Laser Excitation Spectroscopy Research Articles

Imaging moiré excited states with photocurrent tunnelling microscopy.

Moiré superlattices provide a highly tuneable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators to moiré excitons. Scanning tunnelling microscopy has played a key role in probing microscopic behaviours of the moiré correlated ground states at the atomic scale. However, imaging of quantum excited states in moiré heterostructures remains an outstanding challenge. Here we develop a photocurrent tunnelling microscopy technique that combines laser excitation and scanning tunnelling spectroscopy to directly visualize the electron and hole distribution within the photoexcited moiré exciton in twisted bilayer WS2. The tunnelling photocurrent alternates between positive and negative polarities at different locations within a single moiré unit cell. This alternating photocurrent originates from the in-plane charge transfer moiré exciton in twisted bilayer WS2, predicted by our GW-Bethe-Salpeter equation calculations, that emerges from the competition between the electron-hole Coulomb interaction and the moiré potential landscape. Our technique enables the exploration of photoexcited non-equilibrium moiré phenomena at the atomic scale.

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

First detection and analysis of an electronic spectrum of vanadium hydride: The D5Π-X5Δ (0,0) band.

The D5Π-X5Δ (0,0) band of vanadium hydride at 654nm has been recorded by laser excitation spectroscopy and represents the first analyzed spectrum of VH in the gas phase. The molecules were generated using a hollow cathode discharge source, with laser-induced fluorescence detected via the D5Π-A5Π (0,0) transition. All five main (ΔΩ = ΔΛ) subbands were observed as well as several satellite ones, which together create a rather complex and overlapped spectrum covering the region 15 180-15 500 cm-1. The D5Π state displays the effects of three strong local perturbations, which are likely caused by interactions with high vibrational levels of the B5Σ- and c3Σ- states, identified in a previous multiconfigurational self-consistent field study by Koseki et al. [J. Phys. Chem. A 108, 4707 (2004)]. Molecular constants describing the X5Δ, A5Π, and D5Π states were determined in three separate least-squares fits using effective Hamiltonians written in a Hund's case (a) basis. The fine structure of the ground state is found to be consistent with its assignment as a σπ2δ, 5Δ electronic state. The fitted values of its first-order spin-orbit and rotational constants in the ground state are A=36.537815cm-1 and B = 5.7579(13)cm-1, the latter of which yields a bond length of R0=1.72122 Å. This experimental value is in good agreement with previous computational studies of the molecule and fits well within the overall trend of decreasing bond length across the series of 3d transition metal monohydrides.

Laser excitation spectroscopy of beryllium heat treatment in synthetic ruby

Ruby is a gem variety of corundum (α-Al2O3) that is colored red by trivalent chromium (Cr3+) substituting for Al3+ in the corundum structure. Recently, the beryllium (Be) heat treatment has been practiced as a technique to enhance the color of ruby. The ruby will turn orange to be an orange sapphire as a result of the enhancement. Some advanced technics to identify the Be heat-treated orange sapphire have been developed such as laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary ion mass spectrometry (SIMS) and laser-induced breakdown spectroscopy (LIBS). We studied the effect of Be atoms in the corundum structure using the red, green, blue and purple laser excitation spectroscopy with the wavelengths of 634 nm, 545 nm, 452 nm and 408 nm respectively. The fluorescence spectra were analyzed for unheated, normal heat treatment and Be heat treatment synthetic ruby samples. The fluorescence intensity of the Be-heated samples was lowest compared to the unheated and normal treated ones for both high and low Cr content.

The effect of one-atom substitution on the photophysical properties and electron spin polarization: Intersystem crossing of compact orthogonal perylene/phenoxazine electron donor/acceptor dyad.

