Absorption Line Shape Research Articles

Towards perfect NMR: Spin-echo versus perfect-echo building blocks.

The development of new tools to improve the quality of nuclear magnetic resonance (NMR) spectra is a challenging task. The concept of "perfect NMR" includes the design of robust pulse sequences that allow an investigator to obtain undistorted pure in-phase signals, with pure absorption lineshapes that are free of phase anomalies derived from undesired J modulations. Here, alternative NMR building blocks to the spin-echo that are based on a general double SE module, known as a perfect-echo, are reviewed. Several implementations to minimize/remove unwanted dispersive contributions in homonuclear and heteronuclear NMR experiments are described and illustrated with some examples of broad interest for small molecules.

Role of tunneling induced coherence in modulation of absorption and dispersion in a quantum dot molecule with symmetrical coupling configuration

Coherent control of optical effects are studied in a quantum dot molecule comprising triple quantum dots with the consideration of symmetric tunneling induced coupling. We have analyzed the origin of such coherence effects in terms of perturbative influence of the array of tunneling induced coherence terms associated with the semiclassical density matrix element relating to optically induced exciton generation. It has been shown that the coherence terms can significantly modify the absorptive and dispersive line shapes due to the change of condition of weak tunneling to moderate tunneling limited by the coherence dephasing effect. We have illustrated the evolution of Fano-like resonance controlling the absorptive line shapes under the conditions mentioned above. We have shown how, depending upon the variation of gate-biasing voltage, dispersion like features evolve in the absorptive case, while absorption-peak like features originate in case of dispersion. This is due to the appearance of non-linear Fano resonance inherent to this model. Our results highlight the appearance of a negative refractive index like pattern in the dispersive response of the line shape under such conditions.

Exciton-polaron spectral structures in two-dimensional hybrid lead-halide perovskites

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. Here, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. We present a phenomenological, yet quantitative framework to simulate excitonic absorption lineshapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by $35\pm5$\,meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the lineshape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Our work highlights the complexity of excitonic properties in this class of nanostructured materials.

Optimal iodine absorption line applied for spaceborne high spectral resolution lidar.

Spaceborne high spectral resolution lidar (HSRL) provides a wide range of observations, e.g., measurements of aerosol backscattering and extinction coefficients and aerosol depolarization ratio with high accuracy, which are of great significance to the study of air pollution monitoring and climate change. With different cells and finger temperatures, the transmittance of the different absorption lines of the iodine vapor filter at 532nm wavelength was measured. The 1064nm fundamental frequency pulse energy and the 532nm frequency-doubled pulse energy output of different seeder laser wavelengths were measured. Based on the relationship among the laser output power, the absorption line shape of the iodine vapor filter, and the atmospheric model, the echo power was calculated and compared. The 1110 iodine absorption line was selected as the optimized filter for the HSRL, which could increase in 22% and 14% efficiency compared with the traditional 1109 line, and a new proposed 1105 line at the 532nm HSRL channel at 5km altitude with an enhanced aerosol model, respectively.

Rigid versus Flexible Protein Matrix: Light-Harvesting Complex II Exhibits a Temperature-Dependent Phonon Spectral Density.

Dynamics-function correlations are usually inferred when molecular mobility and protein function are simultaneously impaired at characteristic temperatures or hydration levels. In this sense, excitation energy transfer in the photosynthetic light-harvesting complex II (LHC II) is an untypical example because it remains fully functional even at cryogenic temperatures relying mainly on interactions of electronic states with protein vibrations. Here, we study the vibrational and conformational protein dynamics of monomeric and trimeric LHC II from spinach using inelastic neutron scattering (INS) in the temperature range of 20-305 K. INS spectra of trimeric LHC II reveal a distinct vibrational peak at ∼2.4 meV. At temperatures above ∼160 K, however, the inelastic peak shifts toward lower energies, which is attributed to vibrational anharmonicity. A more drastic shift is observed at about 240 K, which is interpreted in terms of a "softening" of the protein matrix along with the dynamical transition. Monomeric LHC II exhibits a higher degree of conformational mobility at physiological temperatures, which can be attributed to a higher number of solvent-exposed side chains at the protein surface. The effects of the changes in protein dynamics on the spectroscopic properties of LHC II are considered in comparative model calculations. The absorption line shapes of a pigment molecule embedded into LHC II are simulated for the cases of (i) a rigid protein matrix, (ii) a protein matrix with temperature-dependent spectral density of protein vibrations, and (iii) temperature-dependent electron-phonon coupling strength. Our findings indicate that vibrational and conformational protein dynamics affect the spectroscopic (absorption) properties of LHC II at physiological temperatures.

