Ions in Intense Femtosecond Laser Fields Jarlath McKenna MSci Project10th 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 atomic ions?Why study strong field ionisation of positive 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 interactions?What happens in high intensity Laser 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 interactions?What happens in high intensity Laser 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 interactions?What happens in high intensity Laser 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 Ionisation Potential range ( x ) x0x0 Atomic Potential well V 0 ( x ) Potential well Electric field  e-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 Ionisation As barrier is lowered, it’s width decreases. Increased probability of electron tunnelling Potential range ( x ) x0x0 Atomic Potential well V 0 ( x ) Potential well Electric field  e-e- e-e-

Over-the-barrier RegimeOver-the-barrier Regime Field Ionisation Electron is free to escape the atom Potential range ( x ) x0x0 Atomic Potential well V 0 ( x ) Potential well Electric field  e-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 Ionisation Potential range ( x ) x0x0 Atomic Potential well V 0 ( x ) Potential well Electric field  e-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 Ionisation A A + A 2+ Non-Sequential: –Ionisation takes place in a single step A A 2+

CORE  A Recollision Model Atomic core with outer shell of electrons Electric field strength from laser pulse expels an electron

CORE  A+A+ Recollision 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  A 2+ Recollision 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 45 o 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) Radius (mm) z Slit Laser beam Intensity Selective Scanning or Z-scan

r Z Value (mm) Radius (mm) z Slit Laser beam Intensity Selective Scanning Laser Intensity is –Lorentzian along z direction –Gaussian in radial r direction Scan with a 0.5mm aperture or Z-scan

r Z Value (mm) Radius (mm) z Slit Laser beam Intensity Selective Scanning Laser Intensity is –Lorentzian along z direction –Gaussian in radial r direction Scan with a 0.5mm aperture or Z-scan

r Z Value (mm) Radius (mm) z Slit Laser beam Intensity Selective Scanning Laser Intensity is –Lorentzian along z direction –Gaussian in radial r direction Scan with a 0.5mm aperture or Z-scan

r Z Value (mm) Radius (mm) z Slit Laser beam Intensity Selective Scanning Laser Intensity is –Lorentzian along z direction –Gaussian in radial r direction Scan with a 0.5mm aperture or Z-scan

Uses a half-wave plate energy selector technique By rotating the angle of polarisation , the intensity is given by I  = I 0 cos 2  Intensity Scan   /2 Polaroid /2 Plate Slow 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 C 2+ ion production Z scan displays the classic Gaussian volume shape Shoulder feature is indicative of a secondary process at a lower threshold intensity Shoulder feature

Determines the ion production volume at saturation For saturated regime; Ion yield  Interaction volume r Z Value (mm) Radius (mm) z Slit Laser beam Saturated Volume Method Rayleigh range z 0 =  0 2 / Waist radius  0 =2f /  D  z -aperture size I s -Saturation intensity IsIs

Theoretical Volume fit to Z-scan of C 2+ Volume method only works well for ‘over-the-barrier’ ionisation –It doesn’t describe the tunneling ionisation regime at low intensities

Intensity Scan for C 2+ Production Two distinguishable regions to the results: 1. Low intensity curve indicating C 2+ production from the C + metastable state. 2. High intensity curve indicating production from C + groundstate.

Intensity Scan for C 2+ 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 C + GS – C 2+ GS C + MS – C 2+ GS C + MS – C 2+ MS MS –Metastable GS -Groundstate

Intensity Scan for C 2+ Production 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 Ne 2+ Production Best fit includes ionisation to states which require the spin flip of an electron Ne + GS – Ne 2+ GS Ne + 4 P – Ne 2+ GS Ne + 4 P – Ne 2+ 1 S Ne + 4 P – Ne 2+ 1 D SUM

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 C 3+ 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 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…. …..et al