A perylene (Pery)-phenoxazine (PXZ) compact orthogonal electron donor/acceptor dyad was prepared to study the relationship between the molecular structures and the spin-orbit charge transfer intersystem crossing (SOCT-ISC), as well as the electron spin selectivity of the ISC process. The geometry of Pery-PXZ (80.0°) is different from the previously reported perylene-phenothiazine dyad (Pery-PTZ, 91.5°), although there is only one atom variation for the two dyads. Pery-PXZ shows a high singlet oxygen quantum yield (84%). Femtosecond transient absorption spectra indicate that the charge separation (CS, faster than 120 fs) is faster than the Pery-PTZ analog (CS, 250 fs) and charge recombination (CR, i.e., SOCT-ISC, 5.98 ns) of Pery-PXZ is slower than the Pery-PTZ analog (CR, 0.9 ns). The intrinsic triplet state lifetime of Pery-PXZ is 242 µs vs the lifetime of 181 µs for the Pery-PTZ analog. Moreover, the triplet state lifetime of Pery-PXZ in the solid polymer matrix is extended to 4.45 ms, which indicates that the triplet state of Pery-PXZ in fluid solution is deactivated not only by the triplet-triplet annihilation effect but also by other factors such as vibration coupled relaxation. Interestingly, with pulsed laser excited time-resolved electron paramagnetic resonance spectroscopy, the electron spin polarization (ESP) pattern of the triplet state of the current dyad is opposite to that of Pery-PTZ. These results demonstrated the rich electron spin chemistry of the ISC of compact electron donor/acceptor dyads, e.g., the ESP is dependent on not only the molecular geometry but also the structure of the electron donor (or acceptor).

Identification of Dy^{3+}/Dy^{2+} as Electron Trap in Persistent Phosphors.

Laser excitation and x-ray spectroscopy are combined to settle a long-standing question in persistent luminescence. A reversible electron transfer is demonstrated, controlled by light and showing the same kinetics as the persistent luminescence. Exposure to violet light induces charging by oxidation of the excited Eu^{2+} while Dy^{3+} is simultaneously reduced. Oppositely, detrapping of Dy^{2+} occurs at ambient temperature or by infrared illumination, yielding afterglow or optically stimulated luminescence, respectively.

Connecting all electronic states of TaH: Observation of five new weak bands

Five new weak bands in the electronic spectrum of gaseous tantalum hydride (TaH) have been recorded and rotationally analyzed by laser excitation spectroscopy. Due to common upper or lower states, the measurement of these bands has provided absolute term energies for all 18 known electronic states of the molecule. A global least-squares fit to all new and previously recorded excitation bands has produced an accurate set of term energies and rotational constants for the electronic states. Refined Ω-doubling constants are also reported as well as absolute e/f parity assignments for the Ω-doubled [16.1]1 state.

Electronic coupling and spin-orbit charge transfer intersystem crossing (SOCT-ISC) in compact BDP-carbazole dyads with different mutual orientations of the electron donor and acceptor.

In order to study the spin-orbit charge transfer induced intersystem crossing (SOCT-ISC), Bodipy (BDP)-carbazole (Cz) compact electron donor/acceptor dyads were prepared. Charge transfer (CT) emission bands were observed for dyads showing strong electronic coupling between the donor and the acceptor (coupling matrix elements VDA, 0.06 eV-0.18 eV). Depending on the coupling magnitude, the CT state of the dyads can be either dark or emissive. Equilibrium between the 1LE (locally excited) state and the 1CT state was confirmed by temperature-dependent fluorescence studies. Efficient ISC was observed for the dyads with Cz connected at the meso-position of the BDP. Interestingly, the dyad with non-orthogonal geometry shows the highest ISC efficiency (ΦΔ = 58%), which is different from the previous conclusion. The photo-induced charge separation (CS, time constant: 0.7 ps) and charge recombination (CR, ∼3.9 ns) were studied by femtosecond transient absorption spectroscopy. Nanosecond transient absorption spectroscopy indicated that the BDP-localized triplet state was exceptionally long-lived (602 µs). Using pulsed laser excited time-resolved electron paramagnetic resonance spectroscopy, the SOCT-ISC mechanism was confirmed, and we show that the electron spin polarization of the triplet state is highly dependent on the mutual orientation of the donor and acceptor. The dyads were used as triplet photosensitizers for triplet-triplet-annihilation (TTA) upconversion, and the quantum yield is up to 6.7%. TTA-based delayed fluorescence was observed for the dyads (τDF = 41.5 µs). The dyads were also used as potent photodynamic therapy reagents (light toxicity of IC50 = 0.1 µM and dark toxicity of IC50 = 70.8 µM).

Luminescent and Chiroptical Properties of 1 : 1 Eu (III) : Tetracycline Species Probed by Circularly Polarized Luminescence.

