Near-infrared Laser Field Research Articles

Effect of orbital symmetry on atomic ionization in near-infrared laser fields

Effect of orbital symmetry on atomic ionization in near-infrared laser fields

Excitation dynamics in molecule resolved by internuclear distance driven by the strong laser field.

Rydberg-state excitation of stretched model molecules subjected to near-infrared intense laser fields has been investigated based on a fully quantum model (QM) proposed recently and the numerical solutions of time-dependent Schrödinger equation (TDSE). Given the good agreement between QM and TDSE, it is found that, as the molecules are stretched, the electron tends to be trapped into low-lying Rydberg-states after its ionization from the core, which can be attributed to the shift of the ionization moments corresponding to maximum excitation populations. Moreover, the n-distribution is broadened for molecules with increasing internuclear distance, which results from the change of momentum distribution of emitted electrons. Analysis indicates that both of the above phenomena are closely related to the interference effect of electronic wave packets emitted from different nuclei. Our study provides a more comprehensive understanding of the molecular excitation in intense laser fields, as well as a means of possible applications to related experimental observations.

Strong-field photoionization/dissociation of vinyl bromide in near-infrared and ultraviolet femtosecond laser fields

Ionization of polyatomic molecules in strong femtosecond laser fields is a pivotal process that may induce a variety of highly nonlinear molecular physical phenomena. We performed an experimental study on photoionization and dissociation of vinyl bromide molecules in 800 nm or 400 nm femtosecond strong laser fields using a time-of-flight mass spectrometer. The dependence of the ion yields on laser intensity and polarization was investigated, and the results are compared in different wavelength laser fields. The kinetic energy releases from Coulomb explosion of vinyl bromide, and the angular distributions of different fragment ions were obtained. The mechanism of strong-field ionization/dissociation of vinyl bromide in different laser fields is discussed based on the experimental results.

Angle-Resolved Time-of-Flight Electron Spectrometer Designed for Femtosecond Laser-Assisted Electron Scattering and Diffraction

We developed an apparatus for measuring kinetic energy and two-dimensional angular distributions of femtosecond laser-assisted electron scattering (LAES) signals with a high detection efficiency, consisting of a photocathode-type ultrashort pulsed electron gun, a gas injection nozzle, an angle-resolved time-of-flight analyzer, and a time-and-position sensitive electron detector. We also established an analysis method for obtaining the kinetic energy and two-dimensional angular distributions of scattered electrons from raw data of their flight times and the detected positions at the detector recorded using the newly developed apparatus. From the measurement of the LAES processes of Ar atoms in a femtosecond near-infrared intense laser field, we obtained a two-dimensional angular distribution image of the LAES signals and showed that the detection efficiency of the LAES signals was raised by a factor of 40 compared with that achieved before in 2010.

Photoionization and Photofragmentation Dynamics of I2 in Intense Laser Fields: A Velocity-Map Imaging Study.

The photoionization and photofragmentation dynamics of I2 in intense femtosecond near-infrared laser fields were studied using velocity-map imaging of cations, electrons, and anions. A series of photofragmentation pathways originating from different cationic electronic states were observed following single ionization, leading to I+ fragments with distinct kinetic energies, which could not be resolved in previous studies. Photoelectron spectra indicate that these high-lying dissociative states are primarily produced through nonresonant ionization from several molecular orbitals (MO) of the neutral. The photoelectron spectra also show clear signatures of resonant ionization pathways (Freeman resonances) to low-lying bound ionic states via Rydberg states of the neutral moiety. To investigate the role of these Rydberg states further, we imaged anionic products (I-) formed through ion-pair dissociations of neutral molecules excited to these Rydberg states by the intense femtosecond laser pulse. Collectively, these results shed significant new light on the complex dynamics of I2 molecules in intense laser fields and on the important role of neutral Rydberg states in a full description of strong-field phenomena in molecules.

Ionization and ultrafast hydrogen migration of methylamine in few-cycle intense near-infrared laser fields

Double ionization and ultrafast hydrogen migration of methylamine induced by a few-cycle intense near-infrared laser pulse are investigated by the coincidence momentum imaging method. By reference to the results obtained by DFT calculations, the peak profiles in the kinetic energy release distributions have been assigned to the singlet and triplet decomposition channels of CH3NH22+. The anisotropy in the angular distribution of the ejection direction of the fragment ions is interpreted based on the symmetries of the (7a’) HOMO orbital. By the pump–probe measurements, it is revealed that the ultrafast hydrogen migration in CH3NH2+ proceeds within ∼ 30 fs.

