Ultrafast Laser Pulses Research Articles

Hybrid thermo-physical modelling and experimentation of ultrafast laser-based fabrication of polycrystalline core–amorphous shell nitinol nanoparticles

Ultrafast lasers pulses induced ablation of nickel-titanium (NiTi) alloy substrates immersed in deionized water yielded a high-throughput of polycrystalline core - amorphous shell NiTi nanoparticles (NPs) owing to the high pulse energy of femtosecond (fs) lasers. In this study, the alloy NPs were fabricated by varying the laser fluences from 15 to 45 J/cm2. The polycrystalline core - amorphous shell NiTi were confirmed by selected area diffraction patterns via High resolution – Transmission Electron microscopy. Theoretical investigations were carried out via hybrid simulation approach of molecular dynamics simulation and analytical modelling. Molecular dynamics study revealed the spatial and temporal evolution of electron and lattice temperature of NiTi melt pool due to ablation at various laser fluences. The subsequent temporal dynamics of melt-pool temperature along the depth revealed the phase separation via spinodal decomposition process after 40 picoseconds post irradiation of NiTi with ultrafast laser pulse. The analytical modelling incorporating phase explosion theory estimated the thermo-physical expansion of ablated chunks and subsequent fragmentation of NiTi melt pool in liquid–vapor co-existing phase. The parameters of mathematical modelling were evaluated in-conjunction with the experimental results of size distribution of NPs at the laser fluence of 15 J/cm2. Later, the hybrid mathematical model was employed to predict the size of NiTi NPs ablated at higher laser fluences of 30 J/cm2 and 45 J/cm2 and the results were experimentally verified.

Response of the mechanical and chiral character of ethane to ultra-fast laser pulses.

A pair of simulated left and right circularly polarized ultra-fast laser pulses of duration 20 femtoseconds that induce a mixture of excited states are applied to ethane. The response of the electron dynamics is investigated within the next generation quantum theory of atoms in molecules (NG-QTAIM) using third-generation eigenvector-trajectories which are introduced in this work. This enables an analysis of the mechanical and chiral properties of the electron dynamics of ethane without needing to subject the C-C bond to external torsions as was the case for second-generation eigenvector-trajectories. The mechanical properties, in particular, the bond-flexing and bond-torsion were found to increase depending on the plane of the applied laser pulses. The bond-flexing and bond-torsion, depending on the plane of polarization, increases or decreases after the laser pulses are switched off. This is explainable in terms of directionally-dependent effects of the long-lasting superpositions of excited states. The chiral properties correspond to the ethane molecule being classified as formally achiral consistent with previous NG-QTAIM investigations. Future planned investigations using ultra-fast circularly polarized lasers are briefly discussed.

Low Threshold and Robust Ultrafast Pulses from Freestanding‐Growth 2D Quaternary BiCuSeO

AbstractSaturable absorber is a key component in ultrafast pulsed laser systems, which are pivotal in ultrafast physics, high‐speed communication, etc. 2D materials appear to be superior candidates as saturable absorbers for compact pulsed fiber lasers in recent years due to their wide band response and ease of integration. However, the integration of mode‐locked devices inevitably involves contamination or polymer in the interface between the layered saturable absorber and the fiber, which contributes to a high start‐up threshold for the ultrafast pulsed lasers. This study develops an on‐fiber femtosecond laser with 2D narrow‐gap semiconducting BiCuSeO as a saturable absorber by a fully dry integration process thanks to the controllable growth of freestanding quaternary BiCuSeO nanoflakes. The corresponding thermodynamic growth mechanism is systemically revealed by electron microscopy and bind‐energy calculation. An electrostatic‐force‐assisted transfer method is developed to avoid damage or chemical contamination. The BiCuSeO‐based femtosecond laser shows decent output performance (pulse width, signal‐noise ratio and repetition rate are 395 fs, 61 dB, 17.1 MHz) with an ultra‐low threshold of 30 mW. The strategy of controllable growth of free‐standing 2D material facilitates a clean interface between the saturable absorber and fibers, which is favorable for high‐performance ultrafast photonic devices.