This study investigates the significantly different luminescent and chiroptical properties of tetracycline (TC) when coordinated to Eu(III). The approach involves understanding the 1) speciation of TC and 2) conformation and species formed between Eu(III) and TC in a ratio of 1 : 1 in a dimethylformamide (DMF) solution and as a function of the pH value. By identifying the conformational changes of the various 1 : 1 Eu(III) : TC species, the results from this study explain information on the local microenvironment about the Eu(III) metal center. In particular, 5 D0 ←7 F0 Eu(III) laser excitation spectroscopy was employed to distinguish the different types of species found in solution in order to understand the interaction between Eu(III) and TC. On the other hand, circularly polarized luminescence (CPL) spectroscopy was used to understand the structural changes within the 1 : 1 Eu(III) : TC complex that could be related to the chirality of the Eu(III)-containing species. The CPL spectrum serves as a "fingerprint" to indicate the conformational changes within the 1 : 1 Eu(III) : TC complex as a result of the chiroptical signal arising from the various Eu(III) : TC species.

Luminescence properties of red-emitting Mn2+-Activated Na2Mg5Si12O30 phosphors

Na2Mg5(1-x)Si12O30:5xMn2+(x = 0.005, 0.01, 0.03,0.05,0.07 and 0.09) red phosphors were synthesized by solid state reactions. The formation of a single phase compound is verified through the X-ray diffraction (XRD) studies. The excitation and emission spectra, and decay curves of Na2Mg5Si12O30:Mn2+ are investigated by optical and laser excitation spectroscopy in the temperature range 7–520 K. The strong red emission peaking at 600 nm is observed due to the spin forbidden transition 4T1 → 6A1 of Mn2+ ions. At high Mn2+ concentration, new side bands appear at lower energy side at around 670 nm which can be ascribed to the appearance of the formation of new Mn2+ centers. In addition, the thermal quenching behavior is investigated to evaluate the commercial value of Na2Mg5Si12O30:Mn2+ as red-emitting materials.

Energy transfer and luminescence quenching of Cr3+-doped LiGaW2O8

Luminescence properties of the Cr3+-doped LiGaW2O8 crystals are investigated by laser excitation spectroscopy. The powder samples for various Cr3+ concentration were prepared by high-temperature solid state reaction. The temperature behaviors of the emission intensity and the decay time of the self-activated LiGaW2O8 host are discussed. The emission spectra and decay times of LiGa1-xW2O8:xCr3+ are measured under UV excitation in the temperature range 10–300 K. The luminescence quenching of LiGaW2O8:Cr3+ as functions of temperature and Cr3+-concentration is explained by structural and spectral characteristics.

Optical spectroscopy of Tb 3+ ions doped NaCa(PO 3 ) 3 phosphors

Luminescence properties of NaCa(PO3)3 doped with Tb3+ ions are investigated by optical and laser excitation spectroscopy. The phosphors were prepared by solidstate reaction method The X-ray diffraction(XRD) was used to analyze the crystal structure and the crystallinity of the samples. The excitation and emission spectra and decay curve of NaCa(PO3)3:Tb3+(0.01 ~ 30mol%) were measured at room temperature. The f - d band of Tb3+ is observed in the excitation spectra of NaCa(PO3)3:Tb3+ in the wave length region 205 ~ 245 nm. Strong emission lines due to the 5D4 → 7FJ transition and weak emission lines due to the 5D4 → 7FJ transition are observed in the emission spectra of NaCa(PO3)3:Tb3+. The energy transfer and cross relaxation between Tb3+ ions are discussed in NaCa(PO3)3:Tb3+ in the emission spectra and life time.

Eu3+ -Activated Gd8 V2 O17 : Energy Transfer, Luminescence, and Temperature-Dependence Characteristics.

A new phosphor of the type Gd8(1-x) Eu8 x V2 O17 (x=0-1.0) was synthesized through the solid-state reaction ceramics method. A pure phase formation was verified by using X-ray powder diffraction measurements. The luminescence of Gd8 V2 O17 :Eu3+ was investigated through optical and laser excitation spectroscopy. The luminescence curves were investigated in the temperature region 10 to 300 K. Gd8 V2 O17 shows a self-activated luminescence under excitation with UV- and near-UV light. The spectra, the decay lifetimes, and the thermal stability of Gd8(1-x) Eu8 x V2 O17 (x=0.005-1.0) strongly depend on both the Eu3+ concentration and the temperature. The tunable luminescence is realized by controlling the Eu3+ -doping level to adjust the host energy-transfer efficiency from the VO43- groups to the Eu3+ activators. At low Eu3+ concentrations (<30 mol %), the intensity and lifetime show an unusual change with an increase of the temperature from 10-300 K, that is, the luminescence experiences a straightforward enhancement. The energy transfer from the VO43- group to the Eu3+ ions could be accelerated with an increase of the temperature resulting in an unusual enhancement of the Eu3+ luminescence and lifetime. However, the emission of the Eu3+ ions decreased for highly Eu-doped samples (>30 mol %) with an increase of the temperature. The luminescence mechanism was discussed on the basis of the charge-transfer band of the Eu3+ ions, the doping concentration, and the proposed microstructures in the lattices.

