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8/3/2019 Chris Calvert- Ionisation of Carbon in Intense Laser Fields
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Ionisation of Carbon inIonisation of Carbon in
Intense Laser FieldsIntense Laser Fields
Chris Calvert - MSci Project
Supervisor Prof. Ian Williams
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Prof Ian Williams
Dr B. Srigengan
Mr M. Suresh
Mr Jarlath McKenna
Prof Roy Newell
Dr Will Bryan
Miss Elizabeth English
Mr Joseph Wood
Dr Andrew Langley
Dr Edwin Divall
Dr Chris Hooker
Dr Edmund Turcu
Ionisation of Carbon inIonisation of Carbon in
Intense Laser FieldsIntense Laser Fields
Chris Calvert - MSci Project
In collaboration with
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Ionisation in Intense Laser fieldsIonisation in Intense Laser fields
Motivation
Theory
Experimental InvestigationExperimental Investigation Experimental Setup and Method
Modelling the experiment
OverviewOverview
Previous work on CPrevious work on C++ IonsIons
- Motivation for current research
Results and AnalysisResults and Analysis
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Why investigate Ionisation in Intense Laser fields?
Specific Ionisation dynamics. Multiple Ionisation, Rescattering. More details on these later
Why Carbon? Spin Forbidden C+ C2+ transition?
Multiple Ionisation/Rescattering? explain later
MotivationMotivation
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Main processesMain processes
Single Photon Ionisation
Multi Photon Ionisation (MPI) Field Ionisation
Non sequential Ionisation
Non sequential Ionisation
Rescattering Model
Increased
Laser Intensity
Ionisation in Laser FieldsIonisation in Laser Fields
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Ground State
Ionisation Energy
Yh
p eXhX qq 1R
Low Intensity Process
Single Photon IonisationSingle Photon Ionisation
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p eXhnX qq 1R
Ground State
Ionisation Energy
Yh
Virtual Excited
States
Real Excited State
Yh
Yh
YhFor MPI to occur,
successive photons must
arrive within the lifetime of
the preceding intermediate
excited states.
Higher Intensity Process
Multi Photon IonisationMulti Photon Ionisation
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High Intensities > 1014 W/cm2
Quasi static model
Potential experienced by
the electron arises from
the resultant of the
Atomic potential well +
instantaneous effect of
Laser field.
PotentialV(x)
Range ofPotentialx
e-
Atomic potential well (no external field)
Electric field of the
Laser tends towards
the magnitude of the
atomic field.
Field IonisationField Ionisation
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Laser field effectively
lowers the potential
barrier experienced
by electron
PotentialV(x)
Range ofPotentialx
e-
Electric Field of
Laser
Resultant Potential experienced by the electron = Atomic potential well+
instantaneous effect of Laser field
Intensities > 1014 W/cm2
Field IonisationField Ionisation
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PotentialV(x)
Range ofPotentialx
e-
Electric Field of
LaserTunnelling Ionisation-Barrier width becomes finite
-Electron can escape by
Quantum Mechanical
tunnelling.
Resultant Potential experienced by the electron = Atomic potential well+
instantaneous effect of Laser field
Intensities > 1014 W/cm2
Field IonisationField Ionisation
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PotentialV(x)
Range ofPotentialx
e-
Electric Field of
Laser
Resultant Potential experienced by the electron = Atomic potential well+
instantaneous effect of Laser field
Over the Barrier
Ionisation-The resultant potential barrier
experienced by the electron is
lower than the energy of the
electron state.
-The electron is no longer boundby the resultant effective potential
and is free to escape.).(
22
)(4
9)( 104
uaZ
VI
eVipcmWs v!
