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The interaction of laser pulses with condensed matter forms the basis of light-wave-driven electronics potentially enabling tera- and petahertz switching rate applications. Carrier control using near- and midinfrared pulses is appealing for integration into existing platforms. Toward this end, a fundamental understanding of the complexity of phenomena concerning sub-band-gap driven semiconductors such as high harmonic generation, carrier excitation due to multiphoton absorption, and interband tunneling as well as carrier-carrier interactions due to strong acceleration in infrared transients is important. Here, stimulated emission from polycrystalline ZnO thin films for pump wavelengths between 1.2 \ensuremath{\mu}m (1 eV) and 10 \ensuremath{\mu}m (0.12 eV) is observed. Contrary to the expected higher intensity threshold for longer wavelengths, the lowest threshold pump intensity for stimulated emission is obtained for the longest pump wavelength corroborating the importance of collisional excitation upon intraband electron acceleration.

The generation of high order harmonics from femtosecond mid-IR laser pulses in ZnO has shown great potential to reveal new insight into the ultrafast electron dynamics on a few femtosecond timescale. In this work we report on the experimental investigation of photoluminescence and high-order harmonic generation (HHG) in a ZnO single crystal and polycrystalline thin film irradiated with intense femtosecond mid-IR laser pulses. The ellipticity dependence of the HHG process is experimentally studied up to the 17th harmonic order for various driving laser wavelengths in the spectral range 3&ndash, 4 &micro, m. Interband Zener tunneling is found to exhibit a significant excitation efficiency drop for circularly polarized strong-field pump pulses. For higher harmonics with energies larger than the bandgap, the measured ellipticity dependence can be quantitatively described by numerical simulations based on the density matrix equations. The ellipticity dependence of the below and above ZnO band gap harmonics as a function of the laser wavelength provides an efficient method for distinguishing the dominant HHG mechanism for different harmonic orders.

Multiple filamentation of terawatt femtosecond Ti:sapphire laser beams in air is experimentally investigated for intermediate external focusing conditions with the numerical apertures $\mathrm{NA}={10}^{\ensuremath{-}2}$ and $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The electron density distribution in a multifilament plasma channel is measured by means of transversal interferometry. The results confirm the existence of superdense filaments (superfilaments). It is also found that the maximum electron density in filaments differs by an order of magnitude ($\ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and $\ensuremath{\sim}9\ifmmode\times\else\texttimes\fi{}{10}^{16}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$) for the two values of NA. This demonstrates high sensitivity of the multiple filamentation regime to focusing conditions. The interferometric results are compared with the spatially resolved measurements of the luminescence intensity from the multifilament plasma channel to verify the accuracy of the widely used luminescence technique.

We experimentally investigate dissociation of ${\mathrm{H}}_{2}^{+}$ in the (near-)zero proton energy region. Based on our experimental results we conclude that the underlying mechanism for the production of protons with such low energy in strong two-color and broadband laser fields is a stimulated Raman-scattering-based zero-photon-dissociation process, ${\mathrm{ZPD}}_{\text{stR}}$, taking place on the electronic ground state. It is furthermore shown that in the (near-)zero energy region the asymmetry in proton ejection induced by asymmetric laser fields is due to the interplay of several processes, rather than only pathway interferences, with vibrational trapping (or bond hardening) taking a key role.

We present experimental evidence of ultra-high energy density plasma states with the keV bulk electron temperatures and near-solid electron densities generated during the interaction of high contrast, relativistically intense laser pulses with planar metallic foils. The bulk electron temperature and density have been measured using x-ray spectroscopy tools; the temperature of supra-thermal electrons traversing the target was determined from measured bremsstrahlung spectra; run-away electrons were detected using magnet spectrometers. The measured electron energy distribution was in a good agreement with results of Particle-in-Cell (PIC) simulations. Analysis of the bremsstrahlung spectra and results on measurements of the run-away electrons showed a suppression of the hot electrons production in the case of the high laser contrast. By application of Ti-foils covered with nm-thin Fe-layers we demonstrated that the thickness of the created keV hot dense plasma does not exceed 150 nm. Results of the pilot hydro-dynamic simulations that are based on a wide-range two-temperature EOS, wide-range description of all transport and optical properties, ionization, electron and radiative heating, plasma expansion, and Maxwell equations (with a wide-range permittivity) for description of the laser absorption are in excellent agreement with experimental results. According to these simulations, the generation of keV-hot bulk electrons is caused by the collisional mechanism of the laser pulse absorption in plasmas with a near solid step-like electron density profile. The laser energy firstly deposited into the nm-thin skin-layer is then transported into the target depth by the electron heat conductivity. This scenario is opposite to the volumetric character of the energy deposition produced by supra-thermal electrons., 15 pages, 10 figures