Sodium-doped InP/ZnSeS/ZnS quantum dots as a saturable absorber for passive Q-switched fiber lasers

In this work, sodium-doped InP/ZnSeS/ZnS (InP–Na) QDs was fabricated by thermal injection method for the first time. The nonlinear optical properties were first demonstrated by realizing a modulation depth of 7.85% and saturable intensity of 2.2 MW cm−2 by two-detector method. The Q-switching operations were carried out by integrating InP–Na QDs as saturable absorber (SA) in a fiber laser ring cavity system. Q-switched laser pulses with a pulse width of 139 ns were generated. The ability of effective pulse width compression can be attributed to the short exciton recovery time of SA, as confirmed by transient absorption and time-resolved photoluminescence. This is the first demonstration of Na-doped InP/ZnSeS/ZnS QDs and using it as SA for generating ultrafast laser pulses. Therefore, this study extends the research scope of InP-based QDs in optical communications, nonlinear photonics and ultrafast photonics.

On-target delivery of intense ultrafast laser pulses through hollow-core anti-resonant fibers.

We report the flexible on-target delivery of 800 nm wavelength, 5 GW peak power, 40 fs duration laser pulses through an evacuated and tightly coiled 10 m long hollow-core nested anti-resonant fiber by positively chirping the input pulses to compensate for the anomalous dispersion of the fiber. Near-transform-limited output pulses with high beam quality and a guided peak intensity of 3 PW/cm2 were achieved by suppressing plasma effects in the residual gas by pre-pumping the fiber with laser pulses after evacuation. This appears to cause a long-term removal of molecules from the fiber core. Identifying the fluence at the fiber core-wall interface as the damage origin, we scaled the coupled energy to 2.1 mJ using a short piece of larger-core fiber to obtain 20 GW at the fiber output. This scheme can pave the way towards the integration of anti-resonant fibers in mJ-level nonlinear optical experiments and laser-source development.

Ultrafast Laser Pulse Induced Transient Ferrimagnetic State and Spin Relaxation Dynamics in Two-Dimensional Antiferromagnets.

We employ real-time time-dependent density functional theory (rt-TDDFT) and ab initio nonadiabatic molecular dynamics (NAMD) to systematically investigate the ultrafast laser pulses induced spin transfer and relaxation dynamics of two-dimensional (2D) antiferromagnetic-ferromagnetic (AFM/FM) MnPS3/MnSe2 van der Waals heterostructures. We demonstrate that laser pulses can induce a ferrimagnetic (FiM) state in the AFM MnPS3 layer within tens of femtoseconds and maintain it for subpicosecond time scale before reverting to the AFM state. We identify the mechanism in which the asymmetric optical intersite spin transfer (OISTR) effect occurring within the sublattices of the AFM and FM layers drives the interlayer spin-selective charge transfer, leading to the transition from AFM to FiM state. Furthermore, the unequal electron-phonon coupling of spin-up and spin-down channels of AFM spin sublattice causes an inequivalent spin relaxation, in turn extending the time scale of the FiM state. These findings are essential for designing novel optical-driven ultrafast 2D magnetic switches.

Progress and Prospects in Optical Ultrafast Microscopy in the Visible Spectral Region: Transient Absorption and Two-Dimensional Microscopy.

Ultrafast optical microscopy, generally employed by incorporating ultrafast laser pulses into microscopes, can provide spatially resolved mechanistic insight into scientific problems ranging from hot carrier dynamics to biological imaging. This Review discusses the progress in different ultrafast microscopy techniques, with a focus on transient absorption and two-dimensional microscopy. We review the underlying principles of these techniques and discuss their respective advantages and applicability to different scientific questions. We also examine in detail how instrument parameters such as sensitivity, laser power, and temporal and spatial resolution must be addressed. Finally, we comment on future developments and emerging opportunities in the field of ultrafast microscopy.

Nitrogen-doped graphene quantum dots for femtosecond ultrafast pulsed fiber lasers

Nitrogen-doped graphene quantum dots (N-GQDs) are a new class of quantum dot with unique electronic properties, whose electrons transport and biochemical properties have been extensively studied. However, the optical properties of N-GQDs, especially their nonlinear optical response and applications in ultrafast photonics and photoelectronic, need to be further explored. The nonlinear optical response and ultrafast carrier dynamics of N-GQDs in an aqueous solution are systematically investigated in this work. N-GQDs have an ultrafast carrier cooling delay of about 143 fs in the visible range. In the study of the nonlinear optical response of N-GQDs, the saturable absorption is transformed into reverse saturable absorption as the incident pulse intensity increases, which makes N-GQDs an effective saturable absorber for ultrashort pulse generation in mode-locked Er-doped fiber lasers. Our results may lead to further investigations into the optical properties of N-GQDs and demonstrate potential opportunities for ultrafast photonic applications.