Over the Barrier
Ionisation-The Saturation Intensity (Is)
corresponding to an Ionisation
potential Vip can be calculated
classicallyto be
Intensities > 1014 W/cm2
Field IonisationField Ionisation
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p
eXX
1 p
eXX
21
Sequential Ionisation Two Succesive steps
Non Sequential Ionisation Simultaneous removal of two electrons
p eXX 22
Can be explained by Re-collision orRe-scattering
Theory (Corkum 1993)
Double IonisationDouble Ionisation
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A primary electron is ejected from the atom by the laser pulse. This
can be modelled quantum mechanically by tunnelling ionisation.
This free electron is then regarded classically as a free particle as it
proceeds towards the continuum..
Laser Field
Electron ReElectron Re--ScatteringScattering
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As the electron passes through the ion it can collisionally remove a
second electron.
Both electrons have enough energy to escape the atomic core and
double ionisation is observed
Electron ReElectron Re--ScatteringScattering
The oscillating nature of the laser field can cause the electron to be
driven back towards the atomic core.
Laser Field
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Corkum predicted
As Ellipticity (e) of the incident radiation increases the probability of the firstelectron being driven back into the ion decreases probability of non sequential
ionisation decreases
Therefore, forLinearly polarised light (e=0), there is a high probability
ofNon-sequential ionisation occurring. ForCircularly Polarised light
(e=1) this probability is small.
Electron ReElectron Re--ScatteringScattering
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Ion Beam
Laser
Products of
Interaction
The approach in this project is to focus Intense Femtosecond Laser pulses
across an Ion Beam source and detect the resulting ionisation products.
Experimental Investigation of IonisationExperimental Investigation of Ionisation
in Intense Laser Fieldsin Intense Laser Fields
Only two groups in the world use this Ion Beam approach method!
Other groups use Gas targets
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ASTRA Laser SystemIon Beam Apparatus
QUB Atomic and Molecular
Division
Rutherford Appleton Laboratories
(RAL), Oxfordshire
Experimental Investigation of IonisationExperimental Investigation of Ionisation
in Intense Laser Fieldsin Intense Laser Fields
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Ion Beam ApparatusIon Beam Apparatus
Analyser plates
Interaction Region
DischargeIon source
Selection Magnet
Gas SourceNeedle Valve
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Laser SystemLaser System
ASTRA (TA1)
Central Laser Facility,
Rutherford Appleton Laboratories (RAL) Oxfordshire.
Lens Translation Stage
Interaction Region
Laser Path
Ti:Sapphire,
50-55fs Pulses,800nm, 10Hz, Pulse Energies ~20mJ
Peak Intensity ~ 5*1016 W/cm2
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z axis
r
Ionisation
Products
The peak laser intensity (Io) occurs at the focal point. The intensity
distribution changes along the z-axis.
The z-dependence (and thus Intensity dependence) of the Ion yield can
be found by varying the z-position of the Laser-Ion beam interaction.
or Intensity Selective Scan (ISS)
Laser
Ion Beam
Focal Point
The ZThe Z--Scan ExperimentScan Experiment
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In practice the Laser Focus is moved with respect to the Ion beam by
use of planar convex lens mounted on translation stage.
This is carried out along the z-axis in steps of 0.25mm. 500
measurements (or shots) are taken at each z-position and the
corresponding ion yields are recorded.
z
r
Laser
Ionisation Products
Ion Beam
The ZThe Z--Scan ExperimentScan Experiment
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z axis
r
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0 1 2 3 4 5 6
z position (mm)
Integrated
Ion
Yield
(arb.units)
High intensity at
focal point but
small interaction
volume
Good compromise between
Intensity and Interaction Volume
Interaction Volume is
large but Intensities
are no longer high
enough to cause
Ionisation
ZZ--Scan of CScan of C2+2+ Ion YieldIon Yield(Using Circularly Polarised Light)
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The Intensity distribution of a focussed Laser beam is Lorentzian along the
direction of beam travel, z, and Gaussian along the radial direction, r.
