Photoexcitation Cross Section Research Articles

New perspectives on traps and radiative recombination centers for optically stimulated luminescence in LiF:Mg,Cu,P

LiF:Mg,Cu,P (MCP) is well-known for its high thermoluminescence (TL) sensitivity and is widely used for TL dosimetry. Following recent studies, highlighting its optically stimulated luminescence (OSL), the properties of MCP have been studied for OSL-based 2D and 3D dosimetry. Here, we have measured the spectrally resolved OSL emission as a function of stimulation time, identifying the peak of the emission band to be at ∼355 nm during the early stimulation period before shifting to peaking at ∼382 nm during the remaining part of the experiment. By assuming the spectra to originate from radiative recombination centers with Gaussian emission bands, we identified two recombination centers with emission centered at ∼355 nm (RC1) and ∼394 nm (RC2), respectively. From the OSL decay curves, we have identified two dominating types of OSL traps (fast and slowly decaying OSL traps). We provide indicative values for the photoexcitation cross section of these OSL traps. The fast-decaying OSL traps were found to predominantly emit OSL through RC1. By depleting the fast-decaying OSL traps before TL measurements, we have identified these traps to be associated with a previously undetected TL peak at ∼87 ° C (1 °C/s heating rate), located between the TL peaks usually denoted as peaks one and two. We further found that the features related to the fast-decaying OSL traps were not observable for powder samples without a heat treatment at ∼560–700 °C. Pellet samples show these features without the heat treatment, but could potentially have undergone a similar treatment during sintering.

Evolution of L -shell photoabsorption of the molecular-ion series SiHn+ ( n=1,2,3 ): Experimental and theoretical studies

We report on complementary laboratory and theoretical investigations of the $2p$ photoexcitation cross sections for the molecular-ion series $\mathrm{Si}{{\mathrm{H}}_{n}}^{+}$ ($n=1,2,3$) near the $L$-shell threshold. The experiments used an electron cyclotron resonance (ECR) plasma molecular-ion source coupled with monochromatized synchrotron radiation in a merged-beam configuration. For all three molecular ions, the $\mathrm{S}{\mathrm{i}}^{2+}$ decay channel appeared dominant, suggesting similar electronic and nuclear relaxation patterns involving resonant Auger and dissociation processes, respectively. The total yields of the $\mathrm{S}{\mathrm{i}}^{2+}$ products were recorded and put on absolute cross-section scales by comparison with the spectrum of the $\mathrm{S}{\mathrm{i}}^{+}$ parent atomic ion. Interpretation of the experimental spectra ensued from a comparison with total photoabsorption cross-sectional profiles calculated using ab initio configuration interaction theoretical methods inclusive of vibrational dynamics and contributions from inner-shell excitations in both ground and valence-excited electronic states. The spectra, while broadly similar for all three molecular ions, moved towards lower energies as the number of screening hydrogen atoms increased from one to three. They featured a wide and shallow region below $\ensuremath{\sim}107\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ due to $2p\ensuremath{\rightarrow}{\ensuremath{\sigma}}^{*}$ transitions to dissociative states, and intense and broadened peaks in the $\ensuremath{\sim}107--113\ensuremath{-}\mathrm{eV}$ region merging into sharp Rydberg series due to $2p\ensuremath{\rightarrow}n\ensuremath{\delta},n\ensuremath{\pi}$ transitions converging on the ${L}_{\mathrm{II},\mathrm{III}}$ limits above $\ensuremath{\sim}113\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. This overall spectral shape is broadly replicated by theory in each case, but the level of agreement does not extend to individual resonance structures. In addition to the fundamental interest, the work should also prove useful for the understanding and modeling of astronomical and laboratory plasma sources where silicon hydride molecular species play significant roles.

An almost knowledge-free approach to XPS intensity evaluation where use of atomic photoemission cross sections suffices for yielding material-specific inelastic background

