Introduction to TDAONBGMFUVP Edward Gash 11 Dec 2003

Introduction to TDAONBGMFUVP Edward Gash 11 Dec 2003 Introduction to ‘Time-dependent absorption of naphthalene buffer gas mixtures following UV photolysis’

Outline Introduction Experimental Results What? Why?

Introduction absorption laser light at 650 nm naphthalene UV laser pulses In this experiment, measured is the absorption of laser light at 650 nm by a naphthalene buffer gas mixture that as been photolysed by UV laser pulses. Naphthalene Naphthalene is a polycyclic aromatic hydrocarbon. White crystalline solid. Commonly known as mothballs. Vapour pressure at room tempature is mbar. C 10 H 8

Introduction absorption laser light at 650 nm UV laser pulses In this experiment, measured is the absorption of laser light at 650 nm by a naphthalene buffer gas mixture that as been photolysed by UV laser pulses. UV laser pulses UV laser pulses: the excimer laser in the lab produces pulses of UV light at 308nm. EX-700 pulsemaster XeCl laser. Maximum pulse energy of 100 mJ Pulse length of 15 ns. Usually run at a repetition rate of Hz. Can be triggered internally or with pulses from the computer.

Introduction absorption laser light at 650 nm In this experiment, measured is the absorption of laser light at 650 nm by a naphthalene buffer gas mixture that as been photolysed by UV laser pulses. Laser light at 650 nm Laser light at 650 nm: this is produced by a dye laser. Hyperdye 700 laser, containing DCM. The dye in the laser is ‘pumped’ using the excimer laser. A grating in the dyelaser allows the wavelength to be tuned. DCM allows tuning from nm. Maximum output energy ~5mJ; maximum repetition rate 20 HZ. Spectral resolution is 0.3 cm -1.

Introduction absorption In this experiment, measured is the absorption of laser light at 650 nm by a naphthalene buffer gas mixture that as been photolysed by UV laser pulses. absorption absorption: the technique used for measuring the absorption is ‘Cavity Ring- down Spectroscopy’ - CRDS

Introduction absorption absorption: the technique used for measuring the absorption is ‘Cavity Ring- down Spectroscopy’ - CRDS Light coupled into an optically stable cavity. Light emerging through the rear of the cavity decays exponentially, the characteristic decay time is called the ring-down time. The ring-down time 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 CRDS is intensity independent. It has a long effective path length. Extremely sensitive – can measure absorption coefficents of cm -1. Conventional absorption experiments can only measure absorption coefficents of cm -1. Applicable over a large spectral range.

Experiment Dye Laser Excimer Laser 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

Results 5.Measure the absorption as a function of time. time absorption 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 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 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

Type III Type III(c) 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 (b) or (c).

Buffer Gas Pressure What determines the response Type that is observed? Buffer Gas Pressure Type I responseType II response or + Ar He Ne mbar

Questions? 1)What are we measuring? 2)Why are these responses happening?

Questions? 1)What are we measuring? 1) What are we not measuring? Not naphthalene, not buffer gas, not photolysed buffer gas No response when the photolysis pulses are unfocussed Response caused by multiphoton excitation of naphthalene buffer gas mixture

Mutiphoton 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 state.

Mutiphoton excitation of 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 vibrational level of the ground state – these may be ionisted by 2 photons. The molecule can absorb more photons from the metastable state.

Mutiphoton excitation of 1234photons 3 photon absorption is again resonance enhanced and ionises the naphthalene. There is also a chance that the naphthalene ion may isomerise to the azulene ion. +

Mutiphoton excitation of 1234photons 4 or 5 photon absorption the ion may fragment. H and C 2 H 2 are the most likely fragments to be lost

Other considerations The energy dependance of some of the responses, suggests that the absorbing compound is not a direct result of photolysis. The photolysis products may react to produce the absorbing compund. Absorption may not be the mechanism for removing light from the cavity; the light may also be scattered from particles formed following photolysis. The naphthalene cation and azulene are known to absorb at this wavelength.

Questions? 2)Why are these responses happening? Is it due to a non-linear chemical reaction? Is it due to a physical process such as convection currents? ?.

Nonlinear chemical reactions Not common in the gas phase 1 st report of a chemical oscillator was by Waterford scientist Robert Boyle in late 1600s, describing a gas phase system. Feedback Fundamental to all nonlinear chemical systems is feedback. A product or intermediate must influence the rate of an earlier step. Feedback can be thermal or chemical.

Nonlinear chemical reactions Chemical system 1 No feedback. S is present in excess. Can be solved analytically Q + S A rate = k 0 A B rate = k u A + 2B 3B rate = k 1 B C rate = k 2 Type I response

Nonlinear chemical reactions Chemical system 1 Assume Q and S depend on the amount of photolysis pulses. How does the this model’s response change with the number of pulses? Q + S A rate = k 0 A B rate = k u A + 2B 3B rate = k 1 B C rate = k 2

Nonlinear chemical reactions Chemical system 1 The height of the response is proportional to the number of pulses squared. Q + S A rate = k 0 A B rate = k u A + 2B 3B rate = k 1 B C rate = k 2 The decay rate is linearly proportional to the number of pulses. Type I responses This simple chemical system may be used to model Type I responses

Nonlinear chemical reactions Chemical system 2 Cubic autocatalysis step. Can’t be solved analytically, but may be modelled. Q + S A rate = k 0 A B rate = k u A + 2B 3B rate = k 1 B C rate = k 2

Nonlinear chemical reactions Type II(a) response Type II(b) response Period increases in model. Exponential decay following oscillations No physical justification for model No mechanism for removing S Can only describe 1 component oscillations Observed responses are consistant with a nonlinear chemical system

Other Options Soot formation Periodic precipitation Convection currents Heterogeneous process ?.

Other factors Stirring the gas mixture removes oscillations. Stirring the gas mixture during photolysis changes the response. etc. Increasing the temperature at low pressure lowers the height of the response. Increasing the temperature during,or immediately after, photolysis can induce a Type II response. Same initial conditions can give rise to different responses. Changing the repetition rate of the photolysis pulse can effect the result. The height of Type I responses depend exponentially on the energy of the photolysis pulses. During photolysis the absorption increases exponentially.