International audienceA strong laser field may tunnel ionize a molecule from several orbitals simultaneously, forming an attosecond electron–hole wavepacket. Both temporal and spatial information on this wavepacket can be obtained through the coherent soft X-ray emission resulting from the laser-driven recollision of the liberated electron with the core. By characterizing the emission from aligned N 2 molecules, we demonstrate the attosecond contributions of the two highest occupied molecular orbitals. We determine conditions where they are disentangled in the real and imaginary parts of the emission dipole moment. This allows us to carry out a tomographic reconstruction of both orbitals with angstrom spatial resolution. Their coherent superposition provides experimental images of the attosecond wavepacket created in the ionization process. Our results open the prospect of imaging ultrafast intramolecular dynamics combining attosecond and angstrom resolutions

A theoretical investigation is presented that examines the wavelength scaling from near-visible (0:8 m) to midinfrared (2 m) of the photoelectron distribution and high harmonics generated by a ''single'' atom in an intense electromagnetic field. The calculations use a numerical solution of the timedependent Schrödinger equation (TDSE) in argon and the strong-field approximation in helium. The scaling of electron energies ( 2 ), harmonic cutoff ( 2 ), and attochirp ( ÿ1 ) agree with classical mechanics, but it is found that, surprisingly, the harmonic yield follows a ÿ5-6 scaling at constant intensity. In addition, the TDSE results reveal an unexpected contribution from higher-order returns of the rescattering electron wave packet. DOI: 10.1103/PhysRevLett.98.013901 PACS numbers: 42.65.Ky, 32.80.Fb, 32.80.Rm A method for producing bursts of high-energy photoelectrons and photons has its origin in the fundamental interaction of an ''isolated'' atom with an intense, lowfrequency (nonresonant) electromagnetic field. The foundations of these processes are based on scaling metrics [1] and quasiclassical physics [2,3] which are linked to a tunnel ionized continuum electronic wave packet being ''coherently'' driven back into the core by the intense oscillating field. It is not surprising that under these conditions the inherent characteristics of the intense optical field, such as amplitude, phase, duration and frequency, are intimately connected to the observables. Numerous investigations have exploited the utility of the field parameters, for example, to produce isolated attosecond pulses [4,5] or to realize the time domain analogy of the Young's double slit experiment [6]. However, the wavelength has been largely ignored due, in part, to the limitation imposed by ultrafast amplifier technology which confines experiments to near-visible wavelengths, e.g., 0:8 m for titanium sapphire media. Nonetheless, the above principles suggest that scaling the intense laser-atom interaction towards longer wavelength will result in the realization of more energetic particles, many-body interactions and shorter bursts of attosecond (1 as 10 ÿ18 s) light. Furthermore, the impetus for understanding the fundamental scaling is also motivated by recent advances in new ultrafast amplifier engineering [7] that will provide the necessary experimental tools.In this Letter, a theoretical investigation is presented that explores the fundamental wavelength scaling of the intense laser-atom interaction. The primary focus is to map the evolving isolated quantum response and the corresponding relationship to the classical behavior as the wavelength is varied from the near-visible to the midinfrared (0:8-2 m). Both numerical solutions of the timedependent Schrödinger equation (TDSE) within a singleactive electron approximation (SAE) [8] and a strong-field approximation (SFA) [9,10] are used. The scaling is investigated using argon and helium since (i) they are prototypical laboratory atoms, (ii) computationally efficient model potentials exist ...