Photoexcitation Spectra Research Articles

Luminescence characterization of titanium related defects in 6H–SiC

Photoluminescence (PL), thermally stimulated luminescence (TSL) and their photo-excitation spectra in hexagonal type SiC were studied between the temperatures of 5 and 300 K. Titanium related TSL glow peak is observed close to 140 K in compensated crystals 6H–SiC with corresponding thermal ionisation energy of a trap close to 170 meV. The spectral distribution of TSL has a maximum at 1.8 eV in 6H–SiC and 2.43 eV in 4H–SiC. A specific structure in the photo-excitation spectrum of PL close to the exciton gap E gx=3.023 eV is revealed, exclusively in 6H–SiC material. This indicates an essential role of excitons in the resonance energy transfer to the Ti-related centers in silicon carbide. The results are discussed in a model of isoelectronic centers composed of either individual Ti atoms or Ti–N pair representing a complex of the electronic trap and recombination center.

Transient photoconductivity spectroscopy of polycrystalline diamond films

Photoexcitation spectra of nonequilibrium charge carriers in polycrystalline diamond films were measured in the visible region (430 — 690 nm) by the contact-less transient photoconductivity technique with a tunable optical parametric oscillator. Preliminary irradiation of samples by intense UV laser pulses resulted in the redistribution of the population of deep trap levels and was accompanied by substantial long-term enhancement of photoconductivity in the spectral band at 2 eV.

Photogeneration of charge carriers in segmented arylamino-PPV derivatives

Stationary photoconductivity has been measured for sandwich cells of three arylamino-PPV derivatives between gold and aluminum electrodes. Photoaction spectra recorded under forward bias resemble well the shape of solid state photoexcitation spectra, indicating that the photocurrent is dominated by bulk photogeneration of charge carriers. Even though the photoaction spectra were similar in shape, maximum charge collection efficiencies depended strongly on the choice of the substituents to the PPV-backbone. Under reverse bias the photoaction spectra depend on illumination conditions. The data are interpreted by the build-up of a negative space charge region by immobile photogenerated electrons. We could identify a contribution by photocurrent multiplication, which is caused by the accumulation of photogenerated electrons near the Al/polymer interface.

Rydberg excitations in rare gas clusters: structure and electronic spectra of Ar* n (3 ≤ n ≤ 25)

Likely candidates are located for the global potential energy minima of Ar* n (3 ≤ n ≤ 25) clusters using the diatomics-in-molecules (DIM) approach. The favoured geometries are found to be different from the structures of Ar+ n and correspond to the trimer Ar*3 bound to the surface of an Ar n−2 core via a common atom. The Ar n−2 core is usually only slightly distorted from its own global potential minimum, although in a few cases it corresponds to a nearby local minimum. Therefore, the ‘magic’ sizes of the excimer systems are predicted to differ from those of the ions and correlate instead with the stability of Ar n−2. The predicted electronic photoabsorption and emission spectra of Ar* n , and photoexcitation spectra of Ar n are discussed in terms of experimental data. Global potential energy minima for neutral Ar n up to n = 55 with the Aziz potential are summarized also; the structure is the same as for the Lennard-Jones potential except at n = 21 where the stabilities of the two lowest Lennard-Jones minima are reversed.

Luminescence of an erbium- and ytterbium-containing complex in porous silicon films

Er3+ and Yb3+ ions are introduced into porous silicon films, stabilized by oxidation in an oxygen plasma, in the form of a gadolinium oxychloride-based luminophor by means of thermal diffusion. An investigation is made of the luminescence and photoexcitation spectra of samples with Er3+ (10 and 30 wt.%), Yb3+ (10 wt.%), and Er3, Yb3++ (10 wt.% each). It is shown that the intense IR luminescence (1.00 and 1.54 μm) is caused by cross-relaxation effects. The most effective excitation of the luminescence has been observed in the UV absorption band of the porous silicon.

