Raman Process Research Articles

Broadband EPR Spectroscopy of the Triplet State: Multi-Frequency Analysis of Copper Acetate Monohydrate.

Electron paramagnetic resonance spectroscopy is a long-standing method for the exploration of electronic structures of transition ion complexes. The difficulty of its analysis varies considerably, not only with the nature of the spin system, but more so with the relative magnitudes of the magnetic interactions to which the spin is subject, where particularly challenging cases ensue when two interactions are of comparable magnitude. A case in point is the triplet system S = 1 of coordination complexes with two unpaired electrons when the electronic Zeeman interaction and the electronic zero-field interaction are similar in strength. This situation occurs in the X-band spectra of the thermally excited triplet state of dinuclear copper(II) complexes, exemplified by copper acetate monohydrate. In this study, applicability of the recently developed low-frequency broadband EPR spectrometer to S = 1 systems is investigated on the analysis of multi-frequency, 0.5-16 GHz, data from [Cu(CH3COO)2H2O]2. Global fitting affords the spin Hamiltonian parameters gz = 2.365 ± 0.008; gy = 2.055 ± 0.010; gx = 2.077 ± 0.005; Az = 64 gauss; D = 0.335 ± 0.002 cm-1; E = 0.0105 ± 0.0003 cm-1. The latter two define zero-field absorptions at ca. 630, 7730, and 10,360 MHz, which show up in the spectra as one half of a sharpened symmetrical line. Overall, the EPR line shape is Lorentzian, reflecting spin-lattice relaxation, which is a combination of an unusual, essentially temperature-independent, inverted Orbach process via the S = 0 ground state, and a Raman process proportional to T2. Other broadening mechanisms are limited to at best minor contributions from a distribution in E values, and from dipolar interaction with neighboring copper pairs. Monitoring of a first-order double-quantum transition between 8 and 35 GHz shows a previously unnoticed very complex line shape behavior, which should be the subject of future research.

Stimulated Raman scattering by circularly polarized quadruple Gaussian laser beam and co-propagating electron beam in plasma

This study investigates the interplay between a co-propagating relativistic electron beam and a quadruple Gaussian laser beam in plasma, focusing on the suppression of stimulated Raman scattering (SRS) growth. The presence of the laser beam induces the excitation of a pair of plasma waves and side-scattered electromagnetic waves. As the side-scattered wave and pump wave couple together, they exert a ponderomotive force on the electron beam and plasma electrons, resulting in an enhancement of the plasma wave amplitude. Nonlinear coupling between the density perturbation in the plasma, induced by the plasma wave, and the pump wave leads to the excitation of a nonlinear current responsible for the growth of the side-scattered electromagnetic wave associated with SRS. Furthermore, the growth rate of SRS is shown to be highly sensitive to the phase matching between the relativistic electron beam and the plasma wave. In cases of phase mismatch, the growth rate experiences a significant reduction. Additionally, the effectiveness of the electron beam in driving the stimulated Raman process is greatly affected by the energy spread of the electron beam. A substantial reduction in effectiveness is observed due to this energy spread.

Rotational wave packet of NO+ created upon strong-field ionization of NO

We theoretically investigate the creation of the rotational wave packet in molecular ions upon the strong-field ionization of diatomic molecules by a femtosecond laser pulse. The rotational excitation of molecular cation is ascribed to the dependence of the strong-field ionization probability on the orientation angle of the molecular axis with respect to the laser polarization direction. By extending the molecular strong-field approximation theory, we calculate the rovibrational state distribution of the ground electronic X state of NO+ after the ionization of NO in the electronic ground X state. We also show that the extent of the rotational excitation of NO+ is enhanced by the stimulated impulsive Raman processes both in neutral NO and in NO+ cation. The resultant time evolution of the rotational wave packet of NO+ is in good agreement with the delay dependent NO2+ yield recorded experimentally in our previous pump–probe study.

