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)