Facile Synthesis of Fe3O4@Au/PPy-DOX Nanoplatform with Enhanced Glutathione Depletion and Controllable Drug Delivery for Enhanced Cancer Therapeutic Efficacy

The complex physiological environment and inherent self-healing function of tumors make it difficult to eliminate malignant tumors by single therapy. In order to enhance the efficacy of antitumor therapy, it is significant and challenging to realize multi-mode combination therapy by utilizing/improving the adverse factors of the tumor microenvironment (TME). In this study, a novel Fe3O4@Au/PPy nanoplatform loaded with a chemotherapy drug (DOX) and responsive to TME, near-infrared (NIR) laser and magnetic field was designed for the combination enhancement of eliminating the tumor. The Fe2+ released at the low pH in TME can react with endogenous H2O2 to induce toxic hydroxyl radicals (·OH) for chemodynamic therapy (CDT). At the same time, the generated Fe3+ could deplete overexpressed glutathione (GSH) at the tumor site to prevent reactive oxygen species (ROS) from being restored while producing Fe2+ for CDT. The designed Fe3O4@Au/PPy nanoplatform had high photothermal (PT) conversion efficiency and photodynamic therapy (PDT) performance under NIR light excitation, which can promote CDT efficiency and produce more toxic ROS. To maximize the cancer-killing efficiency, the nanoplatform can be successfully loaded with the chemotherapeutic drug DOX, which can be efficiently released under NIR excitation and induction of slight acidity at the tumor site. In addition, the nanoplatform also possessed high saturation magnetization (20 emu/g), indicating a potential magnetic targeting function. In vivo and in vitro results identified that the Fe3O4@Au/PPy-DOX nanoplatform had good biocompatibility and magnetic-targeted synergetic CDT/PDT/PTT/chemotherapy antitumor effects, which were much better than those of the corresponding mono/bi/tri-therapies. This work provides a new approach for designing intelligent TME-mediated nanoplatforms for synergistically enhancing tumor therapy.

Dissociative Ionization and Coulomb Explosion of CHBrCl2 in Intense Near-Infrared Femtosecond Laser Fields

We experimentally demonstrate the dissociative photoionization of CHBrCl2 molecules in a femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. The results suggest that the low kinetic energy components are from the dissociative ionization process of single-charged molecular ions. The angular distribution of fragment Cl+ ions can be attributed to the features of dissociative state and molecular configuration, and that of Br+ ions results from the electronic wave-packet evolution and combination of the multi-dissociation processes. The high kinetic energy components are from the Coulomb explosion of multi-charged molecular ions, and the error of the C-Br distance involved in the Coulomb explosion can be explained by the movement of the effective charge center of the polyatomic molecule.

Direct In Situ Measurement of an Ultrashort Pulse Using an Optical Hologram

The dipole response of helium exposed to attosecond extreme ultraviolet (EUV) pulses and a femtosecond near-infrared (NIR) laser field is investigated. The optical interference between the EUV pulse and nonlinear wave-mixing signals leads to fringes in the transmitted spectrum above the single ionization threshold. Perturbation theory suggests that such fringes record the self-convolution of complex-valued NIR pulse in the frequency domain, allowing direct characterization of a broadband optical pulse without the need of reconstruction algorithms. We apply this in situ interferometric scheme to diagnose a few-cycle laser pulse.

Generation of isolated attosecond pulses from atoms driven by optimized two near-infrared pulses and their second harmonic fields

With the rapid development of laser technology, it is possible to control optical waveforms by coherent superposition of electric fields with multiple color components, which creates conditions for generating the ultra-short isolated attosecond pulses (IAP). Based on the strong-field approximation theory, this work focuses on the IAP generated by the optimized multicolor field synthesized by two fundamental near-infrared lasers and their second harmonic fields. The results show that by applying frequency-doubled pulses to the near-infrared laser fields and optimizing the laser parameters, the emission properties of high order harmonics from single atom can be greatly improved, and the nearly attochirp-free harmonic emission can be realized within a certain energy range. As a result, shorter IAPs are obtained. With the consideration of the macroscopic propagation effect of gas, the IAP with a pulse width up to 40 as is generated under appropriate experimental conditions. Finally, the effects of gas pressure on the properties of the high-order harmonic and attosecond pulses are also investigated. This study provides useful theoretical guidance for generating ultra-short IAPs with near-infrared laser pulses in experiment.