Efficient bulk photogeneration of charge carriers and photoconductivity gain in arylamino-PPV polymer sandwich cells

Stationary photoconductivity has been measured for sandwich cells of poly(phenylimino-1,4-phenylene-1,2-ethenylene-2,5-dioctyloxy-1,4-phenylene-1,2-ethenylene-1,4-phenylene) between gold and aluminum electrodes. Films with thicknesses below and above 1 \ensuremath{\mu}m were examined in order to separate photocurrent contributions arising from processes at the polymer/electrode interfaces and the bulk. Spectra recorded under forward bias were almost identical in shape and size for irradiation through either the anode or the cathode indicating that the photocurrent is dominated by bulk photogeneration of charge carriers. Large charge carrier collection efficiencies exceeding those of typical PPV derivatives are derived from these experiments. The photogeneration is clearly enhanced within the tail of the polymer absorption and photoaction spectra recorded under forward bias resemble well the shape of solid-state photoexcitation spectra. We presume that both photoluminescence and photoconductivity in the studied arylamino-PPV compound are dominated by the excitation of states or sites rather deep in the density of states distribution. Under reverse bias the photoaction spectra depend strongly on illumination conditions. For illumination through the positively biased aluminum electrode the photocurrent spectra are almost symbatic with the polymer absorption while antibatic behavior is observed upon illumination through the gold electrode. The data can qualitatively be explained by the buildup of a negative space-charge region by immobile photogenerated electrons. Upon illumination through the positively biased aluminum electrode collection efficiencies of up to 2000% are observed. This is attributed to photocurrent multiplication arising from the accumulation of electrons near the interface, which promotes tunneling of holes into the film.

Effect of different capping environments on the optical properties of CdS nanoparticles in reverse micelles

CdS (E=2.5 eV) nanoparticles were synthesized in reversed micelles and capped with materials of different band gaps and charge compensation ability: Cd(OH)2 (4.0 eV), ZnS (3.2 eV), and CdO (2.3 eV). The preparations allowed the examination and comparison of the efficiencies of confinement of the electron and hole by the various capping materials. The absorption and emission spectra of the capped CdS nanoparticles suggest the following: (1) Efficient capping and strong quantum confinement are observed for CdS particles of small size (<3.8 nm), showing well-resolved photoexcitation spectra; (2) the confinement efficiency improves with capping thickness; (3) although ZnS has smaller band gap than Cd(OH)2, it was more efficient in surface-charge compensation and it is the best capping agent of the three materials; and (4) the similar band gaps of CdO and CdS resulted spectroscopically in strong mixing between the excited states of these materials. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 439–450, 1999

S 2p photoexcitation spectra of SO2 at high resolution

The S 2p photoabsorption spectrum of gas-phase SO2 was studied at high resolution (ΔE≅30 meV) using synchrotron radiation. The spectrum contains excitations to both valence orbitals and Rydberg states. The lowest core-to-valence excitation, S 2p−13b1, was found to be fivefold split, and vibrational subbands originating from the symmetric-stretching mode could be resolved. A Franck–Condon analysis of these five states reveals different geometries depending on the symmetry of the total electronic wave function in the final state. In the Rydberg region, both the spin–orbit splitting (≅1.20 eV) and the ligand-field splitting (≅95 meV) were resolved.

Long-lived electronic-deformation excitations in a polyacetylene chain — the photoexcitation spectrum

It is observed that the photoexcitation efficiency of neutral long-lived states in a trans-polyacetylene chain increases with increasing photon energy of the excitation. The observed effect is given an interpretation according to which the energy of a photon above the optical absorption edge is added to the excitation of the vibrational subsystem, increasing sharply the probability that the chain relaxes into a long-lived deformed neutral state.

