Post on 03-Jan-2016
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Ultrafast Strong Field Ionization of CUltrafast Strong Field Ionization of C++ Ions IonsJ 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, UK3Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France 4Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
The The ResultsResults….….
…….Ionization of C.Ionization of C++ Ions Ions
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.
Using the ISS technique, we observed the production of C2+ ions from the strong field ionization of C+ ions.
ISS for C2+ produced via C+ ionization
Focal Detuning (mm)
0.0 2.0 4.0 6.0 8.0 10.0
Inte
gra
ted
Ion
Yie
ld (
arb
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
C+ 2P - C2+ 1S
C+ 4P - C2+ 1S
C2+ 1S - C3+ 2STotal FitExperiment
Z Position (mm)
-8 -6 -4 -2 0 2 4 6 8 Time-of-flight
metastable
groundstate peaks
The The TheoryTheory….….
…….Strong Field Ionization.Strong Field Ionization
Field Free
Tunnelling
Over-the-Barrier Ionization
i.e. similar to atomic Coulomb field strength
Laser Intensity ~ 1016 Wcm-2
Electric field ~ 2x1011 Vm-1
Alternating electric field gradient lowers the Coulomb barrier
Barrier width decreases hence there is an increased probability of the electron tunnelling through it
If barrier height is lowered below electron potential level, the electron escapes over the barrier
The The TechniqueTechnique….….
…….Intensity Selective Scan.Intensity Selective Scan
ion beam direction
focused laser beam direction
Translate the laser focus with respect to ion beam
Interact with different intensity slices of confocal volume and measure the ionization yield at each position
This technique has the advantage of volume enhanced signal in the low intensity regions
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
Ion beam or aperture
Laser focus
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.
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)
The Experiment….The Experiment….
…….Ion Beam Apparatus.Ion Beam Apparatus
1) Discharge Ion Source
2) Mass Selection Magnet
3) Interaction Region
790nmTi:Sapphire laser 65fs, 20mJ/pulse, 10Hz
Intensity~3x1016 Wcm-2
4) Electrostatic Parallel Plate Analyser
1.4.
Bends a particularmass/charge ionthrough aperture
Separate productions from primaryion beam
1)