Asymmetric manipulations on the coherent Raman sideband generation under molecular modulations

We study the light evolution through a Raman-active medium, where a pair of fields pump molecules near-resonantly and excite the molecular coherence. A synthetic lattice including multiple frequency sidebands via the coherent Raman process is constructed. By varying the density of molecules in the light propagation direction, the system can present a lattice model undergoing dynamic modulations. Further considering the frequency-dependent couplings between synthetic lattice sites, we explore intriguing phenomena, including spectral Bloch oscillation and super Bloch oscillation with asymmetric patterns, which leads to interesting frequency-conversion phenomena with the Raman sideband generations. Our work could be useful for future experiments in coherent Raman sideband processes towards ultrashort pulse generations and higher-frequency conversion.

Raman amplification at 2.2 μm in silicon core fibers with prospects for extended mid-infrared source generation

Raman scattering provides a convenient mechanism to generate or amplify light at wavelengths where gain is not otherwise available. When combined with recent advancements in high-power fiber lasers that operate at wavelengths ~2 μm, great opportunities exist for Raman systems that extend operation further into the mid-infrared regime for applications such as gas sensing, spectroscopy, and biomedical analyses. Here, a thulium-doped fiber laser is used to demonstrate Raman emission and amplification from a highly nonlinear silicon core fiber (SCF) platform at wavelengths beyond 2 μm. The SCF has been tapered to obtain a micrometer-sized core diameter (~1.6 μm) over a length of 6 cm, with losses as low as 0.2 dB cm−1. A maximum on-off peak gain of 30.4 dB was obtained using 10 W of peak pump power at 1.99 μm, with simulations indicating that the gain could be increased to up to ~50 dB by extending the SCF length. Simulations also show that by exploiting the large Raman gain and extended mid-infrared transparency of the SCF, cascaded Raman processes could yield tunable systems with practical output powers across the 2–5 μm range.

In-Situ Low-Frequency SERS Observation at Various Electrochemical Interfaces

Surface-selective vibrational spectroscopy, which is a powerful tool to study electrochemical interfaces, is typically operated in the mid-infrared region; chemical information obtained can give deep insights into electrochemical reactions at the molecular level. However, when the vibrational frequency range is extended into far-infrared or terahertz (THz) region, one can expect to obtain additional information about the interfaces because external vibrations such as surface-adsorbate vibrations and intermolecular vibrations appear in this range. Unfortunately, most of vibrational methods using THz radiation are not suitable for surface-selective observation of buried electrochemical interfaces. On the other hand, surface-enhanced Raman scattering (SERS) is potentially capable of detecting low-frequency vibrations at interfaces. Recently, the detectable frequency range of SERS has been extended into the low-frequency region down to 10 cm-1 [1], and then a vibrational analysis procedure for such low-frequency region has also been developed using a density of states format of SERS spectrum [2-4]. Using this method, for example, we have simultaneously observed both chemical information and molecular mass of surface adsorbates at interfaces [5]. In this talk, we focus on intermolecular interactions such as hydrogen bonding and anion-cation interactions at electrochemical interfaces. We present in-situ low-frequency SERS observation at various electrochemical interfaces under potential control [6].To obtain low-frequency SERS spectra, volume Bragg grating filters were used for both monochromatization of He-Ne laser radiation of 632.8 nm and removal of Rayleigh scattered light. SERS-active Au electrodes were prepared by electrochemical surface roughening in 0.1M KCl solution. For SERS-active Pt electrodes, monoatomic Pt overlayers were formed on the roughened Au surfaces. SERS spectra for both aqueous electrolyte solutions and ionic liquid electrolyte solutions were measured on these electrode surfaces under application of electrochemical potentials. The measured spectra were converted to density states of formats by reducing the Purcell factor, Bose-Einstein thermal factor, and frequency factor.Fig. 1 shows measured and reduced SERS spectra for 0.1M H2SO4/Au interface in the low-frequency region covering both Stokes and anti-Stokes branches. In the measured spectrum, vibrational features are not recognized because the huge background continuum dominates in the spectrum. This background continuum, caused by plasmon-enhanced electronic Raman scattering in the conduction band of Au, can be reduced in the density states of format, leading to considerable improvement in analyzability of vibrational features. Indeed, THz vibrational features are clearly unveiled in the reduced spectrum. Incidentally, the symmetry of the response between the Stokes and anti-Stokes branches in the reduced spectrum indicates that the SERS spectrum is properly converted to the reduced form because the Stokes and anti-Stokes Raman processes are connected due to the time reversal symmetry. Potential-dependence of these THz vibrational features will be discussed in the talk.[1] M. Inagaki, K. Motobayashi, K. Ikeda, J. Phys. Chem. Lett., 8, 4236-4240 (2017).[2] M. Inagaki, T. Isogai, K. Motobayashi, K. -Q. Lin, B. Ren, K. Ikeda, Chem. Sci., 11, 9807-9717 (2020).[3] M. Inagaki, K. Motobayashi, K. Ikeda, Nanoscale, 12, 22988-22994 (2020).[4] R. Kamimura, T. Kondo, K. Motobayashi, K. Ikeda, Phys. Status Solidi B, 259, 2100589 (2022).[5] T. Kondo, M. Inagaki, K. Motobayashi, K. Ikeda, Catal. Sci. Technol., 12, 2670-2676 (2022).[6] T. Isogai, M. Inagaki, M. Uranagase, K. Motobayashi, S. Ogata, K. Ikeda, submitted. Figure 1