Tailoring optical responses of infrared plasmonic metamaterial absorbers by optical phonons.

Here we numerically and experimentally explore the rich phenomena in the optical responses and local electromagnetic fields of a MIM absorber mediated by the dispersive spacer. We first show that the line shape of the spectral absorption is strongly modified by the interaction between the plasmonic resonances and the optical phonons in the silicon dioxide spacer. Importantly, broadening the spectral absorption in the long wave infrared range is achieved by tuning the strength of coupling. Modification to the local electromagnetic field distribution in the epsilon-near-zero region is also numerically studied. The incident-angle dependence and polarization dependence of the broadened absorption spectrum are evaluated. We also show that the spectral broadening mechanism can be generalized to other frequency bands by employing different spacing materials such as silicon nitride and polydimethylsiloxane. Our results can be useful for designing spectrally selective thermal detectors and thermal emitters.

Simulations of absorption and fluorescence lineshapes using the reaction coordinate method

Absorption and fluorescence lineshapes reflect a complex interplay between electronic and vibrational dynamics in molecular aggregates. Entanglement between the electronic system and vibrational bath poses a considerable challenge to accurate spectroscopic simulations. It can be described by accounting for at least some of the bath explicitly. Recently such type of approach formulated in terms of a reaction coordinate master equation was proposed (Iles-Smith et al., 2014). It is based on the reaction coordinate mapping, which permits to identify the most influential collective bath modes and include them into an enlarged system explicitly. Here we extend this reaction coordinate method (RCM) for calculations of absorption and fluorescence lineshapes. We apply the theory to a model dimer and obtain excellent agreement with exact results calculated with hierarchical equations of motion. This suggests that the RCM approach is applicable for simulations of linear and non-linear spectra of molecular aggregates.

Surface States Mediate Triplet Energy Transfer in Nanocrystal-Acene Composite Systems.

Hybrid organic:inorganic materials composed of semiconductor nanocrystals functionalized with acene ligands have recently emerged as a promising platform for photon upconversion. Infrared light absorbed by a nanocrystal excites charge carriers that can pass to surface-bound acenes, forming triplet excitons capable of fusing to produce visible radiation. To fully realize this scheme, energy transfer between nanocrystals and acenes must occur with high efficiency, yet the mechanism of this process remains poorly understood. To improve our knowledge of the fundamental steps involved in nanoparticle:acene energy transfer, we used ultrafast transient absorption to investigate excited electronic dynamics of PbS nanocrystals chemically functionalized with 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) ligands. We find photoexcitation of PbS does not lead to direct triplet energy transfer to surface-bound TIPS-pentacene molecules but rather to the formation of an intermediate state within 40 ps. This intermediate persists for ∼100 ns before evolving to produce TIPS-pentacene triplet excitons. Analysis of transient absorption lineshapes suggests this intermediate corresponds to charge carriers localized at the PbS nanocrystal surface. This hypothesis is supported by constrained DFT calculations that find a large number of spin-triplet states at PbS NC surfaces. Though some of these states can facilitate triplet transfer, others serve as traps that hinder it. Our results highlight that nanocrystal surfaces play an active role in mediating energy transfer to bound acene ligands and must be considered when optimizing composite NC-based materials for photon upconversion, photocatalysis, and other optoelectronic applications.

The modeling of the absorption lineshape for embedded molecules through a polarizable QM/MM approach.

We present a computational strategy to simulate the absorption lineshape of a molecule embedded in a complex environment by using a polarizable QM/MM approach. This strategy is presented in two alternative formulations, one based on a molecular dynamics simulation of the structural fluctuations of the system and the other using normal modes and harmonic frequencies calculated on optimized geometries. The comparison for the case of a chromophore within a strongly inhomogeneous and structured environment, namely the intercalation pocket of DNA, shows that the MD-based approach is able to reproduce the experimental spectral bandshape. In contrast, the static approach overestimates the vibronic coupling, resulting in a much broader band.