This can be modelled by theIsointensity Contourexpression-
-
-
!2
0
2
0
2
2
0
0
1
2
1
),(
zz
rExp
zz
IzrI
[
Io = Peak intensity of Laser,
Beam waist radius o = 2f/d
Rayleigh range zo= o2/
(o and zo are measurements of the focal point of the Laser)
The Saturated VolumeModelThe Saturated VolumeModel
Modelling the Z-Scan
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Expressing r as a function of I and z -
21
2
0
0
2
0
2
0
1
12
-
s!
z
z
I
ILn
z
z
r
n
n
[
The profile of the focussed laser
can be plotted
The region contained in within each contour In has intensities > InThe 3-D profile is generated by rotating these contours through 2
In contour r
z
Laser Direction
The Saturated VolumeModelThe Saturated VolumeModel
Modelling the Z-Scan
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Assume that the current density of the Ion beam is constant
Ion Yield at zi Interaction Volume bounded by Ion
beam and the Is contour
Consider a 1+ to 2+ ionisation process.
Assume that this Ionisation process will occur for all I>Is
The 2+ Ion Yield at zi = the number of 1+ ions bound by the ion beam and
the Is contour
w
The Saturated VolumeModelThe Saturated VolumeModel
Modelling the Z-Scan
Is shell
zi
Laser Direction
r
z
.).(
2
2
)(4
9)( 104
uaZ
VI
eVipcmWs v!
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Original model (El-Zein 2001) used the interaction volume bounded
by a vertical slit and the laser contour.
Assuming Cylindrical Ion beam dimensions, new modifications were
made. A computer program was constructed to calculate the Interaction
Volume at each z position.
Ion Beam Direction
7mm 10mm0mm
7mm 10mm0mm
Ion Beam direction into the screen
Hence a plot can be made to model the z-dependence of the Ion yield
for a given Ionisation process.
Laser Direction
The Saturated VolumeModelThe Saturated VolumeModel
Modelling the Z-Scan
This is the first Ion beam Sat Vol Model
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Previous ZPrevious Z--Scan of CScan of C2+2+ Yield (Feb 2002)Yield (Feb 2002)
0.0E+00
5.0E-12
1.0E-11
1.5E-11
2.0E-11
2.5E-11
3.0E-11
3.5E-11
4.0E-11
4.5E-11
0 2 4 6 8 10 12 14
Z position (mm)
Ion
Yield
(arbitrary
units
Shoulder
+ Yield at large z
values (z > 4mm)
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The Shoulder feature
requires the metastable
transition -
C+ 4Pp C2+ 1S (19eV)
In order to model the Ion yield
But this transition is spin-
forbidden!
Time to investigate this!
The Ion yield from z=1mm
to z=4mm is explained by
the expected groundstate
transition
C+ 2Pp C2+ 1S (24.4eV)
Some C2+ to C3+ depletion
occurs at small z values
C2+ 1Sp C3+ 2S (47.8eV)
AimAim
Detailed experiment to
examine C2+ Z-scan.
Looking for
confirmation of
Metastable transition
+Check for
rescattering
Previous ZPrevious Z--Scan of CScan of C2+2+ Yield (Feb 2002)Yield (Feb 2002)-Modelled by Saturated Volume Fit-
Volume Curve Fit for Z-Scan of C2+
Z Position (mm)
0 2 4 6 8
Volume(
arbitraryu
nits)
0
2e-13
4e-13
6e-13
8e-13
1e-12
1e-12
1e-12
2e-12
2e-12
2e-12
Vol fit C2+
Groundstate
Vol fit C2+
Metastable
Vol fit C3+
Groundstate
Sum of Volume fits
DATA Z-scan
Shoulder
C+Metastable configuration 1s2 2s 2p2
spin orientation oq o o o
C2+Ground configuration 1s2 2s2
spin orientation oq oq
+ Yield at large zvalues (z > 4mm)
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2 Main Z2 Main Z--Scan ResultsScan Results
C2+ Yield
C3+ Yield
Research at RALResearch at RAL
1515thth
1919thth
NovemberNovember
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0
0.05
0.1
0.15
0.2
0.25
0 0.5 1 1.5 2 2.5 3 3.5
z position (mm)
I
ntegrated
Yield
(arb
.units)
Linear
Circular
If rescattering C+ to C3+ occurs, itis a very small effect
Assume that 3+ yield arises from
sequential process
C+ C2+ C3+
Checking forRe-scattering C+ to C3+
Saturated Volume
Model can be
applied
CC3+3+ ZZ--ScanScan
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0
0.05
0.1
0.15
0.2
0.25
0 0.5 1 1.5 2 2.5 3
z position (mm)
Integrated
Yield
(arb.
