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MSci Project 10th December 2001 Ions in Intense Femtosecond Laser Fields Jarlath McKenna Supervisor: Prof. Ian Williams Outline • Introduction / Background – Strong Field Ionisation – Sequential and Non-sequential Ionisation • Experimental Apparatus / Techniques – Z-scan – Intensity scan • Results and Analysis Introduction • Study of the ionisation dynamics of positively charged atomic ions in intense femtosecond Laser fields • Experiments carried out in collaboration with a group from UCL – February 2002 at RAL using the ASTRA laser • Analysis and interpretation of results – familarisation with experimental apparatus Why? • Why study strong field ionisation of positive atomic ions? • All previous experiments have used neutral targets • 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 • What happens in high intensity Laser interactions? – Low intensity: Single Photon Ionisation – Higher intensity: Multiphoton Ionisation – Very high intensity: Field Ionisation • Single Photon Ionisation – Ionisation energy of valence electron is supplied by one photon Ionisation level Ground state • What happens in high intensity Laser interactions? – Low intensity: Single Photon Ionisation – Higher intensity: Multiphoton Ionisation – Very high intensity: Field Ionisation • Multiphoton Ionisation – Ionisation energy of valence electron is supplied by a number of photons Ionisation level Real excited state Virtual excited state Ground state • What happens in high intensity Laser interactions? – Low intensity: Single Photon Ionisation – Higher intensity: Multiphoton Ionisation – Very high intensity: Field Ionisation ATI • Multiphoton Ionisation – Ionisation energy of valence electron is supplied by a number of photons – Above Threshold Ionisation may take place Ionisation level Real excited state Virtual excited state Ground state Field Ionisation • Electric field distorts the atomic potential well Atomic Potential well V0(x) – this lowers the potential barrier seen by an electron in the atom/ion Potential well Electric field e- x0 Potential range (x) Field Ionisation • Electric field distorts the atomic potential well • Tunnelling Regime As barrier is lowered, it’s width decreases. Increased probability of electron tunnelling Atomic Potential well V0(x) – this lowers the potential barrier seen by an electron in the atom/ion Potential well Electric field e- x0 Potential range (x) e- Field Ionisation • Electric field distorts the atomic potential well • Over-the-barrier Regime Potential barrier is lower than electronic state Electron is free to escape the atom Atomic Potential well V0(x) – this lowers the potential barrier seen by an electron in the atom/ion Potential well Electric field e- x0 Potential range (x) Field Ionisation • Electric field distorts the atomic potential well • Dynamic Stark Shift Energy states of electrons are Stark shifted up towards the continuum Dynamic or ac Stark shift because of oscillating E-field of laser Atomic Potential well V0(x) – this lowers the potential barrier seen by an electron in the atom/ion Potential well Electric field e- x0 Potential range (x) Sequential and Non-Sequential Ionisation • Sequential: – Ionisation takes place in a series of steps A+ A A2+ • Non-Sequential: – Ionisation takes place in a single step A A2+ Recollision Model A CORE Electric field strength from laser pulse expels an electron Atomic core with outer shell of electrons Recollision Model A+ CORE During oscillatory motion of E-field, the electron may make multiple returns to the atomic core Electron may collide with a valence electron 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 A2+ CORE Apparatus Primary Beam Collector Charged Fragment Detector Extraction and Focussing Lenses Einzel lens 45o Parallel Plate deflectors Neutral Fragment Detector -ions are accelerated to 1-2 keV Deflection Plates Laser Beam Ion Source -ions produced via discharge Interaction Region Selection Magnet Intensity Selective Scanning or Z-scan • Laser Intensity is • Scan with a 0.5mm aperture r z Radius (mm) – Lorentzian along z direction – Gaussian in radial r direction Laser beam