Ions in Intense Femtosecond Laser Fields Jarlath McKenna MSci Project10th December 2001 Supervisor: Prof. Ian Williams

Ions in Intense Femtosecond Ions in Intense Femtosecond Laser FieldsLaser Fields

Jarlath McKenna

MSci Project 10th December 2001

Supervisor: Prof. Ian Williams

Outline

• Experimental Apparatus / Techniques– Z-scan

– Intensity scan

• Introduction / Background– Strong Field Ionisation

– Sequential and Non-sequential Ionisation

• Results and Analysis

Introduction

• Study of the ionisation dynamics of positively charged atomic ions in intense femtosecond Laser fields

• Analysis and interpretation of results– familarisation with experimental apparatus

• Experiments carried out in collaboration with a group from UCL– February 2002 at RAL using the ASTRA laser

Why?

• Why study strong field ionisation of positive Why study strong field ionisation of positive atomic ions?atomic ions?

• First study using a beam of positive atomic ions– allows study of wider range of species e.g. O+, N+

– compare with results from neutral target

• All previous experiments have used neutral targets

Ground state

Ionisation level• Single Photon IonisationSingle Photon Ionisation

– Ionisation energy of valence electron is supplied by one photon

• What happens in high intensity Laser What happens in high intensity Laser interactions?interactions?– Low intensity: Single Photon Ionisation– Higher intensity: Multiphoton Ionisation– Very high intensity: Field Ionisation

• Multiphoton IonisationMultiphoton Ionisation– Ionisation energy of

valence electron is supplied by a number of photons

Ground state

Ionisation level

Virtual excited state

Real excited state


• Multiphoton IonisationMultiphoton Ionisation– Ionisation energy of

valence electron is supplied by a number of photons

– Above Threshold Ionisation may take place

Ground state

Ionisation level

ATI

Virtual excited state

Real excited state


• Electric field distorts the atomic potential well– this lowers the potential barrier seen by an electron in

the atom/ion

Field IonisationField Ionisation

Potential range (x)x0

Ato

mic

Pot

ent

ial w

ell

V0(x

) Potential wellElectric field

e-

• Tunnelling RegimeTunnelling Regime


the atom/ion


As barrier is lowered, it’s width decreases.

Increased probability of electron tunnelling


Ato

mic

Pot

ent

ial w

ell

V0(x


e- e-

• Over-the-barrier Over-the-barrier RegimeRegime


Electron is free to escape the atom


Ato

mic

Pot

ent

ial w

ell

V0(x


e-Potential barrier is lower than electronic state


the atom/ion

• Dynamic Stark ShiftDynamic Stark Shift



Ato

mic

Pot

ent

ial w

ell

V0(x


e-


the atom/ion

Energy states of electrons are Stark shifted up towards the continuum

Dynamic or ac Stark shift because of oscillating E-field of laser

• Sequential:– Ionisation takes place in a series of steps

Sequential and Non-Sequential Sequential and Non-Sequential IonisationIonisation

A A+ A2+

• Non-Sequential:– Ionisation takes place in a single step

A A2+

CORE

A

Recollision ModelRecollision Model

Atomic core with outer shell of electrons

Electric field strength from laser pulse expels an electron

CORE

A+


During oscillatory motion of E-field, the electron may make multiple returns to the atomic core

Electron may collide with a valence electron

CORE

A2+


Collision with another electron may directly remove the electron or excite it to a higher energy state in which it then tunnels its way through the remaining barrier

Ion Source-ions produced via

discharge

Extraction and Focussing Lenses -ions are accelerated to 1-2 keV

Selection Magnet

Einzel lens

Deflection Plates

Interaction Region

45o Parallel Plate deflectors

Neutral Fragment Detector

Primary Beam

Collector

Charged Fragment Detector

Apparatus

Laser Beam

• Laser Intensity is – Lorentzian along z

direction

– Gaussian in radial r direction

• Scan with a 0.5mm aperture

r

Z Value (mm)

Rad

ius

(mm

)

z

Slit

Laser beam

Intensity Selective ScanningIntensity Selective Scanningor Z-scanor Z-scan

r

Z Value (mm)

Rad

ius

(mm

)

z

Slit

Laser beam

Intensity Selective ScanningIntensity Selective Scanning


direction



or Z-scanor Z-scan

r

Z Value (mm)

Rad

ius

(mm

)

z

Slit

Laser beam



direction



or Z-scanor Z-scan

r

Z Value (mm)

Rad

ius

(mm

)

z

Slit

Laser beam



direction



or Z-scanor Z-scan

r

Z Value (mm)

Rad

ius

(mm

)

z

Slit

Laser beam



direction



or Z-scanor Z-scan

• Uses a half-wave plate energy selector technique

• By rotating the angle of polarisation , the intensity is given by I = I0cos2

Intensity ScanIntensity Scan

/2 Polaroid

/2 PlateSlow

Fast

Laser

Results and Analysis

• Z-scan and Intensity scan results for ionisation of positively charged ions: C+, Ne+, He+, Kr+

• Model the results using theoretical approaches– Volume fit for saturation ionisation– ADK tunneling model

• Suggest explanations for some of the main features of the results

Z Scan results for CZ Scan results for C2+2+ ion production ion production

• Z scan displays the classic Gaussian volume shape

• Shoulder feature is indicative of a secondary process at a lower threshold intensity

Production of C2+ ions from a C+ laser beam as a function of focusing lens position (z)

Z Position (mm)

0 2 4 6 8 10 12 14

Inte

grat

ed Io

n Y

ield

(ar

b.)