units)
This saturated volume fit for C2+ to C3+ can now be used to give the depletion
of C2+ ions when modelling C2+ Z-scan
C2+ to C3+ fit (production)
C3+ to C4+ fit (depletion)
Ion Yield (circ)
CC3+3+ ZZ--ScanScan
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0
0.05
0.1
0.15
0.2
0.25
0 0.5 1 1.5 2 2.5 3
z position (mm)
Integrated
Yield
(arb
.units)
This saturated volume fit for C2+ to C3+ can now be used to give the depletion
of C2+ ions when modelling C2+ Z-scan
C2+ to C3+ fit (production)
C3+ to C4+ fit (depletion)
Ion Yield (circ)
Saturated Volume Fit
CC3+3+ ZZ--ScanScan
Tunnelling and MPI
occur at large
values.
Sat Vol only works
forOTB ionisation.
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Slight difference between Linear and circular signal around z=2mm
Experimental factor
As before assume little or no rescattering
0
0.5
1
1.5
2
0 1 2 3 4 5 6z position (mm)
Integrated
yiel
d
(arb.units)
Linear
Circular
CC2+2+ ZZ--ScanScan
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Shoulder not observed. Metastable transition absent?
Applying the Saturated Volume Model to find out what
transitions occur
0
0.5
1
1.5
2
0 1 2 3 4 5 6z position (mm)
Integrated
yield
(arb.units)
Linear
Circular
CC2+2+ ZZ--ScanScan
! NO SHOULDER !
+ No Yield at large
z values (z > 4mm)
0.0E+00
5.0E-12
1.0E-11
1.5E-11
2.0E-11
2.5E-11
3.0E-11
3.5E-11
4.0E-11
4.5E-11
0 5 10 15Z (mm)
Ion
Yield(arbitraryunits)
Shoulder
Previous Z-Scan
Yield at z>4mm
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0
0.5
1
1.5
2
0 1 2 3 4 5
z position (mm)
Integrated
yield
(arb.units)
C2+ to C3+ fit
(depletion)C+ to C2+ ground state
(production)
Ion Yield (circ)
CC2+2+ ZZ--ScanScan
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0
0.5
1
1.5
2
0 1 2 3 4 5
z position (mm)
Integrated
yield
(arb.units)
C2+ to C3+ fit
(depletion)C+ to C2+ fit (production)
Ion Yield (circ)
Saturated
Volume fit
CC2+2+ ZZ--ScanScan
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0
0.5
1
1.5
2
0 1 2 3 4 5
z position (mm)
Integrated
yield
(arb.units)
C2+ to C3+ fit
(depletion)C+ to C2+ fit (production)
Ion Yield (circ)
Saturated
Volume fit
Metastable transition
(10% pop)
CC2+2+ ZZ--ScanScan
Metastable transition is clearly not viable in this model
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0
0.5
1
1.5
2
0 1 2 3 4 5
z position (mm)
Integrated
yield
(arb.units)
C2+ to C3+ fit
(depletion)C+ to C2+ ground
(production)
Ion Yield (circ)
Saturated
Volume Fit
Best fit is
N
o metastable, spin flip transition !
CC2+2+ ZZ--ScanScan
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Experimental Artefact in previous Carbon data?
Uncertainties.
In Applying Sat Vol fits. Peak Intensity, pulse length,relating Vip to Is. Tunnelling, MPI?
