Exciton binding energies of six polythiophene-derived polymers were studied through using a nonempirical, optimally tuned range-separated (RS) functional combining with the polarizable continuum model (PCM). We demonstrate that this approach predicts ionization energies (IE), electron affinities (EA), transport gaps, optical gaps, and exciton binding energies of six different polymer chains in both vacuum and solid (dielectric medium) with accuracy comparable to many-body perturbation theory within the GW approximation and Bethe–Salpeter equation (BSE). Furthermore, the behavior of exciton binding energy versus dielectric constant was also reasonably described by the PCM-tuned RS functional, whereas the conventional functionals such as PBE, B3LYP, M062X, and nontuned LC-ωPBE completely fail. We believe our method provides for a reliable and computationally efficient tool for future investigation of efficiency-enhancing mechanism and molecular design in organic solar cells.

Proposed in theory and then their existence confirmed, anion–π interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion–π interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl−, Br−, I−, linear thiocyanate SCN−, trigonal planar nitrate NO3−, pyramidic iodate IO3−, and tetrahedral sulfate SO42−). The binding energies of the resultant gaseous 1:1 complexes (1·Cl−, 1·Br−, 1·I−, 1·SCN−, 1·NO3−, 1·IO3− and 1·SO42−) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl−, NO3−, IO3− with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol−1, but only about 40% of that between 1 and SO42−. Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion–π binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion–π binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion–π interactions and has potential to provide quantitative guest–host molecular binding strengths and unravel fundamental insights in specific anion recognitions.

We theoretically demonstrate the effect of the intensity ratio of the two-color laser field on the terahertz generation based on a transient photocurrent model. We show that the terahertz generation depends on the intensity ratio of the two-color laser field at a given total laser intensity, and the optimal intensity ratio for the maximum terahertz generation will decrease with the increase of the total laser intensity. We also show that the final ionization degree can be used to explain the optimal intensity ratio change at different laser intensities. Furthermore, we utilize the increasing rate of electron density and the electron drift velocity to illustrate the physical mechanism of the maximum terahertz radiation generation.

The multi-photon dissociation and Coulomb explosion of ethyl bromide C2H5Br under near-infrared (800 nm) femtosecond laser field are experimentally investigated by a DC-sliced ion imaging technique. The sliced images of fragment ions C2H5+, Br+, CH3+, CH2Br+, H2+ and C2H3Br+ are obtained, and their dissociative pathways are assigned by observing their corresponding kinetic energy release (KER) and angular distribution. It is shown that low-KER components of these fragment ions result from multi-photon dissociation of singly charged parent ion C2H5Br+, while high-KER components come from Coulomb explosion of doubly charged parent ion C2H5Br2+. It is also shown that the precursor species [C2H5+…Br+] has a longer lifetime than [C2H3Br+…H2+] and [CH3…CH2Br+]. In addition, the probable H2 and H2+ elimination channels are theoretically simulated by Gaussian 09 software packages, and the results show that the former is an asynchronous process while the latter is a synchronous process.

Uncorrelated position and velocity distribution of the electron bunch at the photocathode from the residual energy greatly limit the transverse coherent length and the recompression ability. Here we first propose a femtosecond pulse-shaping method to realize the electron pulse self-compression in ultrafast electron diffraction system based on a point-to-point space-charge model. The positively chirped femtosecond laser pulse can correspondingly create the positively chirped electron bunch at the photocathode (such as metal–insulator heterojunction), and such a shaped electron pulse can realize the self-compression in the subsequent propagation process. The greatest advantage for our proposed scheme is that no additional components are introduced into the ultrafast electron diffraction system, which therefore does not affect the electron bunch shape. More importantly, this scheme can break the limitation that the electron pulse via postphotocathode static compression schemes is not shorter than the excitation laser pulse due to the uncorrelated position and velocity distribution of the initial electron bunch.

The oxidation power of permanganates (MnO4–) is known to be strongly dependent on pH values, and is greatly enhanced in acidic solutions, in which, for example, MnO4– can even oxidize Cl– ions to produce Cl2 molecules. Although such dependence has been ascribed due to the different reduced states of Mn affordable in different pH media, a molecular level understanding and characterization of initial redox pair complexes available in different pH solutions is very limited. Herein, we report a comparative study of [MnO4]− and [MnO4·Sol]− (Sol = H2O, KCl, and HCl) anion clusters by negative ion photoelectron spectroscopy (NIPES) and theoretical computations to probe chemical bonding and electronic structures of [MnO4·Sol]− clusters, aimed to obtain a microscopic understanding of how MnO4– interacts with surrounding molecules. Our study shows that H2O behaves as a solvent molecule, KCl is a spectator bound by pure electrostatic interactions, both of which do not influence the MnO4– identity in their respective clusters. In contrast, in [MnO4·HCl]−, the proton is found to interact with both MnO4– and Cl– with appreciable covalent characters, and the frontier MOs of the cluster are comprised of contributions from both MnO4– and Cl– moieties. Therefore, the proton serves as a chemical bridge, bringing two negatively charged redox species together to form an intimate redox pair. By adding more H+ to MnO4–, the oxygen atom can be taken away in the form of a water molecule, leaving MnO4– as an electron deficient MnO3+ species, which can subsequently oxidize Cl– ions.