We use orthogonally polarized two-color (OTC) laser pulses to separate quantum paths in the multiphoton ionization of Ar atoms. Our OTC pulses consist of 400 and 800 nm light at a relative intensity ratio of 10∶1. We find a hitherto unobserved interference in the photoelectron momentum distribution, which exhibits a strong dependence on the relative phase of the OTC pulse. Analysis of model calculations reveals that the interference is caused by quantum pathways from nonadjacent quarter cycles.

We present an experimental study of X-ray generation from nanostructured ZnO targets. Samples of different morphology ranging from nanowires to polished surfaces are irradiated by relativistically intense femtosecond laser pulses. X-ray emission of plasma is generated by 45 fs 130 mJ laser pulses at 400 nm with picosecond temporal contrast better than 10−9 interacting with an array of ZnO nanowires. The measured spectra indicate the existence of highly ionized states of Zn (up to He-like Zn). The obtained flux of ∼1010 photons per laser shot at the neutral Zn Kα energies around 8.65 keV and at the Zn Heα energies around 9 keV is almost 3 times higher for nanostructured targets compared to the reference polished sample and implies 10−4 conversion efficiency from the laser energy to the total energy of the emitted X-ray photons.

Ultrafast coherent phonon dynamics in ZnO is studied via high-order harmonic generation by intense mid-IR laser pulses. We show, the phonon dynamic is very different after excitation in the tunnel and multiphoton regime.

Electron correlation in sequential double ionization of helium is studied experimentally with the ``attoclock'' technique. Control of the two-electron emission dynamics is demonstrated on the time scale of a half laser cycle.

The dynamics of sequential two-electron emission from helium is extracted from measured momentum distributions. Emission in between the field-maxima and on sub-laser-cycle times is identified. The corresponding structures are roughly reproduced by a semi-classical model.

We report on the observation of subcycle interferences of electron wave packets released during strong field ionization of $\text{H}_{2}$ with cycle-shaped two-color laser fields. With a reaction microscope we measure three-dimensional momentum distributions of photoelectrons correlated with either $\text{H}_{2}^{+}$ or protons within different energy ranges generated by dissociation of $\text{H}_{2}^{+}$ . We refer to these different types of photoelectrons as channels. Our results show that the subcycle interference structures of electron wave packets are very sensitive to the cycle shape of the two-color laser field. We explain this behavior by the dependence of the ionization time within an optical cycle on the shape of the laser field cycle. The subcycle interference structures can be further used to obtain insight into the subcycle dynamics of molecules during strong field interaction.

We demonstrate control over the localization of high-lying Rydberg wave packets in argon atoms with phase-locked orthogonally polarized two-color laser fields. With a reaction microscope, we measure ionization signals of high-lying Rydberg states induced by a weak dc field and blackbody radiation as a function of the relative phase between the two-color fields. We find that the dc-field-ionization yield of high-lying Rydberg argon atoms oscillates with the relative two-color phase with a period of $2\ensuremath{\pi}$ while the photoionization signal by blackbody radiation shows a period of $\ensuremath{\pi}$. Accompanying simulations show that these observations are a clear signature of the asymmetric localization of electrons recaptured into very elongated (low angular momentum) high-lying Rydberg states after conclusion of the laser pulse. Our findings thus open an effective pathway to control the localization of high-lying Rydberg wave packets.

