Despite their broad implications for phenomena such as molecular bonding or chemical reac tions, our knowledge of multi electron dynamics is limited and their theoretical modelling remains a most difficult task. From the experimental side, it is highly desirable to study the dynamical evolution and interaction of the electrons over the relevant timescales, which extend into the attosecond regime. Here we use near single cycle laser pulses with well defined electric field evolution to confine the double ionization of argon atoms to a single laser cycle. The measured two electron momentum spectra, which substantially differ from spectra recorded in all previous experiments using longer pulses, allow us to trace the correlated emission of the two electrons on sub femtosecond timescales. The experimental results, which are discussed in terms of a semiclassical model, provide strong constraints for the development of theories and lead us to revise common assumptions about the mechanism that governs double ionization.

The lifetime of interatomic Coulombic decay (ICD) [L. S. Cederbaum et al., Phys. Rev. Lett. 79, 4778 (1997)] in Ne2 is determined via an extreme ultraviolet pump-probe experiment at the Free-Electron Laser in Hamburg. The pump pulse creates a 2s inner-shell vacancy in one of the two Ne atoms, whereupon the ionized dimer undergoes ICD resulting in a repulsive Ne+(2p(-1))-Ne+(2p(-1)) state, which is probed with a second pulse, removing a further electron. The yield of coincident Ne+-Ne2+ pairs is recorded as a function of the pump-probe delay, allowing us to deduce the ICD lifetime of the Ne2(+)(2s(-1)) state to be (150±50)  fs, in agreement with quantum calculations.

Subfemtosecond control of the breaking and making of chemical bonds in polyatomic molecules is poised to open new pathways for the laser-driven synthesis of chemical products. The break-up of the C-H bond in hydrocarbons is an ubiquitous process during laser-induced dissociation. While the yield of the deprotonation of hydrocarbons has been successfully manipulated in recent studies, full control of the reaction would also require a directional control (that is, which C-H bond is broken). Here, we demonstrate steering of deprotonation from symmetric acetylene molecules on subfemtosecond timescales before the break-up of the molecular dication. On the basis of quantum mechanical calculations, the experimental results are interpreted in terms of a novel subfemtosecond control mechanism involving non-resonant excitation and superposition of vibrational degrees of freedom. This mechanism permits control over the directionality of chemical reactions via vibrational excitation on timescales defined by the subcycle evolution of the laser waveform.

A precise, real-time, single-shot carrier–envelope phase (CEP) tagging technique for few-cycle pulses was developed and combined with cold-target recoil-ion momentum spectroscopy and velocity-map imaging to investigate and control CEP-dependent processes with attosecond resolution. The stability and precision of these new techniques have allowed for the study of intense, few-cycle, laser-matter dynamics with unprecedented detail. Moreover, the same stereo above-threshold ionization (ATI) measurement was expanded to multi-cycle pulses and allows for CEP locking and pulse-length determination. Here we review these techniques and their first applications to waveform characterization and control, non-sequential double ionization of argon, ATI of xenon and electron emission from SiO2 nanospheres.

The velocity map recorded in above-threshold ionization of xenon at 800 nm exhibits a distinct carpetlike pattern of maxima and minima for emission in the direction approximately perpendicular to the laser polarization. The pattern is well reproduced by a numerical solution of the time-dependent Schrödinger equation. In terms of the simple-man model and the strong-field approximation, it is explained by the constructive and destructive interference of the contribution of the long and the short orbit. Strictly perpendicular emission is caused by ionization at the two peaks of the laser field per cycle, which results in a 2ħω separation of the above-threshold ionization rings.

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to manipulate the movement of nuclei with tailored light within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely, acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics is monitored using coincident 3D momentum imaging spectroscopy and described with a widely applicable quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wave packet by the intense off-resonant laser field. DOI: 10.1103/PhysRevLett.116.193001 The rearrangement of hydrocarbon bonds via the migration of a hydrogen atom can result in major deformations of molecular architecture and, thus, alter the molecule's chemical properties. Examples include keto-enol tautomerism where the migration of a proton changes an aldehyde into an alcohol. Isomerization reactions of that kind have been the subject of numerous studies [1,2]. Of particular interest was to determine the so-called isomerization time, which has been measured to be within several tens of fs in small hydrocarbons [3][4][5][6]. The phenomenon has also been observed in larger molecules, such as protonated triglycine [7]. Tracing of the hydrogen migration from different locations within the molecule has been made possible via isotope labeling; see, e.g., Refs. [8][9][10]. The ability to exert control over the migration could lead to advancement in topics such as the efficiency of catalytic reactions [11] and combustion reactions regarding fuel and energy research [12]. Furthermore, light-induced control of hydrogen migration may open new reaction pathways which cannot materialize by other means.Despite its direct relevance to applied chemistry, studies regarding the control of the hydrogen migration process have been scarce and have been limited to theory [13,14] for a long time. However, recent progress has been made in coherently controlling isomerization reactions using fundamental parameters of ultrafast strong-field laser sources. Xie et al. varied the pulse duration and intensity to explore the isomerization of ethylene [15] and reported control of the total fragmentation yields of various hydrocarbons [16].Here, we demonstrate steering of the direction of the hydrogen migration using the electric field waveform of intense few-cycle laser pulses. This approach goes beyond earlier work on toluene [17] and methanol [18] using twocolor pulses with a duration of tens of fs. In contrast, the duration of our few-cycle laser pulses is significantly shorter than the time scale of the isomerization dynamics, therefore, avoiding charge-resonance-enhanced ionization [19] occurring at large internuclear distances [20]. Moreover, an influence of electron localization-assisted enhanced ionization on the dissociation reactions, recently ...

