Persistent quantum interfering electron trajectories

The emission of above-ionization-threshold harmonics results from the recombination of two electron wavepackets moving along a "short" and a "long" trajectory in the atomic continuum. Attosecond pulse train generation has so far been attributed to the short trajectory, attempted to be isolated through targeted trajectory-selective phase matching conditions. Here, we provide experimental evidence for the contribution of both trajectories to the harmonic emission, even under phase matching conditions unfavorable for the long trajectory. This is finger printed in the interference modulation of the harmonic yield as a function of the driving laser intensity. The effect is also observable in the sidebands yield resulting from the frequency mixing of the harmonics and the driving laser field, an effect with consequences in cross-correlation pulse metrology approaches.Comment: 13 pages, 3 figure

Two-Electron Effects in the Multiphoton Ionization of Magnesium with 400 nm 150 fs Pulses

The multiphoton ionization and photoelectron spectra of magnesium were studied at laser intensities of up to 6x10^{13} Wcm^{-2} using 150 fs laser pulses of a wavelength of 400 nm. The results indicated that a variety of different ionization mechanisms played a role in both types of spectra. A theoretical model describing the processes is presented and the routes to ionization are identified. The work demonstrates the significance of the two-electron nature of the atom in interpreting the experimental results.Comment: 14 pages, 9 figures, submitted to Physical Review

Disclosing intrinsic molecular dynamics on the 1-fs scale through extreme-ultraviolet pump-probe measurements

Through frequency up-conversion of polarization-shaped, femtosecond laser pulses nonlinearly interacting with xenon atoms, energetic, broadband, coherent, XUV continuum radiation is generated. By exploiting the thus-formed short-duration XUV pulses, all the optically allowed excited states of H2 are coherently populated. Nuclear and electronic 1-fs-scale dynamics are subsequently investigated by means of XUV-pump-XUV-probe measurements, which are compared to the results of ab initio calculations. The revealed dynamics reflects the intrinsic molecular behavior, as the XUV probe pulse hardly distorts the molecular potentialThis work is supported in part by the European Commission programs ATTOFEL, CRISP, Laserlab Europe, the European COST Actions MPI1203-SKO and CM1204 XLIC, and the Greek funding program NSRF. A.P. and F.M. acknowledge allocation of computer time by CCC-UAM and BSC Mare Nostrum, and financial support from the Advanced Grant of the European Research Council XCHEM (No. 290853), the European grant MC-RG ATTOTREND, the MICINN Project (No. FIS2010-15127), and the ERA-Chemistry Project (No. PIM2010EEC-00751)

Multiphoton ionization of V, Cr, Y, La, and Ir through 0.5-ps ultraviolet laser pulses

The two-photon ionization of the metal atoms V, Cr, Y, La, and Ir sputtered by an Ar-ion gun from solid targets has been investigated using linearly polarized laser pulses of 500- fs duration and 248.6-nm (KrF) wavelength in combination with mass spectroscopy that is free of volume effects. A near-resonant ionization process has to be attributed to all five atoms. By fitting analytical solutions of a rate equation model describing the temporal evolution of the ionization process, we can determine absolute values of the one-photon cross sections and saturation intensities

Phase-Control of Photoabsorption in Optically Dense Media

We present a self-consistent theory, as well as an illustrative application to a realistic system, of phase control of photoabsorption in an optically dense medium. We demonstrate that, when propagation effects are taken into consideration, the impact on phase control is significant. Independently of the value of the initial phase difference between the two fields, over a short scaled distance of propagation, the medium tends to settle the relative phase so that it cancels the atomic excitation. In addition, we find some rather unusual behavior for an optically thin layer.Comment: 5 pages, 3 figures, submitted to PR