Chris Calvert- Ionisation of Carbon in Intense Laser Fields

8/3/2019 Chris Calvert- Ionisation of Carbon in Intense Laser Fields

1/55

Ionisation of Carbon inIonisation of Carbon in

Intense Laser FieldsIntense Laser Fields

Chris Calvert - MSci Project

Supervisor Prof. Ian Williams


2/55

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


3/55

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


4/55

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


5/55

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


6/55

Ground State

Ionisation Energy

Yh

p eXhX qq 1R

Low Intensity Process

Single Photon IonisationSingle Photon Ionisation


7/55

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


8/55

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


9/55

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



10/55

PotentialV(x)

Range ofPotentialx

e-

Electric Field of

LaserTunnelling Ionisation-Barrier width becomes finite

-Electron can escape by

Quantum Mechanical

tunnelling.






11/55

PotentialV(x)

Range ofPotentialx

e-

Electric Field of

Laser



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




12/55

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


13/55

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


14/55

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


The oscillating nature of the laser field can cause the electron to be

driven back towards the atomic core.

Laser Field


15/55

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.



16/55

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


17/55

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


18/55

Ion Beam ApparatusIon Beam Apparatus

Analyser plates

Interaction Region

DischargeIon source

Selection Magnet

Gas SourceNeedle Valve


19/55

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


20/55

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


21/55

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


22/55

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)


23/55

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


24/55

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




25/55

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



Is shell

zi

Laser Direction

r

z

.).(

2

2

)(4

9)( 104

uaZ

VI

eVipcmWs v!


26/55

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



This is the first Ion beam Sat Vol Model


27/55

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)


28/55

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)


29/55

2 Main Z2 Main Z--Scan ResultsScan Results

C2+ Yield

C3+ Yield

Research at RALResearch at RAL

1515thth

1919thth

NovemberNovember


30/55

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


31/55

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


32/55

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.


33/55

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


34/55

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


35/55

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


36/55

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


37/55

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


38/55

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


39/55

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?


40/55

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.


41/55

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


42/55

Many thanks to

Prof Ian WilliamsDr B Srigengan

Mr M Suresh

Mr J McKenna

+ UCL and RAL Collaboration+ UCL and RAL Collaboration

AcknowledgementsAcknowledgements


43/55

Thank you for listeningThank you for listening


44/55

Questions?Questions?


45/55

Polarisation of Light


46/55

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


47/55

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-


48/55

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


Electric VectorEo

Intensity Io

Interaction Beam

Linearly polarised

Intensity Io

Electric VectorEo

Plate4

P

Optical Axis

Measured Beam

Linearly polarised


Electric VectorEo

Intensity Io

Interaction Beam

Linearly polarised

Intensity Io

Electric VectorEo

Plate4

P

Optical Axis

Measured Beam

Linearly polarised


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


49/55

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


Electric VectorEo

Intensity Io

Interaction Beam


Electric Vector

Intensity

45o

2

oE

2

0I

Plate4

P

Optical Axis

Measured Beam

Linearly polarised


Electric VectorEo

Intensity Io

Interaction Beam


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


50/55


51/55

Laser Field

Summary

Electron Re-scattering


52/55


53/55

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


54/55

CC++ transitionstransitions

Ground state

Ground state

Metastable

Metastable

Ground state


55/55

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.

Chris Calvert- Ionisation of Carbon in Intense Laser Fields

Documents

Transcript of Chris Calvert- Ionisation of Carbon in Intense Laser Fields