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?

IntroductionIn this experiment, measured is the absorptionabsorption of laser light at 650 laser light at 650 nmnm by a naphthalenenaphthalene buffer gas mixture that as been photolysed by UV UV laser pulseslaser pulses.

Naphthalene Naphthalene is a polycyclic aromatic hydrocarbon.

• White crystalline solid.

• Commonly known as mothballs.

• Vapour pressure at room tempature is 0.008 mbar.

C10H8

IntroductionIn this experiment, measured is the absorptionabsorption of laser light at 650 laser light at 650 nmnm by a naphthalene buffer gas mixture that as been photolysed by UV UV laser pulseslaser pulses.

UV laser pulsesUV 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 10-20Hz.

• Can be triggered internally or with pulses from the computer.

IntroductionIn this experiment, measured is the absorptionabsorption of laser light at 650 laser light at 650 nmnm by a naphthalene buffer gas mixture that as been photolysed by UV laser pulses.

Laser light at 650 nmLaser 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 610-690 nm.

• Maximum output energy ~5mJ; maximum repetition rate 20 HZ.

• Spectral resolution is 0.3 cm-1.

IntroductionIn this experiment, measured is the absorptionabsorption of laser light at 650 nm by a naphthalene buffer gas mixture that as been photolysed by UV laser pulses.

absorptionabsorption: the technique used for measuring the absorption is ‘Cavity Ring-down Spectroscopy’ - CRDS

Introductionabsorptionabsorption: 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.

Introductionabsorptionabsorption: 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 10-8 cm-1. Conventional absorption experiments can only measure absorption coefficents of 10-6 cm-1.

•Applicable over a large spectral range.

Experiment

Dye LaserDye Laser

Excimer LaserExcimer 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

abso

rptio

n

Results

5. Measure the absorption as a function of time.

time

abso

rptio

n

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) response Type 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 PressureWhat determines the response Type that is observed?

Buffer Gas Pressure

Type I response Type II responseor

+

Ar

He

Ne

0 10 20 30 40 50 60 70 80 90 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

1 2 3 4 photon• 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 state.

Mutiphoton excitation of

1 2 3 4 photons• 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

1 2 3 4 photons• 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

1 2 3 4 photons• 4 or 5 photon absorption the ion may fragment.

• H and C2H2 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 reactionsNot common in the gas phase

• 1st 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 reactionsChemical system 1

• No feedback.

• S is present in excess.

• Can be solved analytically

Q + S A rate = k0

A B rate = ku

A + 2B 3B rate = k1

B C rate = k2

Type I response

Nonlinear chemical reactionsChemical 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 = k0

A B rate = ku

A + 2B 3B rate = k1

B C rate = k2

Nonlinear chemical reactionsChemical system 1

• The height of the response is proportional to the number of pulses squared.

Q + S A rate = k0

A B rate = ku

A + 2B 3B rate = k1

B C rate = k2

• 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 reactionsChemical system 2

• Cubic autocatalysis step.

• Can’t be solved analytically, but may be modelled.

Q + S A rate = k0

A B rate = ku

A + 2B 3B rate = k1

B C rate = k2

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 OptionsSoot 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.