New methods are emerging that aim to image chemical reactions as they occur, using X-ray diffraction, electron diffraction or laser-induced recollision. But none of these methods offer spectral selection, which allows a laser pulse with light of one wavelength to initiate a reaction, and a second pulse with another, appropriately selected wavelength to monitor the reacting molecules. Worner et al. now show that this apparent limitation offers exciting opportunities for recollision-based high-harmonic spectroscopy: due to the coherent nature of the attosecond high-harmonic pulse generation, unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale. High-harmonic spectroscopy thus seems ideally suited to measure coupled electronic and nuclear dynamics in fast photochemical reactions, or to characterize short-lived transition states. New methods are emerging that aim to image chemical reactions as they occur using X-ray diffraction, electron diffraction or laser-induced recollision, but spectral selection cannot be used to monitor the reacting molecules for these methods. These authors show that this apparent limitation offers opportunities for recollision-based high-harmonic spectroscopy, in which unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale. The study of chemical reactions on the molecular (femtosecond) timescale typically uses pump laser pulses to excite molecules and subsequent probe pulses to interrogate them. The ultrashort pump pulse can excite only a small fraction of molecules, and the probe wavelength must be carefully chosen to discriminate between excited and unexcited molecules. The past decade has seen the emergence of new methods that are also aimed at imaging chemical reactions as they occur, based on X-ray diffraction1, electron diffraction2 or laser-induced recollision3,4—with spectral selection not available for any of these new methods. Here we show that in the case of high-harmonic spectroscopy based on recollision, this apparent limitation becomes a major advantage owing to the coherent nature of the attosecond high-harmonic pulse generation. The coherence allows the unexcited molecules to act as local oscillators against which the dynamics are observed, so a transient grating technique5,6 can be used to reconstruct the amplitude and phase of emission from the excited molecules. We then extract structural information from the amplitude, which encodes the internuclear separation, by quantum interference at short times and by scattering of the recollision electron at longer times. The phase records the attosecond dynamics of the electrons, giving access to the evolving ionization potentials and the electronic structure of the transient molecule. In our experiment, we are able to document a temporal shift of the high-harmonic field of less than an attosecond (1 as = 10−18 s) between the stretched and compressed geometry of weakly vibrationally excited Br2 in the electronic ground state. The ability to probe structural and electronic features, combined with high time resolution, make high-harmonic spectroscopy ideally suited to measuring coupled electronic and nuclear dynamics occurring in photochemical reactions and to characterizing the electronic structure of transition states.

Two-body dissociation processes of H2C@CH–CH@CH 2þ 2 induced by an intense laser field were investi

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescale in ethylene and acetylene molecules, following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique, and found that both the lifetime of the long-lived ethylene dication leading to the delayed deprotonation and the relative channel strength of the delayed deprotonation compared to the prompt one have no evident dependence on the laser pulse duration and the laser peak intensity. Quantum chemical simulations suggest that such delayed fragmentation originates from the tunneling of near-dissociation-threshold vibrational states through a dissociation barrier on a dication electronic state along C--H stretching. Such vibrational states can be populated through strong field double ionization induced vibrational excitation on an electronically excited state in the case of ethylene, and through intersystem crossing from electronically excited states to the electronic ground state in the case of acetylene.

We experimentally and theoretically demonstrate channel-selective control over strong-field induced fragmentation of a polyatomic molecule, acetylene, using impulsive laser alignment as the control mechanism.

Using orthogonally polarized two-color laser fields on neon and coincidence momentum imaging we gain access to the Coulomb influence in single ionization on sub-cycle times, and demonstrate a strong electron-electron anti-correlation in double ionization.

Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on the OTC field shape. © 2014 American Physical Society.

We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.

The two-dimensional sub-cycle-time to electron momentum mapping provided by orthogonal two-color laser fields is applied to photoelectron spectroscopy. Using neon as the example we gain experimental access to the dynamics of emitted electron wave packets in electron momenta spectra measured by coincidence momentum imaging. We demonstrate the opportunities provided by this time-to-momentum mapping by investigating the influence of the parent ion on the emitted electrons on laser-sub-cycle times. It is found that depending on their sub-cycle birth time the trajectories of photoelectrons are affected differently by the ion's Coulomb field.

