eV Photon Energy Research Articles

The electronic spectra of trifluoroacetic acid and chlorodifluoroacetic acid in the 4.5 – 10.8 eV photon energy region

The electronic spectra of trifluoroacetic acid and chlorodifluoroacetic acid in the 4.5 – 10.8 eV photon energy region

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH3, CH2, and CH Fragments.

Under irradiation of a vacuum ultraviolet (VUV) photon, methane dissociates and yields multiple fragments. This photochemical behavior is not only of fundamental importance but also with wide-ranging implications in several branches of science. Despite that and numerous previous investigations, the product channel branching is still under debate, and the underlying dissociation mechanisms remain elusive. In this study, the photofragment imaging technique was exploited for the first time to map out the momentum and anisotropy parameter distributions of the CH3, CH2, and CH fragments at the 118 nm photolysis wavelength (10.48 eV photon energy). In conjunction with previously reported results of the H atom fragment at 121.6 nm (10.2 eV), a complete set of product channel branching in both two-body and three-body fragmentations is accurately determined. We concluded that extensive nonadiabatic transitions partake in the processes with two-body fragmentations accounting for more than 90% of overall photodissociation, for which the channel branching values for CH2 + H2 and CH3 + H are about 0.66 and 0.25, respectively. Careful kinematic analysis enables us to untangle the intertwined triple fragmentations into the CH2(X̃ 3B1 and ã 1A1) + H + H and CH(X2Π) + H + H2 channels and to evidence their underlying sequential (or stepwise) mechanisms. With the aid of electronic correlation and prior theoretical calculations of the potential energy surfaces, the most probable or dominant dissociation pathways are elucidated. Comparisons with fragmentary reports in the literature on various photochemical aspects are also documented, and discrepancies are clarified. This comprehensive study benchmarks the VUV photochemistry of methane and advances our understanding of this important process.

First-principles calculations on electronic, mechanical and optical properties of ScMC2 (M = Ti, V, Cr, and Mn) ternary carbides

First-principles calculations have been employed to investigate the physical properties of scandium-based ternary carbides, denoted as ScMC2 (M = Ti, V, Cr, Mn). All the studied carbides demonstrate both structural and mechanical stability. Mechanical anisotropy and optical properties are reported for the first time to enhance their importance on an industrial scale. The calculated mechanical parameters are correlated with the previously reported results to check the reliability of the current work. Poisson's and Pugh ratios indicate that ScMC2 (M = Ti, V, Cr) compounds are brittle whereas ScMnC2 is ductile. Thermal shock resistance is also investigated which suggests that ScMC2 carbides could be used as TBC's materials. DOS and optical characteristics reflect the metallic behavior of studied carbides. Optical properties indicate the existence of anisotropy in ScMC2 carbides up to 10 eV photon energy. The reflectance spectra suggest that ScMnC2 could be used to reduce solar heating.

Resonant photoemission studies on Fe-Ni alloys

This paper investigates the electronic structure of Fe1−xNix (x = 0.32, 0.36, 0.4, 0.5) alloys employing high resolution photoelectron spectroscopy using synchrotron radiation. The valence bands and the core levels have been recorded at various on-resonance and off-resonance photon energies of Fe and Ni. An anti-resonance dip observed in the valence bands taken at Fe 3p → 3d resonance lies closer to the Fermi level compared to that observed at Ni 3p → 3d resonance, indicating larger density of itinerant Fe 3d electrons. Whereas, Ni 3d states are comparatively less itinerant in these alloys. The hybridization effects increase with increase of Fe content in the alloy system. The intensity of Ni L3M4,5M4,5 Coster-Cronig Auger transition observed with 853 eV (Ni 2p → 3d resonance) photon energy, drops down drastically for the Invar alloy (Fe0.64Ni0.36) as compared to other Fe-Ni alloys due to the existence of Ni in more than one local chemical surroundings as expected for Invar alloy. The electron-electron interaction energy (Udd) of Ni 3d shell is found to decrease with the decrease of Ni concentration. The 3p and 3s core levels of Fe and Ni recorded at 707 eV (Fe 2p → 3d resonance) are overwhelmed by various intense resonant Auger features such as Fe L3M2,3M2,3, Fe L3M2,3M4,5 and Fe L3M1M4,5. The 1S state of Fe L3M2,3M2,3 is greatly enhanced in the Invar alloy as compared to other alloys. The microscopic compositional in-homogeneity present in Invar alloy plays a crucial role in the screening of holes in Ni sites but does not contribute in screening the holes in Fe sites.