Observation of laser-assisted electron-impact ionization in ultrashort intense laser fields

A laser-assisted ($e, 2e$) [LA($e, 2e$)] process of Ar in an ultrashort (1.2 ps) near-infrared intense laser field, $\mathrm{Ar}+{e}^{\ensuremath{-}}+h\ensuremath{\nu}\ensuremath{\rightarrow}{\mathrm{Ar}}^{+}+{e}^{\ensuremath{-}}+{e}^{\ensuremath{-}}$, was observed by coincidence measurements of the fast electron scattered forward within the small-angle range and the slow electron emitted to the wide-angle range. The spectrum of the sum of the kinetic energies of fast and slow electrons exhibited a shoulder structure at the one-photon energy shift from the ($e, 2e$) peak for the Ar $3p$ orbital, which was unambiguously assigned to the LA($e, 2e$) process with the one-photon energy gain. The determined triply differential cross section of the LA($e, 2e$) process with one-photon gain from the laser field was approximately twice as large as a theoretical estimate and this enhancement was regarded as an evidence of the formation of light-dressed states of Ar.

High-lying Rydberg atoms surviving intense laser fields

In this work, we have experimentally and theoretically investigated the high-lying Rydberg state excitation for noble gas atoms subject to an intense near-infrared laser field. To obtain the signal of these high-lying Rydberg atoms which are further ionized by a constant electric field $({F}_{c})$, coincident detection of the photoelectrons and photoions with well-chosen arrival times is performed. Based on a fitting to the time dependence of the coincidently measured photofragment yields with a semiempirical formula, we can extract a principal quantum number distribution (PQND) of the population of the excited states in a range closely related to the strength of ${F}_{c}$. The extracted valid PQND is in qualitative agreement with a semiclassical model calculation. Our work thus provides a method to extract the PQND and study the ultrafast high-lying Rydberg state excitation.

Single-shot imaging of surface molecular ionization in nanosystems

Abstract Using single-shot velocity map imaging technique, explosion imaging of different ion species ejected from 50 nm SiO2 nanoparticles are obtained excitedly by strong near-infrared and ultraviolet femtosecond laser fields. Characteristic momentum distributions showing forward emission of the ions at low excitation intensities and shock wave behaviors at high intensities are observed. When the excitation intensity is close to the dissociative ionization threshold of the surface molecules, the resulting ion products can be used to image the instant near-field distributions. The underlying dynamics of shock formation are simulated by using a Coulomb explosion model. Our results allow one to distinguish the ultrafast strong-field response of various molecular species in nanosystems and will open a new way for further exploration of the underlying dynamics of laser-and-nanoparticle interactions.

Exploring recollision of ultrafast electrons from photoelectron momentum distributions using single-cycle near-infrared laser pulses

The electron scattering process has been investigated by analyzing the interference structure in the photoelectron momentum distribution (PMD) of a hydrogen atom exposed to a single-cycle linearly polarized near-infrared laser field, based on the numerical solution of the full-dimensional time-dependent Schr\"odinger equation and the Coulomb correlative classical trajectory simulation. The interference pattern in the PMD is closely related to the form of the ultrashort pulse which is dominated by the carrier-envelope phase. A fish-bone-like pattern appears in the PMD using the sine electric field and a spider-like pattern appears using the cosine electric field. These interference structures reflect the scattering process. It is found that the stripe density of the spider-like pattern is mainly dominated by the recollision time of scattering electron trajectories, i.e., the longer the recollision time, the greater the stripe density. Therefore, the photoelectron interference pattern can be used to understand the ionization and scattering processes, and identify these processes on the attosecond time scale.

Two-color symmetry breaking in laser-based secondary neutral mass spectrometry.

An alternative method of analysis for laser-based secondary neutral mass spectrometry is introduced. By mixing a fundamental 1240 nm wavelength of the near-infrared laser field with an intensity of 1 × 1015 W/cm2 with its second harmonic generation wavelength of 620 nm, control is attained over the ion yield in secondary neutral mass spectrometry for a native silver surface and a surface covered with silver nanoparticles. The ion yields of selected species in the mass spectra, such as a C2 H4 O2 surfactant fragment ion and the Ag ion, are enhanced or suppressed by a factor of 8 or 23, respectively, in comparison with single color post-ionization. In addition, a significant suppression of ion fragmentation is demonstrated for both the silver sample and silver nanoparticles. Periodic variation of the ion yield with a modulation depth of 4% is observed depending upon the relative phase between the fundamental and second harmonic generated fields. These variations are explained in terms of tunneling ionization with symmetry-broken fields. An additional degree of freedom in a form of additional second color is introduced in laser-based secondary neutral mass spectrometry. Ion yield control is extended in a form of symmetry-broken fields.