We experimentally investigate the ionization mechanism behind the formation of remarkably high charge states observed in the laser-pulse-induced fragmentation of different hydrocarbon molecules by Roither et al. [Phys. Rev. Lett. 106, 163001 (2011)], who suggested enhanced ionization occurring at multiple C-H bonds as the underlying ionization mechanism. Using multiparticle coincidence momentum imaging we measure the yield of multiply charged fragmenting ethylene and acetylene molecules at several intensities and pulse durations ranging from the few-cycle regime to 25 fs. We observe, at constant intensity, a strong increase of the proton energy with increasing laser pulse duration. It is shown that this is caused by a strong increase in the yield of highly charged parent molecular ions with pulse duration. Based on experimental evidence we explain this increase by the necessary population of precursor states in the parent ion that feature fast C-H stretch dynamics to the critical internuclear distance, where efficient ionization via enhanced ionization takes place. For increasing pulse duration these precursor ionic states are more efficiently populated, which leads in turn to a higher enhanced-ionization probability for longer pulses. Our work provides experimental evidence for the existence of a multiple-bond version of enhanced ionization in polyatomic molecules.

Strong narrow-band coherent emission lines of the \(\mathrm{B}^{2} \varSigma_{\mathrm{u}} ^{+} \mbox{--} \mathrm{X}^{2} \varSigma_{\mathrm{g}} ^{+}\) transition of \(\mathrm{N}_{2} ^{+}\) in air and the A2 Π u–X2 Π g transition of \(\mathrm{CO}_{2} ^{+}\) in CO2 gas are observed from filaments generated by intense infrared femtosecond laser pulses (1150–2000 nm

We experimentally demonstrated control of chemical bond break of ethylene in intense laser fields with employing both electronic dynamics and nuclear vibrational dynamics.

Angstrom and attosecond control of free electron wave packets is one of the pinnacles of attosecond science. Orthogonally polarized two-color (OTC) laser fields allow to control the motion of field-ionizing electronic wave packets both in time and space[1]. In OTC pulses time and space are connected and thus an attosecond time scale is established in the polarization plane for both the emitted and the re-colliding wave packets[2,3]. © 2013 IEEE., 2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013, 12 May 2013 through 16 May 2013, Munich

We present experimental and numerical investigation of a new filamentation regime of mid-infrared femtosecond pulses in solids. Efficient emission of dispersive waves without fundamental spectrum broadening and continuum generation is observed in this regime.

We report an experimental proof and full characterization of laser generation in molecular nitrogen in an argon-nitrogen gas mixture remotely excited at a distance above 2 m in a femtosecond laser filament. Filamentation experiments performed with near-infrared, 1-\ensuremath{\mu}m-wavelength and midinfrared, 4-\ensuremath{\mu}m-wavelength short-pulse laser sources show that mid-IR laser pulses enable radical enhancement of filamentation-assisted lasing by N${}_{2}$ molecules. Energies as high as 3.5 \ensuremath{\mu}J are achieved for the 337- and 357-nm laser pulses generated through the second-positive-band transitions of N${}_{2}$, corresponding to a 0.5$%$ total conversion efficiency from midinfrared laser energy to the energy of UV lasing.

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wavepackets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of pre-determining fragmentation and isomerization reactions in polyatomic molecules on sub-femtosecond time-scales.

We analyze the formation of the low-energy structure (LES) in above-threshold ionization spectra first observed by Quan et al.\ \cite{quan09} and Blaga et al.\ \cite{blaga09} using both quasi-classical and quantum approaches. We show this structure to be largely classical in origin resulting from a two-dimensional focusing in the energy-angular momentum plane of the strong-field dynamics in the presence of the atomic potential. The peak at low energy is strongly correlated with high angular momenta of the photoelectrons. Quantum simulations confirm this scenario. Resulting parameter dependences agree with experimental findings \cite{quan09,blaga09} and, in part, with other simulations \cite{liu10,yan10,kast11}., 12 pages, 6 figures

Attosecond Physics builds on the properties of electron wave packets created by strong-field ionization close to the maxima of the field oscillations which are subsequently driven by the strong laser field on a sub-cycle time scale [1]. These wave packets can lead to interference structures in energy or momentum space. While interference of wave packets created during different field cycles (inter-cycle) give rise to the well-known above-threshold ionization (ATI) peaks, interference of wave packets that are created during the same laser cycle (intra-cycle) can lead to a hologram of the binding potential [2,3].

