Introduction to PhD research
Edward Gash 30 June 2004
Introduction to PhD research
Edward Gash 30 June 2004
Investigation into the role ofnaphthalene cations in theinterstellar medium
Original motivation
Diffuse interstellar bands (DIB)
Search for responsible compounds
The origins are unknown. The cations of polycyclic aromatichydrocarbons (PAHs) e.g. naphthalene, have been proposedto be responsible for some DIBs.
Absorption lines observed when light from a distant starpasses through an interstellar cloud.
Ref: Jenniskens und Désert
Original motivation
Absorption Spectrum
Preliminary experiment
Surprising results
To determine possible cation yields, the absorption at650 nm of a naphthalene-argon mixture wasinvestigated following UV ionisation.
Need a spectrum of cold, isolated, ions to compare the with theDIBs 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 orderto study this extraordinary phenomenon.
Introduction to PhD research
Edward Gash 30 June 2004
Unusual dynamic absorption ofnaphthalene-buffer gas mixturesfollowing UV photolysis
• Introduction
• Experimental
• Results
• Discussion
• Conclusion
Outline
Introduction
In thisexperiment,measured is theabsorption at650 nm of anaphthalenebuffer gasmixturefollowingmulti-photonUV excitation.
Absorption
absorption: the technique used for measuringthe absorption is ‘Cavity Ring-downSpectroscopy’ - CRDS
• Light coupledinto an opticallystable cavity.
• Emerging lightdecaysexponentially, thecharacteristic decaytime is called thering-down time,CRD.
• CRD depends onthe reflectivity ofthe mirrors and theabsorption in thecavity.
Introduction
Absorption
absorption: the technique used for measuringthe absorption is ‘Cavity Ring-downSpectroscopy’ - CRDS
•Highly sensitive~10-8 cm-1.Conventionalabsorptionexperiments~10-6 cm-1.
(Can be seen ashaving a longeffective pathlength)
• Independent offluctuations in lightsource intensity.
Introduction
Absorption
absorption: the technique used for measuringthe absorption is ‘Cavity Ring-downSpectroscopy’ - CRDS
Introduction
Absorption
In thisexperiment,measured is theabsorption at650 nm of anaphthalenebuffer gasmixturefollowingmulti-photonUV excitation.
absorption: the technique used for measuringthe absorption is ‘Cavity Ring-downSpectroscopy’ - CRDS
naphthalene is a polycyclic aromatichydrocarbon.
• White crystalline solid.
• Commonly used in mothballs.
• Vapour pressure at room temperature is0.08 mbar.
C10H8
Introduction
Naphthalene
In thisexperiment,measured is theabsorption at650 nm of anaphthalenebuffer gasmixturefollowingmulti-photonUV excitation.
Introduction
In thisexperiment,measured is theabsorption at650 nm of anaphthalenebuffer gasmixturefollowingmulti-photonUV excitation.
Mutli-photon excitation of
1
2
3
4
photon
• 1 photon absorption excitesthe molecule to the 180 state ofS1.
• ~2 % of the naphthalenemolecules with undergointersystem crossing to thetriplet manifold.
• The molecule can absorbmore photons from themetastable T1 state.
Introduction
1
2
3
4
photons
• 2 photon absorption isresonance enhanced. It leavesthe molecule just below theionisation threshold.
• At 298 K, some of thenaphthalene will begin in anexcited vibrational level of theground state – these may beionised by 2 photons.
• The molecule can absorbmore photons from themetastable T1 state.
Multi-photon excitation of
Introduction
1
2
3
4
photons
• 3 photon absorption is againresonance enhanced and maylead to ionisation.
+
+
• There is also a chance that thenaphthalene ion may isomeriseto the azulene ion.
Multi-photon excitation of
Introduction
1
2
3
4
photons
• 4 or 5 photon absorption theion may fragment.
• H and C2H2 are the mostlikely fragments.
Mutli-photon excitation of
Introduction
Dye-laser(Rhod 101)
PMT
WLM
JM
Shutter
Iris
Excimer Laser(XeCl)
Lens
Quartz Window
HR Mirror
HR Mirror
Filters
Absorption
Photolysis
Quartz Window
Oscilloscope
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 absorptionas a function of time.
time
absorption
Experimental
Procedure
Buffer gas
5. Measure the absorptionas a function of time.
Type II response
time
absorption
Type I response
Type III response
Type I: Growth-decay responses
The responses observed are dividedinto 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 dividedinto 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 I
Results
Type II
Type II(b) response
Type I: Growth-decay responses
The responses observed are dividedinto 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 dividedinto 3 types.
Type II: Oscillating responses
Type III: Complex responses
Type III(b) response
3 classes of Type III response
Most Type III responsesare either (a) or (b).