Nonequilibrium phases of ultracold bosons with cavity-induced dynamic gauge fields

Gauge fields are a central concept in fundamental theories of physics, and responsible for mediating long-range interactions between elementary particles. Recently, it has been proposed that dynamical gauge fields can be naturally engineered by photons in composite, neutral quantum gas-cavity systems using suitable atom-photon interactions. Here we comprehensively investigate nonequilibrium dynamical phases appearing in a two-leg bosonic lattice model with leg-dependent, dynamical complex tunnelings mediated by cavity-assisted two-photon Raman processes. The system constitutes a minimal dynamical flux-lattice model. We study fixed points of the equations of motion and their stability, the resultant dynamical phase diagram, and the corresponding phase transitions and bifurcations. Notably, the phase diagram features a plethora of nonequilibrium dynamical phases including limit-cycle and chaotic phases. In the end, we relate regular periodic dynamics (i.e., limit-cycle phases) of the system to time crystals.

Self-consumed strategy to reuse cathode residue for Zn stabilization in spent batteries: Structural properties and metal stabilization effect

Due to ecotoxicity, zinc (Zn) as a heavy metal from electronic waste (e-waste) has been a source of pollution to soil and water for several decades. This study proposes a solution to this serious environmental problem via a self-consumed strategy to stabilize Zn in anode residues. This unique method uses cathode residues from spent zinc-manganese oxide (Zn-Mn) batteries as a stabilized matrix via thermal treatment. More specifically, the strategy incorporates zinc metal into a chemically durable matrix comprised of a lattice of AB2O4 compounds. Results demonstrate that 5–20 wt% of anode residue were fully incorporated into the cathode residue to form a Mn3-xZnxO4 solid solution after sintering at 1300 ℃ for 3 h. The lattice parameters of the Mn3-xZnxO4 solid solution reveal an approximately linear decreasing evolution with the addition of anode residue. To determine the occupancy of Zn in the crystal structure of the products, we used Raman and Rietveld refinement processes; the results reveal that Mn2+ in the 4a site was gradually replaced by Zn2+. We then used a prolonged toxicity leaching procedure to evaluate the Zn stabilization effect after phase transformation; this showed that the Zn leachability of sintered anode-doped cathode sample was over 40 folds lower than that of untreated anode residue. Therefore, this study presents an economical and effective strategy for mitigating the presence of heavy metal pollutants derived from e-waste.

Structural and Magnetization Dynamics of Borohydride-Bridged Rare-Earth Metallocenium Cations.

