Photoelectrochemical Impedance Spectroscopy Research Articles

Effective degradation of tetracycline under visible light by microwave-assisted reflux synthesis of ZnO/MoS2/rGO ternary nanocomposites

ZnO@MoS2 and conducting rGO based ternary nanocomposites were synthesized by a bottom-up microwave-assisted reflux method and effectively investigated for the photocatalytic degradation of tetracycline hydrochloride (TCH) antibiotic. The XRD, EDAX and XPS results corroborates the formation of nanostructure composite with high phase purity. The ZnO nanorods are well grafted on the curled configuration of layered MoS2 and thin surfaced rGO sheets as visualised from HR-TEM analysis. Upon addition of rGO, there is a significant decrease in the bandgap and improved visible light response in the ternary nanocomposites is elucidated by UV–visible Diffuse Reflectance Spectroscopy (UV-DRS) analysis. The suppression in photo-induced electron/hole pair recombination and allowing easy charge transport of ZnMG-50 were evidenced from photoluminescence (PL) and photo-electrochemical impedance (photo-EIS) spectroscopy. From BET and BJH analysis, the enhancement in surface area and pore volume by loading of rGO (SBET = 264.794 m2/g and 0.548 cm3/g). The optimized ZnMG-50 nano-photocatalyst possess a superior photocatalytic performance with higher removal efficiency of 92.18 % was achieved at pH level 10, catalyst dosage 40 mg and TCH concentration 20 ppm within 180 min under illumination. The degradation process compiles a pseudo-first order reaction with a kinetic rate constant of 0.01365 min−1. The nonstop generation of hydroxyl (˙OH) and superoxide (O2˙-) reactive species were beneficial for free electrons in the active participation, which remains as a core factor in the degradation process. The detailed study on ZnMG-50 nanocomposite substantiated to be an efficient noble metal-free synthetic catalyst for enhanced photocatalytic degradation of antibiotics.

Combined Effect of Underlayer and Deposition Solution to Optimize the Alignment of Hematite Photoanodes.

This study investigates the optimization of hematite (α-Fe2O3) photoanodes for enhanced photoelectrochemical (PEC) performance and reproducibility, which are crucial for photocatalytic applications. Despite hematite's potential, hindered by inherent limitations, significant improvements were realized by introducing a titanium dioxide (TiO2) underlayer and ethanol-modified deposition. The influence of the deposition methods was understood by potential-dependent photoelectrochemical impedance spectroscopy analysis. The introduction of the TiO2 underlayer effectively increased the density of states, preferable for the electron transport in the bulk hematite, and the ethanol deposition on a TiO2 underlayer led to a stable surface state formation (S1 state) for the photoexcited hole transfer. This analysis illuminated the intricate interplay between electron transport in the bulk and photogenerated hole transfer at the solution interface, thereby facilitating smoother charge transfer. These findings underscore the viability of surface engineering and meticulous process optimization in addressing critical challenges in photocatalyst development.

A kinetics-based approach to the steady-state and impedance response of photoelectrodes

A kinetics-based analytical model is developed to describe the steady-state and impedance response of photoelectrodes in the case where charge carrier build-up modifies the potential distribution across the semiconductor-electrolyte junction. The predictions of the steady-state model are compared with numerical calculations. It is shown that carrier build up should accelerate interfacial hole transfer by increasing the potential drop across the Helmholtz layer. This effect may explain observations of higher reaction orders with respect to hole concentration during photoelectrochemical oxygen evolution. The analytical model is extended to photoelectrochemical impedance spectroscopy (PEIS). The calculations indicate that interpretation of PEIS measurements becomes more complicated if high illumination intensities are used. The modeling of the analytically calculated PEIS response using equivalent circuits is discussed with the aim of understanding the physical significance of the resistive and capacitive elements.