A general procedure for peak intensity evaluation on the inelastic background of X-ray photoelectron spectra (XPS) has been developed, which is made up of tasks in three nested levels, level I, II and III. The task in level I is a simple background optimization for the case when the inelastic mean free path (IMFP, denoted λ(E)) and other constraints are given in advance [M. Jo, Surf. Interface Anal., 2003, 35, 729–737]. In order to find appropriate constraints, a batch job, which is a series of single jobs with systematically varied constraints, is constructed and iteratively solved in level II by improving the constraints from the viewpoint of how successfully the background is subtracted. It is found that, by the task in level II, for polycrystal Cu metal, the spectrum after inelastic background subtraction still contains a baseline component proportional to E−k, where E (>50eV) is the electron kinetic energy and the exponent k around 2.4. This baseline is attributed to the high-energy tail of primary excitation spectrum of true secondary electron emission. The relative core intensities thus determined are in good agreement with the Scofield’s photoexcitation cross sections. The relation of core and Auger peak intensities is consistent as demonstrated by the fact that the sum of Cu 2p and 2s equals that of all Cu LMM peaks. The level III calculation is designed for the general case where λ is not available before the analysis. The relative energy dependence of λ except for the absolute value can be estimated by iterating the level II tasks with updated λ(E), based on the assumption that the peak intensities should be proportional to Scofield’s values. The relative deviation of thus determined λ(E) in level III from those by the well-known calculation (Tanuma, Powell, and Penn, TPP) is less than 1.7% between E=600 and 2000eV if their values at 800eV are assumed to coincide. The normalized peak shapes and relative intensities after background subtraction in this level are also very similar to those obtained in level II starting with λ(E) by TPP that is currently known to be reliable. These findings indicate that an almost knowledge-free analysis, which is free from the detail of the solid’s properties such as chemical composition and nevertheless retains the material-dependent results, became possible for the first time, as long as one keeps away from the discussion of absolute intensity.

Experimental and theoretical study of the [formula omitted] ([formula omitted]) [formula omitted] excited states of D2: Absolute absorption cross sections and branching ratios for ionization, dissociation and fluorescence

The absolute absorption cross sections for photoexcitation of D2 to N=0 Rydberg levels have been measured at a resolution of 0.001nm in the 123,500–135,000cm−1 spectral range. The experimental energies and line intensities are compared with ab initio multichannel quantum defect (MQDT) calculations of the n⩾4npσ1Σu+N=0 levels of D2. The calculations provide theoretical values of level positions, line intensities and autoionization widths. They are based on quantum–mechanical clamped-nuclei potential energy curves and dipole transition moments available for the lowest Rydberg members of the npσ series. The overall agreement between experiment and theory is good. The decay dynamics of the excited N=0 Rydberg levels is discussed based on the observed quantum yields for ionization, dissociation and fluorescence, and is compared with the yields previously observed in the analogous states of the H2 isotopomer.

Effective photoexcitation cross section of115In(γ,γ′)115mIn from photoactivation data

Photoexcitation yields of the ^115mIn metastable state were measured with Bremsstrahlung gamma beams over a range of endpoint energies between 4.5 and 18 MeV. An effective cross section of the ^115In(gamma,gamma')^115mIn photoexcitation cross section was determinedto reproduce the data. This cross section is built from a cross section calculated with the TALYS code, to which an enhancement is added at about 8.5 MeV.

Understanding photoexcitation dynamics in a three-step photoionization of atomic uranium and measurement of photoexcitation and photoionization cross sections

Photoexcitation dynamics in a three-step photoionization of atomic uranium has been investigated using time-resolved two-color three-photon and delayed three-color three-photon photoionization signals. Investigations are carried out in an atomic beam of uranium coupled to a high-resolution time-of-flight mass spectrometer using three tunable pulsed dye lasers. Dependence of both the signals on the second-step laser photon fluence is studied. Excited-level-to-excited-level photoexcitation cross section and photoionization cross section from the second excited level are simultaneously determined by analyzing the two-color three-photon and three-color three-photon photoionization signals using population rate equation model. Using this methodology, photoexcitation and photoionization cross sections at seven values of the second-step laser wavelength have been measured. From the measured values of the photoexcitation cross sections, we have obtained excited-level-to-excited-level transition probabilities and compared these with the values reported in the literature.

Evolution from Rydberg gas to ultracold plasma in a supersonic atomic beam of Xe

A Rydberg gas of xenon, entrained in a supersonic atomic beam, evolves slowly to form an ultracold plasma. In the early stages of this evolution, when the free-electron density is low, Rydberg atoms undergo long-range -mixing collisions, yielding states of high orbital angular momentum. The development of high- states promotes dipole–dipole interactions that help to drive Penning ionization. The electron density increases until it reaches the threshold for avalanche. Ninety μs after the production of a Rydberg gas with the initial state, , a 432 V cm−1 electrostatic pulse fails to separate charge in the excited volume, an effect which is ascribed to screening by free electrons. Photoexcitation cross sections, observed rates of -mixing, and a coupled-rate-equation model simulating the onset of the electron-impact avalanche point consistently to an initial Rydberg gas density of .