The structure and magnetic properties of the bimetallic borohydride-bridged dysprosocenium compound [{(η5-Cpttt)(η5-CpMe4t)Dy}2(μ:κ2:κ2-BH4)]+[B(C6F5)4]- ([3Dy][B(C6F5)4]) are reported along with the solution-phase dynamics of the isostructural yttrium and lutetium analogues (Cpttt is 1,2,4-tri(tert-butyl)cyclopentadienyl, CpMe4t is tetramethyl(tert-butyl)cyclopentadienyl). The synthesis of [3M][B(C6F5)4] was accomplished in the 2:1 stoichiometric reactions of [(η5-Cpttt)(η5-CpMe4t)Dy(BH4)] (2M) with [CPh3][B(C6F5)4], with the metallocenes 2M obtained from reactions of the half-sandwich complexes [(η5-Cpttt)M(BH4)2(THF)] (1M) (M = Y, Dy, Lu) with NaCpMe4t. Crystallographic studies show significant lengthening of the M···B distance on moving through the series 1M, 2M, and 3M, with essentially linear {M···B···M} bridges in 3M. Multinuclear NMR spectroscopy indicates restricted rotation of the Cpttt ligands in 3Y and 3Lu in solution. The single-molecule magnet (SMM) properties of [3M][B(C6F5)4] are characterized by Raman and Orbach processes, with an effective barrier of 533(18) cm-1 and relaxation via the second-excited Kramers doublet. Although quantum tunneling of the magnetization (QTM) was not observed for [3M][B(C6F5)4], it was, surprisingly, found in its magnetically dilute version, which has a very similar barrier of Ueff = 499(21) cm-1. Consistent with this observation, slightly wider openings of the magnetic hysteresis loop at 2 K are found for [3M][B(C6F5)4] but not for the diluted analogue. The dynamic magnetic properties of the dysprosium SMMs and the role of exchange interactions in 3Dy are interpreted with the aid of multireference ab initio calculations.

Anti-Stokes Raman Scattering of the Smallest Carbon Nanowires Made of Long Linear Carbon Chains Inserted inside Single-Walled Carbon Nanotubes

The smallest carbon nanowires, i.e., single-walled carbon nanowires (SWCNWs, LLCCs@SWCNTs) made of long linear carbon chains (LLCCs, sp) inserted inside single-walled carbon nanotubes (SWCNTs, sp2), were successfully mass produced recently, making the refined strong Stokes Raman characterization of LLCCs@SWCNTs possible. This work reports the observation of anti-Stokes Raman scattering of LLCCs@SWCNTs with normal Raman spectroscopy, and their Stokes and anti-Stokes Raman scattering at a wide temperature range of 80–850 K has been systematically studied. It was found that the normalized intensity of L-band at 1835 cm–1 of LLCCs@SWCNTs decreased as the temperature increased, which obeys a quadratic polynomial fitting for Stokes and cubic fitting for anti-Stokes peaks. The absolute intensity of Stokes peaks decreased and that of anti-Stokes increased with the increasing temperature. Meanwhile, a softening of Stokes and anti-Stokes peaks was observed as the temperature increased. Furthermore, both the high-energy longitudinal phonon (G+) of SWCNTs hosting LLCCs and the L-band originating from LLCCs inside SWCNTs exhibit a quasi-linear variation with temperature, having temperature coefficients of −0.0192 and −0.0296 cm–1·K–1 for Stokes and 0.0200 and 0.0334 cm–1·K–1 for anti-Stokes, respectively. The linear temperature dependence of peak positions of L-band implies that the crystal, vibrational, and electronic structures of LLCCs inside SWCNTs can be easily tuned via the temperature. This work paves the way for studying the mechanism of Raman process and fundamental properties of both LLCCs/carbyne and SWCNWs (LLCCs@SWCNTs), as well as their potential applications in nanoscale temperature monitoring.