0.0

2.0e-10

4.0e-10

6.0e-10

8.0e-10

1.0e-9

1.2e-9

1.4e-9

1.6e-9

Shoulder feature

• Determines the ion production volume at saturation

• For saturated regime; Ion yield Interaction volume

r

Z Value (mm)

Rad

ius

(mm

)

z

Slit

Laser beam

Saturated Volume MethodSaturated Volume Method

sI

zI

z

zzzV

)(ln1

2)( 0

2

0

20

Rayleigh range z0=02/

Waist radius 0=2f/D z -aperture size

Is -Saturation intensity

Is

Theoretical Volume fit to Z-scan of CTheoretical Volume fit to Z-scan of C2+2+

• Volume method only works well for ‘over-the-barrier’ ionisation– It doesn’t describe the tunneling ionisation regime at low intensities

Volume Curve Fit for Z-Scan of C2+

Z Position (mm)

0 2 4 6 8

Vo

lum

e (

arb

itra

ry u

nit

s)

0

2e-13

4e-13

6e-13

8e-13

1e-12

1e-12

1e-12

2e-12

2e-12

2e-12

Vol fit C2+ GroundstateVol fit C2+ MetastableVol fit C3+ GroundstateSum of Volume fitsDATA Z-scan

Intensity Scan for CIntensity Scan for C2+2+ Production Production

• Two distinguishable regions to the results:

1. Low intensity curve indicating C2+ production from the C+ metastable state.

2. High intensity curve indicating production from C+ groundstate.

Laser Intensity Scan for production of C2+ from C+

Laser Intensity (Wcm-2)

1e+14 1e+15 1e+16

Inte

gra

ted

Ion

Yie

ld (

arb

. un

its)

1e-13

1e-12

1e-11

1e-10

1e-9

1e-8

Intensity Scan for CIntensity Scan for C2+2+ Production Production

• ADK Tunneling Model

1. ADK is a quasi-static tunneling method which models ionisation rate w

2. Provides a probability of tunnel ionisation as a function of the intensity of the alternating E-field

Laser Intensity Scan for production of C2+ from C+


1e+14 1e+15 1e+16

Inte

gra

ted

Ion

Yie

ld (

arb

. un

its)

1e-13

1e-12

1e-11

1e-10

1e-9

1e-8

C+ GS – C2+ GSC+ MS – C2+ GSC+ MS – C2+ MS

MS –MetastableGS -Groundstate

Intensity Scan for CIntensity Scan for C2+2+ Production ProductionLaser Intensity Scan for production of C2+ from C+


1e+14 1e+15 1e+16

Inte

gra

ted

Ion

Yie

ld (

arb

. un

its)

1e-13

1e-12

1e-11

1e-10

1e-9

1e-8

1. ADK is a quasi-static tunneling method which models ionisation rate w

2. Provides a probability of tunnel ionisation as a function of the intensity of the alternating E-field

• ADK Tunneling Model

SUM

Intensity Scan for NeIntensity Scan for Ne2+2+ Production ProductionLaser Intensity Scan for production of Ne2+ from Ne+

Laser Intensity (W/cm2)

1e+13 1e+14 1e+15 1e+16

Inte

gra

ted

Io

n Y

ield

(a

rb. u

nit

s)

1e-13

1e-12

1e-11

1e-10

1e-9

1e-8

• Best fit includes ionisation to states which require the spin flip of an electron

Ne+ GS – Ne2+ GSNe+ 4P – Ne2+ GSNe+ 4P – Ne2+ 1SNe+ 4P – Ne2+ 1DSUM

• At low intensity there is the apparent onset of non-sequential ionisation processes

• The best physical model for these non-sequential processes is the ‘recollision model’

Non-Sequential Ionisation in CNon-Sequential Ionisation in C3+3+

Laser Intensity Scan for the production of C3+ form C+

Laser Intensity (W/cm2)

1e+14 1e+15 1e+16

Inte

gra

ted

Ion

Yie

ld (

arb

. un

its)

1e-12

1e-11

1e-10

ADK fit

Summary• In an intense Laser field, atoms and ions are

ionised via field ionisation– distortion of Coulomb potential by E-field of laser

– sequential and non-sequential ionisation processes

• Experimental techniques employed are the z-scan and intensity scan

• ADK and Saturated Volume models appear to work well– suggestion of spin-flips due to magnetic field effects

• Multiply charged positive ions

• Limit the interaction volume for the intensity scan studies

Future

February 2003, 4 week experimental run at RALFebruary 2003, 4 week experimental run at RAL

• Compare results from positive ion target to neutral target

• Repeat some of the results from previous run

Acknowledgements

• Prof. Ian Williams

• (Dr) Gail Johnston

• Dr B. Srigengan

• Dr Jason Greenwood

Many thanks to….Many thanks to….

……....et alet al

Documents

Ions in Intense Femtosecond Laser Fields Jarlath McKenna MSci Project10th December 2001 Supervisor: Prof. Ian Williams