Phthalocyanine‐Based 2D Polymer for Ultrafast Nonlinear Optical Application

AbstractStrong ultrafast nonlinear optical (NLO) responses in near‐infrared (NIR) regions and favorable solution processability are two urgent requirements for practical NLO applications, especially for optical limiting (OL) applications. However, simultaneously optimizing these properties remains significant challenging. In this study, the successful synthesis of a phthalocyanine‐based 2D polymer (Pc‐2DP) is reported; its structure features extended π‐electrons delocalization space constructed by topologically integrating the phthalocyanine units into the 2D framework. The as‐prepared few‐layered Pc‐2DP not only exhibits power‐ and wavelength‐dependent NLO properties toward 35 fs ultrafast pulsed laser, but it also demonstrates excellent ultrafast reverse saturable absorption (RSA) responses in NIR regions. The nonlinear absorption coefficients of Pc‐2DP dispersion reach up to 0.12 and 0.19 cm GW−1 at 800 and 1550 nm, respectively, which are much higher than those of the typical MXene (Ti3C2TX) and molybdenum disulfide with hexagonal structure (2H‐MoS2). The obtained NLO devices exhibit remarkable OL performance at 800 nm and, particularly, in the telecommunication region (1300 to 1550 nm), which is highly desired but seldom reported. The study demonstrates that the as‐prepared Pc‐2DP featuring a highly conjugated aromatic framework is a very promising OL material and that this research provides a new strategy for developing high‐performance 2D organic NLO polymers.

Single-pulse, reference-free, spatiotemporal characterization of ultrafast laser pulse beams via broadband ptychography.

Ultrafast laser pulse beams are four-dimensional, space-time phenomena that can exhibit complicated, coupled spatial and temporal profiles. Tailoring the spatiotemporal profile of an ultrafast pulse beam is necessary to optimize the focused intensity and to engineer exotic spatiotemporally shaped pulse beams. Here we demonstrate a single-pulse, reference-free spatiotemporal characterization technique based on two colocated synchronized measurements: (1) broadband single-shot ptychography and (2) single-shot frequency resolved optical gating. We apply the technique to measure the nonlinear propagation of an ultrafast pulse beam through a fused silica window. Our spatiotemporal characterization method represents a major contribution to the growing field of spatiotemporally engineered ultrafast laser pulse beams.

MP-CITDSE: A set of ab-initio programs for the simulation of hydrogenic and helium-like atom-laser interactions

MP-CITDSE is a package of programs which solves the time-dependent Schrödinger equation for hydrogenic and helium-like atomic systems interacting with an ultra-short laser pulse (of attosecond or femtosecond duration). The output of the computations – with some minimal processing – may be used to calculate excited populations, single and double ionisation yields, kinetic, angular and radial electron distributions, and harmonic yields. For the Helium atom, a full account of the inter-electronic correlation effects is included via a configuration interaction approach; for the time propagation of the wavefunction a spectral basis expansion on the eigenstates of the field-free Hamiltonian is used; for this reason post-processing of the expansion coefficients just after the pulse lead to simple formulas for quantities of experimental interest. Program summaryProgram Title: MP-CITDSECPC Library link to program files:https://doi.org/10.17632/gv4zxmfdx3.1Developer's repository link:https://github.com/aforembs/MP-CITDSELicensing provisions: GPLv3Programming language: C++Nature of problem: The ultra-fast high-intensity laser pulses of modern experimental sources require an ab-initio theoretical treatment, as the characteristics of such pulses are generally incompatible with the established Lowest-order Perturbation Theory (LOPT) approximation. Such a treatment can be quite cumbersome to not only implement, but also describe in a clear, intuitive manner. Its inherent complexity ensures that even in its simplest form, an ab-initio TDSE solver can easily consist of several thousand lines of code. As with any project of such scope a significant amount of effort is required to keep the code up to date, running on several platforms, compilers and operating systems. As such, while the numerical problem tackled by MP-CITDSE is that of the simulation of hydrogenic and helium-like atom-laser interactions on a single node computer/workstation; its main challenges are longevity and expandability. By doing away with unnecessary abstractions within the code, using a modern standard of a widely used and performant language (C++ 17) and hosting the source code on a publicly available GitHub repository we hope that the programs provided herein prove themselves useful to many current and future AMO Physicists.Solution method: The atomic systems are considered confined in a spherical box of finite size. In the one-electron case (hydrogen) the eigenstates are calculated in a spherical coordinate system by the direct diagonalisation of the field-free Hamiltonian matrix representation on a B-splines piecewise polynomial basis expansion of the radial part of the eigenfunctions; for the two-electron case (helium) the numerically calculated one-electron eigenstates are angularly coupled to form a zero-order (uncorrelated) two-electron numerical basis; then the latter basis is further coupled via a configuration-interaction approach to eventually calculate the helium's two-electron eigenstates.Following these steps, for both systems (hydrogen and helium) the computation proceeds by expanding the time-dependent wavefunction on the corresponding field-free eigenstate basis; thus, we end up to a system of first-order ordinary differential equations (ODEs) in time for the expansion (time-dependent) coefficients; the dynamical parameters for the ODEs time propagation, namely the eigenenergies and dipole matrix elements are calculated only once prior to the ODEs propagation [2]. The structure of the propagating matrix is block tridiagonal.Additional comments including restrictions and unusual features: This paper serves as the definitive reference for the MP-CITDSE code. In this version we solve the ODEs using a Runge-Kutta-Felnberg (RKF) algorithm. Also we have restricted the code to treat hydrogen and helium but with very little effort (non-relativistic) hydrogenic and helium-like atomic systems can also be simulated. The programs are supplied with two methods of compilation, via make and cmake to ensure greater portability.