Coulomb explosion and dissociative ionization of 1,2-dibromoethane are experimentally investigated in a near-infrared (800 nm) femtosecond laser field by dc-slice imaging technology. The sliced images of the fragment ions C2H4Br+, Br+, C2H4+, Br2+ and CH2Br+ are obtained, and their corresponding kinetic energy releases (KERs) and angular distributions are calculated. It is confirmed that the high-KER components come from Coulomb explosion of 1,2-C2H4Br22+, while the low-KER components come from dissociative ionization of 1,2-C2H4Br2+. Furthermore, the dissociation pathway leading to C2H4+ and Br2 is theoretically simulated, and the results show that the singly charged precursor overcomes an energy barrier to dissociate via an asynchronous concerted mechanism after undergoing isomerization.

•Br2+ and C2H4+ ejections from 1,2-dibromoethane is observed.•Br2+ ejections is a synchronous concerted elimination.•Br2+ and C2H4+ results from the two-body CE of the doubly charged parent ion 1,2-DBE2+.•Br2+ and C2H4+ are dissociated after surpassing the energy barrier of a TS structure.Concerted elimination pathway leading to Br2+ and C2H4+ from 1,2-dibromoethane molecule has been investigated in 800 nm femtosecond laser field by dc-slice imaging technology. The kinetic energy release and angular distributions of Br2+ and C2H4+ demonstrate that Br2+ results from the two-body Coulomb explosion of the doubly charged parent ion. Ab initio calculations show that the doubly charged precursor overcomes a small energy barrier (0.18 eV) and then dissociates into Br2+ and C2H4+ through a synchronous concerted elimination mechanism. Moreover, the relative yield of the Br2+ channel is obtained, and it remains about 3.4% when the laser intensity exceeds 1.0 × 1014 W/cm2.Graphical abstractConcerted elimination pathway leading to Br2+ and C2H4+ results from the two-body Coulomb explosion of from 1,2-dibromoethane molecule in 800 nm femtosecond laser field via a TS structure. In the TS structure, the dihedral angle Φ(BrCCBr) decreased to 72°, both C–Br bonds are elongated to 1.94 Å, and thus the distance between two Br atoms is shortened to 3.50 Å. After surpassing the energy barrier (TS), the doubly charged precursor dissociates into Br2+ and C2H4+ through CE process. The Br2+ elimination is a synchronous concerted reaction.

Molecular species with electron affinities (EAs) larger than that of the chlorine atom (3.6131 eV) are superhalogens. The corresponding negative ions, namely, superhalogen anions, are intrinsically very stable with high electron binding energies (EBEs) and widely exist as building blocks of bulk materials and ionic liquids. The most common superhalogen anions proposed and experimentally confirmed to date are either ionic salts or compact inorganic species. Herein, we report a new class of superhalogen species, a series of tetracoordinated organoboron anions [BL4]− (L = phenyl (1), 4-fluorophenyl (2), 1-imidazolyl (3), L4 = H(pyrazolyl)3 (4)) with bulky organic ligands covalently bound to the central B atom. Negative ion photoelectron spectroscopy (NIPES) reveals that all of these anions possess EBEs higher than that of Cl– with the adiabatic/vertical detachment energy (ADE/VDE) of 4.44/4.8 (1), 4.78/5.2 (2), 5.08/5.4 (3), and 4.59/4.9 eV (4), respectively. First-principles calculations confirmed high EBEs of [BL4]− and predicted that these anions are thermodynamically stable against fragmentation. The unraveled superhalogen nature of these species provides a molecular basis to explain the wide-ranging applications of tetraphenylborate (TPB) (1) and trispyrazolylborate (Tp) (4) in many areas spanning from industrial waste treatment to soft material synthesis and organometallic chemistry.

Femtosecond-induced resonance-enhanced multi-photon ionization (REMPI) photoelectron spectroscopy of the Rydberg states is faced with low spectral resolution due to the broadband laser spectrum. In this paper, we theoretically demonstrate that a high-resolution (2 + 1) REMPI photoelectron spectrum of the Rydberg states in a sodium (Na) atom can be obtained by shaping a femtosecond laser pulse with a spectral phase step modulation. Our results show that, by using a phase-shaped femtosecond laser pulse, some narrowband holes or peaks in the REMPI photoelectron spectrum can be created, and the positions of these holes or peaks are correlated with the eigenenergies of the Rydberg states. Thus, by observing these holes or peaks in the REMPI photoelectron spectrum, both the high-resolution REMPI photoelectron spectrum and the energy-level diagram of the Rydberg states can be obtained.

In this paper, a theoretical model is proposed to investigate the molecular rotational state populations pumped by multiple laser pulses through an impulsive Raman process based on second-order perturbation theory and an analytical solution for the dependence of the rotational state populations on the time delays and the relative amplitudes of the multiple laser pulses has been achieved. The results indicate that the molecular rotational state populations can be controlled by precisely manipulating the time delays and the relative amplitudes, which can be significantly enhanced or completely suppressed, and so the molecular rotational wave packet and field-free molecular alignment can be efficiently manipulated.