Laser measurement of the photodetachment cross section ofH−at the wavelength 1064 nm

A method is described to measure photoexcitation cross sections, relying on the expected behavior of the signal in the saturated regime, when excitation is provided by a Gaussian light beam. The method is implemented on a negative ion beam, with a single-mode pulsed Nd:YAG laser, to make a laser measurement of the photodetachment cross section of ${\mathrm{H}}^{\ensuremath{-}}$, at the wavelength 1064 nm. This cross section is of importance both as a photodetachment cross section of the most elementary negative ion and as a key parameter for the production of fast neutral ${\mathrm{H}}^{0}$ or ${\mathrm{D}}^{0}$ atoms, by photodetachment of accelerated anions. The obtained value $4.5(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{2}$ is greater than the one known from older measurements and most ab initio calculations.

Comparison of two-color holography among different doped LiNbO3 crystals at low intensities

The effects of material and experimental parameters on the nonvolatile two-color holographic recording space charge field and sensitivity for different doped LiNbO3:Fe crystals have been studied theoretically based on a two-center model. When the direct electron transfer between the deep-trap centers and the shallow-trap centers was considered, the near-stoichiometric LiNbO3:Fe is confirmed theoretically to be of bigger space charge field and higher recording sensitivity than the LiNbO3:Fe:Mn and LiNbO3:Cu:Ce in the low intensity region. A further improvement of the recording sensitivity can be achieved by doping concentration, thermal reduction treatment of Fe, appropriate gating and recording wavelengths with large photo-excitation cross sections.

Estimation of IMFP except for a constant factor using only XPS background-optimized peak intensities and cross sections

A method for estimation of inelastic mean-free path (IMFP or λ) of an unknown material except for an arbitrary proportionality constant c, i.e. λ0(E) in λ(E)=cλ0(E), where E is the electron kinetic energy, has been proposed through iteration of background optimization on an XPS spectrum. The dependence of the i-th core peak intensity pi on its energy Ei given by background optimization is proportional to the product of theoretical photoexcitation cross sections σi and the asymmetry factor ai after analyzer transmission correction. Therefore, any non-parallel behavior between pi and (σi·ai) is ascribed to λ(E). This enables the estimation of λ0(E) using pi and (σi·ai), by assuming the initial λ to be constant, and then repeating background optimization and λ update. Between 700 and 1500 eV kinetic energy (KE) for Au metal, where λ by TPP [Tanuma et al, Surf. Interface Anal., 43,689(2011)], denoted as λTPP, is known to be well approximated by a straight line, λ converges to λTPP within 2.4 % if c is chosen so that both values coincide at 800 eV. This is remarkably satisfactory because only relative peak intensities are involved in a usual practical analysis and because what is requested for surface analysis is to analyze the sample whose properties including λ are yet to be known.

Soft-X-ray ARPES facility at the ADRESS beamline of the SLS: concepts, technical realisation and scientific applications

Soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) with photon energies around 1 keV combines the momentum space resolution with increasing probing depth. The concepts and technical realisation of the new soft-X-ray ARPES endstation at the ADRESS beamline of SLS are described. The experimental geometry of the endstation is characterized by grazing X-ray incidence on the sample to increase the photoyield and vertical orientation of the measurement plane. The vacuum chambers adopt a radial layout allowing most efficient sample transfer. High accuracy of the angular resolution is ensured by alignment strategies focused on precise matching of the X-ray beam and optical axis of the analyzer. The high photon flux of up to 10(13) photons s(-1) (0.01% bandwidth)(-1) delivered by the beamline combined with the optimized experimental geometry break through the dramatic loss of the valence band photoexcitation cross section at soft-X-ray energies. ARPES images with energy resolution up to a few tens of meV are typically acquired on the time scale of minutes. A few application examples illustrate the power of our advanced soft-X-ray ARPES instrumentation to explore the electronic structure of bulk crystals with resolution in three-dimensional momentum, access buried heterostructures and study elemental composition of the valence states using resonant excitation.

Three-Dimensional Electron Realm inVSe2by Soft-X-Ray Photoelectron Spectroscopy: Origin of Charge-Density Waves

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.

The role of daughter nuclei excitation in a structure of photonuclear resonances

The results of microscopic calculations of dipole resonances in 32S and 34S nuclei obtained on the basis of a “particle-finite nucleus state” using the spectroscopic factors of a nucleon pick-up reaction are presented. The conditions for the applicability of this approach to obtaining a realistic picture of a photo-excitation cross section are discussed.

Measurement of photoexcitation cross-sections of uranium by saturation method

We report the measurement of photoexcitation cross-sections of three first-step uranium transitions (0 → 16900.38 cm −1, 0 → 17361.89 cm −1 and 620 → 17361.89 cm −1) using saturation method. These measurements were performed on a resonance ionization mass spectrometry (RIMS) set-up consisting of Nd:YAG-pumped dye lasers, a reflectron time-of-flight mass spectrometer and high-temperature atomic vapour source. The uranium vapours were excited and photoionized by two-colour, three-photon photoionization scheme using Nd:YAG-pumped dye laser system. The resultant photoion signal was monitored as a function of dye laser fluence used for first step excitation to measure the excitation cross-section values. A new approach was adopted to overcome the large uncertainties associated with such measurements. With this approach the cross-section of transitions whose value is already reported in the literature was measured as a bench mark. By normalizing the measured value to the reported value, a scaling factor was derived. This scaling factor was used to scale up the cross-section values of other transitions measured by this method.