Study of photo-induced charge transfer properties in Cs3Bi2I9/BiFeO3/TiO2 film

Inorganic lead-free perovskite Cs3Bi2I9 has attracted extensive attention due to its enhanced stability and environmental friendliness. However, the photon-to-current conversion of Cs3Bi2I9 perovskites is not efficient, partly due to inefficient electron-hole separation. In this manuscript, BiFeO3 was deposited at the interface of Cs3Bi2I9/TiO2 as the interface modifier by sol–gel method, and the photocurrent and responsiveness of the Cs3Bi2I9/BiFeO3/TiO2 were significantly higher than those of the Cs3Bi2I9/TiO2 based device. Kelvin probe force microscope, photoelectrochemical impedance spectroscopy and in-situ surface photovoltage spectra demonstrated that a self-polarized electric field of BiFeO3 contributed to the separation of photogenerated charge carriers at the Cs3Bi2I9/TiO2 interface. By analyzing the surface photovoltage phase spectra via a vector model method, complicated surface photovoltage processes were identified, and a detailed relationship between photocurrent generation and transfer characteristic of the photogenerated charge carriers have been discussed.

Electrostatic [FeFe]-hydrogenase-carbon nitride assemblies for efficient solar hydrogen production.

The assembly of semiconductors as light absorbers and enzymes as redox catalysts offers a promising approach for sustainable chemical synthesis driven by light. However, achieving the rational design of such semi-artificial systems requires a comprehensive understanding of the abiotic-biotic interface, which poses significant challenges. In this study, we demonstrate an electrostatic interaction strategy to interface negatively charged cyanamide modified graphitic carbon nitride (NCNCNX) with an [FeFe]-hydrogenase possessing a positive surface charge around the distal FeS cluster responsible for electron uptake into the enzyme. The strong electrostatic attraction enables efficient solar hydrogen (H2) production via direct interfacial electron transfer (DET), achieving a turnover frequency (TOF) of 18 669 h-1 (4 h) and a turnover number (TON) of 198 125 (24 h). Interfacial characterizations, including quartz crystal microbalance (QCM), photoelectrochemical impedance spectroscopy (PEIS), intensity-modulated photovoltage spectroscopy (IMVS), and transient photocurrent spectroscopy (TPC) have been conducted on the semi-artificial carbon nitride-enzyme system to provide a comprehensive understanding for the future development of photocatalytic hybrid assemblies.

A comparative study of H2O2 production over g-C3N4 photocatalysts made from different sources

The photocatalytic performance of graphitic carbon nitride (g-C3N4) in H2O2 production has been constantly studied, however, whether g-C3N4 made from different raw materials possess distinct photocatalytic ability is not fully explored. In this work, a comparative study of g-C3N4 made from urea, melamine, thiourea, and dicyandiamide respectively under the same thermal process condition was carried out to distinguish their photocatalytic activities. By determining the yield of H2O2 produced under the same light conditions, the ability of the above four g-C3N4 photocatalysts was compared and the amount of H2O2 produced within 5 h was 55.8, 112.7, 101, and 134 µmol/L respectively. Photoelectrochemical and electrochemical impedance spectroscopy measurements indicated that g-C3N4 photocatalysts made from melamine and dicyandiamide with higher H2O2 production had higher photocurrent and lower charge transfer impedance. This might relate to the differences in material crystallinity and the band gap, which influenced their effective carrier concentration.

Exploring the role of Ti3C2TxMXene in photoelectrochemical water splitting over solution-processed BiVO4 photoanode

Transition metal carbides, nitrides, or carbonitrides known as MXenes commonly improve the efficiency of photo(electro)catalysts, but their role is still not fully explored. Herein, we developed a novel and facile solution-processing method to fabricate BiVO4 and Ti3C2Tx-modified BiVO4 (Ti3C2Tx/BiVO4) photoanodes to explore the role of Ti3C2Tx MXene in photoelectrochemical water splitting. The charge transfer and recombination kinetics were studied by transient open-circuit potential, transient photocurrent, photoelectrochemical impedance spectroscopy, and intensity-modulated photocurrent spectroscopy. It was demonstrated that Ti3C2Tx forms a Schottky-like junction at the BiVO4/Ti3C2Tx interface and that it acts as a hole transport/storage layer. The Ti3C2Tx layer creates a built-in electric field at the surface that increases the band bending and space charge layer width, eventually enhancing the charge separation efficiency. Modification of Ti3C2Tx/BiVO4 with a water oxidation catalyst such as cobalt phosphate (Co-Pi) was essential to deliver the maximum photocurrent. For instance, the Co-Pi/Ti3C2Tx/BiVO4 photoanode delivered 4.72 mA/cm−2 at 1.23 VRHE, which is 2- and 11-fold higher than those of Ti3C2Tx/BiVO4 and pristine BiVO4 photoanodes, respectively. This study provides a practical protocol for fabricating Ti3C2Tx/BiVO4 and reveals the role of MXene in photoelectrochemical water splitting.

Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoOx-loaded bismuth vanadate.

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoOx) loaded on bismuth vanadate (BiVO4) to investigate the impact of CoOx on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V5+ in BiVO4 promotes the formation of a surface state on CoOx that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoOx-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoOx(OH)y layer dominates over single sites. Therefore, CoOx is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.

Effect of Interfacial SiOx Defects on the Functional Properties of Si-Transition Metal Oxide Photoanodes for Water Splitting.

The transfer of photogenerated charges through interfaces in heterojunction photoanodes is a key process that controls the efficiency of solar water splitting. Considering Co3O4/SiOx/Si photoanodes prepared by physical vapor deposition as a representative case study, it is shown that defects normally present in the native SiOx layer dramatically affect the onset of the photocurrent. Electron paramagnetic resonance indicates that the signal of defects located in dangling bonds of trivalent Si atoms at the Si/SiOx interface vanishes upon vacuum annealing at 850 °C. Correspondingly, the photovoltage of the photoanode increases to ≈500 mV. Similar results are obtained for NiO/SiOx/Si photoanodes. Photoelectrochemical analysis and impedance spectroscopy (in solution and in the solid state) indicate how the defect annealing modifies the Co3O4/SiOx/Si junction. This work shows that defect annealing at the solid-solid interface in composite photoanodes strongly improves the efficiency of charge transfer through interfaces, which is the basis for effective solar-to-chemical energy conversion.

High‐Performance Silver‐Doped Porous CuBi2O4 Photocathode Integrated with NiO Hole‐Selective Layer for Improved Photoelectrochemical Water Splitting

AbstractCuBi2O4(CBO) has received considerable attention owing to its ideal optical bandgap and positive photocurrent onset potential. However, CBO photocathodes exhibit poor charge carrier separation and transfer across the conducting substrate interface. Herein, a systematic incorporation of Ag+‐cations into nanosized porous CBO network (ACBO) using a simple pulsed‐electrodeposition method. In ACBO photocathode, the Ag+‐ions replace the Bi3+‐ions, thereby building an increased hole concentration, which further signifies the photogenerated electron‐hole separation. Additionally, introducing a low‐cost NiO hole‐selective layer between ACBO and the conducting substrate enables a back‐interface‐aided hole‐extraction and electron blocking, resulting in an improved charge transfer across the back interface. Compared with an unmodified CBO, the NiO/ACBO photocathode exhibits a three‐fold enhanced photocurrent performance. This enhances the photocurrent originating from the incorporation of a substantial amount of Ag+‐ions into the CBO structure, leading to an increased acceptor density as well as the formation of an appropriate hole‐selective layer across the back‐contact. The absorption percentage, time‐resolved photoluminescence, and photoelectrochemical impedance spectroscopy measurements unveil the potential light harvesting, charge separation, and transfer characteristics of the NACBO photoelectrode, respectively. Through this systematic study, an efficient and simple strategy is determined for developing ternary metal oxide‐based photocathode/photoanode systems for sustainable energy applications.