Niobic tellurite glass doped by silver chloride nanocrystal was prepared with the melting-quenching and heat treatment method, and the self-trapped exciton absorption band of the silver chloride nanocrystal was observed at 532 nm in the UV-visible absorption spectrum. The glass structure characteristics were investigated by Raman spectroscopy, and the mechanism of self-trapped exciton was analyzed by Jahn-Teller model. Its optical limiting was measured with 532 nm picosecond laser pulses, and the corresponding nonlinear absorption coefficient was measured with open-aperture Z-scan. The experimental results showed that optical limiting at 532 nm was attributed to free carrier absorption between the self-trapped state and the continuum band.

Degenerate four-wave mixing measurements, using the 35 ps pulses at 532 nm, have been employed to investigate the third-order nonlinear optical parameters of two chromium tricarbonyl complexes η6-bonded to 3-amino-9-ethylcarbazole at either the NH2-substituted aryl ring (1) or the unsubstituted ring (2) and their precursor 3-amino-9-ethylcarbazole (AECz). The second-order hyperpolarizability Y of the compounds 1 and 2 were found to be 42.9×10−31 and 35.9×10−31 esu, respectively, approximately one order of magnitude greater than AECz. The relation between the molecular structure and second-order hyperpolarizability of the compounds 1 and 2 was explored in detail based on the three-level model and the density functional theory (DFT) calculation. The theoretical results indicate that the spatial distribution of electron density has the profound role in the third-order nonlinear optical properties.

The thin film of a heptamethine cyanine chromophore HC was prepared by spin-coating technique. Its surface morphology and linear optical property were characterized by atomic force microscopy (AFM) and UV-visible absorption spectroscopy. The results show that HC molecules are arranged in a well-ordered H-aggregate type. The third-order nonlinear optical properties of the spin-coating film were also measured by degenerate four-wave mixing (DFWM) measurement. Enhanced third-order nonlinear susceptibility can be attributed to molecular aggregation effects, and the corresponding mechanism was dealt with by collective electronic oscillator (CEO) approach.

Vibrational photon echo decay in the CeO2-doped TeO2–Nb2O5–ZnO glass is observed with near-infrared femtosecond laser pulse at the temperature of 7.5 and 300 K. The photon echo signals exhibit an initial fast decay followed by an exponential decay. The anharmonic frequency splitting (Δ), the dephasing rate (γ), the dephasing time (Td) and the linewidth (Γ) are obtained. The results show that the dephasing time of the ν = 0 → 1 transition is dominated by the vibration state lifetime T1l, and the dephasing time of the ν = 0  → 2 transition is determined by both the vibration state lifetime T2l and the pure dephasing time T2d∗.

Proposed in theory and then their existence confirmed, anion–π interactions have been recognized as new and important non-covalent binding forces. Despite extensive theoretical studies, numerous crystal structural identifications, and a plethora of solution phase investigations, anion–π interaction strengths that are free from complications of condensed-phase environments have not been directly measured in the gas phase. Herein we present a joint photoelectron spectroscopic and theoretical study on this subject, in which tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic, was used as a charge-neutral molecular host to probe its interactions with a series of anions with distinctly different shapes and charge states (spherical halides Cl−, Br−, I−, linear thiocyanate SCN−, trigonal planar nitrate NO3−, pyramidic iodate IO3−, and tetrahedral sulfate SO42−). The binding energies of the resultant gaseous 1:1 complexes (1·Cl−, 1·Br−, 1·I−, 1·SCN−, 1·NO3−, 1·IO3− and 1·SO42−) were directly measured experimentally, exhibiting substantial non-covalent interactions with pronounced anion-specific effects. The binding strengths of Cl−, NO3−, IO3− with 1 are found to be strongest among all singly charged anions, amounting to ca. 30 kcal mol−1, but only about 40% of that between 1 and SO42−. Quantum chemical calculations reveal that all the anions reside in the center of the cavity of 1 with an anion–π binding motif in the complexes' optimized structures, where 1 is seen to be able to self-regulate its cavity structure to accommodate anions of different geometries and three-dimensional shapes. Electron density surface and charge distribution analyses further support anion–π binding formation. The calculated binding energies of the anions and 1 nicely reproduce the experimentally estimated electron binding energy increase. This work illustrates that size-selective photoelectron spectroscopy combined with theoretical calculations represents a powerful technique to probe anion–π interactions and has potential to provide quantitative guest–host molecular binding strengths and unravel fundamental insights in specific anion recognitions.

In this paper, a theoretical model is proposed to investigate the molecular rotational state populations pumped by multiple laser pulses through an impulsive Raman process based on second-order perturbation theory and an analytical solution for the dependence of the rotational state populations on the time delays and the relative amplitudes of the multiple laser pulses has been achieved. The results indicate that the molecular rotational state populations can be controlled by precisely manipulating the time delays and the relative amplitudes, which can be significantly enhanced or completely suppressed, and so the molecular rotational wave packet and field-free molecular alignment can be efficiently manipulated.