Dephasing by optical phonons in GaN defect single-photon emitters

Single-photon defect emitters (SPEs), especially those with magnetically and optically addressable spin states, in technologically mature wide bandgap semiconductors are attractive for realizing integrated platforms for quantum applications. Broadening of the zero phonon line (ZPL) caused by dephasing in solid state SPEs limits the indistinguishability of the emitted photons. Dephasing also limits the use of defect states in quantum information processing, sensing, and metrology. In most defect emitters, such as those in SiC and diamond, interaction with low-energy acoustic phonons determines the temperature dependence of the dephasing rate and the resulting broadening of the ZPL with the temperature obeys a power law. GaN hosts bright and stable single-photon emitters in the 600–700 nm wavelength range with strong ZPLs even at room temperature. In this work, we study the temperature dependence of the ZPL spectra of GaN SPEs integrated with solid immersion lenses with the goal of understanding the relevant dephasing mechanisms. At temperatures below ~ 50 K, the ZPL lineshape is found to be Gaussian and the ZPL linewidth is temperature independent and dominated by spectral diffusion. Above ~ 50 K, the linewidth increases monotonically with the temperature and the lineshape evolves into a Lorentzian. Quite remarkably, the temperature dependence of the linewidth does not follow a power law. We propose a model in which dephasing caused by absorption/emission of optical phonons in an elastic Raman process determines the temperature dependence of the lineshape and the linewidth. Our model explains the temperature dependence of the ZPL linewidth and lineshape in the entire 10–270 K temperature range explored in this work. The ~ 19 meV optical phonon energy extracted by fitting the model to the data matches remarkably well the ~ 18 meV zone center energy of the lowest optical phonon band (E_{2}(low)) in GaN. Our work sheds light on the mechanisms responsible for linewidth broadening in GaN SPEs. Since a low energy optical phonon band (E_{2}(low)) is a feature of most group III–V nitrides with a wurtzite crystal structure, including hBN and AlN, we expect our proposed mechanism to play an important role in defect emitters in these materials as well.

Modulational instability and soliton trains in optical fiber media with real and imaginary Raman gains

We consider a higher-order nonlinear Schrödinger amplitude equation with real and imaginary Raman gain terms. This equation is shown to govern the propagation of femtosecond pulses in optical fiber media. We investigate on the modulational instability of continuous wave solution of this amplitude equation and show that the gain exist in both the anomalous and normal dispersion regimes, provided the complex intrapulse Raman terms are considered. However, when the dissipation induced by Raman process in the optical fiber is neglected; periodic bright solitary waves can only be identified in the anomalous dispersion regime. Nonlinear periodic trains of optical pulses are analytically obtained using the ansatz technique which incorporates the direct integral method. The observation of an optical shock front moving at the group velocity, is attributed to the Raman dissipation process in the optical fiber. Finally, results of numerical simulations equally underscores the influence of Raman gain terms on the evolution of optical soliton trains.

A Chain of Vertex-Sharing {CoIII2CoII2}n Squares with Single-Ion Magnet Behavior

A new mixed-valence one-dimensional coordination polymer of formula {[CoII(MeOH)2][(μ-NC)2CoIII(dmphen)(CN)2]2}n·2nH2O (1) was obtained by reacting the Ph4P[CoII(dmphen)(CN)3] metalloligand (dmphen = 2,9-dimethyl-1,10-phenanthroline and Ph4P+ = tetraphenylphosphonium ion) with cobalt(II) acetate tetrahydrate. The structural analysis shows the formation of a neutral 4,2-ribbon-like chain of vertex-sharing cyanido-bridged {CoIII2CoII2} squares in which the metalloligand underwent an oxidation process and a reorganization to form {CoIII(dmphen)(CN)4}− linkers that coordinate to the [CoII(MeOH)2]2+ units through single cyanido ligands. Both cobalt(II) and Co(III) cations are six-coordinated in distorted octahedral environments. The shortest intrachain distance between the paramagnetic cobalt(II) ions is 7.36 Å, a value which is shorter than the shortest interchain one (10.36 Å). Variable-temperature (1.9–300 K) static (dc) magnetic measurements for 1 indicate the occurrence of magnetically isolated high-spin cobalt(II) ions with a D value of +67.0 cm−1. Dynamic alternating current (ac) magnetic measurements between 2.0–13 K reveal that 1 exhibits slow magnetic relaxation under non-zero applied dc fields, being thus a new example of field-induced SIM with easy-plane magnetic anisotropy. Theoretical calculations by CASSCF/NEVPT2 on 1 support the results from magnetometry. The relaxation of the magnetization occurs in the ground state under external dc fields through a two-phonon Raman process and one intra-Kramers mechanism.