The dissociation of an H + 2 molecular-ion beam by linearly polarized, carrier-envelope-phase-tagged 5 fs pulses at 4×10 14 W/cm 2 with a central wavelength of 730 nm was studied using a coincidence 3D momentum imaging technique. Carrier-envelope-phase-dependent asymmetries in the emission direction of H + fragments relative to the laser polarization were observed. These asymmetries are caused by interference of odd and even photon number pathways, where net-zero photon and 1-photon interference predominantly contributes at H + +H kinetic energy releases of 0.2 -0.45 eV, and net-2-photon and 1-photon interference contributes at 1.65 -1.9 eV. These measurements of the benchmark H + 2 molecule offer the distinct advantage that they can be quantitatively compared with ab initio theory to confirm our understanding of strong-field coherent control via the carrier-envelope phase. PACS numbers: XXXOne ultimate goal of ultrafast, strong-field laser science is to coherently control chemical reactions [1][2][3]. A prerequisite to achieving this goal is to understand the control mechanisms and reaction pathways. To this end, tailoring the electric field waveform of few-cycle laser pulses to control reactions and uncover the underlying physics has become a powerful tool [4][5][6]. It has been applied to the dissociative ionization of H 2 and its isotopologues [7][8][9][10][11][12] and has recently been extended to more complex diatomic molecules, such as CO [13][14][15], and to small polyatomic molecules [16,17].Conceptually, one of the most basic features of a fewcycle laser pulse to control is the carrier-envelope phase (CEP). When the laser's electric field is written as E(t) = E 0 (t) cos(ωt + φ), E 0 (t) is an envelope function, ω is the carrier angular frequency, and φ is the CEP. In fact, all of the few-cycle waveform experiments cited above used the CEP as the control parameter.For example, Kling et al. used 5 fs, 1.2×10 14 W/cm 2 pulses with stabilized CEP to dissociatively ionize D 2 and found asymmetries in the emission direction of D + ions for kinetic energy releases (KER) above 6 eV [7,8]. The diminished dissociation signal in a circularly polarized laser field indicated that recollision played a role. Recollision entails a tunnel-ionized electron undergoing a collision with its parent ion after acceleration by the oscillating laser field [18,19]. The energy exchange between the laser-driven electron and the parent ion can promote the D + 2 to the 2pσ u excited state. Coupling of the 2pσ u and 1sσ g states [20] on the trailing edge of the laser pulse during the dissociation of D + 2 was suggested as the explanation for the CEP-dependent asymmetry [7,8].Another example comes from Kremer et al. who exposed an H 2 target to 6 fs, 4.4×10 14 W/cm 2 CEPstabilized laser pulses and observed asymmetries for KER values between 0.4 and 3 eV [9] -energies they attributed to bond softening (BS) [21] and not electron recollision, which has higher KER. They proposed that the initial ionization of H 2 generates a coherent wav...

The electric field of a laser pulse can be described as Here we report the first method permitting absolute CEP detection with a solid-state detector applicable in ambient conditions. Recently, we have shown that the strong electric field of an intense, linearly-polarized, visible/near-infrared (VIS/NIR), few-cycle laser pulse can rapidly increase the (ac) conductivity of a solid insulator, allowing electric currents to be induced and switched with the field of visible light [22]. In these experiments, we exposed amorphous silicon dioxide (bandgap g 9 eV E ≈ ) to a strong, controlled electric field ( ) F t of a few-cycle pulse with a carrier photon energy of ∆ is a consequence of dispersive pulse broadening inside the glass wedges. However, in our experiments P ( ) Q l ∆ was still detectable above the noise level for values of 400 µm l ∆ > , corresponding to a pulse duration of more than 9 fs (FWHM of the time-dependent cycle-averaged intensity). Subsequently, PQ was calibrated with respect to the absolute CEP of the laser pulse via stereo-ATI measurements performed with identical pulses [4]. After the measurement of P ( ) Q l ∆ with the solid-state device, a mirror was inserted into the beam path, deflecting the 5 pulses into a stereo-ATI apparatus located -together with the solid-state detector -in the same vacuum chamber (Fig. 1). Here, the CEP of the incident laser pulse was detected by analyzing the kinetic energy distribution of electrons that are photoemitted from Xe atoms, see Methods Summary. An uncertainty due to a Gouy phase shift in both foci can be neglected since in both experiments, the sample was placed exactly in the region of the highest laser intensity.We set 17 different propagation lengths l ∆ , ranging from 21.5 µm − to 27.5 µm +. For each of them, 500 single-shot stereo-ATI measurements were performed. Because consecutive laser pulses had a CEP-shift of π , which is only required for the accurate detection of P ( ) Q l ∆ , only spectra from odd-numbered pulses were considered for the stereo-ATI measurements. As shown in [4], CEϕ can then be reconstructed by calculating two asymmetry parameters ( , ) X Y by integrating the averaged time-of-flight spectra L,R TOF ( ) n t of the electrons photoemitted from Xe atoms by the intense few-cycle VIS/NIR pulses in two different regions. The parametric plot of ( , ) X Y in Fig. 2(a) was obtained by calculating, for each. The photoelectron spectra L,R TOF ( ) n twere measured with the left (L) and right (R) micro-channel plates (MCPs) of the set-up in Fig. 1 We have compared the results of the solid-state-based phase retrieval with the predictions of two quantum mechanical models. The first model, which was earlier employed in Ref.[24] to describe the ultrafast increase in conductivity of SiO 2 nanojunctions, is based on the nearestneighbor tight-binding approximation. The second model, presented in detail in Ref.[25], describes quantum dynamics in a one-dimensional pseudopotential (see the Methods Summary for details). In both models, the electric fi...