M Krug et al 2009 New J. Phys. 11 105051 doi:10.1088/1367-2630/11/10/105051
M Krug1, T Bayer1, M Wollenhaupt1, C Sarpe-Tudoran1, T Baumert1,4, S S Ivanov2 and N V Vitanov2,3,4
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We present a joint experimental and theoretical study on strong-field photo-ionization of sodium atoms using chirped femtosecond laser pulses. By tuning the chirp parameter, selectivity among the population in the highly excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways enabling control are identified by simultaneous ionization and measurement of photoelectron angular distributions employing the velocity map imaging technique. Free electron wave packets at an energy of around 1 eV are observed. These photoelectrons originate from two channels. The predominant 2 + 1 + 1 resonance enhanced multi-photon ionization (REMPI) proceeds via the strongly driven two-photon transition 4s←←3s, and subsequent ionization from the states 5p, 6p and 7p whereas the second pathway involves 3 + 1 REMPI via the states 5f and 6f. In addition, electron wave packets from two-photon ionization of the non-resonant transiently populated state 3p are observed close to the ionization threshold. A mainly qualitative five-state model for the predominant excitation channel is studied theoretically to provide insights into the physical mechanisms at play. Our analysis shows that by tuning the chirp parameter the dynamics is effectively controlled by dynamic Stark shifts and level crossings. In particular, we show that under the experimental conditions the passage through an uncommon three-state 'bow-tie' level crossing allows the preparation of coherent superposition states.
32.80.Fb Photoionization of atoms and ions
32.60.+i Zeeman and Stark effects
32.80.Rm Multiphoton ionization and excitation to highly excited states
Issue 10 (October 2009)
Received 8 May 2009
Published 30 October 2009
M Krug et al 2009 New J. Phys. 11 105051
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