Tunability of the optical absorption and refractive index changes in step-like and parabolic quantum wells under external electric field

In this paper, the effects of an external electric field on the energy transitions and optical absorption lineshape in step and parabolic GaAs quantum wells are theoretically investigated. The energy separations between the three lowest energy levels E1, E2 and E3 and their corresponding wavefunctions are calculated by solving the Schrödinger equation. Our theoretical findings show that the intersubband transitions and the optical absorption in the step-like quantum well can be tuned and controlled by an applied electric field (F). Especially the optical absorption coefficient α12(ω) presents a blue shift by increasing the intensity of F. Contrary to the case of step-like quantum well, the energy transition and the optical absorption coefficient α12(ω) are less sensitive to the applied electric field F in the case of parabolic quantum well. In the latter case α12(ω) presents a small red-shift variation. The obtained behavior in the optical absorption coefficient and refractive index changes can be used to design and fabricate new optical devices such as optical modulators.

Computational Modeling of Exciton-Bath Hamiltonians for Light Harvesting 2 and Light Harvesting 3 Complexes of Purple Photosynthetic Bacteria at Room Temperature.

Light harvesting 2 (LH2) complex is the primary component of the photosynthetic unit of purple bacteria that is responsible for harvesting and relaying excitons. The electronic absorption line shape of LH2 contains two major bands at 800 and 850 nm wavelength regions. Under low light conditions, some species of purple bacteria replace LH2 with light harvesting 3 (LH3), a variant form with almost the same structure as the former but with distinctively different spectral features. The major difference between the absorption line shapes of LH2 and LH3 is the shift of the 850 nm band of the former to a new 820 nm region. The microscopic origin of this difference has been the subject of some theoretical/computational investigations. However, the genuine molecular level source of such a difference is not clearly understood yet. This work reports a comprehensive computational study of LH2 and LH3 complexes so as to clarify different molecular level features of LH2 and LH3 complexes and to construct simple exciton-bath models with a common form. All-atomistic molecular dynamics simulations of both LH2 and LH3 complexes provide detailed molecular level structural differences of bacteriochlorophylls (BChls) in the two complexes, in particular, in their patterns of hydrogen bonding (HB) and torsional angles of the acetyl group. Time-dependent density functional theory calculation of the excitation energies of BChls for structures sampled from the MD simulations suggests that the observed differences in the HB and torsional angles cannot fully account for the experimentally observed spectral shift of LH3. Potential sources that can explain the actual spectral shift of LH3 are discussed, and their magnitudes are assessed through fitting of experimental line shapes. These results demonstrate the feasibility of developing simple exciton-bath models for both LH2 and LH3, which can be employed for large-scale exciton quantum dynamics in their aggregates.

Multi-property isotropic intermolecular potentials and predicted spectral lineshapes of collision-induced absorption (CIA), collision-induced light scattering (CILS) and collision-induced hyper-Rayleigh scattering (CIHR) for H2[sbnd]Ne, −Kr and −Xe

Quantum mechanical lineshapes of collision-induced absorption (CIA), collision-induced light scattering (CILS) and collision-induced hyper-Rayleigh scattering (CIHR) at room temperature (295 K) are computed for gaseous mixtures of molecular hydrogen with neon, krypton and xenon. The induced spectra are detected using theoretical values for induced dipole moment, pair-polarizability trace and anisotropy, hyper-polarizability and updated intermolecular potentials. Good agreement is observed for all spectra when the literature and the present potentials which are constructed from the transport and thermo-physical properties are used.

Current developments in homonuclear and heteronuclear J-resolved NMR experiments.

Two-dimensional J-resolved (Jres) NMR experiments offer a simple, user-friendly spectral representation where the information of coupling constants and chemical shifts are separated into two orthogonal frequency axis. Since its initial proposal 40years ago, Jres has been the focus of considerable interest both in improving the basic pulse sequence and in its successful application to a wide range of studies. Here, the latest developments in the design of novel Jres pulse schemes are reviewed, mainly focusing on obtaining pure absorption lineshapes, minimizing strong coupling artifacts, and also optimizing sensitivity and experimental measurements. A discussion of several Jres versions for the accurate measurement of a different number of homonuclear (JHH ) and heteronuclear (JCH ) coupling constants is presented, accompanied by some illustrative examples.