Hot carrier photocurrent induced by 0.92 eV photon energy radiation in a Si solar cell

Absorption of the below-bandgap solar radiation and direct pre-thermalizational impact of a hot carrier (HC) on the operation of a single-junction solar cell are ignored by the Shockley-Queisser theory. The detrimental effect of the HC is generally accepted only via the thermalization-caused heating of the lattice. Here, the authors demonstrate experimental evidence of the HC photocurrent induced by the below-bandgap 0.92 eV photon energy radiation in an industrial silicon solar cell. The carriers are heated both through direct free-carrier absorption and by residual photon energy remaining after the electron-hole pair generation. The polarity of the HC photocurrent opposes that of the conventional generation photocurrent, indicating that the total current across the p-n junction is contingent upon the interplay between these two currents. A model of current-voltage characteristics analysis allowing us to obtain a reasonable value of the HC temperature was also proposed. This work is remarkable in two ways: first, it contributes to an understanding of HC phenomena in photovoltaic devices, and second, it prompts discussion of the HC photocurrent as a new intrinsic loss mechanism in solar cells.

The Near‐Infrared‐II Optical Tuning and Electronic Engineering of MoS2 Bilayers Doped by Halogen Elements

The improvement of optical absorption behavior of the second near‐infrared region (NIR‐II, 1000–1350 nm) based on new function materials is significant for the development of biomedicine and optical imaging. Therefore, the electronic structures and optical absorption of 2H phase MoS2 doped by F, Cl, Br, and I atoms with various doping concentrations are calculated, results show that halogen elements introduced extra transition allowed bands which is about 1.0 eV wide to 2H‐MoS2, and the increasing of impurity concentration can trigger strong spin splits. The transformation of electronic structures leads to 2H‐MoS2 metallic behaviors, improves transition efficiency, and lowers photon energy loss. More importantly, strong light–mass interaction is verified and breaks the energy level degeneracy, makes absorption peaks at 0.6–1.0 eV photon energy region, successfully extends the work region of 2H‐MoS2 to NIR‐II window, greatly improves the optical response and application potential in the photoelectric field. These results not only prove the potential of halogen elements as dopants in bandgap engineering of 2H‐MoS2 but also may indicate a direction for seeking new generation function materials that own excellent performance in the NIR‐II window, which is of great significance to promote the development of optoelectronics and bioimaging applications.

Bright continuously tunable vacuum ultraviolet source for ultrafast spectroscopy

Ultrafast electron dynamics drive phenomena such as photochemical reactions, catalysis, and light harvesting. To capture such dynamics in real-time, femtosecond to attosecond light sources are extensively used. However, an exact match between the excitation photon energy and a characteristic resonance is crucial. High-harmonic generation sources are advantageous in terms of pulse duration but limited in spectral tunability in the vacuum ultraviolet range. Here, we present a monochromatic femtosecond source continuously tunable around 21 eV photon energy utilizing the second harmonic of an optical parametric chirped pulse amplification laser system to drive high-harmonic generation. The unique tunability of the source is verified in an experiment probing the interatomic Coulombic decay in doped He nanodroplets across the He absorption bands. Moreover, we achieved intensities sufficient for driving collective processes in multiply excited helium nanodroplets, which have been previously observed only at free electron lasers.

Dissociative photoionization of acetaldehyde in the 10.2-19.5 eV VUV range.

The valence-shell dissociative photoionization of acetaldehyde has been investigated by means of the photoion photoelectron coincidence technique in conjunction with tuneable synchrotron radiation. The experimental results consist of threshold photoelectron spectra for the parent ion and for each fragment ion in the 10.2-19.5 eV photon energy range, along with (ion, e) kinetic energy coincidence diagrams obtained from measurements at fixed photon energies. The results are complemented by high-level ab initio calculations of potential energy curves as a function of the C-H bond distance. The nudged elastic band (NEB) method has been employed to connect the parent ion Franck-Condon region to the formation of the HCO+, CH3+ and CH4+ ion fragments. Appearance energies have been determined for six fragment ions with an improved accuracy, including two fragmentation channels, which to the best of our knowledge have not been reported previously, i.e. the formation of CH2CO+, lying at 13.10 ± 0.05 eV, and the formation of CH2+ at 15.1 ± 0.1 eV. Based on both experimental and theoretical results, the dissociation dynamics following ionization of acetaldehyde into the different fragmentation channels are discussed.

