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High-harmonic spectroscopy of solids is a powerful tool, which provides access to both electronic structure and ultrafast electronic response of solids, from their band structure and density of states, to phase transitions, including the emergence of the topological edge states, to the PetaHertz electronic response. However, in spite of these successes, high harmonic spectroscopy has hardly been applied to analyse the role of coherent femtosecond lattice vibrations in the attosecond electronic response. Here we study coherent phonon excitations in monolayer graphene to show how high-harmonic spectroscopy can be used to detect the influence of coherent lattice dynamics, particularly longitudinal and transverse optical phonon modes, on the electronic response. Coherent excitation of the in-plane phonon modes results in the appearance of sidebands in the spectrum of the emitted harmonic radiation. We show that the spectral positions and the polarisation of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high harmonic spectroscopy -- the combination of sub-femtosecond to tens of femtoseconds temporal resolution -- to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids., 15 pages, 4 figures

High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS$_2$, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5 eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multi-band recombination dipoles.

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.

Bound states in the continuum (BICs) are lossless states predicted in quantum mechanics in 1929, and for the last decade, received special attention in optics and photonics as a versatile tool to achieve giant quality factors (Q factors) [1] . For ordered arrays of subwavelength meta-atoms, metasurfaces, with broken in-plane symmetry, it was shown that Q factor of quasi-BICs can be unambiguously controlled by variation of the unit cell asymmetry parameter [2] .

Bound states in the continuum (BICs) represent dark modes trapped in the radiation continuum. BICs received significant attention in optics and photonics as a simple tool to achieve giant quality factors by transforming them into quasi-BICs. Here, we report the observation of high-harmonic generation in dielectric metasurfaces hosting BICs. The metasurface is composed of a square lattice with parallel Si bars of a slightly different width placed on a transparent substrate. The structure is engineered to support a quasi-BIC in the mid-IR with a high quality factor. We tune the metasurface asymmetry to enable the optimal coupling condition that provide the highest high-harmonic generation efficiency. In the experiment, we demonstrate the generation of odd optical harmonics from the 3rd to the 11th order in the BIC regime and study their polarization dependence. We measure the dependence of the high-harmonic signal on the input intensity. The concept of metasurfaces with highly localized light boosted by BIC resonances provides a new degree of freedom to control experimentally strong nonlinear optical response.

High harmonic generation (HHG) and electron excitation by intense mid-IR pulses upon photo-carrier doping have been investigated. We observe a damped HHG and increased electron excitation process as a function of the free electron density.

High harmonic generation (HHG) versus the crystal orientation of a monolayer MoS2 is investigated experimentally. It is found that the laser wavelength and the multiband electronic structure plays an important role in HHG process.

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.

The expansion of a laser plasma created during irradiation of a solid target with intense femtosecond pulse is studied. The combined use of interferometry and depolarization measurements allowed us to reconstruct the spatial distribution of the plasma density and revealed the development of small-scale instabilities accompanied by megagauss magnetic field generation. The nature of the observed instabilities is discussed.

Through the utilization of optical bound states in the continuum supported by resonant Si metasurfaces in the mid-IR spectral range, we demonstrate the generation of odd optical harmonics, from the 3rd to the 11th order.

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.

A generation of optical harmonics by circularly polarized light, which is in general forbidden in an isotropic medium, was recently observed in molecular gases where molecules were aligned and oriented by another linearly polarized laser pulse [1] or by pulses with twisted polarization [2]. In order to generate nth-harmonic with circularly polarized light, a molecule has to "absorb" n photons, having the same handedness, and to emit a single nth harmonic photon while fulfilling the energy and the momentum conservation laws, meaning a necessity to excite the molecule to appropriate rotational state. One can do this by means of intrapulse rotational Raman scattering, which prepares the molecule in the excited state through a subsequent absorption of (n-1) same-handed photons [1]. All previously reported experimental studies on circular harmonic generation in molecular gases were performed with visible/near-IR drivers, in either gas jets or in standing cells and with pre-alignment of molecules by an additional laser pulses. Here we report on the studies of self-action of elliptically polarized mid-IR pulses on the generation of low-order harmonics in filaments ignited in air and in argon, representing a molecular and an atomic gas respectively. The harmonics were driven by 20-mJ, sub-100 fs mid-IR pulses, centred at 3.9 pm wavelength. In the experiments filamentation was assisted by focusing light with a spherical mirror of ROC=-2500 mm into an open-end tube, through which Ar gas was flown at slight overpressure. Polarization of the pulses was controlled by a broadband quarter wave plate (QWP). Spectra of 5th and 7th harmonics were recorded after the filaments with a grating spectrometer (HR4000, OceanOptics). Polarization state of the generated harmonics was analysed with a Glan-Taylor polarizer.