Slow magnetic relaxation and photoluminescence behaviors of dilanthanide complexes bearing N8O2 donor macrocyclic ligand

On account of the complicated magnetic exchange interactions between lanthanide ions, binuclear lanthanide complexes have broad application prospect in the field of single-molecule magnets. Therefore, it is necessary to develop reasonable bridging ligands to manipulate the directional assembly of binuclear lanthanide complexes. Herein, we selected the macrocyclic ligand LN8O2 to build up two new dilanthanide complexes [Ln2(LN8O2) (OpyO)2(H2O)2](NO3)2 (1-Ln, Ln = Dy, Tb; LN8O2 = hexamethyl-tetraaza-dioxe-dipyrazolacycloicosaphane-2,9,12,19-tetraene; OpyOH = 2-pyridinol-1-oxide). Dynamic magnetic studies show that 1-Dy exhibits slow relaxation behavior under a 1 kOe applied field. Further fitting analysis of relaxation times gives the effective energy barrier of 38.2 cm−1, and reveals that the slow magnetic relaxation behavior is dominated by the Orbach and Raman processes. High-resolution luminescence emission spectrum indicates the energy gap of 36.8 cm−1 between the ground state and the first excited state, consistent with the magnetic measurement results. 1-Tb exhibits brilliant characteristic green light emission under UV light excitation. The absolute quantum yield of 1-Tb is 44.8%, and its first-order fitted decay lifetime is 779.21 μs at room temperature. This study provides the way for directional construction of high-performance molecular materials with magnetic and optical dual-function.

Plasmonic volcano-like fiber-optic probe for Raman enhancement.

Light-matter interaction is a fascinating topic extensively studied from classical theory, based on Maxwell's equations, to quantum optics. In this study, we introduce a novel, to the best of our knowledge, silver volcano-like fiber-optic probe (sensor 1) for surface-enhanced Raman scattering (SERS). We employ the emerging quasi-normal mode (QNM) method to rigorously calculate the Purcell factor for lossy open system responses, characterized by complex frequencies. This calculation quantifies the modification of the radiation rate from the excited state e to ground state g. Furthermore, we use and extend a quantum mechanical description of the Raman process, based on the Lindblad master equation, to calculate the SERS spectrum for the plasmonic structure. A common and well-established SERS probe, modified by a monolayer silver nanoparticle array, serves as a reference sensor (sensor 2) for quantitatively predicting the SERS performance of sensor 1 using quantum formalism. The predictions show excellent consistency with experimental results. In addition, we employ the FDTD (finite-difference time-domain) solver for a rough estimate of the all-fiber Raman response of both sensors, revealing a reasonable range of SERS performance differences compared to experimental results. This research suggests potential applications in real-time, remote detection of biological species and in vivo diagnostics. Simultaneously, the developed FDTD and quantum optics models pave the way for analyzing the response of emitters near arbitrarily shaped plasmonic structures.

Probing symmetry-breaking defects in polished graphitizable sp2 carbons using angle-resolved polarized Raman scattering

Raman scattering is commonly used to quantify defects in $s{p}^{2}$ carbons, mostly based on the values of the double-resonant D-band intensity. It is well known that this peculiar band undergoes changes when the $s{p}^{2}$ carbon is polished. However, the physical process behind this phenomenon is unclear so far. In this paper, we provide a deep understanding of the fundamental optical properties of the polishing-induced defects by combining angle-resolved polarized Raman scattering and theoretical calculations. We address the question regarding the types of these additional defects that participate in the intervalley double-resonance Raman process and contribute to the spatially inhomogeneous increase of the relative D-band intensity. The study was performed on anthracene-based coke pyrolyzed at $2000\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$ and uses highly oriented pyrolytic graphite with known crystallographic orientations as a model material. The results shed light on the types of defects as well as the D-band peculiar behavior as a function of the spatial orientation of graphene layers dictated by polishing with respect to the fixed direction of the incident and scattered light. Moreover, the insignificant change in ${I}_{2\mathrm{D}}/{I}_{\mathrm{G}}$, the width and the position of the G band after polishing, and the insensitivity of less ordered carbons to this process are also discussed.