Exploring the deposition pathway in the notch region of double-graded bandgap ACIGS solar cells

The bandgap widening of Ag-alloyed Cu(In,Ga)Se2 (ACIGS) thin films poses a significant challenge in enhancing their photocurrent. This study demonstrates correlations between element diffusion behavior and notch-point formation in ACIGS films, which ultimately influence the resulting current circuit voltage and fill factor. Our findings delineate a novel approach for fabricating a suitably tailored band gap grading in ACIGS, which serves as a bottom subcell in tandem configuration. This was achieved by modulating a wide range of process temperatures from 340 to 470 °C, which were measured by pyrothermal, to assess the GGI profile during deposition. Experiments were undertaken to variate the second-stage conditions, striving to sustain the photocurrent, mitigate defects, and enhance crystallinity, achieving an impressive performance of 17.7 % without applying post-deposition treatments or anti-reflection coatings. The efficiencies surpassing 8 % were achieved as we measured these cells under a 715 nm long-pass filter, which corresponds to the bandgap of a typical top cell in tandem applications (1.73 eV photon energy). These results emphasize the significance of understanding the deposition temperature and its impact on the properties of ACIGS films, paving the way for further advancements in solar-cell technology.

Broad spectral response to photon energy unlimited by Schottky barrier from NiSi/Si junction

Recent advances in plasmonic absorption devices offer a novel approach for extending the range of silicon light detection. This is achieved by exploiting the Schottky interface energy barrier, which overcomes the limitation of the 1.12 eV photon energy conversion imposed by the silicon intrinsic bandgap. Internal photoemission has been widely accepted as the primary emission mechanism, which is mostly limited by the Schottky barrier height. However, we have discovered that carriers with energy lower than the Schottky barrier can generate electrical responses through the photothermal effect, breaking this restriction. In this study, we closely investigate two forms of photoresponses, namely photoelectric and photothermal, on a NiSi thin film Schottky photodetector, considering various factors such as temperature, applied bias, incident power, and wavelength regime. Without photothermal assistance, the device achieves a responsivity of 0.41 mA/W to a 1550 nm laser. Under bias-driven conditions, highly energized hot carriers generate a 2.18 μA photoelectric response, while a 0.861 μA photothermal response is simultaneously observed under 6 mW 1550 nm laser illumination. Through systematic investigation, we advance the mechanism of both responses by combining the hot carrier model and the Schottky barrier diagram. Finally, we experimentally obtain a 4.85 μA sheer photothermal response to a 0.475 mW 4-μm wavelength signal with 10.21 mA/W responsivity. It is expected that the device will also show responses to longer wavelengths.

First-principle calculations to investigate electronic and optical properties of carbon-doped silicon

Silicon (Si) is a highly promising candidate for the applications of photovoltaics. However, the bandgap constraints, low conductivity, and bounded photon energy absorption make the applications restricted. Herein, we investigated the optimized structural, electronics and optical characteristics of carbon (C) doped Si by first-principle calculations. The analysis of the various doping illustrates that doping of the C atom in Si adds extra stability by reducing the Si–Si bond length from 2.3 Å to 2.002 Å Si–C bond length which leads to the shrinkage of lattice parameters. In addition, the bandgap of Si is reduced from 1.13 eV to 0.2 eV if a 14.28 % C-doping is employed. As the C-doping is reduced to 6.66 %, the bandgap increases to 0.87 eV with a transition from indirect to direct bandgap. Further reduction in C-doping varies the bandgap from 0.87 eV to 1.13 eV. The density of states calculations show that for a low C-doping, the transitions are mainly due to Si-3p and Si-3s orbitals in the valence band which leads to a higher energy level, while for higher C-doping, the C atom 2p orbital contribution causes a reduction in bandgap and addition of stability in the doped structures. The conductivity of the Si can be increased by utilizing higher C-doping which reduces hole effective mass by 46 % and electron effective mass by 24 %. Optical absorption of Si can be enhanced at a higher spectral range while adopting a higher C-doping, which shifts the absorption peak shapes from spikes to flat and extends to 7 eV photon energy.