Fe-Ce/Layered Double Hydroxide Heterostructures and Their Derived Oxides: Electrochemical Characterization and Light-Driven Catalysis for the Degradation of Phenol from Water

Fe-Ce/layered double hydroxides (LDHs) were synthesized via a facile route by exploiting the "structural memory" of the LDH when the calcined MgAlLDH and ZnAlLDH were reconstructed in the aqueous solutions of FeSO4/Ce(SO4)2. XRD analysis shows the formation of heterostructured catalysts that entangle the structural characteristics of the LDHs with those of Fe2O3 and CeO2. Furthermore, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, TG/DTG, SEM/EDX and TEM results reveal a complex morphology defined by the large nano/microplates of the reconstructed LDHs that are tightly covered with nanoparticles of Fe2O3 and CeO2. Calcination at 850 °C promoted the formation of highly crystallized mixed oxides of Fe2O3/CeO2/ZnO and spinels. The photo-electrochemical behavior of Fe-Ce/LDHs and their derived oxides was studied in a three-electrode photo-electrochemical cell, using linear sweep voltammetry (LSV), Mott-Schottky (M-S) analysis and photo-electrochemical impedance spectroscopy (PEIS) measurements, in dark or under illumination. When tested as novel catalysts for the degradation of phenol from aqueous solutions, the light-driven catalytic heterojunctions of Fe-Ce/LDH and their derived oxides reveal their capabilities to efficiently remove phenol from water, under both UV and solar irradiation.

Photoelectrochemical CO2 Reduction at a Direct CuInGaS2/Electrolyte Junction.

Photoelectrochemical (PEC) CO2 reduction has received considerable attention given the inherent sustainability and simplicity of directly converting solar energy into carbon-based chemical fuels. However, complex photocathode architectures with protecting layers and cocatalysts are typically needed for selective and stable operation. We report herein that bare CuIn0.3Ga0.7S2 photocathodes can drive the PEC CO2 reduction with a benchmarking 1 Sun photocurrent density of over 2 mA/cm2 (at -2 V vs Fc+/Fc) and a product selectivity of up to 87% for CO (CO/all products) production while also displaying long-term stability for syngas production (over 44 h). Importantly, spectroelectrochemical analysis using PEC impedance spectroscopy (PEIS) and intensity-modulated photocurrent spectroscopy (IMPS) complements PEC data to reveal that tailoring the proton donor ability of the electrolyte is crucial for enhancing the performance, selectivity, and durability of the photocathode. When a moderate amount of protons is present, the density of photogenerated charges accumulated at the interface drops significantly, suggesting a faster charge transfer process. However, with a high concentration of proton donors, the H2 evolution reaction is preferred.

Roles of Surface States in Cocatalyst-Loaded Hematite Photoanodes for Water Oxidation

Photoelectrochemical water oxidation over bare hematite (α-Fe2O3) involved physical–chemical processes through surface states. It is found that the surface state with a higher oxidative energy (S1) served as a reaction intermediate for water oxidation, while the other surface state with a lower oxidative energy (S2) was basically catalytically inactive and induced charge recombination. Since a cocatalyst is commonly loaded on α-Fe2O3 to enhance water oxidation, the physical–chemical processes between the surface states and the cocatalyst are supposedly important but remain unclear. In the present study, we elucidated such processes by employing photoelectrochemical impedance spectroscopy. We found that, in a CoPi-loaded α-Fe2O3, S1 no longer served as an intermediate for water oxidation and was mostly passivated. At the same time, S2 became the key intermediate for water oxidation by serving as a hole reservoir, prolonging the hole lifetime, and finally transferring holes to CoPi. This study reveals the surface chemistry for optimizing cocatalyst-loaded α-Fe2O3, which is different from that for bare α-Fe2O3.

Degradation of Methylparaben Using Optimal WO3 Nanostructures: Influence of the Annealing Conditions and Complexing Agent

In this work, WO3 nanostructures were synthesized with different complexing agents (0.05 M H2O2 and 0.1 M citric acid) and annealing conditions (400 °C, 500 °C and 600 °C) to obtain optimal WO3 nanostructures to use them as a photoanode in the photoelectrochemical (PEC) degradation of an endocrine disruptor chemical. These nanostructures were studied morphologically by a field emission scanning electron microscope. X-ray photoelectron spectroscopy was performed to provide information of the electronic states of the nanostructures. The crystallinity of the samples was observed by a confocal Raman laser microscope and X-ray diffraction. Furthermore, photoelectrochemical measurements (photostability, photoelectrochemical impedance spectroscopy, Mott-Schottky and water-splitting test) were also performed using a solar simulator with AM 1.5 conditions at 100 mW·cm-2. Once the optimal nanostructure was obtained (citric acid 0.01 M at an annealing temperature of 600 °C), the PEC degradation of methylparaben (CO 10 ppm) was carried out. It was followed by ultra-high-performance liquid chromatography and mass spectrometry, which allowed to obtain the concentration of the contaminant during degradation and the identification of degradation intermediates. The optimized nanostructure was proved to be an efficient photocatalyst since the degradation of methylparaben was performed in less than 4 h and the kinetic coefficient of degradation was 0.02 min-1.