Multiparticle coincidence imaging reveals laser-driven ejection of energetic protons via concerted Coulomb explosions from unexpectedly high molecular charge states. The underlying mechanism is studied with laser pulses from 4.5 to 27 fs in duration.

We demonstrate self-referenced wavefunction retrieval of a valence electron wavepacket during its creation by strong-field ionization with a sub-10-attosecond precision, based on a distinct separation of interferences arising at different time-scales.

We use cycle-sculpted two-color waveforms to drive electronic wavepackets generated by strong-field ionization from helium, neon, and argon gas atoms and analyze their momentum spectra measured by electron-ion coincidence momentum spectroscopy. Varying the relative phase of the two colors allows to sculpt the ionizing field and hence to control the emission times and motion of the wavepackets on an attosecond timescale. Using semiclassical calculations, we investigate the influence of the ionic Coulomb field onto the motion of emitted electronic wavepackets. We further show that the measured electron momentum spectra contain interference patterns created by pairs of electron wavepackets that are released within a single laser-field cycle. We experimentally distinguish these subcycle interference structures from above-threshold ionization (ATI) peaks and argue that they can be used to extract the subcycle phase evolution of the laser-driven complex bound-state wavefunction.

By controlling the electric field of strong laser pulses wave packets created by field ionization can be steered in space and time and can be used to coherently probe the parent ion on the attosecond and angstrom scale. In addition to the actual photon or electron emission signal caused by the probe, two important processes influence the observed result: (i) ionization and (ii) wave packet propagation due to the combined force of the laser electric field and the ionic binding potential. The ionic Coulomb potential is often considered only a weak perturbation to the dominant driving force of the laser field. Quantitative statements, however, can only be made when the influence of the ionic long range Coulomb potential is properly accounted for [1–3].

Using the tools of femtosecond X-ray diffraction [1], electron diffraction [2] and laser-induced recollision [3], scientists are developing new methods to image chemical reactions as they occur. However, all of these methods face a common problem - only a small fraction of the molecules can be excited with an ultrashort pulse. When this problem was confronted in femtochemistry, a careful selection of the probe wavelength provided a way to discriminate excited molecules from unexcited molecules. Spectral selection is not available for any of these new methods. © 2011 IEEE., 2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011, 22 May 2011 through 26 May 2011, Munich

Using multi-particle coincidence detection we are able to show that carbon-hydrogen molecules exposed to moderate laser-intensities can completely disintegrate from high charge states by a concerted emission of all protons with high kinetic energies.

Ultrafast proton migration in 1,3-butadiene in an intense laser field (40 fs, 4.5 × 10(14) W cm(-2)) is investigated by using Coulomb explosion coincidence momentum imaging. The spatial distribution maps of a migrating proton reconstructed for the two three-body Coulomb explosion pathways, C(4)H(6)(3+)→ H(+) + CH(3)(+) + C(3)H(2)(+) and C(4)H(6)(3+)→ H(+) + C(2)H(+) + C(2)H(4)(+), reveal that two protons migrate within a 1,3-butadiene molecule, prior to the three body decomposition.

We have experimentally detected optical harmonics that are generated due to a tunneling-ionization-induced modulation of the electron density. The optical signature of electron tunneling can be isolated from concomitant optical responses by using a noncollinear pump-probe setup. Whereas previously demonstrated tools for attosecond metrology of gases, plasmas, and surfaces rely on direct detection of charged particles, detection of the background-free time-resolved optical signal, which uniquely originates from electron tunneling, offers an interesting alternative that is especially suited for systems in which free electrons cannot be directly measured.

Using coincidence momentum spectroscopy we show that the external laser field’s properties driving the internal molecular dynamics have negligible influence on the decision to follow a given fragmentation pathway in three-body fragmentation of polyatomic molecules.

We investigate attosecond electron tunnelling dynamics in a time-resolved pump–probe experiment on the generation of higher-order harmonics that are emitted due to a stepwise plasma concentration change occurring with each half-cycle of the pump-field.