Study of the Graphene Oxide Nanosheets Synthesized from Pencil Electrode Using Electrochemical Method and Solar Energy as a Source of Power

Graphene Oxide nanosheets were synthesized using electrochemical exfoliation of pencil electrodes in aqueous solution containing 2% of magnesium sulfate salt. A solar panel of 20V was used as a power supply to turn the synthesis into a green method. Several measurements were carried out to investigate the product, namely: Raman scattering, X-ray diffraction, photoluminescence, UV-VIS-NIR spectroscopy, and FTIR spectroscopy. Raman scattering showed the existence of both D-band and G-band, which are an indication of graphene oxide existence. The D-to-G band ratio was 1.5. The results of X-ray showed a diffraction peak at 2θ = 20.7° corresponding to the space distance between graphene oxide nanosheets and a diffraction peak at 2θ = 26.25° corresponding to the short range interplanar spacing. Photoluminescence showed two peaks of emission (at wavelengths 355nm & 701nm) regarding to the π-π* transition. UV-VIS-NIR spectroscopy exhibited a 4.2eV photon energy absorption peak corresponding to the aromatic C=C bonds and 5.4eV photon energy absorption peak corresponding to the carbonyl groups. FTIR showed peaks related to hydroxyl groups, hydrogen-bonded OH groups of COOH, and functional groups such as C-OH (1375 cm-1), and C-O (1039 cm-1). FTIR results approved that graphene oxide nanosheets are functionalized. TEM images show that the synthesized graphene oxide is single-, double-, and multi-layer stacks with an amount of impurities.

High-precision room temperature Fe opacity measurements at 1000-2000eV photon energies

Prior measurements of room temperature (cold) Fe opacity have errors as high as ±10 % and a spread in values that exceeds the uncertainties. These data, along with current cold opacity databases, were used for comparison with experimental solar Fe opacity. The solar Fe opacity is expected to be lower than cold Fe opacity in the ∼1500–2000 eV photon energy range. However, results from this comparison contradicted this assumption and prompted an investigation of the precision and accuracy of cold Fe opacity measurements. Cold Fe opacity is determined here with high precision using transmission measurements of Fe foils at three characteristic line energies in the soft X-ray range (1000–2000 eV). The present opacities are determined with ≲1 % overall uncertainties. This is achieved through precise measurements of transmission and areal density which are related to opacity by the Beer-Lambert Law. Transmission is measured with overall individual uncertainties of ≲1 % and is obtained through simultaneous measurements of attenuated and unattenuated spectra using an X-ray source and a Bragg crystal spectrometer. The required areal density is indepen-dently measured using two different techniques: Rutherford Backscattering Spectroscopy and a recently-developed technique using calibrated cold absorption in the 3–17 keV range. The final areal density used in the opacity determination is an average between both methods. The measured opacity at 1188 eV is in agreement with current opacity databases but is higher by around 5–10 % at the highest photon energy (1924 eV) and lower by around 2–4 % at the lowest photon energy (1012 eV). A caveat is that opacity accuracy is linearly dependent on the areal density. We performed the first direct comparison between the two areal density methods which revealed a 7–11 % discrepancy. If one of the methods is proven correct in future studies, it may impact the opacity accuracy reported here since we use the average of the two methods. This result affects the solar Fe opacity measurements as they also rely on the accuracy of these areal density measurement techniques.

DFT Study about the Effect of Doping on the Properties of GaSb Material and Designing of High‐Efficiency Infrared Photodetector

The gallium antimonide (GaSb) material has very attractive electronic and optoelectronic properties which are suitable for next‐generation infrared (IR) photodetector applications. In this work, properties of undoped GaSb material such as density of states, bandstructure, electron density, absorption coefficient, dielectric function, refractive index, and extinction coefficient are calculated using density‐functional theory (DFT). Moreover, the effects of doping with Ge, Sn, and Zn elements on these properties of GaSb material are investigated. It is found that undoped GaSb material exhibits a direct gap of ≈0.72 eV. Among different doping elements, Ge‐doped GaSb produces a very significant enhancement in optical properties. The Ge‐doped GaSb demonstrates a four times higher absorption coefficient in comparison to undoped GaSb in the IR region at 0.8 eV photon energy. GaSb‐based photodetector device is designed using the Solar Cell Capacitance Simulator (SCAPS) 1D tool. The efficiency of the designed photodetector with optimum thicknesses and doping of different layers is found to be improved from 21.34% to 25.91% after incorporating the absorption data set obtained from the DFT calculations. Additionally, the photodetector with optimum parameters demonstrates maximum responsivity of value ≈0.31 A W−1. In the previous findings, it is demonstrated that GaSb is a very suitable material for next‐generation IR photodetector applications.

