Introduction to PhD research Edward Gash 30 June 2004

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

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. Absorption lines observed when light from a distant star passes through an interstellar cloud. Ref: Jenniskens und Désert

Original motivation Absorption Spectrum Preliminary experiment Surprising results To determine possible cation yields, the absorption at 650 nm of a naphthalene-argon mixture was investigated following UV ionisation. 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. An unexpected time-dependent absorption was observed. The original objectives of the project were revised in order to study this extraordinary phenomenon.

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

Introduction Experimental Results Discussion Conclusion Outline

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

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

naphthalene is a polycyclic aromatic hydrocarbon. White crystalline solid. Commonly used in mothballs. Vapour pressure at room temperature is 0.08 mbar. C 10 H 8 Introduction Naphthalene In this experiment, measured is the absorption at 650 nm of a naphthalene buffer gas mixture following multi-photon UV excitation.

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

Mutli-photon excitation of 1234photon 1 photon absorption excites the molecule to the state of S 1. ~2 % of the naphthalene molecules with undergo intersystem crossing to the triplet manifold. The molecule can absorb more photons from the metastable T 1 state. Introduction

1234photons 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 T 1 state. Multi-photon excitation of Introduction

1234photons 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. Multi-photon excitation of Introduction

1234photons 4 or 5 photon absorption the ion may fragment. H and C 2 H 2 are the most likely fragments. Mutli-photon excitation of Introduction

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

Experimental Procedure

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

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

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

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

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

No response -Laser fluence No naphthalene No buffer gas Not focussed No focus: ~ 1 GWm -2 No response Beam focussed Spherical: ~10 6 GWm -2 Cylindrical: ~10 3 GWm -2 Response measured Multi-photon process Results

What determines the type of response seen? Type I responseType II response or + Ne He Ar mbar Buffer gas pressure Results Buffer gas pressure

Type I Parameters of Type I responses H H – the height of the response k d – the rate of decay t max – the time of the maximum kdkd t max Reproducible if naphthalene was allowed to equilibrate. How these parameters depend on the experimental variables was investigated. In particular, Number of pulses, N p Energy of pulses, E p Results

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

Chemical system 1 Can be solved analytically. S is present in excess. Q + S A rate = k 0 sq A B rate = k u a A + 2B 3B rate = k 1 B C rate = k 2 b Results b = k u a - k 2 b.

Chemical system 1 Q + S A rate = k 0 sq A B rate = k u a A + 2B 3B rate = k 1 B C rate = k 2 b Results Chemical system 1 The height of the response is proportional to the number of pulses squared. H = q 01 s 01 k 0 k 2 -1 N p 2 The decay rate is linearly proportional to the number of pulses. k d = s 01 k 0 N p Assume the initial concentration of Q and S depend linearly on the number of photolysis pulses.

Chemical system 1 Q + S A rate = k 0 sq A B rate = k u a A + 2B 3B rate = k 1 B C rate = k 2 b Results Chemical system 1 The height of the response is proportional to the number of pulses squared. H = q 01 s 01 k 0 k 2 -1 N p 2 The decay rate is linearly proportional to the number of pulses. k d = s 01 k 0 N p Assume the initial concentration of Q and S depend linearly on the number of photolysis pulses.

Chemical system 1 Q + S A rate = k 0 sq A B rate = k u a A + 2B 3B rate = k 1 B C rate = k 2 b Results Chemical system 1 Feedback All nonlinear chemical systems require feedback. A product or intermediate must influence the rate of an earlier step. b = k u a - k 2 b.

. Chemical system 2 Cubic autocatalysis step. Can’t be solved analytically, but may be solved numerically. Q + S A rate = k 0 sq A B rate = k u a A + 2B 3B rate = k 1 ab 2 B C rate = k 2 b Results b = k u a - k 2 b + k 1 ab 2. Chemical system 2

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

Type II Parameters of Type II responses 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. Results

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

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

Timescale Discussion N P 1 P 2 ……. Q+S A B C Timescale (a) ns (b) ms-s (c) s UV

Timescale (a) Discussion N P 1 P 2 ……. Q+S A B C Timescale (a) ns (b) ms-s (c) s UV 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, C 2 H 2, C 4 H 2, H 2, C 3 H 3, C 7 H 5 +, C 10 H 6 +, C 6 H 6 +, C 8 H 6 +, C 10 H photons: C 2 H, C 4 H 3, C 6 H 3 +, C 4 H 2 +, C 7 H 3 +, C 5 H 3 +, C 4 H 4 +, C 6 H 5 +, C 6 H 4 +, C 8 H 5 +, C 7 H 5 +, C 10 H 6 + … The magnitude of response measured suggests the excess naphthalene plays a major role in producing B. Timescale (a) – single excitation. What is produced in the photolysis pulse?

Timescale (a) Discussion N P 1 P 2 ……. Q+S A B C Timescale (a) ns (b) ms-s (c) s UV Expected: If H is proportional to the concentration of the multi-photon photolysis products, H  E p n n is an integer related to the number of photons in the photolysis process. Observed: H increases exponentially with E p. H  e cEp Timescale (a) – single excitation. What is produced in the photolysis pulse?

Timescale (b) Discussion N P 1 P 2 ……. Q+S A B C Timescale (a) ns (b) ms-s (c) s 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.

Timescale (c) Discussion N P 1 P 2 ……. Q+S A B C Timescale (a) ns (b) ms-s (c) s 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.

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

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