Peculiarities of measuring fluorescence decay times by a streak camera for ps-TALIF experiments in reactive plasmas

We present a detailed methodology for (i) correctly configuring a streak camera to capture raw picosecond two-photon absorption laser induced fluorescence signals (ps-TALIF) of H-atom in low- and atmospheric-pressure plasmas, and (ii) properly processing the recorded raw experimental data with a dedicated mathematical signal processing method to infer actual ps-TALIF signals of H-atom. The goal is the accurate determination of the decay time of the recorded ps-TALIF signals of H-atom. A ps-laser is used to excite atomic hydrogen produced in both plasmas and the raw fluorescence signals are detected by the streak camera using different time windows/ranges (TR). It is shown that the choice of the TR affects the shapes and the decay times of the recorded raw TALIF signals. This is defined as the instrumental function of the streak camera and has a Gaussian profile as determined by recording the ultrafast laser pulse at different TR. To remove this instrumental distortion and extract the actual shape of the TALIF signals, the captured raw TALIF signals were fitted using the mathematical procedure developed in this study, which involved an exponentially modified Gaussian function. The application of our methodology leads to more reliable measurements of hydrogen atoms decay times after respecting the following acquisition conditions: (i) the TR of the streak camera should be sufficiently large to capture the complete (raw) TALIF signal, and (ii) the time width of the instrumental function of the streak camera should be as small as possible compared to the actual decay time of the fluorescence, while ensuring an optimal signal-to-noise ratio. This work demonstrates the remarkable potential of the combination of ps-TALIF and streak cameras in state-of-the-art optical plasma diagnostics.

Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.

Assessing and Quantifying Thermodynamically Concomitant Degradation during Oxygen Evolution from Water on SrTiO3

The oxygen evolution reaction (OER) from water, while more stable on transition metal oxide surfaces than others, has nonetheless proved to be concomitant with charge-induced surface degradation. Since heterogeneous and nanostructured electrodes are often used and with a large excitation area, the degradation can be difficult to quantify. Here, we utilize single crystalline SrTiO3, highly efficient photoexcitation of the OER, and a focused laser to spatially define the degradation. A repetitive, ultrafast laser pulse above the band gap energy is employed, which allows for highly varied exposure of the surface using different scan methods. It also connects the work to the OER and its time-resolved mechanisms. By characterizing the degradation using optical spectroscopy and electron microscopy, the material dissolution constitutes an upper bound of 6% of the charge passed in a pH 13 electrolyte, while for pH 7, it reaches 23%; the pH dependence is anticorrelated with the ultrafast population of trapped charge. Although a minority component, the remarkable consistency of the 6% upper bound in the pH 13 electrolyte across a large range of linearly increasing degradation volumes and changing electrode composition defines a dominant lattice dissolution reaction as thermodynamically concomitant with the OER. Along with the pH dependence, the elemental composition of the degraded layer quantified by energy-dispersive and photoelectron and absorption X-ray spectroscopy suggests the relevance of certain chemical cation redeposition reactions. Altogether, using spatially and temporally defined photoexcitation of a crystalline surface provides a means to quantify semiconducting transition metal oxide degradation during the OER and constricts its mechanisms.

Intraoperative glioma characterization using time-resolved fluorescence spectroscopy.