Reflectance anisotropies of polycrystalline Ce[formula omitted] Gd[formula omitted] O[formula omitted]/ Si[formula omitted] interfaces

Ce1−xGdxO2−x/2 thin films were deposited by spin coating on oxidized Si(001) substrates. Two strain regimes are observed by μ− Raman spectroscopy: for x< 0.1 the films are tensile strained, whereas a strain component of the wave number shift of −71.2x cm−1 compensates with a 32.6x cm−1 bond component for x> 0.1. Reflectance anisotropy spectroscopy (RAS) measurements were carried out on these films in which interface-related optical anisotropies signatures appearing in the 1.5 eV to 3.6 eV photon energy range are found dependent on the incorporation of Gd in the lattice. To explain the observed RAS signatures with a complex reflectance three-phase model, we resorted to spectroscopic ellipsometry (SE) to retrieve the interface optical anisotropies (IOA) related to the changes in refractive index, <Δn>, between interface formed by both the oxidized Si(001) and polycrystalline CeO2:Gd films, and to assess the CeO2 energy gap variation from 3.2 to 2.8 eV in the 0 <x< 0.4 composition range. This study renders the combination of RAS, Raman, and SE versatile tools to optimize the growth parameters during the fabrication of devices based on polycrystalline CeO2 on a quantitative basis.

Demonstration of thermal limit mean transverse energy from cesium antimonide photocathodes

The mean transverse energy (MTE) of electrons emitted from cathodes is a critical parameter that determines the brightness of electron beams for applications, such as x-ray free electron lasers, particle colliders, and ultrafast electron scattering experiments. Achieving a MTE close to the thermal limit is a key step toward realizing the full potential of electron sources in these applications. Cesium antimonide (Cs3Sb) is a technologically important material with a long history of use in photon detection and electron sources. The smallest MTE of electrons photoemitted from Cs3Sb has always been appreciably greater than the thermal limit and was attributed to surface non-uniformities. In this work, we present comprehensive measurements of the photoemission electron energy spectra (PEES), quantum efficiency, and MTE from Cs3Sb in a wide photoexcitation energy range from 1.5 to 2.3 eV. Our PEES measurements demonstrate a notably low photoemission threshold of around 1.5 eV, which is in contrast with the previously perceived threshold of 1.8–2.0 eV. Moreover, we show that the MTE at this threshold of 1.5 eV nearly converges to the thermal limit at 300 K. At 1.8 eV, the MTE measured is 40 meV, which is comparable to the previously reported value. We conclude that this MTE value at 1.8 eV photon energy is not due to surface roughness effects as previously believed, but is a direct consequence of the excess energy.

Double Photoionization of Nitrosyl Chloride by Synchrotron Radiation in the 24-70 eV Photon Energy Range.

The behavior of nitrosyl chloride (ClNO) exposed to ionizing radiation was studied by direct probing valence-shell electrons in temporal coincidence with ions originating from the fragmentation process of the transient ClNO2+. Such a molecular dication was produced by double photoionization with synchrotron radiation in the 24-70 eV photon energy range. The experiment has been conducted at the Elettra Synchrotron Facility of Basovizza (Trieste, Italy) using a light beam linearly polarized with the direction of the polarization vector parallel to the ClNO molecular beam axis. ClNO molecules crossing the photon beam at right angles in the scattering region are generated by effusive expansion and randomly oriented. The threshold energy for the double ionization of ClNO (30.1 ± 0.1 eV) and six dissociation channels producing NO+/Cl+, N+/Cl+, N+/O+, O+/Cl+, ClN+/O+, NO+/Cl2+ ion pairs, with their relative abundance and threshold energies, have been measured.

