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Two-color (800-and 400-nm) short (45-fs) linearly polarized pulses are used to ionize and dissociate CO and NO. The emission of C q + , N q + , and O + fragments indicates that the higher ionization rate occurs when the peak electric field points from C to O in CO and from N to O in NO. This preferred direction is in agreement with that predicted by Stark-corrected strong-field-approximation calculations.

Abstract:We study the evolution of nuclear wave packets launched in molecular nitrogen, oxygen and carbon monoxide by intense 8fs infrared pulses.We use velocity map imaging to measure the momentum of the ion fragments when these wave packets are interrogated by a second such pulse after a variable time delay. Both quasi-bound and dissociative wave packets are observed. For the former, measurements of bound-state oscillations are used to identify the participating states and in some cases extract properties of the relevant potential energy surfaces. Vibrational structure is resolved in both energy and oscillation-frequency for the cations of oxygen and carbon monoxide, displaying the same quantum wave packet motion in both energy and time domains. In addition, vibrational structure is seen in the dication of carbon 2 monoxide in a situation where the energy resolution by itself is inadequate to resolve the structure.

We experimentally demonstrate that atomic orbital parity mix interferences can be temporally controlled on an attosecond time scale. Electron wave packets are formed by ionizing argon gas with a comb of odd and even high-order harmonics, in the presence of a weak infrared field. Consequently, a mix of energy-degenerate even and odd parity states is fed in the continuum by one-and two-photon transitions. These interfere, leading to an asymmetric electron emission along the polarization vector. The direction of the emission can be controlled by varying the time delay between the comb and infrared field pulses. We show that such asymmetric emission provides information on the relative phase of consecutive odd and even order harmonics in the attosecond pulse train. DOI: 10.1103/PhysRevLett.109.083001 PACS numbers: 32.80.Rm, 32.80.Qk, 42.65.Ky Coherent control of electron dynamics in atoms and molecules is a fascinating perspective in laser physics, with promising implications for many different branches of scientific and engineering research. The recent development of x-or extreme-ultraviolet (XUV) light pulses in the attosecond time scale has opened up new avenues for experimentalists to achieve an effective control over electronic dynamics. Both single attosecond pulses (SAPs) and attosecond pulse trains (APTs) emerge as very promising tools to manipulate the electronic charge distribution in molecules [1][2][3]. Despite these successful proof-ofprinciple experiments, attosecond control of the electron dynamics is, nevertheless, still in its infancy. This is mainly because the usefulness of such attosecond pulses is limited by the degree to which they can be synthesized and characterized. In particular, a precise measurement of the phases of the frequency components is required as they determine the ultimate length and shape of these pulses. The literature is rich with experimental studies of the characteristics of APTs and SAPs [4][5][6][7][8][9][10][11]. Most of them are based on the conversion of the attosecond pulse into electron wave packets, through photoionization of atoms, in the presence of an IR field to get access to the relative phases of the frequency components making up the pulse.This scheme, in turn, can be employed to control the electron emission from atoms and molecules. Mauritsson et al. reported an electron scattering imaging technique, based on a sequence of attosecond pulses used to release electrons into a sufficiently strong IR field to guide them back to their parent ions exactly once per laser cycle [12]. A recent theoretical study also suggested combining an APT and a much weaker IR field than in the previous study, as an efficient means for generating strong asymmetric emission of continuum electrons along the direction of the laser polarization [13]. The authors showed that interference between one-and two-photon transitions can produce a large asymmetry in the angular distribution of the photoelectrons though the separate contributions of the two paths have no asymmetries.In this...

Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C 60 fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization. DOI: 10.1103/PhysRevLett.114.123004 PACS numbers: 33.80.Eh, 31.15.xv, 42.50.Hz, 71.20.Tx Electrons determine the forces on the nuclei in molecules. Tuning the nonequilibrium electronic dynamics before the onset of significant nuclear motion opens new routes for tailoring chemical reactivity. For few-cycle optical pulses, varying the phase between the envelope and the field amplitude [carrier-envelope phase (CEP)] can be used to control electronic dynamics induced in molecules during the interaction with the pulse [1][2][3]. Electronic dynamics are typically probed indirectly by recording molecular fragmentation patterns of dissociative (ionization) channels exploiting the coupling between the electronic and nuclear degrees of freedom . The analysis of the fragmentation patterns is usually complex-even for simple diatomic molecules-and quickly becomes prohibitively complicated for large polyatomic molecules because of the large number of fragmentation channels [3]. Angularly resolved photoionization by ultrashort laser pulses has been advocated for probing the electronic dynamics before the onset of significant nuclear motion (see, e.g., [27][28][29][30][31] [36,37], and efficient high-harmonic generation [38][39][40]. The ionization and fragmentation of C 60 have been investigated extensively in the past (see, e.g., [35,[41][42][43][44][45][46][47]). C 60 is very stable and is one of the few molecular systems for which the ionization energy is smaller than the lowest fragmentation threshold. Therefore it is an ideal system for probing electronic dynamics, and, when suitably excited, the electronic density oscillates on a nanometer scale. Moreover, in the experiments reported here, the pulse duration is short enough to avoid significant thermionic emission that occurs for longer pulse durations of hundreds of femtoseconds to nanoseconds [48,49]. In this Letter, we demonstrate the control over transient electronic dynamics in a large polyatomic system, the C 60 fullerene, and we find that the angular distribution of direct photoelectrons reflects the spatial localization of the electronic wave packet at about the time of ionization.The electron emission from C 60 as a function of the CEP is recorded with phase-tagged velocity-map imaging (VMI) [50]. Details of the experimental setup are contained in the Supplemental Material [51]. The few-cycle laser pulses are focused into the VM...

The transition between two distinct mechanisms for the laser-induced field-free orientation of CO molecules is observed via measurements of orientation revival times and subsequent comparison to theoretical calculations. In the first mechanism, which we find responsible for the orientation of CO up to peak intensities of 8 × 10(13) W/cm(2), the molecules are impulsively oriented through the hyperpolarizability interaction. At higher intensities, asymmetric depletion through orientation-selective ionization is the dominant orienting mechanism. In addition to the clear identification of the two regimes of orientation, we propose that careful measurements of the onset of the orientation depletion mechanism as a function of the laser intensity will provide a relatively simple route to calibrating absolute rates of nonperturbative strong-field molecular ionization.

Light-field driven electron localization in deuterium molecules in intense near single-cycle laser fields is studied as a function of the laser intensity. The emission of D + ions from the dissociative ionization of D 2 is interrogated with single-shot carrier-envelope phase (CEP)-tagged velocity map imaging. We explore the reaction for an intensity range of (1.0-2.8) × 10 14 W cm −2 , where laser-driven electron recollision leads to the population of excited states of D 2 +. Within this range we find the onset of dissociation from 3σ states of D 2+ by comparing the experimental data to quantum dynamical simulations including the first eight states of D 2 + . We find that dissociation from the 3σ states yields D + ions with kinetic energies above 8 eV. Electron localization in the dissociating molecule is identified through an asymmetry in the emission of D + ions with respect to the laser polarization axis. The observed CEP-dependent asymmetry indicates two mechanisms for the population of 3σ states: (1) excitation by electron recollision to the lower excited states, followed by laser-field excitation to the 3σ states, dominating at low intensities, and (2) direct excitation to the 3σ states by electron recollision, playing a role at higher intensities.

A novel design for a velocity-map imaging (VMI) spectrometer with high resolution over a wide energy range surpassing a standard VMI design is reported. The main difference to a standard three-electrode VMI is the spatial extension of the applied field using 11 electrodes forming a thick-lens. This permits measurements of charged particles with higher energies while achieving excellent resolving power over a wide range of energies. Using SIMION simulations, the thick-lens VMI is compared to a standard design for up to 360 eV electrons. The simulations also show that the new spectrometer design is suited for charged-particle detection with up to 1 keV using a repeller-electrode voltage of-30 kV. The experimental performance is tested by laserinduced ionization of rare gases producing electrons up to about 70 eV. The thick-lens VMI is useful for a wide variety of studies on atoms, molecules and nanoparticles in intense laser fields and highphoton-energy fields from high-harmonic-generation or free-electron lasers.