2072 Background: Extent of surgical resection significantly improves progression-free and overall survival in glioma patients. Due to the infiltrative nature of gliomas, tumor margin detection is often difficult, and current intraoperative visualization methods are limited. Time-resolved fluorescence spectroscopy (TRFS) has the potential of differentiating glioma from normal brain tissue, thereby maximizing extent and safety of resection. Methods: Twenty-seven preoperative patients diagnosed with glioma met inclusion criteria for an IRB-approved study aimed to assess the accuracy of TRFS relative to tissue histopathology for differentiating glioma from normal cortex (NC), normal white matter (NWM), and white matter with edema (WME). A multi-channel TRFS system was developed for in vivo brain tissue and tumor characterization during glioma surgery. A custom-designed fiber optic probe was used to deliver ultrafast laser pulses to tissue regions of interest and collect fluorescence signals, with simultaneous registration to intraoperative neuronavigation. The fluorescence light was collected using a photomultiplier tube and digitized across six wavelength bands. Biopsies were taken at each measurement site for evaluation by a blinded neuropathologist to determine the accuracy of TRFS, which was calculated using parameters derived from the recorded fluorescence pulse and used to characterize tissue signatures. Results: During surgical resection of 27 patients, 162TRFS measurements were collected in vivo followed by biopsy of the same tissue. Samples from brain tumors (39 low grade gliomas (LGG, WHO grade I to III), 35 high grade gliomas (HGG, WHO grade IV)) were compared with 47 samples of normal brain tissue (NC and NWM). The time-resolved system was able to differentiate LGG from normal tissue with 97% sensitivity and 93% specificity (93% PPV and 98% NPV). When all tumor grades were tested, the sensitivity dropped to 91% (96% PPV and 86% NPV), which we believe reflects the heterogeneity of HGG. HGG alone showed a sensitivity of 83% and specificity of 93% (91% PPV and 88% NPV). Normal cortex had the fastest decays across all wavelength bands among different tissue types, WME had longer decays compared to other tissue types at all wavelengths, and HGG had a variable but predictably different fluorescence decay pattern. Conclusions: TRFS is an intraoperative tool that can distinguish glioma from normal brain tissue and has the potential to maximize safe surgical resection.

Toward strong nonlinear optical absorption properties of perovskite films via porphyrin axial passivation

Defects within perovskite layers are considered to be a primary factor that inhibits nonlinear optical (NLO) performance, and mitigating or eliminating them is indispensable for the implementation of high-performance perovskite-based photonic and energy devices. Herein, we propose a novel defect modulation strategy through binding the functional groups at the axial positions of porphyrins with the defects of perovskites. Two axially-coordinated porphyrin molecules (TiOPr and SnOHPr) were utilized as effective functional additives to passivate the defects and control perovskite crystallization, while the role of axial functional groups of porphyrins has been systematically studied. Z-scan measurement results demonstrate that porphyrin-treated perovskite films exhibit prominently enhanced optical absorption nonlinearities under ultrafast femtosecond (fs) laser excitation at 800 nm in comparison to the MAPbI3 perovskite film, the NLO absorption coefficient (β) values for the modified films are approximately one or two orders of magnitudes superior to that of the pristine film. This significant improvement in NLO performance likely stems from the increased photoinduced ground-state dipole moment of the perovskite film treated by porphyrin during the process of two-photon absorption (TPA), and their photoinduced charge/energy transfer between perovskite and porphyrin. Particularly, the MAPbI3/SnOHPr film displays the largest β values (636.92–6621.42 cm GW−1) among all measured samples at different irradiation energies and a low optical limiting threshold of 5 mJ cm−2, which may profit from the dual-functional passivation effect of SnOHPr onto perovskite, suggesting its great potential as optical limiters. These observations strongly indicate that porphyrin-axial passivated perovskite films are outstanding material candidates for optical limiting applications towards ultrafast fs laser pulses in the near-infrared region. Our work affords a feasible paradigm for constructing high-performance NLO perovskite materials through porphyrin-axial passivation of defects.

Coherent generation and control of tunable narrowband THz radiation from a laser-induced air-plasma filament.

We report on the proof-of-principle experiment of generating carrier-envelope phase (CEP)-controllable and frequency-tunable narrowband terahertz (THz) radiation from an air-plasma filament prescribed by the beat of a temporally stretched two-color laser pulse sequence. The pulse sequence was prepared by propagating the fundamental ultrafast laser pulse through a grating stretcher and Michelson interferometer with variable inter-arm delay. By partially frequency-doubling and focusing the pulse sequence, an air-plasma filament riding a beat note was created to radiate a THz wave with primary pulse characteristics (center frequency and CEP) under coherent control. To reproduce experimental results and elucidate complex nonlinear light-matter interaction, numerical simulation has been performed. This work demonstrates the feasibility of generating coherently controlled narrowband THz wave with high tunability in laser-induced air plasma.