Thermoluminescence from CO-doped solid Ar

Cameron bands luminescence from a new thermoluminescence (TL) system - CO-doped solid Ar - has been investigated using selective excitation by synchrotron radiation and thermally stimulated luminescence methods. The photoexcitation and TL emission spectra of the Cameron bands are measured. The activation energies and the frequency factors of charge traps are determined by numerical fitting of the TL emission spectrum.

Excitonic and Molecular Properties of the Triplet T 1-State in Diphenylpolyene Single Crystals

Excitonic properties of the excited triplet state in single crystals of diphenylpolyenes all-trans(1, 4-diphenyl)-1, 3-butadiene (DPB) and all-trans(1, 6-diphenyl)-1, 3, 5-hexatriene (DPH) are derived from T 1←S 0 photoexcitation spectra at low temperatures. The 0-0-transition is split into 2 (DPH monoclinic) and 4 (DPB orthorhombic) Davydov components in zero-field. Measurements with an applied external magnetic field made accessible the respective fine-structure components and their assignment to the principal axes of the fine-structure tensor. In the case of DPB orthorhombic the determination of the excitonic D* and E* fine-structure parameters was possible due to the small linewidths occuring in the spectra. The intensity ratios of the Davydov components were evaluated for different polarizations of the excitation light with respect to the crystal axes. The conclusion is drawn that in both DPH and DPB the transition moment is oriented mainly perpendicular to the molecular plane and that the total symmetry of the T 1-state is Au . The vibrational structure in the photoexcitation spectra was assigned by using selectively deuterated crystal samples for comparison. Since the vibrations involving the phenyl endgroup are absent in the spectra of DPH and carry only very weak intensity in the spectra of DPB it is concluded that the T 1-excitation energy is localized mainly on the polyene chain.

Photoluminescence and photoexcitation spectra of porous silicon subjected to anodic oxidation and etching

The photoluminescence and photoexcitation spectra of porous silicon films with an initial porosity of 50–60%, produced on single crystals of p-type silicon and subjected to anodic oxidation and chemical etching, are studied. The existence of an amorphous phase in the etched porous silicon is found not to affect the photoluminescence spectrum of porous silicon. Features of the photoexcitation spectra before and after etching, as well as the evolution of the photoluminescence and photoexcitation spectra after etching, can be interpreted in terms of a uniform quantization model that includes elastic stresses in the silicon crystals of the porous silicon.

Luminescence of porous silicon films with an europium-containing complex

We obtained porous silicon films modified at room temperature by an Eu3+-containing polymer complex. The most intense photoluminescence of Eu3+ implanted in the porous silicon was observed at the wavelengths of 611, 618, 691, and 704 nm. In this case, the intensity of the intrinsic photoluminescence of strongly irradiated specimens of porous silicon decreased, while the intensity of weakly emitting films multiply increased. An investigation of the photoexcitation spectra made it possible to establish the effect of Eu3+-containing complexes on the mechanism underlying the excitation of photoluminescence of porous silicon.

A theoretical simulation of the 1s→2π excitation and deexcitation spectra of the NO molecule

We present ab initio calculations for the N and O 1s→2π photoexcitation spectra and the deexcitation electron spectra which result in the decay of these core excited states. The photoexcitation spectra are simulated with potential energy curves and transition dipole moments calculated with the multiconfiguration coupled electron pair approximation method. The energy lifetime width of the core excited states, which influences the form of these excitation spectra via the broadening of the vibrational progressions, was calculated by the one center approximation. The theoretical spectra are compared to recent experimental data. The deexcitation electron spectra, which monitor the autoionization of the core excited states, are reproduced by a combination of valence configuration interaction calculations to obtain the relative energies and the one center approximation which yields decay rates to the singly charged final states. Furthermore, we included lifetime vibrational interference effects using the recently proposed moment method of Cederbaum and Tarantelli [J. Chem. Phys. 98, 9691 (1993)]. The calculated deexcitation spectra compare extremely favorably with experimental data.