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 examine mid-IR light bullets generated in ambient air. 2-optical cycle pulses confined in space are generated in filamentation regime. Few-fold solitonic self-compression is achieved for strongly chirped mid-IR pulses.

We generate fully controllable fs multimillijoule pulse bursts with the energy handling, throughput efficiency and frequency resolution substantially exceeding that achievable in spatial-light-modulator and interferometric techniques. The demonstrated proof-of-concept experiments include coherent control of nitrogen-ion emission via multiple-pulse excitation and generation of tunable narrowband THz pulses via optical rectification.

The Multiplexed Structured Image Capture (MUSIC) technique is used to demonstrate single-shot multiframe passive imaging, with a nanosecond difference between the resulting images. This technique uses modulation of light from a scene before imaging, in order to encode the target’s temporal evolution into spatial frequency shifts, each of which corresponds to a unique time and results in individual and distinct snapshots. The resulting images correspond to different effective imaging gate times, because of the optical path delays. Computer processing of the multiplexed single-shot image recovers the nanosecond-resolution evolution. The MUSIC technique is used to demonstrate imaging of a laser-induced plasma. Simultaneous single-shot measurements of electron numbers by coherent microwave scattering were obtained and showed good agreement with MUSIC characterization. The MUSIC technique demonstrates spatial modulation of images used for passive imaging. This allows multiple frames to be stacked into a single image. This method could also pave the way for real-time imaging and characterization of ultrafast processes and visualization, as well as general tracking of fast objects.

Physical review / X Expanding access X 9(2), 021029 (2019). doi:10.1103/PhysRevX.9.021029, We report on experimental results in a new regime of relativistic light-matter interaction employing midinfrared (3.9-mu m wavelength) high-intensity femtosecond laser pulses. In the laser-generated plasma, electrons reach relativistic energies already for rather low intensities due to the fortunate lambda(2) scaling of the kinetic energy with the laser wavelength. The lower intensity efficiently suppresses optical field ionization and creation of the preplasma at the rising edge of the laser pulse, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. Numerical simulations, which are in a good agreement with experimental results, suggest that approximate to 80% of the incident laser energy has been absorbed resulting in a long-living, key-temperature, high-charge-state plasma with a density more than 3 orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate., Published by APS, College Park, Md.

U.S. Air Force Office of Scientific Research under MURI [FA9550-16-1-0013]; Laserlab-Europe Project [HIJ-FSU002344]; Friedrich-Schiller-Universitat Jena; University of Arizona

Plasma density spatial distribution at superfilamentation regime of femtosecond laser radiation in ambient air was measured by precision interferometry technique. It was shown that at weak focusing (f-number ∼125) the filaments formed before the focus converged near the focus and formed superfilament without changing the plasma density while at stronger focusing (f-number ∼50) the plasma density in each filament increased several times.

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 present a detailed investigation of X-ray emission from both flat and nanowire zinc oxide targets irradiated by 60 fs 5 &times, 1016 W/cm2 intensity laser pulses at a 0.8 &micro, m wavelength. It is shown that the fluence of the emitted hard X-ray radiation in the spectral range 150&ndash, 800 keV is enhanced by at least one order of magnitude for nanowire targets compared to the emission from a flat surface, whereas the characteristic K&alpha, line emission (8.64 keV) is insensitive to the target morphology. Furthermore, we provide evidence for a dramatic increase of the fast electron flux from the front side of the nanostructured targets. We suggest that targets with nanowire morphology may advance development of compact ultrafast X-ray sources with an enhanced flux of hard X-ray emission that could find wide applications in highenergy density (HED) physics.

We report results of experimental investigation on polarization evolution and losses in linearly and circularly polarized mid-IR filaments in air under different focusing conditions.

