Probing H_{2} Double Ionization with Bicircular Laser Fields.

We present a kinematically complete study on strong-field double ionization of H_{2} molecules in two-color bicircular laser fields. The releasing times of electrons and protons are recorded with the double-hand attoclock. We observe the relative emission angles of two electrons oscillate with the kinetic energy release of protons, indicating the internal concerted four-body fragmentation. Using a three-dimensional molecular semiclassical ensemble model, we have disentangled the attosecond correlated electron emission in H_{2} double ionization. This work reveals the strong electron-nuclear coupling in the molecular bond breaking and may open up a new approach to experimentally accessing the intramolecular electron and bond dynamics with bicircular fields.

Ultrafast Imaging of Jahn-Teller Distortion and the Correlated Proton Migration in Photoionized Cyclopropane.

The Jahn-Teller (JT) distortion is one of the fundamental processes in molecules and condensed phase matters. For photoionized organic molecules with high symmetry, the JT effect leads to geometric instability in certain electron configurations and thus has a significant effect on the subsequent isomerization and proton migration processes. Utilizing the femtosecond pump-probe Coulomb explosion method, we probe the isomerization dynamics process of a monovalent cyclopropane cation (C3H6+) caused by proton migration and reveal the relationship between proton migration and JT distortion. We found that the C3H6+ cation evolves from the D3h symmetric equilateral triangle geometry either to the acute triangle via two elongated C-C bonds (JT1) or to the obtuse triangle via a single elongated C-C bond (JT2). The JT1 pathway does not involve proton migration, while the JT2 pathway drives proton migration and can be mapped into the indirect dissociation channel of Coulomb explosion. The time-resolved experiment indicates that the delay time between those two JT pathways can be as large as ∼600 fs. After the JT distortion, the cyclopropane cation undergoes a subsequent structural evolution, which brings a greater variety of dissociation channels.

Sub-Second Long Lifetime Triplet Exciton Reservoir for Highly Efficient and Stable Organic Light-Emitting Diode.

In organic light-emitting diode (OLED), achieving high efficiency requires effective triplet exciton confinement by carrier-transporting materials, which typically have higher triplet energy (ET) than the emitter, leading to poor stability. Here, an electron-transporting material (ETM), whose ET is 0.32 eV lower than that of the emitter is reported. In devices, it surprisingly exhibits strong confinement effect and generates excellent efficiency. Additionally, the device operational lifetime is 4.9 times longer than the device with a standard ETM, 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl) phenyl (whose ET 0.36 eV is higher than the emitter). This anomalous finding is ascribed to the exceptionally long triplet state lifetime (≈0.2 s) of the ETM. It is named as long-lifetime triplet exciton reservoir effect. The systematic analysis reveals that the long triplet lifetime of ETM can compensate the requirement for high ET with the help of endothermic energy transfer. Such combination of low ET and long lifetime provides equivalent exciton confinement effect and high molecular stability simultaneously. It offers a novel molecular design paradigm for breaking the dilemma between high efficiency and prolonged operational lifetime in OLEDs.

Spatiotemporal imaging and shaping of electron wave functions using novel attoclock interferometry.

Electrons detached from atoms by photoionization carry valuable information about light-atom interactions. Characterizing and shaping the electron wave function on its natural timescale is of paramount importance for understanding and controlling ultrafast electron dynamics in atoms, molecules and condensed matter. Here we propose a novel attoclock interferometry to shape and image the electron wave function in atomic photoionization. Using a combination of a strong circularly polarized second harmonic and a weak linearly polarized fundamental field, we spatiotemporally modulate the atomic potential barrier and shape the electron wave functions, which are mapped into a temporal interferometry. By analyzing the two-color phase-resolved and angle-resolved photoelectron interference, we are able to reconstruct the spatiotemporal evolution of the shaping on the amplitude and phase of electron wave function in momentum space within the optical cycle, from which we identify the quantum nature of strong-field ionization and reveal the effect of the spatiotemporal properties of atomic potential on the departing electron. This study provides a new approach for spatiotemporal shaping and imaging of electron wave function in intense light-matter interactions and holds great potential for resolving ultrafast electronic dynamics in molecules, solids, and liquids.

