Ultrafast Strong Field Ionization of C + Ions

1. Ultrafast Strong Field Ionization of C+ Ions J McKenna, M Suresh, I M G Johnston, B Srigengan, I D Williams 1 E M L English, S L Watson, W Bryan, T A J Goodworth, W R Newell 2 S Carles, J L LeGarrec, S D LePicard, J B A Mitchell 3 C J Hooker, E J Divall, A J Langley 4 1Dept. Physics, Queen’s University Belfast, Belfast BT7 1NN, UK 2Dept. Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 3Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France 4Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK The intense field ionization of a pre-cursor C+ ion target is investigated experimentally for the first time. Using 65fs, 790nm laser pulses, focal intensity > 1016 Wcm-2, we study the single ionization dynamics by employing the Intensity Selective Scanning technique. Both ionization of the groundstate and metastable population is observed with the latter interpreted as due to the occurrence of an electron spin-flip event. The Experiment…. The Technique…. In these experiments, we use the volume restricted Intensity Selective Scan (ISS) technique2,3) to measure the ionization signal as a function of focused laser intensity 1) Discharge Ion Source focused laser beam direction 2) Mass Selection Magnet Bends a particular mass/charge ion through aperture 4. 1. ion beam direction This technique has the advantage of volume enhanced signal in the low intensity regions Intensity ~3x1016 Wcm-2 Separate product ions from primary ion beam  Interact with different intensity slices of confocal volume and measure the ionization yield at each position 3) Interaction Region Laser focus 4) Electrostatic Parallel Plate Analyser Translate the laser focus with respect to ion beam Ion beam or aperture 790nm Ti:Sapphire laser 65fs, 20mJ/pulse, 10Hz ….Ion Beam Apparatus ….Intensity Selective Scan 1) The Theory…. The Results…. Using the ISS technique, we observed the production of C2+ ions from the strong field ionization of C+ ions. Laser Intensity ~ 1016 Wcm-2  Field Free Electric field ~ 2x1011 Vm-1 groundstate peaks i.e. similar to atomic Coulomb field strength metastable Alternating electric field gradient lowers the Coulomb barrier Time-of-flight -8 -6 -4 -2 0 2 4 6 8 Tunnelling Z Position (mm) Barrier width decreases hence there is an increased probability of the electron tunnelling through it The observed metastable transition (red curve) corresponds to the spin forbidden transition, C+4P  C2+1S  This is the strong signature of an electron spin-flip event. Over-the-Barrier Ionization If barrier height is lowered below electron potential level, the electron escapes over the barrier ….Strong Field Ionization ….Ionization of C+ Ions References: 1) J. B. Greenwood et al, Phys. Rev. Lett., 88, 233011 (2002) 2) P. Hansch et al, Phys. Rev. A, 54, R2559 (1996) 3) A. A. A. El-Zein et al, Phys. Scripta, T92, 119 (2001)