Optical tuning of dielectric properties of La0.7Sr0.3MnO3/SrTiO3 superlattices in the terahertz range.

Two (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices with different superlattice period but the same total thickness were deposited on LaAlO3 substrates by pulsed laser deposition. Dielectric properties of these samples were investigated by means of terahertz time-domain spectroscopy (THz-TDS) under external continuous wave green laser excitation and optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. Experimental results show that the real part of the permittivity for both superlattices increases significantly with increasing green laser pump power, which indicates the decrease of the plasma frequency, along with the increase of the electron scattering rate, soft mode eigenfrequency and oscillator strength in the Drude-Lorentz model. Furthermore, it's observed that the insulating superlattice exhibits a more significant dielectric tunability than the metallic superlattice. Besides, the carrier lifetime of superlattices is much shorter than the La0.7Sr0.3MnO3 thin film in the OPTP measurements, indicating that the electrons excited in the La0.7Sr0.3MnO3 layers may be trapped by the defects located in the interfaces of La0.7Sr0.3MnO3 and SrTiO3 or the SrTiO3 layers. With the optical field-induced tunability of dielectric properties, (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices show great potential in the actively tunable devices in the THz range.

Luminescence properties of sodalite-type Zn4B6O13:Mn2+

Luminescence properties of sodalite Zn4B6O13:Mn2+ are investigated by optical and laser excitation spectroscopy in the temperature range 10 – 300 K. The samples of Zn4B6O13:Mn2+ are prepared in the carbon reducing atmosphere. The green emission at 540 nm are observed in Zn4B6O13:Mn2+ for various Mn2+ concentration. Optimum Mn2+ concentration and critical distances between Mn2+ ions in Zn4B6O13 are obtained by the luminescence intensity as functions of Mn2+ concentration. The decays of Mn2+ emission depend strongly on Mn2+ concentration due to the energy transfer and energy diffusion in Zn4B6O13:Mn2+. The fine structures consisting of the intense zero phonon line (ZPL) and weak vibrational sidebands are observed at low temperature. The relevant mechanism of excitation bands, ZPL, vibrational sidebands and the bandwidth as functions of temperature are interpreted in detail by energy level diagram of the 3d5 transition metal ions and configuration coordinated diagram. The high thermal stability of the Zn4B6O13:Mn2+ phosphor are observed in the temperature range 10–300 K.

Dynamics of Histone H3 Tails in Mononucleosomes Studied by Single-Molecule FRET and MD Simulations

The nucleosome, consisting of two copies of each of the four histone proteins H2A, H2B, H3, H4, and a 147bp long DNA, is the basic unit of chromatin compaction and key to gene regulation. The N-terminal tails of the four core histone proteins and the C-terminal tail of H2A are protruding from the core particle in a disorder manner. Even though, the nucleosome has been widely studied in the field of epigenetics, surprisingly little is known about the actual conformation of the tails and their dynamics. Here we focus on the conformation and dynamics of the H3 N-terminal tail at various NaCl concentrations and linker DNA length in experiments and MD simulations. Salt dependent conformational changes were measured by FRET with reconstitute Widom 601 mononucleosomes with a DNA length of 170 bp, 190 bp or 210 bp. Nucleosomes were labeled on the H3 tail at position K9 and at various positions on the intranucleosomal DNA or linker DNA. Both, bulk experiments using microplate scanning FRET spectroscopy (µpsFRET), and single molecule experiments using alternating laser excitation (ALEX) or multidimensional confocal fluorescence spectroscopy with pulsed interleaved excitation (MFD-PIE) were performed. Our results confirm the previously described binding preference of the H3 tail towards the linker DNA at low salt concentrations as well as an influence of the linker DNA length on the H3 tail:DNA interaction. At higher salt concentrations (> 600 mM), when nucleosome disassembly occurs, interaction of the H3 tail with the linker DNA is disrupted and interaction with the inner DNA gyre is favorized. The designed constructs enable us for the first time to our knowledge to monitor conformational changes of the H3 tail, which underpins the proposed dynamic nature of the H3 tail.

