We report experiments where hydrogen molecules were dissociatively ionized by an attosecond pulse train in the presence of a near-infrared field. Fragment ion yields from distinguishable ionization channels oscillate with a period that is half the optical cycle of the IR field. For molecules aligned parallel to the laser polarization axis, the oscillations are reproduced in two-electron quantum simulations, and can be explained in terms of an interference between ionization pathways that involve different harmonic orders and a laser-induced coupling between the 1s g and 2p u states of the molecular ion. This leads to a situation where the ionization probability is sensitive to the instantaneous polarization of the molecule by the IR electric field and demonstrates that we have probed the IR-induced electron dynamics with attosecond pulses. The prospect of observing and controlling ultrafast electron dynamics in molecular systems is the basis of the current interest to apply attosecond (1 as ¼ 10 À18 s) laser pulses to physical chemistry. Since the first demonstration of attosecond pulses [1,2], pioneering experiments have demonstrated their potential in atoms [3,4], solid state systems [5], and, most recently, molecules [6], where interest has been stimulated by numerical studies which suggest that an electronic (i.e., attosecond or fewfemtosecond) time scale may be important in fundamental chemical processes [7,8]. The inherent multielectron nature of the electron dynamics in many molecular systems is a formidable challenge to theoreticians and experimentalists alike, and requires the development of novel theoretical and experimental techniques.Attosecond pump-probe spectroscopy is based on the generation of attosecond light pulses by high harmonic generation. Presently, attosecond pulses exist as attosecond pulse trains (APTs) [1] and as isolated attosecond pulses [2]. The first application of attosecond pulses to follow rapid electron dynamics in a molecule revealed that the dissociative ionization of hydrogen by a two-color extreme-ultraviolet ðXUVÞ þ IR field results in a localization of the bound electron in the molecular ion that depends with attosecond time resolution on the time delay between the attosecond XUV pulse and the IR laser pulse [6]. This could be observed via an asymmetry of the ejected fragments in the laboratory frame, i.e., after the dissociation was complete [9]. A similar experimental result was also obtained using an APT [10]. In these experiments the attosecond pulses initiated electron dynamics that was subsequently addressed by an IR pulse. A next challenge is to use attosecond pulses as a probe of ultrafast molecular electron dynamics. In this Letter we do so by investigating how a moderately intense IR field influences the electronic states that are accessed in photoionization of hydrogen using an APT.In the experiment, an XUV APT (with two pulses per IR cycle) and a 30 fs FWHM 780 nm (IR) pulse (3 Â 10 13 W=cm 2 ) with identical linear polarization were collinearly propagated and...

We demonstrate that dissociative ionization of O 2 can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field. Our experiments reveal a dependence of both the branching ratios between a range of electronic states and the fragment angular distributions on the extreme ultraviolet (XUV) to IR time delay. The observations go beyond adiabatic propagation of dissociative wave packets on IR-induced quasistatic potential energy curves and are understood in terms of an IR-induced coupling between electronic states in the molecular ion.

We present an experimental and theoretical investigation of the angular distributions of electrons ejected in aligned molecules by extreme ultra-violet ionization using a high harmonic generation (HHG) source. Impulsive alignment in O 2 , N 2 and CO molecules was achieved using a near-IR laser pulse and the photoelectron angular distribution after ionization by a fs harmonic comb composed of harmonic H11 to H29 (17.5-46 eV) was recorded at the maximum of both alignment and anti-alignment. The experiment reveals signatures that are specific for the electronic orbitals that are ionized as well as the onset of the influence of the molecular structure and is well reproduced by theoretical calculations based on the multichannel Schwinger configuration interaction method.

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