Type III
Results
Experimental variables
• Investigate how the responses measureddepend 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
No response
No response
Beam focussed
Spherical: ~106 GWm-2
Cylindrical: ~103 GWm-2
Response measured
Multi-photon process
Results
What determines the type of response seen?
Type I response
Type II response
or
+
Ne
He
Ar
0
10
20
30
40
50
60
70
80
90 mbar
Buffer gas pressure
Results
Buffer gas pressure.
75
7.5
18
Type I
Parameters of Type I responses
H
• H – the height of the response
• kd – the rate of decay
• tmax – the time of the maximum
kd
tmax
Reproducible if naphthalene wasallowed to equilibrate.
How these parameters dependon the experimental variableswas investigated.
In particular,
• Number of pulses, Np
• Energy of pulses, Ep
Results
Number of pulses, Np
At a fixed pressure, the variation in the Type I response parameterswith Np was determined.
H Np2
Except in helium photolysed using aspherical lens H Npa, 2 < a < 2.8
Results
kd = O + m Np
For all buffer gases and lenses.
Slope is linearly proportional to P.
kd,Np
Chemical system 1
Chemical system 1
• Can be solved analytically.
• S is present in excess.
Q + S A rate = k0sq
A B rate = kua
A + 2B 3B rate = k1
B C rate = k2b
Results
b = kua - k2b
.
Chemical system 1
Q + S A rate = k0sq
A B rate = kua
A + 2B 3B rate = k1
B C rate = k2b
Results
Chemical system 1
• The height of the response is proportionalto the number of pulses squared.
H = q01s01k0k2-1Np2
• The decay rate is linearly proportional tothe number of pulses.
kd = s01k0Np
Assume the initial concentration of Q and S dependlinearly on the number of photolysis pulses.
Chemical system 1
Q + S A rate = k0sq
A B rate = kua
A + 2B 3B rate = k1
B C rate = k2b
Results
Chemical system 1
• The height of the response is proportionalto the number of pulses squared.
H = q01s01k0k2-1Np2
• The decay rate is linearly proportional tothe number of pulses.
kd = s01k0Np
Assume the initial concentration of Q and S dependlinearly on the number of photolysis pulses.
Chemical system 1
Q + S A rate = k0sq
A B rate = kua
A + 2B 3B rate = k1
B C rate = k2b
Results
Chemical system 1
Feedback
• All nonlinear chemical systems require feedback.
• A product or intermediate must influence the rate of an earlier step.
b = kua - k2b
.
b = kua - k2b
.
Chemical system 2
• Cubic autocatalysis step.
• Can’t be solved analytically, butmay be solved numerically.
Q + S A rate = k0sq
A B rate = kua
A + 2B 3B rate = k1ab2
B C rate = k2b
Results
b = kua - k2b + k1ab2
.
Chemical system 2
Nonlinear chemical reactions
• Shows all classes of Type IIresponses
• Period increases in model.
• Exponential decay followingoscillations.
• Doesn’t describe oscillationswith 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 pressureincreases 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 P1 P2 ……. Q+S A B C
Timescale (a) ns (b) ms-s (c) s
UV
UV
UV
Timescale (a)
Discussion
N P1 P2 ……. Q+S A B C
Timescale (a) ns (b) ms-s (c) s
UV
UV
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, C2H2, C4H2, H2, C3H3, C7H5+, C10H6+, C6H6+, C8H6+, C10H7+
5 photons: C2H, C4H3, C6H3+, C4H2+, C7H3+, C5H3+, C4H4+, C6H5+, C6H4+, C8H5+, C7H5+, C10H6+…
The magnitude of response measured suggests the excessnaphthalene plays a major role in producing B.
Timescale (a) – single excitation.
What is produced in the photolysis pulse?
Timescale (a)
Discussion
N P1 P2 ……. Q+S A B C
Timescale (a) ns (b) ms-s (c) s
UV
UV
UV
Expected:
If H is proportional to the concentration of themulti-photon photolysis products,
H Epn
n is an integer related to the number of photons inthe photolysis process.
Observed:
H increases exponentially with Ep.
H ecEp
Timescale (a) – single excitation.
What is produced in the photolysis pulse?
Timescale (b)
Discussion
N P1 P2 ……. Q+S A B C
Timescale (a) ns (b) ms-s (c) s
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.
Timescale (c)
Discussion
N P1 P2 ……. Q+S A B C
Timescale (a) ns (b) ms-s (c) s
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 fromsmall particle.
• Spectrum needed to determine the responsible compound(s).
• Preliminary measurements using IBBCEA.
Conclusion
Conclusion
Variation with experimental variables
First closed gas phase chemical oscillator
Responses classified
Buffer gas pressure
This fascinating system will prove to be a valuablesource 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