Circularly polarized luminescence and performance modulation of chiral europium-titanium (Eu2Ti4)-oxo clusters.

The designed synthesis of chiral luminescent molecules with excellent circularly polarized luminescence (CPL) performance and high quantum yield (QY) levels has attracted great interest but remains very challenging. Herein, we report three pairs of chiral europium-titanium-oxo clusters featuring both modest CPL characteristics and high QY levels (up to 79%), which can be regulated by switching between different ligand substituents.

Probing the orbital angular momentum of intense vortex pulses with strong-field ionization.

With the rapid development of femtosecond lasers, the generation and application of optical vortices have been extended to the regime of intense-light-matter interaction. The characterization of the orbital angular momentum (OAM) of intense vortex pulses is very critical. Here, we propose and demonstrate a novel photoelectron-based scheme that can in situ distinguish the OAM of the focused intense femtosecond optical vortices without the modification of light helical phase. We employ two-color co-rotating intense circular fields in the strong-field photoionization experiment, in which one color light field is a plane wave serving as the probing pulses and the other one is the vortex pulses whose OAM needs to be characterized. We show that by controlling the spatial profile of the probing pulses, the OAM of the vortex pulses can be clearly identified by measuring the corresponding photoelectron momentum distributions or angle-resolved yields. This work provides a novel in situ detection scenario for the light pulse vorticity and has implications for the studies of ultrafast and intense complex light fields with optical OAM.

Preparation of a Lanthanide-Titanium Oxo Cluster-Polymer Composite by CuI -Catalyzed Click Chemistry.

Incorporating metal clusters within the skeleton of the organic polymers through a click reaction cannot only effectively prepare cluster-polymer composites, but also effectively avoid the cluster aggregation. Herein, an azide-containing lanthanide-titanium oxo cluster of Eu8 Ti10 -N3 (Eu8 Ti10 -N3 =[Eu8 Ti10 (µ3 -O)14 (H2 O)4 (OAc)2 (tbba)30 (paza)4 (THF)2 ]⋅4 THF⋅8 H2 O (1), Htbba=4-tert-butylbenzoic acid, Hpaza=4-azidobenzoate, HOAc=acetic acid, THF=tetrahydrofuran) through an in situ solvothermal reaction of 4-azidobenzoic acid and 4-tert-butylbenzoic acid. Reaction of 1 with PEG (PEG=methoxypoly(ethyleneglycol)alkyne, 2000 g mol-1 ) through CuI -catalyzed click chemistry generates a lanthanide-polymer composite of Eu8 Ti10 -N3 @PEG (2). Investigation with IR, 1 H NMR and ICP-OES of 2 indicates that the structural integrity of 1 is maintained in 2. Study of the luminescent properties of 1 and 2 reveals that the quantum yield of 1 itself basically remains unchanged in 2. Significantly, the formation of 2 cannot only effectively prevent the cluster 1 from aggregation, but also greatly enhance its solubility and adhesion to the substrate. Owing to the solubility and adhesion of luminescent materials being the key to their practical application, present work is thus of great significance for the development of metal cluster-polymer composite luminescent materials.

Lanthanide-Titanium Oxo Clusters as the Luminescence Sensor for Nitrobenzene Detection.

A luminescent lanthanide-titanium oxo cluster of Eu2Ti4(µ2-O)2(µ3-O)4(phen)2(tbza)10·4CH3CN (1, Eu2Ti4-phen-tbza, phen = 1,10-phenanthroline, Htbza = 4-tert-butylbenzoic acid) was prepared through the reaction of phen, Htbza, Eu(Ac)3·xH2O, and Ti(OiPr)4 in acetonitrile. Its overall absolute quantum yield is 65.4% in solid state and 30.2% in CH2Cl2, and the detection limit of 1 for the nitrobenzene (NB) is 10.5 ppb. When the concentration of NB is 40 ppm, the luminescence quenching of 1 can be observed with the naked eye. Time-resolved excited-state decay measurements indicate that the static quenching process is dominated across the NB concentration of 0-9 ppm. The distinguishable shifts in 1H NMR spectra of NB together with 1 confirm the presence of π···π stacking interactions between the organic ligands in 1 and the NB, which plays a key contribution for the quenching of luminescence.