Improving charge transfer in phosphate ion-modified hematite photoanode via decoration with gold nanostructures

Herein, we report a unique synergistic behavior between the phosphate ions (Pi) layer and the plasmonic gold nanostructures to enhance the PEC performance of the hematite photoanode. Under such a combined effect, the lifetime of the photogenerated holes is enhanced by a factor of 3-fold relative to that of pristine hematite. The Au/Pi–hematite photoanode illustrates a stable photocurrent of 1.32 mA cm–2 at 1.23 VRHE under standard illumination conditions, which is 3.1, 2.1, and 1.7 times higher than those of pristine hematite, Au/hematite, and Pi–modified hematite photoanodes, respectively. The enhanced photocurrent of Au/Pi–hematite is attributed to the improved hole lifetime and charge transfer kinetics as confirmed by transient photocurrent (TPC), photoelectrochemical impedance spectroscopy (PEIS), and intensity-modulated photocurrent spectroscopy (IMPS) measurements. Au nanostructures have a dual function. They enhance the light absorption ability and more importantly improve the charge transfer efficiency.

Enhancement of photoelectrochemical performance of Bismuth vanadate (BiVO4)-Based photoanode by building phase-junction configurations

To enhance the photo-electrochemical (PEC) water splitting efficiency of BiVO4-based photoanode, we designed phase-junction configurations of monoclinic scheelite (MS), tetragonal scheelite (TS) and tetragonal zircon (TZ) phases of BiVO4. With aid of the phase-junctions, we were able to achieve drastic improvements of the PEC performance. We successfully confirmed using observed X-ray diffraction (XRD) and Raman spectra that the target films were well fabricated. In addition, we could demonstrate using a time-resolved spectroscopic technique and a photo-electrochemical impedance spectroscopy (PEIS) that the enhanced water splitting efficiency could be attributed to the improvement of the charge separation efficiency.

Bacteriochlorin aggregates as dopant-free hole-transporting materials for perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted great interests not only of academic field but also of industrial world toward practical applications in the last decade due to their extremely low production cost and excellent photo-physical properties. Hole-transporting layer (HTL) as an integral component of PSC plays an important role in elevating its performance. In this work, five bacteriochlorophyll- a derivatives characterized by different C17-propionate residue were prepared and their J- type aggregates were investigated as HTLs of PSCs. It was confirmed that the nature of the introduced C17-propionate side chains have negligible effects on the electrochemical properties of main dye unit, such as the electronic absorption spectra and energy levels. However, the different lipophilicity, hydrophilicity, or hydrophobicity of chain characters affect the solubility of each dye component, resulting in diverse surface morphologies of their aggregate films as revealed by AFM observations. Furthermore, the five solid films prepared by spin-coating method showed discrepant charge extraction and transport abilities as HTLs, which were supported by both photo luminescence spectra and electrochemical impedance spectroscopy measurement results and were concluded to be the fundamental reason of the different PSC performance. • Five bacteriochlorophyll- a derivatives were investigated as HTLs of PSCs. • PSC employing BChl-2 bearing a dodecyl ester as an HTL gives the highest PCE. • Charge transfer process and interfacial contact are the essential factors for HTLs.