Pressure broadening and frequency shift of the D1 and D2 lines of K in the presence of Ne and Kr

We present the results of pressure broadening and frequency shift of K D1 and D2 lines in presence of 1–4 amg of Neon gas and 1–5 amg of Krypton gas by laser absorption spectroscopy. Both pressure broadening and frequency shift are linearly related to gas density with high accuracy. The asymmetry of the absorption line shape caused by van der Waals potential was first found in the near-line wings of large density Kr in the experiment. We have also investigated the temperature dependence of the pressure broadening and frequency shift in a range of 353–403 K in Neon and 373–417 K in Krypton and compared the results of the pressure broadening and frequency shift with previous values.

Magnetized Disk Winds in NGC 3783

Abstract We analyze a 900 ks stacked Chandra/HETG spectrum of NGC 3783 in the context of magnetically driven accretion-disk wind models in an effort to provide tight constraints on the global conditions of the underlying absorbers. Motivated by the earlier measurements of its absorption measure distribution (AMD) indicating X-ray-absorbing ionic columns that decrease slowly with decreasing ionization parameter, we employ 2D magnetohydrodynamic (MHD) disk wind models to describe the global outflow. We compute its photoionization structure along with the wind kinematic properties, allowing us to further calculate in a self-consistent fashion the shapes of the major X-ray absorption lines. With the wind radial density profile determined by the AMD, the profiles of the ensemble of the observed absorption features are determined by the two global parameters of the MHD wind; i.e., disk inclination and wind density normalization n o . Considering the most significant absorption features in the ∼1.8–20 Å range, we show that the MHD wind is best described by cm−3 and . We argue that winds launched by X-ray heating or radiation pressure, or even MHD winds but with steeper radial density profiles, are strongly disfavored by data. Considering the properties of Fe K-band absorption features (i.e., Fe xxv and Fe xxvi), while typically prominent in the active galactic nucleus X-ray spectra, they appear to be weak in NGC 3783. For the specific parameters of our model obtained by fitting the AMD and the rest of the absorption features, these features are found to be weak, in agreement with observations.

Explicit extraction of absorption peak positions, widths and heights using higher order derivatives of total shape spectra by nonparametric processing of time signals as complex damped multi-exponentials

Sole lineshape estimation of non-parametrically computed higher-order derivatives of spectral envelopes in different modes (complex, real, imaginary, magnitude) is investigated. The processed time signals are sums of complex attenuated exponentials (harmonics). The peak parameters of the derivative spectra are directly connected to those of the customary (non-derivative) absorption lineshapes. Crucially, this permits the reconstruction of the latter from the former parameters (the latter are sought since they are unknown, whereas the former are extractable from the derivative envelopes). The cross-checking relationships of the lineshapes for the magnitude modes with the real and imaginary parts of the complex-valued envelopes (total shape spectra) are established. The explicit procedure and the necessary analytical expressions are presented for reconstruction of the exact locations, widths and heights of all the retrieved physical resonances (spectral peaks). These facets are illuminated in the derivative fast Padé transform (dFPT) using its non-parametric version, i.e. without solving the quantification problem (no polynomial rooting, no tackling of eigen-value problems, etc.). Two kinds of illustrations for derivative spectra are reported. One deals with the general Breit-Wigner resonance formula and its first three derivatives. The other is concerned with the dFPT in clinical diagnostics of relevance to breast cancer detection by magnetic resonance spectroscopy. A systematic parallel between these two examples is drawn to highlight, in a stepwise manner, the role of paramount importance played by derivative lineshapes, especially for disentangling overlapping resonances that invariably plague all quantitative analyses of spectra.

Fluorescence and Excited-State Conformational Dynamics of the Orange Carotenoid Protein.

