Time-Resolved Holography with Photoelectrons

Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes 1-6 . Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field 7-9 , enabling experiments in the molecular rather than the laboratory frame 10-12 . However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited 13 and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation 14 and the preparation of state-selected molecules using a hexapole 2 . As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to cos θ = −0.74 and cos 2 θ = 0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography 11 of heteronuclear molecules.The outcome of molecular collision experiments is strongly affected by the angular anisotropies in the initial molecular axis distribution. In bimolecular and molecule-surface collisions, collision cross-sections sensitively depend on the relative arrangement of the collision partners 1,2 . Likewise, photon-molecule collisions such as X-ray diffraction and photodissocation experiments aimed at the elucidation of molecular structure or photochemical activity depend on and can benefit from angular confinement of the sample 3,4 . The two most important moments of the molecular axis distribution are the 'alignment' ( cos 2 θ ) and 'orientation' ( cosθ ), where θ is the angle between the molecular axis and a reference axis.First attempts to orient and align molecules relied on electrostatic fields. A hexapole electric field can be used to stateselect polar molecules and orient them through their first-order Stark effect 2,5 using a moderate field strength. The orientation is limited by the selected state. 'Brute-force orientation' uses a strong homogeneous electrostatic field and relies on the second-and higher-order Stark effect 6 . It requires molecules with a large dipole moment and extremely high electric-field strengths.As a part of extensive efforts aimed at achieving laser-controlled chemistry 15-17 , laser-controlled alignment 8 has attracted considerable attention. Suitably chosen laser fields can exert torques on molecules, exploiting the interaction of the laser field with the molecular polarizability. Both adiabatic alignment, where molecules are exposed to a slowly varying laser field 18 , and non-adiabatic (impulsive) alignment, where molecules align after receiving a kick by a short laser pulse 7 , have been succe...

We report experimental results on x-ray diffraction of quantum-state-selected and strongly aligned ensembles of the prototypical asymmetric rotor molecule 2,5-diiodobenzonitrile using the Linac Coherent Light Source. The experiments demonstrate first steps toward a new approach to diffractive imaging of distinct structures of individual, isolated gas-phase molecules. We confirm several key ingredients of single molecule diffraction experiments: the abilities to detect and count individual scattered x-ray photons in single shot diffraction data, to deliver state-selected, e.g., structural-isomer-selected, ensembles of molecules to the x-ray interaction volume, and to strongly align the scattering molecules. Our approach, using ultrashort x-ray pulses, is suitable to study ultrafast dynamics of isolated molecules.

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...