Unifying Tunneling Pictures of Strong-Field Ionization with an Improved Attoclock.

We demonstrate a novel attoclock, in which we add a perturbative linearly polarized light field at 400 nm to calibrate the attoclock constructed by an intense circularly polarized field at 800 nm. This approach can be directly implemented to analyze the recent hot and controversial topics involving strong-field tunneling ionization. The generally accepted picture is that tunneling ionization is instantaneous and that the tunneling probability synchronizes with the laser electric field. Alternatively, recently it was described in the Wigner picture that tunneling ionization would occur with a certain of time delay. We unify the two seemingly opposite viewpoints within one theoretical framework, i.e., the strong-field approximation (SFA). We illustrate that both the instantaneous tunneling picture and the Wigner time delay picture that are derived from the SFA can interpret the measurement well. Our results show that the finite tunneling delay will accompany nonzero exit longitudinal momenta. This is not the case for the instantaneous tunneling picture, where the most probable exit longitudinal momentum would be zero.

Universal Description of the Attoclock with Two-Color Corotating Circular Fields.

We experimentally measure the laser-intensity-dependent photoelectron momentum distributions (PMDs) of Ar atoms with two-color (ω+2ω) corotating circularly polarized fields. The interference patterns on PMDs reveal complex structures with respect to the laser intensity ratio. The main above-threshold ionization peaks and sidebands on PMD distribute oppositely when the fundamental field is much weaker than the second-harmonic field, and the PMD reveals a characteristic single-lobe distribution when the two colors have comparable intensities. Using strong-field approximation, we analytically explain how the interference pattern on PMD evolves with respect to the relative laser intensity. By analyzing the interference pattern, we reveal the phase difference and the temporal evolution of the emitting electron wave packets. We show that, when monitoring the intensity ratio, the double-pointer attoclock geometry with corotating circular fields can be universally mimicked as the spatially rotating temporal double-slit experiments with the variable slit width, which can be used to probe and control strong-field ionization.

Energy- and Momentum-Resolved Photoelectron Spin Polarization in Multiphoton Ionization of Xe by Circularly Polarized Fields.

We perform a joint experimental and theoretical study on momentum- and energy-resolved photoelectron spin polarization in multiphoton ionization of Xe atoms by circularly polarized fields. We experimentally measure the photoelectron momentum distributions of Xe atoms in circularly polarized near-infrared (800 nm) and ultraviolet (400 nm) light, respectively. We analyze the momentum- and energy-resolved photoelectron spin polarization by comparing the experimental photoelectron momentum distributions with the simulations, although we cannot derive the spin polarization solely from the experiment. We show that the use of circularly polarized ultraviolet light at 400 nm can create better than 90% spin polarization with focal volume effect considered, which enables the separation of the spin states by momentum gating. This paves the way to produce high-degree spin-polarized electron sources from strong-field multiphoton ionization.

Revealing the Sub-Barrier Phase using a Spatiotemporal Interferometer with Orthogonal Two-Color Laser Fields of Comparable Intensity.

We measure photoelectron momentum distributions of Ar atoms in orthogonally polarized two-color laser fields with comparable intensities. The synthesized laser field is used to manipulate the oscillating tunneling barrier and the subsequent motion of electrons onto two spatial dimensions. The subcycle structures associated with the temporal double-slit interference are spatially separated and enhanced. We use such a spatiotemporal interferometer to reveal sub-barrier phase of strong-field tunneling ionization. This study shows that the tunneling process transfers the initial phase onto momentum distribution. Our work has the implication that the sub-barrier phase plays an indispensable role in photoelectron interference processes.

Near-Infrared Emitters: Stepwise Assembly of Two Heteropolynuclear Clusters with Tunable Ag(I):Zn(II) Ratio.