Intensity modulated photocurrent spectroscopy to investigate hidden kinetics at hybrid perovskite–electrolyte interface

The numerous assorted accounts of the fundamental questions of ion migration in hybrid perovskites are making the picture further intricate. The review of photo-induced ion migration using small perturbation frequency domain techniques other than impedance spectroscopy is more crucial now. Herein, we probe into this by investigating perovskite–electrolyte (Pe–E) and polymer-aqueous electrolyte (Po–aqE) interface using intensity modulated photocurrent spectroscopy (IMPS) in addition to photoelectrochemical impedance spectroscopy (PEIS). We reported that the electronic-ionic interaction in hybrid perovskites including the low-frequency ion/charge transfer and recombination kinetics at the interface leads to the spiral feature in IMPS Nyquist plot of perovskite-based devices. This spiral trajectory for the perovskite-electrolyte interface depicts three distinct ion kinetics going on at the different time scales which can be more easily unveiled by IMPS rather than PEIS. Hence, IMPS is a promising alternative to PEIS. We used Peter’s method of interpretation of IMPS plot in photoelectrochemistry to estimate charge transfer efficiency ({Q}_{ste}) from the Rate Constant Model. The {Q}_{ste} at low-frequency for Pe–E interface exceeds unity due to ion migration induced modified potential across the perovskite active layer. Hence, ion migration and mixed electronic-ionic conductivity of hybrid perovskites are responsible for the extraordinary properties of this material.

Enhancing Oxygenic Photosynthesis by Cross-Linked Perylenebisimide “Quantasomes”

As the natural-born photoelectrolyzer for oxygen delivery, photosystem II (PSII) is hardly replicated with man-made constructs. However, building on the “quantasome” hypothesis (Science1964, 144, 1009−101117811607), PSII mimicry can be pared down to essentials by shaping a photocatalytic ensemble (from the Greek term ”soma” = body) where visible-light quanta trigger water oxidation. PSII-inspired quantasomes (QS) readily self-assemble into hierarchical photosynthetic nanostacks, made of bis-cationic perylenebisimides (PBI2+) as chromophores and deca-anionic tetraruthenate polyoxometalates (Ru4POM) as water oxidation catalysts (Nat. Chem.2019, 11, 146−15330510216). A combined supramolecular and click-chemistry strategy is used herein to interlock the PBI-QS with tetraethylene glycol (TEG) cross-linkers, yielding QS-TEGlock with increased water solvation, controlled growth, and up to a 340% enhancement of the oxygenic photocurrent compared to the first generation QS, as probed on 3D-inverse opal indium tin oxide electrodes at 8.5 sun irradiance (λ > 450 nm, 1.28 V vs RHE applied bias, TOFmax = 0.096 ± 0.005 s–1, FEO2 > 95%). Action spectra, catalyst mass-activity, light-management, photoelectrochemical impedance spectroscopy (PEIS) together with Raman mapping of TEG-templated hydration shells point to a key role of the cross-linked PBI/Ru4POM nanoarrays, where the interplay of hydrophilic/hydrophobic domains is reminiscent of PSII-rich natural thylakoids.

Surface State-Assisted Delayed Photocurrent Response of Au Nanocluster/TiO2 Photoelectrodes.

Gold nanoclusters (NCs) can be used as sensitizers to extend the absorption capabilities of TiO2 as photoelectrodes. However, the adsorption of NCs also creates additional surface states on the TiO2 surface, which gives rise to intricacies in the understanding of various interfacial phenomena occurring in NC-sensitized TiO2. One of the complexities that have recently been discovered is the size-dependent hole-transfer mechanism. In this work, we reveal another anomalous behavior in the hole-transfer process that the hole scavenging ability of the electrolyte also plays a role in determining the hole-transfer mechanism in the NC-TiO2 system, which is unprecedented in other photoelectrode systems. In the presence of an efficient hole scavenger (Na2SO3), the hole transfer in Au18-TiO2 occurs directly through the highest occupied molecular orbital (HOMO) of Au18 NCs. However, in the presence of a less efficient hole scavenger (ethylenediaminetetraacetic acid), hole transfer in Au18-TiO2 does not occur through the HOMO and shifts to surface state-assisted hole transfer. Due to surface state charging, this surface state-assisted hole-transfer mechanism results in delayed photocurrent response in Au18-TiO2. Evidence for this exotic hole-transfer mechanism shift is provided by photoelectrochemical electrochemical impedance spectroscopy, and its implications are discussed.