Dynamic surface stress field of the pure liquid-vapor interface subjected to the cyclic loads.

We demonstrate a methodology for computationally investigating the mechanical response of a pure molten lead surface system to the lateral mechanical cyclic loads and try to answer the following question: how does the dynamically driven liquid surface system follow the classical physics of the elastic-driven oscillation? The steady-state oscillation of the dynamic surface tension (or excess stress) under cyclic load, including the excitation of high-frequency vibration mode at different driving frequencies and amplitudes, was compared with the classical theory of a single-body driven damped oscillator. Under the highest studied frequency (50GHz) and amplitude (5%) of the load, the increase of in (mean value) dynamic surface tension could reach ∼5%. The peak and trough values of the instantaneous dynamic surface tension could reach (up to) 40% increase and (up to) 20% decrease compared to the equilibrium surface tension, respectively. The extracted generalized natural frequencies seem to be intimately related to the intrinsic timescales of the atomic temporal-spatial correlation functions of the liquids both in the bulk region and in the outermost surface layers. These insights uncovered could be helpful for quantitative manipulation of the liquid surface using ultrafast shockwaves or laser pulses.

Scoring molecular wires subject to an ultrafast laser pulse for molecular electronic devices.

A nonionizing ultrafast laser pulse of 20-fs duration with a peak amplitude electric-field ±E = 200 × 10-4 a.u. was simulated. It was applied to the ethene molecule to consider its effect on the electron dynamics, both during the application of the laser pulse and for up to 100 fs after the pulse was switched off. Four laser pulse frequencies ω = 0.2692, 0.2808, 0.2830, and 0.2900 a.u. were chosen to correspond to excitation energies mid-way between the (S1 ,S2 ), (S2 ,S3 ), (S3 ,S4 ) and (S4 ,S5 ) electronic states, respectively. Scalar quantum theory of atoms in molecules (QTAIM) was used to quantify the shifts of the C1C2 bond critical points (BCPs). Depending on the frequencies ω selected, the C1C2 BCP shifts were up to 5.8 times higher after the pulse was switched off compared with a static E-field with the same magnitude. Next generation QTAIM (NG-QTAIM) was used to visualize and quantify the directional chemical character. In particular, polarization effects and bond strengths, in the form of bond-rigidity vs. bond-flexibility, were found, for some laser pulse frequencies, to increase after the laser pulse was switched off. Our analysis demonstrates that NG-QTAIM, in partnership with ultrafast laser irradiation, is useful as a tool in the emerging field of ultrafast electron dynamics, which will be essential for the design, and control of molecular electronic devices.

Mapping extended reaction coordinates in photochemical dynamics

Laser-based experiments facilitate numerous strategies for interrogating the complex non-adiabatic dynamics operating in the excited states of molecules. Measurements may be broadly separated into frequency- and time-resolved variants, with a combination of different approaches (with different associated observables) typically being required to reveal a complete mechanistic picture. In the former category, quantum state-resolved information may often be obtained using narrow linewidth lasers. This provides detailed information relating to the starting point on the photochemical reaction coordinate (via the absorption spectrum) and the asymptotic endpoints (i.e. the photoproducts). Direct observation of the intermediate pathways connecting these two limits is often not possible, however, due to the inherently long temporal duration of the laser pulses relative to the typical timescales of non-adiabatic energy redistribution processes. It is therefore desirable to obtain complementary information that monitors real-time evolution along the reaction coordinate as excited state population traverses the potential energy landscape. This may be achieved in time-resolved pump–probe experiments conducted using ultrafast (i.e. sub-picosecond) laser pulses. The use of valence state photoionization for the probe step is a commonly employed methodology and has proved highly instructive in revealing subtle mechanistic details of key energy redistribution pathways operating in many different molecular systems. One frequent limitation here, however, is the restricted “view” along the reaction coordinate(s) connecting the initially prepared excited states to various photoproducts. Guided by examples drawn from recent work using time-resolved photoelectron imaging, this review will discuss such issues in detail and highlight some strategies that potentially help overcome them – with particular emphasis placed on the advantages of projecting as deeply as possible into the ionization continuum. The role of complementary measurements using other spectroscopic techniques and the importance of high-level supporting theory to guide data interpretation will also be reinforced.