Two 3d-4d heteropolynuclear clusters with Ag-Zn ratios of 9:2 and 9:4 were stepwise constructed from a robust nonanuclear silver cluster. Their crystal structures consist of a common bucket-shaped [Ag9(mba)9](9-) (H2mba = 2-mercaptobenzoic acid) core with different numbers of Zn(II) connected by different exo-oriented carboxylates. Most fascinating is the observation of emission (∼703 nm) in the near-infrared (NIR) region at 300 K that may be compared to the related Ag9Zn3 cluster with aliphatic polyamine as auxiliary ligand that emits from the visible (∼580 nm). The shift is associated with the change of ligand field of the 2,2'-bipyridine. The emission intensity and lifetime were dramatically enhanced along with the slight bathochromic shift upon cooling from 300 K to 80 K. The results raise two significant issues: (a) the structural and electronic effects of the secondary metal binding to the metalloligand and the factors influencing the heteropolynuclear cluster assembly and (b) the use of NIR fluorescence, introduced by integrating two luminophores into one heteropolynuclear entity, in detecting free-moving zinc in biological systems both in vivo and in vitro.

Hierarchical Assembly of a {Mn(II)15Mn(III)4} Brucite Disc: Step-by-Step Formation and Ferrimagnetism.

In search of functional molecular materials and the study of their formation mechanism, we report the elucidation of a hierarchical step-by-step formation from monomer (Mn) to heptamer (Mn7) to nonadecamer (Mn19) satisfying the relation 1 + Σn6n, where n is the ring number of the Brucite structure using high-resolution electrospray ionization mass spectrometry (HRESI-MS). Three intermediate clusters, Mn10, Mn12, and Mn14, were identified. Furthermore, the Mn19 disc remains intact when dissolved in acetonitrile with a well-resolved general formula of [Mn19(L)x(OH)y(N3)36-x-y](2+) (x = 18, 17, 16; y = 8, 7, 6; HL = 1-(hydroxymethyl)-3,5-dimethylpyrazole) indicating progressive exchange of N3(-) for OH(-). The high symmetry (R-3) Mn19 crystal structure consists of a well-ordered discotic motif where the peripheral organic ligands form a double calix housing the anions and solvent molecules. From the formula and valence bond sums, the charge state is mixed-valent, [Mn(II)15Mn(III)4]. Its magnetic properties and electrochemistry have been studied. It behaves as a ferrimagnet below 40 K and has a coercive field of 2.7 kOe at 1.8 K, which can be possible by either weak exchange between clusters through the anions and solvents or through dipolar interaction through space as confirmed by the lack of ordering in frozen CH3CN. The moment of nearly 50 NµB suggests Mn(II)-Mn(II) and Mn(III)-Mn(III) are ferromagnetically coupled while Mn(II)-Mn(III) is antiferromagnetic which is likely if the Mn(III) are centrally placed in the cluster. This compound displays the rare occurrence of magnetic ordering from nonconnected high-spin molecules.

Three-body fragmentation of CO2 driven by intense laser pulses.

Dissociative ionization dynamics were studied experimentally for CO2 driven by intense laser pulses. Three-dimensional momentum vectors of correlated atomic ions were obtained for each three-body fragmentation event using triple ion coincidence measurement. Newton diagram demonstrated that three-body fragmentation of CO2 (n+) (n = 3-6) can occur through Coulomb explosion process and sequential fragmentation process depending on the fragmentation channels. The experimental data from these two processes were disentangled by using correlation diagram of correlated ions. Based on the accurate Coulomb explosion data, we reconstructed the bond angle distributions of CO2 (n+) at the moment of fragmentation, which are close to that of neutral CO2 before laser irradiation.

Revealing backward rescattering photoelectron interference of molecules in strong infrared laser fields.