Bond-breaking and bond-creation in chemical reactions are preceded and ultimately governed by the intra-molecular dynamics of the electrons. While many concepts of the interaction of strong, short laser pulses with molecules can be studied using the smallest molecule, H 2 , it has been shown that the laser-driven electrons' dynamics in the more chemically (and biologically) relevant polyatomic molecules can lead to remarkable fragmentation behaviour, e.g. [1,2]. Laser induced molecular fragmentation via Coulomb explosion has been studied by coincidence momentum imaging methods, which allow to selectively investigate specific breakup channels and revealed that the fragmentation is in many cases accompanied by remarkably fast hydrogen and/or proton migration, e.g. [3].

In the presence of a high-intensity optical field, electrons are released from atoms on an attosecond time scale. Moreover, in the tunnelling regime, this process displays a strong sensitivity to the carrier-envelope phase (CEP) of a few-cycle light pulse. Tunnelling ionization - a fascinating quantum mechanical phenomenon - leads to a quasi-stepwise increase of free electron density and, as a consequence, of the refractive index of the medium. These steps of the refractive index, corresponding to half-cycles of the driving optical field, impose a transient attosecond phase mask. By scattering probe light off this mask we detect quasi-periodic higher-order harmonics, the spectrum of which, unlike that of the harmonics originating from intrinsic nonlinearity or driven by electron re-collisions, do not depend on the probe intensity and recollision dynamics. The implemented noncollinear pump-probe experimental technique allows optical harmonics generated due to a tunnelling-ionization-induced modulation of the electric current to be spatially separated from the harmonics originating from atomic and ionic nonlinear susceptibilities, enabling background-free time-resolved detection of electron-tunnelling-controlled harmonic spectra and offering an attractive solution for attosecond optical metrology of gases and bulk solids.

Few-cycle light pulses are used to map ultrafast ionization dynamics in the time and frequency domain by all-optical means. Tunneling ionization encodes an attosecond phase mask, suggesting a promising method for attosecond shaping of high-intensity optical fields.

We demonstrate an all-optical technique for mapping sub-cycle tunnel ionization in gas based on a cross-correlation measurement of Brunel-type harmonics detected in the direction of a weak probe pulse to separate them from recollision-driven harmonics.

We report experimental observation of "slow" fragmentation from ethylene molecules on nanosecond or even microsecond timescale after their interaction with femtosecond laser pulses. Our results indicate that such fragmentation process originates from meta-stable high-lying vibrational states of ethylene ions.

We present the theoretical model for a single-pass, discharge-type standoff nitrogen laser initiated by a femtosecond filament in nitrogen gas. The model is based on the numerical solution of the kinetic equation for the electron energy distribution function self-consistently with balance equations for nitrogen species and laser equations. We identify the kinetic mechanisms responsible for a buildup of population inversion in the filament afterglow plasma and determine the dependence of population inversion density and the parameters of nitrogen lasing at a 337 nm wavelength corresponding to the transition between the C3Πu (v = 0) excited and the X1Σg (v = 0) ground electronic states in a nitrogen molecule on the polarization and wavelength of the driver laser pulse used to produce the filament. We show that population inversion is achieved on an ultrafast time scale of ≈10 ps and decays within the time

Single- and double-ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using the COLTRIMS method. We study the influence of the long range Coulomb potential of the parent ion on the final momentum distribution of electrons emitted during single ionization. Furthermore, we investigate nonsequential double ionization in OTC fields and demonstrate that the electron-electron correlation is highly sensitive to the sub-cycle field shape of the OTC pulses, in agreement with recent theoretical predictions., 28th International Conference on Photonic, Electronic and Atomic Collisions, ICPEAC 2013, July 24-30, 2013, Lanzhou

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets.

Using phase-shaped millijoule 1030-nm femtosecond pulses generating a filament in a cell filled with N2, we obtain intense forward UV emission between vibrational manifolds of B 2 Σ and X 2 Σ states of N2 + for optimal pulse sequences. The effect is tentatively ascribed to wave-mixing between intense NIR pulses and weak supercontinuum components, resonant to the UV transitions, whereby a non- instantaneous nonlinear susceptibility is linked to rotational coherence in ions.