Towards a theory of an attosecond transient recorder

Laser-assisted photoemission by a chirped subfemtosecond extreme ultraviolet (xuv) pulse is considered within an exactly solvable quantum-mechanical model. Special emphasis is given to the energy dependence of photoexcitation cross section. The streaked spectra are analyzed within the classical picture of initial time-momentum distribution ${r}_{\text{ini}}(p,t)$ of photoelectrons mapped to the final energy scale. The actual time-momentum distribution in the absence of the probe laser field is shown to be a poor choice for ${r}_{\text{ini}}$, and a more adequate ansatz is suggested. The semiclassical theory offers a simple practically useful approximation for streaked spectra. Its limitations for sufficiently long chirped xuv pulses are established.

Some peculiarities of the energy level, photoelectron and photoexcitation spectra of the atoms with a half-filled shell

The specific features of the atoms with a half-filled shell following from the existence of two classes of its wavefunctions as well as determined by its 2ℓ+2S ground term are considered. The explicit expressions for the photoionization and photoexcitation cross sections for transitions involving such a shell are derived. They are used for a consideration of the spectral regularities of some atoms with p3, d5 and f7 shells.

Photoelectron spectroscopy and secondary ion mass spectrometry characterization of diamond-like carbon films

Photoelectron spectroscopy at different photon energies was used to optimize the photoexcitation cross section for valence band study of diamond-like hydrogenated carbon films. The electronic states of diamond-like carbon (DLC) were studied by synchrotron radiation photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The valence band spectra measured at different excitation energies show the gradual emergence of the p– π band in relation to the sample annealing and ion bombardment amorphization. The p– π band of the annealed DLC was characterized by localized p z states whilst the formation of the amorphous carbon surface was accompanied by appearance of the delocalized p z states in the gap. Secondary ion mass spectrometry and thermal desorption spectroscopy showed that sample annealing was accompanied by hydrogen content decrease.

Ericson Fluctuations in an Open Deterministic Quantum System: Theory Meets Experiment

We provide numerically exact photoexcitation cross sections of rubidium Rydberg states in crossed, static electric, and magnetic fields, in quantitative agreement with recent experimental results. Their spectral backbone underpins a clear transition towards the Ericson regime, associated with a universal, fluctuating behavior of the cross section of strongly coupled, fragmenting quantum systems.

Theoretical studies on nonvolatile two-step, two-color holographic recording sensitivity for LiNbO3:Fe

The effects of material and experimental parameters on the nonvolatile two-step, two-color holographic recording sensitivity for LiNbO3:Fe have been studied theoretically based on a two-center model. We have taken into account the direct electron transfer between the deep-trap center Fe2+/Fe3+ and the shallow-trap center NbLi4+/NbLi5+ due to the tunneling effect. The results show that the direct electron transfer between the Fe2+/Fe3+ and the NbLi4+/NbLi5+ centers plays a key role in the improvement of two-step, two-color holographic recording sensitivity. The near-stoichiometric LiNbO3:Fe has been confirmed theoretically to be of higher recording sensitivity and larger dynamic range than the congruent one in the low intensity region. A further improvement of the recording sensitivity can be achieved by employing appropriate doping concentration and thermal reduction treatment of Fe and/or selecting appropriate gating and recording wavelengths with large photo-excitation cross-sections.

Infrared Quenching of Electroluminescence in ZnS : Mn Thin-Film Electroluminescent Structures

Electroluminescence from thin-film electroluminescent devices is found to be quenched after IR irradiation of the devices in the interval between exciting voltage pulses. The IR irradiation decreases the emission intensity in the spectral range 530–540 nm, while increasing it between 640 and 690 nm. These effects are explained by IR-induced charge exchange between the deep centers due to V S 2+ and V S + sulfur vacancies, an increase in the concentration of the latter vacancies, and the redistribution of the channels of impact excitation of Mn2+ and V S + centers in favor of V S + centers. The cross section and rate of impact excitation of V S + centers, the photoexcitation cross section for V S 2+ centers, the IR radiation absorption coefficient, the internal quantum efficiency of electroluminescence, and the probability of radiative relaxation of Mn2+ centers, as well as the electron multiplication factor in the phosphor layer, are evaluated.