The orange carotenoid protein (OCP) mediates nonphotochemical quenching (NPQ) mechanisms in cyanobacteria. A bound ketocarotenoid serves as a sensor of midvisible light intensity and as a quencher of phycocyanobilin excitons in the phycobilisome. The photochemical mechanism that triggers conversion of the protein from a resting, orange state (OCPO) to an active, red state (OCPR) after optical preparation of the S2 state of the carotenoid remains an open question. We report here that the fluorescence spectrum and quantum yield of the bound carotenoids in OCPO report important details of the motions that follow optical preparation of the S2 state. The fluorescence spectra from OCPO preparations containing 3'-hydroxyechinenone (3hECN) or canthaxanthin (CAN) are markedly mirror asymmetric with respect to the absorption line shape and more than an order of magnitude more intense than for carotenoids in solution. Further, 3hECN exhibits a narrower fluorescence line shape and a larger quantum yield than CAN because its excited-state motions are hindered by a hydrogen bonding interaction between the 3'-hydroxyl group on its β2 ring and Leu37 in the N-terminal domain. These results show that large-amplitude motions of the carotenoid's β2-cyclohexene ring and of the conjugated polyene backbone initiate photochemistry in OCPO.

Transient-absorption phases with strong probe and pump pulses

The quantum dynamics of a system of Rb atoms, modeled by a V-type three-level system interacting with intense probe and pump pulses, are studied. The time-delay-dependent transient-absorption spectrum of an intense probe pulse is thus predicted, simulating pump-probe experiments in which this is preceded or followed by a strong pump pulse. Numerical results are interpreted in terms of an analytical model based on interaction operators, which quantify the transformation undergone by the system under the action of an intense pulse. The oscillating features of the resulting transient-absorption spectra, due to the coupling of several excited states, are thus interpreted in terms of the atomic population and phase changes imposed by the pump and probe pulses. Strong-field-induced phases and their influence on the resulting transient-absorption spectra are thereby investigated for different values of pump and probe intensities and frequencies, focusing on the atomic properties which are encoded in the absorption line shapes for positive and negative time delays.

Absorption linewidth inversion with wavelength modulation spectroscopy**Project supported by the National Natural Science Foundation of China (Grant No. 61505142) and the Tianjin Natural Science Foundation (Grant No. 16JCQNJC02100).

For absorption linewidth inversion with wavelength modulation spectroscopy (WMS), an optimized WMS spectral line fitting method was demonstrated to infer absorption linewidth effectively, and the analytical expressions for relationships between Lorentzian linewidth and the separations of first harmonic peak-to-valley and second harmonic zero-crossing were deduced. The transition of CO2 centered at 4991.25 cm−1 was used to verify the optimized spectral fitting method and the analytical expressions. Results showed that the optimized spectra fitting method was able to infer absorption accurately and compute more than 10 times faster than the commonly used numerical fitting procedure. The second harmonic zero-crossing separation method calculated an even 6 orders faster than the spectra fitting without losing any accuracy for Lorentzian dominated cases. Additionally, linewidth calculated through second harmonic zero-crossing was preferred for much smaller error than the first harmonic peak-to-valley separation method. The presented analytical expressions can also be used in on-line optical sensing applications, electron paramagnetic resonance, and further theoretical characterization of absorption lineshape.

A novel combined experimental and multiscale theoretical approach to unravel the structure of SiC/SiOx core/shell nanowires for their optimal design.

In this work we propose a realistic model of nanometer-thick SiC/SiOx core/shell nanowires (NWs) using a combined first-principles and experimental approach. SiC/SiOx core/shell NWs were first synthesised by a low-cost carbothermal method and their chemical-physical experimental analysis was accomplished by recording X-ray absorption near-edge spectra. In particular, the K-edge absorption lineshapes of C, O, and Si are used to validate our computational model of the SiC/SiOx core/shell NW architectures, obtained by a multiscale approach, including molecular dynamics, tight-binding and density functional simulations. Moreover, we present ab initio calculations of the electronic structure of hydrogenated SiC and SiC/SiOx core/shell NWs, studying the modification induced by several different substitutional defects and impurities into both the surface and the interfacial region between the SiC core and the SiOx shell. We find that on the one hand the electron quantum confinement results in a broadening of the band gap, while hydroxyl surface terminations decrease it. This computational investigation shows that our model of SiC/SiOx core/shell NWs is capable to deliver an accurate interpretation of the recorded X-ray absorption near-edge spectra and proves to be a valuable tool towards the optimal design and application of these nanosystems in actual devices.