Q + S A rate = k0sq A B rate = kua A + 2B 3B rate = k1 B C rate = k2b • The height of the response is proportional to the number of pulses squared. H = q01s01k0k2-1Np2 • The decay rate is linearly proportional to the number of pulses. kd = s01k0Np Chemical system 1 Chemical system 1 Assume the initial concentration of Q and S depend linearly on the number of photolysis pulses.

31. Results Q + S A rate = k0sq A B rate = kua A + 2B 3B rate = k1 B C rate = k2b • The height of the response is proportional to the number of pulses squared. H = q01s01k0k2-1Np2 • The decay rate is linearly proportional to the number of pulses. kd = s01k0Np Chemical system 1 Chemical system 1 Assume the initial concentration of Q and S depend linearly on the number of photolysis pulses.

32. Results Q + S A rate = k0sq A B rate = kua A + 2B 3B rate = k1 B C rate = k2b . b = kua - k2b Chemical system 1 Chemical system 1 • Feedback • All nonlinear chemical systems require feedback. • A product or intermediate must influence the rate of an earlier step.

33. Results Q + S A rate = k0sq A B rate = kua A + 2B 3B rate = k1ab2 B C rate = k2b . b = kua - k2b Chemical system 2 Chemical system 2 . b = kua - k2b + k1ab2 • Cubic autocatalysis step. • Can’t be solved analytically, but may be solved numerically.

34. Results • Shows all classes of Type II responses • Period increases in model. • Exponential decay following oscillations. • Doesn’t describe oscillations with 2 series of peaks. Nonlinear chemical reactions

35. Results Parameters of Type II responses Type II • Period of the response • Trend in the period • Initiation time • Equlibration time • Number of peaks • Maximum absorption • Half-width/Area Type II responses were never reproducible. Only general trends were established. There were always exceptions.

36. Results As the buffer gas pressure increases the Buffer gas pressure • Equlibration time • & • Number of peaks increases.

37. Results Gas mixture stirred during photolysis: Small oscillations can still be seen. Gas mixture stirred after photolysis: Oscillations cease. Stirred gas mixture

38. Discussion Timescale (a) ns(b) ms-s(c) s N P1 P2 ……. Q+S A B C Timescale UV UV UV

39. Discussion Timescale (a) ns(b) ms-s(c) s N P1 P2 ……. Q+S A B C Timescale (a) UV UV UV Timescale (a) – single excitation. What is produced in the photolysis pulse? The magnitude of response measured suggests the excess naphthalene plays a major role in producing B. Many compounds may be created in the pulse: 1 photon: triplet naphthalene 2 photons: naphthalene ion, excited naphthalene 3 photons: naphthalene ion, azulene 4 photons: H, C2H2, C4H2, H2, C3H3, C7H5+, C10H6+, C6H6+, C8H6+, C10H7+ 5 photons: C2H, C4H3, C6H3+, C4H2+, C7H3+, C5H3+, C4H4+, C6H5+, C6H4+, C8H5+, C7H5+, C10H6+…

40. Discussion Timescale (a) ns(b) ms-s(c) s N P1 P2 ……. Q+S A B C Timescale (a) UV UV UV Timescale (a) – single excitation. What is produced in the photolysis pulse? Expected: If His proportional to the concentration of the multi-photon photolysis products, HEpn n is an integer related to the number ofphotons in the photolysis process. Observed: H increases exponentially with Ep. H ecEp

41. Discussion Timescale (a) ns(b) ms-s(c) s N P1 P2 ……. Q+S A B C Timescale (b) UV UV UV Timescale (b) – many pulse excitation. What are the final products following the total number of pulses? Experiments involving repetition rate, continuous excitation, and a large number of pulses show: • UV pulses inhibit the formation of the absorbing species. • Time between pulses affects the production of the absorbing species.

42. Discussion Timescale (a) ns(b) ms-s(c) s N P1 P2 ……. Q+S A B C Timescale (c) UV UV UV Timescale (c) – response timescale. What is being measured? Can’t distinguish between absorption and scattering. • May be caused by aborption of a chemical compound or scattering from small particle. • Spectrum needed to determine the responsible compound(s). • Preliminary measurements using IBBCEA.

43. Conclusion First closed gas phase chemical oscillator Conclusion Responses classified Buffer gas pressure Variation with experimental variables Modelling of responses This fascinating system will prove to be a valuable source of new information on naphthalene and gas-phase oscillators.

44. Acknowledgements & thanks: Prof. Mansfield, Dr. Ruth, Prof. Brint Michael Staak, Sven Fiedler, Laser Spectroscopy Group. Prof. Hese