Simulating two-dimensional correlation spectroscopies with third-order infrared and fifth-order infrared-Raman processes of liquid water.

To investigate the possibility of measuring the intermolecular and intramolecular anharmonic coupling of balk water, we calculate third-order two-dimensional (2D) infrared spectra and fifth-order 2D IR-IR-Raman-Raman spectra expressed in terms of four-body correlation functions of optical observables. For this purpose, a multimode Brownian oscillator model of four interacting anharmonic oscillators strongly coupled to their respective heat baths is employed. The nonlinearity of system-bath interactions is considered to describe thermal relaxation and vibrational dephasing. The linear and nonlinear spectra are then computed in a non-Markovian and nonperturbative regime in a rigorous manner using discretized hierarchical equations of motion in mixed Liouville-Wigner space. The calculated 2D spectra for stretching-bending, bending-librational, stretching-librational, and stretching-translational modes consist of various positive and negative peaks exhibiting essential details of intermolecular and intramolecular mode-mode interactions under thermal relaxation and dephasing at finite temperature.

Rate constants for saturation-recovery EPR and ELDOR of 14N-Spin labels

Saturation-recovery (SR)-EPR can determine electron spin–lattice relaxation rates in liquids over a wide range of effective viscosity, making it especially useful for biophysical and biomedical applications. Here, I develop exact solutions for the SR-EPR and SR-ELDOR rate constants of 14N-nitroxyl spin labels as a function of rotational correlation time and spectrometer operating frequency. Explicit mechanisms for electron spin–lattice relaxation are: rotational modulation of the N-hyperfine and electron-Zeeman anisotropies (specifically including cross terms), spin-rotation interaction, and residual frequency-independent vibrational contributions from Raman processes and local modes. Cross relaxation from mutual electron and nuclear spin flips, and direct nitrogen nuclear spin–lattice relaxation, also must be included. Both the latter are further contributions from rotational modulation of the electron-nuclear dipolar interaction (END). All the conventional liquid-state mechanisms are defined fully by the spin-Hamiltonian parameters; only the vibrational contributions contain fitting parameters. This analysis gives a firm basis for interpreting SR (and inversion recovery) results in terms of additional, less standard mechanisms.

Bound exciton luminescence and phonon sideband analysis of iodine intercalated bulk 2H-MoSe2 crystals

Temperature dependence of the luminescence spectra of MoSe2 crystals intercalated with I2 molecules has been investigated in the temperature range 11–100 K. The spectrum of luminescence, which is caused by the recombination of excitons bound on iodine molecules embedded in the van der Waals gap (vdW), consists of zero-phonon doublet at an energy less by 0.1 eV than the width of the indirect band gap of the host crystal, and its phonon replicas. The distance between the spectral lines of this A-B doublet constitutes Δ AB =5.6 meV. From the temperature dependence of the ratio of the A and B lines intensities, it was found, that the rate of radiative recombination of the exciton state B, which is responsible for the short-wavelength line B (EB = 1.0416 eV), is 76 times higher than the recombination rate of the A state (EA = .0360 eV). Based on a comparative analysis of the structure of the luminescence spectra at different temperatures and the measured Raman spectra, it is shown that the observed nine peaks of the phonon sideband are formed by only two vibrational modes with frequencies νph1 = 144 cm−1 and νph2 = 190 cm−1. The 1st frequency corresponds to the vibrational mode due to the second-order Raman process, and the 2nd – to the local vibrational mode induced by the halogen molecule embedded in the layered crystal structure. Finally, fundamental possibilities provided by the intercalation of halogen molecules in the interface of the van der Waals heterojunctions to modify their electronic properties are considered.

Self-similarity in transient stimulated Raman scattering in CO2-filled hollow-core fiber

We demonstrate experimentally the complex and extensive spectro-temporal dynamics of stimulated Raman scattering in C O 2 -filled hollow-core photonic crystal fiber, pumped by a single infrared microchip laser; namely, the cascading transient Raman process and self-similar behaviors in the regime of higher-order Stokes wave generation. Our measurements are in good agreement with the numerical simulations based on the nonlinear Schrödinger equation.