Modern optical spectroscopy of the atmosphere at high altitudes relies primarily on the powerful and well-established LIDAR technique. A ground-based (or air−/space-borne) laser source is used for standoff linear measurements of the scattering characteristics of molecular, atomic, or particle species in the air in different scattering regimes (Rayleigh, Mie, spontaneous Raman). The incoherent nature of the scattered field measured in the backward direction sets practical limitation on the tracing distance, spatial resolution, and the species-dependent sensitivity threshold. The possibility of standoff initiation of a coherent source of backward-directed radiation in the sky would enable application of different methods of nonlinear optics, such as stimulated Raman scattering, CARS, etc., for the highly sensitive and highly selective spectroscopy of the atmosphere at high altitudes (Hemmer et al. PNAS 108, 3130, 2011; Yuan et al. Laser Phys. Lett. 8, 736, 2011) with (potentially) significantly larger detection range and finer spatial resolution. One of the possibilities to initiate such standoff coherent source of radiation in the atmosphere can be provided by achieving population inversion and laser generation in one of the two main atmospheric constituents, nitrogen (≈78% concentration) and oxygen (≈21% concentration). In principle, both gases allow population inversion and laser generation.

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.

Filamentation of Mid-IR pulses in ambient air is significantly more complicated and challenging as compared to near-IR filaments, due the changing sign of dispersion in the vicinity of 3.6 μm, substantially lower ionization rates and hence plasma density [1], as well as because of the presence of molecular resonances around the atmospheric transparency windows. Experimental observations reveal that despite its low concentration in ambient air, which is only 400–600 ppm (0.04–0.06%), CO2 strongly affects filamentation of 3.9-μm pulses by influencing spectral and temporal dynamics, as well as spatial beam transformations and propagation losses. As a main loss mechanism during filamentation of loosely focused 3.9-μm pulses in ambient air, dynamic absorption by CO2 is identified. Since in the case of loose focusing plasma density in the filament is negligible, pulses during filamentation in both main constituents, O2 and N2, experience pronounced red-sided broadening (Fig.la), which is governed by stimulated Raman scattering. In ambient air, newly generated spectral components roll over CO2 resonant absorption band situated in the vicinity of 4.2-μm and are effectively absorbed, which results in up to 50% filamentation losses. To our surprise, an increase of CO2 concentration to ∼5% level (by exhaling air [3] into 2-m long open-ended tube) leads to complete disappearance of losses.

Light 6(12), e17124 (2017). doi:10.1038/lsa.2017.124, Published by Nature Publishing Group, London

Different regimes of filamentation of multi-millijoule mid-infrared pulses in ambient air can be achieved by varying focusing conditions and chirp of the driving pulses. Dynamic absorption losses are identified as a possible mechanism of observed plasma-less filamentation.

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.

We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

We report on stimulated emission from vertically aligned, vapor transport grown, ZnO nanowire arrays, and pumped by three-photon absorption in intense near-infrared femtosecond laser pulses. In respect to single nanowires, arrays have the advantage of a higher light absorption and emission rate. The intensity and bandwidth of the emitted ultraviolet radiation as a function of the pump intensity is compared for nanowire arrays with different wire lengths, diameters, and spacing. The measured lasing thresholds for all arrays can be well described by the geometry of individual nanowire lasers, showing that coupling effects between the individual emitters in the arrays are negligible, even for the smallest 100 nm diameter wires with an average distance of 200 nm.

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.

We report on the generation of high-energy few-optical-cycle pulses in mid-infrared spectral region via self-compression in transparent dielectrics and filaments as well as via four wave parametric amplification in gas-filled hollow waveguides.

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 study properties of filaments ignited by multi-millijoule, 90-fs mid-IR pulses centered at 3.9 μm. The studies are performed by monitoring plasma density distribution and losses as well as beam profiles at different focusing conditions.

We experimentally investigated a delayed fragmentation of hydrocarbon molecules on nanosecond to microsecond timescale after double ionization by femtosecond laser pulses. Quantum chemical simulations suggest that it originates from meta-stable high-lying vibrational states of dications.

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 report on octave broadband supercontinuum generation from ultraviolet to near infrared wavelengths in the fundamental mode of a gas-filled novel low-loss anti-resonant hollow-core fiber having distinct transmission windows.

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.

We report self-action of 0.5-TW peak-power few-cycle 4-um pulses in air and bulk dielectrics that is strikingly different compared to the case of near-IR drivers. An example of scalable nonlinear self-compression of 20-mJ is highlighted.

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