Slit Z Value (mm) Intensity Selective Scanning or Z-scan • Laser Intensity is • Scan with a 0.5mm aperture r z Radius (mm) – Lorentzian along z direction – Gaussian in radial r direction Laser beam Slit Z Value (mm) Intensity Selective Scanning or Z-scan • Laser Intensity is • Scan with a 0.5mm aperture r z Radius (mm) – Lorentzian along z direction – Gaussian in radial r direction Laser beam Slit Z Value (mm) Intensity Selective Scanning or Z-scan • Laser Intensity is • Scan with a 0.5mm aperture r z Radius (mm) – Lorentzian along z direction – Gaussian in radial r direction Laser beam Slit Z Value (mm) Intensity Selective Scanning or Z-scan • Laser Intensity is • Scan with a 0.5mm aperture r z Radius (mm) – Lorentzian along z direction – Gaussian in radial r direction Laser beam Slit Z Value (mm) Intensity Scan • Uses a half-wave plate energy selector technique Fast /2 Polaroid Laser /2 Plate Slow • By rotating the angle of polarisation , the intensity is given by I = I0cos2 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 C2+ ion production • Z scan displays the classic Gaussian volume shape Production of C2+ ions from a C+ laser beam as a function of focusing lens position (z) 1.6e-9 Integrated Ion Yield (arb.) 1.4e-9 1.2e-9 1.0e-9 Shoulder feature 8.0e-10 6.0e-10 4.0e-10 2.0e-10 0.0 0 2 4 6 8 10 12 14 Z Position (mm) • Shoulder feature is indicative of a secondary process at a lower threshold intensity Saturated Volume Method • Determines the ion production volume at saturation 2 z V ( z ) 0 z 1 2 z 0 2 Rayleigh range z0=02/ I ( z ) 0 ln I s Waist radius 0=2f/D z -aperture size Is -Saturation intensity Laser beam z Radius (mm) • For saturated regime; Ion yield Interaction volume r Is Slit Z Value (mm) Theoretical Volume fit to Z-scan of C2+ Volume Curve Fit for Z-Scan of C 2+ 2e-12 Volume (arbitrary units) 2e-12 Vol fit C 2+ Groundstate Vol fit C 2+ Metastable Vol fit C 3+ Groundstate Sum of Volume fits 2e-12 1e-12 1e-12 DATA Z-scan 1e-12 8e-13 6e-13 4e-13 2e-13 0 0 2 4 6 8 Z Position (mm) • Volume method only works well for ‘over-the-barrier’ ionisation – It doesn’t describe the tunneling ionisation regime at low intensities Intensity Scan for C2+ Production Laser Intensity Scan for production of C2+ from C+ • 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. Integrated Ion Yield (arb. units) 1e-8 1e-9 1e-10 1e-11 1e-12 1e-13 1e+14 1e+15 1e+16 Laser Intensity (Wcm-2) Intensity Scan for C2+ Production Laser Intensity Scan for production of C2+ from C+ 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 Integrated Ion Yield (arb. units) • ADK Tunneling Model 1e-9 1e-10 1e-11 C+ GS – C2+ GS C+ MS – C2+ GS C+ MS – C2+ MS 1e-12 MS –Metastable GS -Groundstate 1e-13 1e+14 1e+15 1e+16 Laser Intensity (Wcm-2) Intensity Scan for C2+ Production Laser Intensity Scan for production of C2+ from C+ 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 Integrated Ion Yield (arb. units) • ADK Tunneling Model 1e-9 1e-10 1e-11 SUM 1e-12 1e-13 1e+14 1e+15 1e+16 Laser Intensity (Wcm-2) Intensity Scan for Ne2+ Production Laser Intensity Scan for production of Ne2+ from Ne+ • Best fit includes ionisation to states which require the spin flip of an electron Integrated Ion Yield (arb. units) 1e-8 1e-9 Ne+ GS – Ne2+ GS Ne+ 4P – Ne2+ GS Ne+ 4P – Ne2+ 1S Ne+ 4P – Ne2+ 1D SUM 1e-10 1e-11 1e-12 1e-13 1e+13 1e+14 1e+15 1e+16 Laser Intensity (W/cm2) Non-Sequential Ionisation in C3+ Laser Intensity Scan for the production of C3+ form C+ • The best physical model for these non-sequential processes is the ‘recollision model’ 1e-10 Integrated Ion Yield (arb. units) • At low intensity there is the apparent onset of non-sequential ionisation processes 1e-11 ADK fit 1e-12 1e+14 1e+15 1e+16 Laser Intensity (W/cm2) 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 Future February 2003, 4 week experimental run at RAL • Multiply charged positive ions • Compare results from positive ion target to neutral target • Repeat some of the results from previous run • Limit the interaction volume for the intensity scan studies Acknowledgements Many thanks to…. • • • • Prof. Ian Williams (Dr) Gail Johnston Dr B. Srigengan Dr Jason Greenwood …..et al