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
• 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
– Above Threshold Ionisation may take place
Ground state
Ionisation level
ATI
Virtual excited state
Real excited state
• 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
• 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
• Electric field distorts the atomic potential well– this lowers the potential barrier seen by an electron in
the atom/ion
Field IonisationField Ionisation
As barrier is lowered, it’s width decreases.
Increased probability of electron tunnelling
Potential range (x)x0
Ato
mic
Pot
ent
ial w
ell
V0(x
) Potential wellElectric field
e- e-
• Over-the-barrier Over-the-barrier RegimeRegime
Field IonisationField Ionisation
Electron is free to escape the atom
Potential range (x)x0
Ato
mic
Pot
ent
ial w
ell
V0(x
) Potential wellElectric field
e-Potential barrier is lower than electronic state
• Electric field distorts the atomic potential well– this lowers the potential barrier seen by an electron in
the atom/ion
• Dynamic Stark ShiftDynamic Stark Shift
Field IonisationField Ionisation
Potential range (x)x0
Ato
mic
Pot
ent
ial w
ell
V0(x
) Potential wellElectric field
e-
• Electric field distorts the atomic potential well– this lowers the potential barrier seen by an electron in
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+
Recollision ModelRecollision Model
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+
Recollision ModelRecollision Model
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
• Laser Intensity is – Lorentzian along z
direction
– Gaussian in radial r direction
• Scan with a 0.5mm aperture
or Z-scanor Z-scan
r
Z Value (mm)
Rad
ius
(mm
)
z
Slit
Laser beam
Intensity Selective ScanningIntensity Selective Scanning
• Laser Intensity is – Lorentzian along z
direction
– Gaussian in radial r direction
• Scan with a 0.5mm aperture
or Z-scanor Z-scan
r
Z Value (mm)
Rad
ius
(mm
)
z
Slit
Laser beam
Intensity Selective ScanningIntensity Selective Scanning
• Laser Intensity is – Lorentzian along z
direction
– Gaussian in radial r direction
• Scan with a 0.5mm aperture
or Z-scanor Z-scan
r
Z Value (mm)
Rad
ius
(mm
)
z
Slit
Laser beam
Intensity Selective ScanningIntensity Selective Scanning
• Laser Intensity is – Lorentzian along z
direction
– Gaussian in radial r direction
• Scan with a 0.5mm aperture
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+
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
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+
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
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
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