Fast transition-edge sensors suitable for photonic quantum computing

Photon-number resolving transition-edge sensors (TESs) with near unity system detection efficiency enable novel approaches to quantum computing, for example, heralding robust Gottesman–Kitaev–Preskill qubit states. Increasing the speed of the detectors increases the rate at which these states can be heralded. In addition, depending on the details of the scheme, faster detectors can reduce the complexities of the hardware implementation. In previous work, we demonstrated that adding a small amount of gold between the tungsten film and silicon substrate can increase thermal conductance and reduce detector recovery time. In that study, the readout electronics imposed limitations on stable biasing conditions of the TES detector, and the TES could only be biased at higher than ideal values. In this report, we demonstrate the operation of the TES illuminated by a heavily attenuated pulsed laser running at 1 MHz repetition rate and examine the limits to adding gold to speed up device recovery times using a higher bandwidth readout system. The best performance was achieved by combining a 15×15μm2 tungsten TES with 5μm3 of gold, which resulted in a recovery time faster than 250 ns, with an energy resolution of 0.25 eV full-width at half maximum at 0.8 eV photon energy.

FlexPES: a versatile soft X-ray beamline at MAX IV Laboratory.

FlexPES is a soft X-ray beamline on the 1.5 GeV storage ring at MAX IV Laboratory, Sweden, providing horizontally polarized radiation in the 40-1500 eV photon energy range and specializing in high-resolution photoelectron spectroscopy, fast X-ray absorption spectroscopy and electron-ion/ion-ion coincidence techniques. The beamline is split into two branches currently serving three endstations, with a possibility of adding a fourth station at a free port. The refocusing optics provides two focal points on each branch, and enables either focused or defocused beam on the sample. The endstation EA01 at branch A (Surface and Materials Science) is dedicated to surface- and materials-science experiments on solid samples at ultra-high vacuum. It is well suited not only to all flavours of photoelectron spectroscopy but also to fast (down to sub-minute) high-resolution X-ray absorption measurements with various detectors. Branch B (Low-Density Matter Science) has the possibility to study gas-phase/liquid samples at elevated pressures. The first endstation of this branch, EB01, is a mobile setup for various ion-ion and electron-ion coincidence techniques. It houses a versatile reaction microscope, which can be used for experiments during single-bunch or multi-bunch delivery. The second endstation, EB02, is based on a rotatable chamber with an electron spectrometer for photoelectron spectroscopy studies on primarily volatile targets, and a number of peripheral setups for sample delivery, such as molecular/cluster beams, metal/semiconductor nanoparticle beams and liquid jets. This station can also be used for non-UHV photoemission studies on solid samples. In this paper, the optical layout and the present performance of the beamline and all its endstations are reported.

Near-infrared study of radiation damage in cerium dioxide

Radiation damage is studied in electron-irradiated cerium dioxide (CeO2) single crystals. Near Infra-red (NIR) spectra are recorded at room temperature between 2800 and 11,000 cm−1 (i.e. ∼0.9–3.57 µm in wavelength and ∼0.35–1.36 eV in photon energy). Measurements were carried out in the transmission mode for the single crystals and diffuse reflectivity mode for sintered CeO2 and (Ce, Gd)O2-x reference un-irradiated samples. The ceria single crystals were irradiated with 1.4-MeV and 2.5-MeV electrons for high fluences up to 3 × 1016 cm−2. Five broad absorption bands centered at 3600, 4100, 4600, 6000, and 7100 cm−1 (i. e. ∼0.44, 0.51, 0.57, 0.74, and 0.88 eV) are deduced from fits for the 2.5-MeV electron irradiation, whereas no absorption band is recorded for the 1.4-MeV electron energy. Those bands are consistent with the broad 0.9-eV band (7260 cm−1, 1.37 µm) in the UV–visible range associated with the change in color from light blue to deep green for the 2.5-MeV energy. Optical absorption arises from point-defect formation by elastic collisions for electron irradiation above the threshold energy near 1.4 MeV corresponding to the onset of cerium atom displacement. No defect bleaching occurs after isothermal annealing at 700 °C. No such bands are recorded for the virgin (Ce, Gd)O2-x samples for 5, 10, and 15 mol% Gd2O3 with an increasing amount of oxygen vacancies. The origin of absorption bands in the NIR range is discussed in relation to the available experimental data and ab-initio calculations of point defects in CeO2 and other dioxides with the cubic fluorite-like structure. Absorption bands are tentatively assigned to electronic transitions involving cerium vacancy levels in the band gap.