Charge transfer transition and energy transfer in Eu3+-doped Gd10V2O20

Eu3+-doped Gd10(1-x)Eu10xV2O20 (x = 0 – 1) phosphors were synthesized via the sol-gel process. The formation of a single phase compound was verified through the X-ray diffraction studies. Luminescence properties of Gd10V2O20:Eu3+ are investigated by optical and laser excitation spectroscopy. The emission and excitation spectra, luminescence decays were measured in the temperature region 7–300K. The strong emission due to the vanadate group of Gd10V2O20:Eu3+ is observed at low temperature. The charge transfer transition of Eu3+ depends strongly on the Eu3+-concentration in Gd10V2O20:Eu3+. The emission intensity as a function of Eu3+ concentration under excitation of charge transfer band is inconsistent with that of the 4f7 states (the 5L6 state) of Eu3+. The energy transfer occurs between two Eu3+ ions at low Eu3+ concentration (< 10mol%), while energy diffusion dominates at high Eu3+ concentration.

Unusual temperature and excitation energy dependences of impurity-trapped excitons in LiBaF3:Eu2+ crystals

Luminescence properties of impurity-trapped excitons (ITEs) in LiBaF3:Eu2+ crystals are investigated by optical and time-resolved laser-excitation spectroscopy. The three excited states of 4f65d, 4f7(6P7/2) and ITE are identified in the excitation and emission spectra. The intensity of ITEs’ emission increases unexpectedly with increasing temperature and excitation energy. The initial intensity of the ITE emission just after pulsed-laser excitation increases continuously with temperature in the temperature range 7–600K. The results are discussed in relation to radiative and nonradiative rates and excitonic processes in this system. The theoretical model that includes the ITE, 4f7(6P7/2) and 4f6(7FJ)5d(eg) states is developed to describe and support the experimental observations.

Thermal quenching and luminescence decay in self-activated La2W3O12

Lanthanum tritungstates, La2W3O12, were prepared by using a conventional high-temperature solid-state reaction. The formation of a single-phase compound with the monoclinic structure of La2W3O12 was verified through X-ray diffraction (XRD) studies. The luminescence propeities of La2W3O12 were investigated by using optical and laser-excitation spectroscopy. The excitation and the emission spectra and the decay curves were measured in the temperature range 7 − 300 K. The charge-transfer transition of WO 4 2− was identified in the excitation and the emission spectra under excitation by ultraviolet radiation. A strong green luminescence was observed at 500 nm with a bandwidth of 4240 cm −1 at temperatures lower than 60 K; then the thermal quenching occurred with increasing temperature. The thermal quenching behaviors of the luminescence intensity and the decay time could be explanined by using a simple thermal quenching model.

Temperature dependent emission characteristics of monoclinic YBO3: Eu3+/Tb3+ phosphor

YBO3:Eu3+/Tb3+ phosphor samples synthesized by modified combustion method are studied in the present work using powder X-ray diffraction, UV–visible absorption spectroscopy, X-ray excited luminescence spectroscopy and optical parametric oscillator (OPO) based laser excited emission spectroscopy. The temperature dependence of luminescence emission is also studied. The structural analysis suggests that the samples possess monoclinic structure with C2/c space group. The emission maximum was excitation wavelength dependent and prominent emission was observed at 593nm (241nm excitation) and 613nm (300nm excitation) for YBO3:Eu3+ samples. The prominent magnetic/ electric (593/613nm) dipole-moment allowed transitions are attributed to the presence of Eu3+ at different sites. For YBO3:Tb3+ phosphor, 543nm emission was prominent and had no impact of the cite symmetry. The increase in PL intensity in Eu3+ doped samples above 225K is associated with the carrier mobility. An energy level scheme showing the positions of the 4f and 5d energy levels of all divalent and trivalent lanthanide ions relative to the valence and conduction band of the YBO3 has been constructed opening the possibility of using YBO3 for other interesting applications.