Photoelectrons ionized from atoms and molecules in a strong laser field are either emitted directly or rescattered by the nucleus, both of which can serve as efficiently useful tools for molecular orbital imaging. We measure the photoelectron angular distributions of molecules (N2, O2 and CO2) ionized by infrared laser pulses (1320 nm, 0.2 ~ 1 × 10(14) W/cm(2)) from multiphoton to tunneling regime and observe an enhancement of interference stripes in the tunneling regime. Using a semiclassical rescattering model with implementing the interference effect, we show that the enhancement arises from the sub-laser-cycle holographic interference of the contributions of the back-rescattering and the non-rescattering electron trajectory. It is shown that the low-energy backscattering photoelectron interference patterns have encoded the structural information of the molecular initial orbitals and attosecond time-resolved dynamics of photoelectron, opening new paths in high-resolution imaging of sub-Ångström and sub-femtosecond structural dynamics in molecules.

Mechanisms of strong-field double ionization of Xe.

We perform a fully differential measurement on strong-field double ionization of Xe by 25 fs, 790 nm laser pulses in intensity region (0.4-3)×10(14) W/cm2. We observe that the two-dimensional correlation momentum spectra along the laser polarization direction show a nonstructured distribution for double ionization of Xe when decreasing the laser intensity from 3×10(14) to 4×10(13) W/cm2. The electron correlation behavior is remarkably different with the low-Z rare gases, i.e., He, Ne, and Ar. We find that the electron energy cutoffs increase from 2.9Up to 7.8Up when decreasing the laser intensities from the sequential double ionization to the nonsequential double ionization regime. The experimental observation indicates that multiple rescatterings play an important role for the generation of high energy photoelectrons. We have further studied the shielding effect on the strong-field double ionization of high-Z atoms.

Unprecedented second-timescale blue/green emissions and iodine-uptake-induced single-crystal-to-single-crystal transformation in Zn(II)/Cd(II) metal-organic frameworks.

The long-persistent phosphorescent metal-organic framework (MOF) is a kind of highly desirable but rare material. Here, two new molecular MOF materials, {[Zn(tipa)Cl]⋅NO3⋅2 DMF}n (1) and {[Cd2(tipa)2Cl4]⋅6 DMF}n (2) (tipa = tri(4-imidazolylphenyl)amine), which have 3D twofold interpenetrated (utp) and 2D noninterpenetrated (kgd) topologies, respectively, are reported. They exhibit unexpected long-persistent emissions yet reported: At 77 K, they persist in glowing after stopping the UV irradiation on a timescale up to seconds at 77 K, which can be detected by the naked eye (ca. 2 s). Compounds 1 and 2 also undergo single-crystal-to-single-crystal (SC-SC) transformations through different routes; a simple anion-exchange route for 1 and a complicated replacement of µ1-Cl(-) ions by DMF molecules accompanying I3(-) captured in the void for 2.

Communication: Determining the structure of the N2Ar van der Waals complex with laser-based channel-selected Coulomb explosion.

We experimentally reconstructed the structure of the N2Ar van der Waals complex with the technique of laser-based channel-selected Coulomb explosion imaging. The internuclear distance between the N2 center of mass and the Ar atom, i.e., the length of the van der Waals bond, was determined to be 3.88 Å from the two-body explosion channels. The angle between the van der Waals bond and the N2 principal axis was determined to be 90° from the three-body explosion channels. The reconstructed structure was contrasted with our high level ab initio calculations. The agreement demonstrated the potential application of laser-based Coulomb explosion in imaging transient molecular structure, particularly for floppy van der Waals complexes, whose structures remain difficult to be determined by conventional spectroscopic methods.

Classical-quantum correspondence for above-threshold ionization.

We measure high resolution photoelectron angular distributions (PADs) for above-threshold ionization of xenon atoms in infrared laser fields. Based on the Ammosov-Delone-Krainov theory, we develop an intuitive quantum-trajectory Monte Carlo model encoded with Feynman's path-integral approach, in which the Coulomb effect on electron trajectories and interference patterns are fully considered. We achieve a good agreement with the measured PADs of atoms for above-threshold ionization. The quantum-trajectory Monte Carlo theory sheds light on the role of ionic potential on PADs along the longitudinal and transverse direction with respect to the laser polarization, allowing us to unravel the classical coordinates (i.e., tunneling phase and initial momentum) at the tunnel exit for all of the photoelectrons of the PADs. We study the classical-quantum correspondence and build a bridge between the above-threshold ionization and the tunneling theory.