Retrieving intracycle interference in angle-resolved laser-assisted photoemission from argon

We report on a combined experimental and theoretical study of XUV ionization of atomic argon in the presence of a near-infrared (NIR) laser field. Using a table-top source of wavelength-selected femtosecond XUV pulses in combination with a velocity map imaging spectrometer we record angle- and energy-resolved photoelectron distributions and simulate the experimental data by solving the time-dependent Schrödinger equation ab initio. In order to compare with the experimental data we average the calculated energy-angle probability distributions over the experimental focal volume for different values of the magnetic quantum number of the photoelectron. This averaging procedure washes out the intracycle interference pattern, which would otherwise be observed in the form of angular modulations of the photoelectron spectra. We recover these modulations experimentally and in the simulations by evaluating the difference between two averaged distributions that are obtained for slightly different NIR laser field intensities.

Multi-ionization of the Cl2 molecule in the near-infrared femtosecond laser field.

The multi-electron ionization and subsequent dissociation of the Cl2 molecule in a near-infrared femtosecond laser field was investigated via the dc-sliced ion imaging technique. The single charged molecular ions, Cl2+, dissociate from two excited states, 2Πu and 2Σg+, with the electrons ionized from the HOMO−1 and HOMO−2 orbital, respectively. For the multi-charged molecular ions, Cl2n+ (n = 2–8), our results showed that the stretch of the inter-nuclear distance benefitted the ionization of the electrons to produce highly-charged molecular ions. In addition, compared with the traditional charge resonance enhanced ionization (CREI) model, the critical distance (Rc) for the Cl2 molecule in our experiment was a short range that depended on the charge state rather than a single point.

Laser-induced deformation of an inner valence molecular orbital in ethanol

SynopsisWe reveal that an intense laser field deforms an inner valence orbital (10a′) in the ethanol molecule. Using photoelectron-photoion coincidence momentum imaging, we measure the recoil-frame photoelectron angular distribution (RFPAD) for the CD2OH+ production channel of CH3CD2OH induced by a circularly polarized intense (I 0 ≈ 8×1013 W/cm2) near-infrared (λ ≈ 795 nm) laser field. Ab initio simulations show that the orbital deformation depends strongly on the laser field direction, and that the measured RFPAD cannot be reproduced without taking the orbital deformation into account.

Direct laser ion acceleration and above-threshold ionization at intensities from 1021 W/cm2 to 3×1023 W/cm2

Calculations on the dynamics of ions and electrons in near-infrared laser fields at intensities up to $3 \times 10^{23}$ W/cm$^2$ are presented. We explore the acceleration of ions in a laser focus by conservation of canonical momentum during ionization events and by the ponderomotive force in the f/1 focal geometry required to reach such intensity. At intensity exceeding 10$^{23}$ W/cm$^2$, highly charged ions are expelled from the laser focus before they can interact with the laser pulse at peak intensity, decreasing the predicted ionization yields of deeply-bound states. We consider the interaction of a tightly-focused, f/1 laser pulse with krypton at an intensity of $3 \times 10^{23}$ W/cm$^{2}$ and a pulse duration of 140 fs. We find that the ions and electrons are accelerated to energies in excess of 2 MeV/nucleon and 1.4 GeV, respectively. Ponderomotive expulsion of the parent ions decreases the total number of ultra-relativistic ATI electrons produced by tunneling ionization from the K-shell states of krypton but does not change their energy spectrum.

Pulse-length effects in strong-field ionization of atoms by co-rotating and counter-rotating bicircular laser pulses

We study strong-field ionization of H and selected rare-gas atoms by solving the three-dimensional time-dependent Schr\odinger equation in the single-active-electron approximation with particular attention to the effect of pulse length on the total ionization yields (TIYs) produced by counter-rotating bicircular (CRBC) lasers compared to co-rotating bicircular (COBC) lasers. To this end, TIYs, photoelectron momentum distributions, and above-threshold ionization spectra are presented for various pulse durations and peak intensities. For pulses containing five or more cycles of the fundamental near-infrared laser field, the CRBC/COBC ratio of TIYs is affected due to ionization via excited states and multiphoton ionization channels. For pulses containing less than five cycles, the CRBC/COBC ratio of TIYs depends on both the frequency combination and number of optical cycles: With the $1\ensuremath{\omega}\text{\ensuremath{-}}2\ensuremath{\omega}$ frequency combination, the ratio of TIYs is enhanced (reduced) for pulses with an odd (even) number of optical cycles. The $1\ensuremath{\omega}\text{\ensuremath{-}}3\ensuremath{\omega}$ frequency combination was considered for pulses containing two or three optical cycles, and for both pulse lengths the CRBC/COBC ratio of TIYs is less than unity.