P. Breger

Resonant multiphoton ionisation of xenon with high-intensity femtosecond pulses

The authors report on the intensity and polarisation dependences of electron energy spectra from multiphoton ionisation of xenon with 120 fs pulses at 615 nm. The spectra show a large number of narrow structures assigned to resonances induced by the AC Stark shift. The structures are suppressed with circular polarisation. The maximum shift is observed for the 6s state which is up-shifted by 1.6 eV. The assignments are based on a lowest-order perturbative calculation of the AC Stark shifts.

Evolution of angular distributions in two-colour, few-photon ionization of helium

Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

Attosecond chirp-encoded dynamics of light nuclei

We study the spectral phase of high-order harmonic emission as an observable for probing ultrafast nuclear dynamics after the ionization of a molecule. Using a strong-field approximation theory that includes nuclear dynamics, we relate the harmonic phase to the phase of the overlap integral of the nuclear wavefunctions of the initial neutral molecule and the molecular ion after an attosecond probe delay. We determine experimentally the group delay of the high harmonic emission from D2 and H2 molecules, which allows us to verify the relation between harmonic frequency and the attosecond delay. The small difference in the harmonic phase between H2 and D2 calculated theoretically is consistent with our experimental results.

Above-threshold ionization contrast and channel closure in argon

Above-threshold ionization (ATI) electron energy spectra of argon with 800 nm, 40 fs pulses display a contrast which goes through a sharp minimum around 6×1013 W cm-2. The contrast change encompasses the whole ATI spectrum including the two plateaux. The results of a full numerical solution of the time-dependent Schrodinger equation and the integration over the interaction volume show that in the low-energy part of the spectrum this effect is due to the closing of the 12-photon ionization channel and the correlated opening of the possibility for 12-photon resonances with a different parity. However, the calculation suggests a different origin for the substructure observed in the plateau.

Contrasted behaviour of Stark-induced resonances in multiphoton ionization of krypton

Electron energy spectra from multiphoton ionization of krypton with 617 nm, 70 fs pulses have been recorded over a large intensity range. Near the appearance intensity, the spectra display Stark-induced resonances with s and d states. Up to about 8*1013 W cm-2, the spectra broaden and the structures are progressively washed out. Above this intensity, a strong resonance emerges, corresponding to the 4f state for which the static detuning is 2.94 eV, a value larger than the photon energy. The influence of a large static detuning and populations of excited states with shifts different from the threshold shift are discussed.

Demonstration of two-color transient pumping in Ni-like silver at 13.9 nm and 16.1 nm: new progress in applications of x-ray lasers

We present new progress in the optimization and understanding of the transient collisional pumping scheme using an ultra-short sub-ps heating pulse. The effect of traveling-wave irradiation in enhancing the lasing output of the 4d-4p Ni-like Ag line is studied in detail. A new irradiation scheme using a frequency-doubled 600 ps pulse to preform a plasma is tested. Strong lasing is also obtained on a new line at 16.05 nm that we identify to a 4f-4d transition in Ni-like Ag. Finally we review our recent work in the development of applications of the 21.2 nm zinc laser for imaging or exciting matter. New experiments include the probing of a plasma by imaging Fresnel interferometry and a first attempts to demonstrate two-photon ionization in a xenon gas.

Imaging molecular orbitals using HHG

High harmonic generation in small aligned molecules provides rich information on their electronic properties, with extreme space and time resolutions.

Physics at high intensities and low frequencies

We discuss multiphoton ionization and harmonic generation processes from atoms subject to intense low‐frequency fields. In this regime, ionization occurs via tunneling through the Coulomb barrier lowered by the field. This yields characteristic (and smooth ) ion intensity dependences and electron spectra. In the same conditions, the atoms radiate many odd harmonics of the pump field, which have a constant amplitude up to an extremely high order (more than one hundred). Thus, we report the observation of the 135th harmonic (160 eV) of a 1053 nm Nd‐glass laser focused in a 40 Torr gas jet of neon. We underline some important differences between the dynamics of these phenomena. Finally, we briefly discuss the influence of phase matching on high order harmonic generation.

Photoionization in combined ultra short XUV and infrared laser pulses

Multi-photon-double ionization of xenon by Ti:Sapphire laser pulses combined with their 25th harmonic has been studied by means of a momentum imaging spectrometer. The determination of the momenta of the emitted photoelectron pair and of its energy and angular correlation gives insight into the various mechanisms leading to double ionization. Although the conditions for non sequential direct multi-photon double ionization are met in the experiment, it is found that two-step sequential processes prevail.

Characterization of Attosecond Pulse Trains

Ultrashort light pulses, as short as 100 as, can be produced from high harmonic generation (HHG) in an atomic gas medium. This major achievement of the last 6 years in high field physics has now been demonstrated in different conditions of medium-field interaction [1].