Attosecond Control in Photoionization of Hydrogen Molecules

Abstract

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...

Time-resolved high harmonic spectroscopy of dynamical symmetry breaking in bi-circular laser fields: the role of Rydberg states

“…(4,5,6) show the symmetry-imposed selection rules for the harmonics of different polarization. We stress again that these rules assume that three emission bursts associated with the three leaves of the driving field are identical, up to rotation and time-delay.…”

“…This light is used to induce, monitor and control the dynamics of electrons in atoms, molecules and solids at their intrinsic timescale [1][2][3][4][5][6]. High harmonic generation also serves as a spectroscopic tool for unraveling the complex multi-electron and coupled electron-nuclear dynamics in molecules [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] and solids [22][23][24][25][26][27].…”

Mapping and controlling ultrafast dynamics of highly excited H2 molecules by VUV-IR pump-probe schemes

“…These experiments have led to the observation of novel ultrafast phenomena in molecules and have allowed for the temporal characterization of known dissociation processes [21]. Moreover, new dissociation channels that had not been predicted previously have been observed [8,16,17]. However, even in rather simple diatomic and triatomic systems, the multitude of degrees of freedom as well as the complexity of the coupled electron and nuclear wave packet motion usually prevent a full theoretical treatment and, hence, limit our ability to fully understand and predict such complex dynamics.…”

“…Recent studies showed how femtosecond VUV and XUV radiation can be used to reveal transient nuclear and electron dynamics in diatomic molecules [8][9][10][11][12] as well as in more complex molecular systems [13,14]. To probe such dynamics, the majority of the pump-probe-style experiments accomplished to date utilize not only pairs of XUV-IR pulses [8,[15][16][17][18], but also a combination of VUV-XUV pulses [19][20][21][22]. These experiments have led to the observation of novel ultrafast phenomena in molecules and have allowed for the temporal characterization of known dissociation processes [21].…”

Ultrafast Relaxation Dynamics of Highly-excited States in N2Molecules Excited by Femtosecond XUV Pulses

“…In the last years, isolated attosecond pulses and trains of attosecond pulses have been used to measure ultrafast electron dynamics in molecules. Sub-femtosecond electron localization after attosecond excitation has been observed in H 2 and D 2 molecules [3]; control of photo-ionization of D 2 and O 2 molecules has been achieved by using attosecond pulse trains (APTs) [4,5]. Femtosecond XUV pulses have been used to investigate electron dynamics in N 2 molecules [6] as well as to measure autoionization processes in O 2 molecules [7].…”