Metastables absent from source?
Contradicts previous evidence. Meta-Meta transition
cant be distinguished from ground ground
Shouldnt be surprised Dipole selection rules(s=0) forbid the transition!
NoMetastable transitionNoMetastable transition Why?Why?
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Ionisation in Intense Laser Fields researched
Ion Beam Modifications made to Saturated Volume Model
-Useful applications for future research
Ionisation of Carbon in Intense Laser fields investigated
-Metastable Transition not observed.
-Data successfully modelled by expected transitions
- Multiple Ionisation from C+ investigated for the 1st time
Mainly sequential suggest negligible contribution from rescattering
- First successful modelling ofC3+ z-scan.
ConclusionConclusion
Very Interesting and enjoyable project.
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Sat Vol Improvements - Ion beam current density
Deconvolution Allow for Tunnelling and MPI
Shorter Pulse times ~10fs allows probing of
stationary molecule.
(Pulse time lower than Vibrational period of molecule e.g. H2 ~20fs.)
Future WorkFuture Work
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Many thanks to
Prof Ian WilliamsDr B Srigengan
Mr M Suresh
Mr J McKenna
+ UCL and RAL Collaboration+ UCL and RAL Collaboration
AcknowledgementsAcknowledgements
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Thank you for listeningThank you for listening
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Questions?Questions?
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Polarisation of Light
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2
U
U
2P Plate
PolaroidLaser
Rotation of plate
2
U
U
2P Plate
PolaroidLaser
Rotation of plate
Half-Wave Plate
Through Polaroid I=Iocos2
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21
3*
321
0 3
3
!
!
n
Z
lin
circ
I
T
IT
I
[
[
>> 1
Linear / Circular matching
ADK theoretical fits show that;
Linear yield at Intensity 1.3*I matches
the Circular yield at I .
-The 0.65 Factor-
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Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 0
Electric VectorEo
Intensity Io
Interaction Beam
Linearly polarised
Intensity Io
Electric VectorEo
Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 0
Electric VectorEo
Intensity Io
Interaction Beam
Linearly polarised
Intensity Io
Electric VectorEo
Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 0
Electric VectorEo
Intensity Io
Interaction Beam
Linearly polarised
Intensity Io
Electric VectorEo
Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 0
Electric VectorEo
Intensity Io
Interaction Beam
Linearly polarised
Intensity Io
Electric VectorEo
Fig /4 Plate Linear transmission
Linear at Io
Interaction beam is at Io
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Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 45o
Electric VectorEo
Intensity Io
Interaction Beam
Circularly polarised
Electric Vector
Intensity
45o
2
oE
2
0I
Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 45o
Electric VectorEo
Intensity Io
Interaction Beam
Circularly polarised
Electric Vector
Intensity
45o
2
oE
2
0I
Plate4
P
Optical Axis
Measured Beam
Linearly polarised
Angle to optical axis = 45o
Electric VectorEo
Intensity Io
Interaction Beam
Circularly polarised
Electric Vector
Intensity
45o
2
oE
2
0I
Fig /4 Plate Linear to Circular
Circular at Io
Interaction beam is actually 0.5Io
So Linear matches at 1.3(0.5I0) = 0.65IoWhen matching to Circular Io
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Laser Field
Summary
Electron Re-scattering
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Laser SystemLaser System
CompressorAuto -correlator
ASTRA (TA1)
Central Laser Facility,
Rutherford Appleton Laboratories(RAL) Oxfordshire.
Ti:Sapphire, 50-55fs Pulses,800nm,
10Hz Pulse Energies ~20mJ
Peak Intensity ~ 5*1016 W/cm2
Laser source
Beam expander
Interaction region
Translation stage
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CC++ transitionstransitions
Ground state
Ground state
Metastable
Metastable
Ground state
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tunnelling
Can occur as long as the time taken for the
electronic wavefunction to tunnel through
the barrier is shorter than half the optical
period of the incident light.