Infrared photoexcitation spectra of conducting polymer/methanofullerene films

We report photoinduced absorption (PIA) and photoinduced reflectance (PIR) spectra of poly(3-octyl thiophene)-methanofullerene films. As a result of the efficient photoinduced intermolecular charge transfer, the PIA and PIR spectra of the composite films are significantly enhanced in magnitude over those in either of the component materials. From the PIA and PIR spectra, the corresponding changes in the complex refractive index, ΔN = Δn(ω)+iΔϰ(ω), are obtained. The results indicate that the PIA spectra are dominated by Δϰ(ω), while the PIR spectra are dominated mainly by Δn(ω). The implications of these photoinduced changes in the index resulting from photoexcitations are discussed in terms of potential optoelectronic and nonlinear optical applications of these materials.

FIR laser spectroscopy of impurities in semiconductors

A concise review of the FIR magnetooptical investigations of the photoexcitation of shallow impurities in semiconductors with the use of the molecular lasers optically pumped with carbon dioxide lasers is presented. The goal of the article is to demonstrate the abilities of the above technique by selecting some typical examples. The primary consideration is given to the photoconductive detection of the photoexcitation, mainly to the photo-thermal ionization technique. It is shown that the high resolution of laser spectroscopy is capable not only of characterizing the shallow impurities themselves, but of giving valuable information on other defects and impurities that the shallow interact with. The illustration of the central cell structure of donor photo-excitation spectra for the purest samples of GaAs is used to demonstrate the ability of the technique. Some examples of mechanisms of line broading are also discussed. Illustrations of effects of Coulomb correlations of charged impurities in compensated semiconductors on the linewidth are provided. A brief description of examples of application of FIR laser spectroscopy to a study of acceptors in II–VI semiconductors, including semimagnetic semiconductors is also included.

Configuration interaction in argon KL resonances.

Curious ``doublet'' resonances in the KL double photoexcitation spectrum of argon are explained as a strong admixture of 3${\mathit{d}}^{2}$ excited states to the principal [1s2p]4${\mathit{p}}^{2}$ excitation. A simulated ``ab initio'' KL photoabsorption spectrum including shake-up and shake-off contributions agrees well with the experimental data.

Photoexcitations in conjugated polymer superlattices

We present evidence of quantum confinement of the photo-generated states in a conjugated block copolymer consisting of regularly alternating thiophenic and phenylenic moieties of various lengths, thus giving rise to a sort of one-dimensional superperiodicity. CW photoinduced spectra (both in the infrared and in the visible) have revealed localization effects when the width of the potential well exceeds five thiophenic rings. A discussion of the new features in the copolymer photoexcitation spectrum with respect to the parent polymers is also presented.

Electron Emission Properties of the Negative Electron Affinity (111)2×1 Diamond-Tio Interface

ABSTRACTWe report photoemission measurements from the (111)2×1 diamond-titanium monoxide (TiO) interface. Submonolayer deposition of TiO on the (111)2×1 diamond surface modifies the electron affinity of the surface from positive to negative. No change in the band bending was observed as a result of the TiO deposition. Total electron yield measurements from the diamond-TiO interface were also performed. The yield spectra, as expected for a negative electron affinity (NEA) surface, have emission thresholds that are in good agreement with the absorption coefficient of diamond. To further understand the emission properties of NEA (111) diamond surfaces we also compare the electron yield photo-excitation spectra of the diamond-TiO interface with the yield spectra of hydrogenated (111)1×1:H diamond surfaces.

Midgap localized states and light emission of porous silicon

Light emissions of porous silicon were studied by photoluminescence (PL) and photoluminescence excitation spectroscopies (PLE). The results suggest the existence of midgap localized states and provide strong evidence of lattice relaxation effect. Visible light emission may be due to the electron transition between the modified conduction band and the surface related localized states with the participation of phonon emissions. The stokes shift between peaks of photoemission and photoexcitation spectra is about 1.2 eV and varies with different preparation and later treatment conditions of the samples.