Photon Energy Dependence Research Articles

Photon energy dependence of circular dichroism in angle-resolved photoemission spectroscopy of Bi2Se3Dirac states

Circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES) has been recently proposed as a sensitive probe of either the spin texture or the orbital momentum texture of helical Dirac fermions at the surface of three-dimensional topological insulators. Recent results obtained on Bi${}_{2}$Te${}_{3}$ point rather towards a final state effect. In this Rapid Communication, we study the prototypical topological insulator Bi${}_{2}$Se${}_{3}$ by CD-ARPES as a function of the photon energy and as a function of thickness. Analysis of the photon energy dependence of CD-ARPES, as well as the existence of a large dichroic signal from massive Dirac fermion states in ulthathin films, confirm that the dichroism is predominantly a final state effect.

Evidence for hybrid surface metallic band in (4 × 4) silicene on Ag(111)

The electronic band structure of monolayer (4 × 4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy dependence experiments. The surface band exhibits a steep linear dispersion around the K¯Ag point and has a saddle point near the M¯Ag point of Ag(111) resembling the π-band dispersion in graphene.

Photon energy dependence of circular dichroism of the Au(111) surface state

Through relativistic photoemission calculations for the Au(111) surface state at the Fermi level, we study the photon energy dependence of circular dichroism. The dichromatic signal (${D}_{S}$) pattern changes 23 times with photon energies between 7 and 100 eV, and we have found 13 different patterns in the ${k}_{\ensuremath{\parallel}}$ map at the Fermi level for the ${D}_{S}$ from the Au(111) surface state with normal incidence light. We show that the photon energy dependence of ${D}_{S}$ is very complex even in the simplest case. The sign change in the circular dichroism as a function of photon energy is related to the relative phases of the complex expansion coefficients of different outgoing partial waves in a time-reversed low-energy electron diffraction state. With off-normal incidence, the $z$ component of the incoming photon field is dominant, and the fine structure seen in the ${D}_{S}$ in the normal incidence case is lost very rapidly, moving from a normal to an off-normal incidence. We also report that the Rashba split surface state of Au(111) has a significant component of $d$-type orbital due to relativistic effects and the computational setup used.

Magneto-optical fingerprints of distinct graphene multilayers using the giant infrared Kerr effect

The remarkable electronic properties of graphene strongly depend on the thickness and geometry of graphene stacks. This wide range of electronic tunability is of fundamental interest and has many applications in newly proposed devices. Using the mid-infrared, magneto-optical Kerr effect, we detect and identify over 18 interband cyclotron resonances (CR) that are associated with ABA and ABC stacked multilayers as well as monolayers that coexist in graphene that is epitaxially grown on 4H-SiC. Moreover, the magnetic field and photon energy dependence of these features enable us to explore the band structure, electron-hole band asymmetries, and mechanisms that activate a CR response in the Kerr effect for various multilayers that coexist in a single sample. Surprisingly, we find that the magnitude of monolayer Kerr effect CRs is not temperature dependent. This unexpected result reveals new questions about the underlying physics that makes such an effect possible.

Source position verification and dosimetry in HDR brachytherapy using an EPID

Accurate treatment delivery in high dose rate (HDR) brachytherapy requires correct source dwell positions and dwell times to be administered relative to each other and to the surrounding anatomy. Treatment delivery inaccuracies predominantly occur for two reasons: (i) anatomical movement or (ii) as a result of human errors that are usually related to incorrect implementation of the planned treatment. Electronic portal imaging devices (EPIDs) were originally developed for patient position verification in external beam radiotherapy and their application has been extended to provide dosimetric information. The authors have characterized the response of an EPID for use with an (192)Ir brachytherapy source to demonstrate its use as a verification device, providing both source position and dosimetric information. Characterization of the EPID response using an (192)Ir brachytherapy source included investigations of reproducibility, linearity with dose rate, photon energy dependence, and charge build-up effects associated with exposure time and image acquisition time. Source position resolution in three dimensions was determined. To illustrate treatment verification, a simple treatment plan was delivered to a phantom and the measured EPID dose distribution compared with the planned dose. The mean absolute source position error in the plane parallel to the EPID, for dwells measured at 50, 100, and 150 mm source to detector distances (SDD), was determined to be 0.26 mm. The resolution of the z coordinate (perpendicular distance from detector plane) is SDD dependent with 95% confidence intervals of ± 0.1, ± 0.5, and ± 2.0 mm at SDDs of 50, 100, and 150 mm, respectively. The response of the EPID is highly linear to dose rate. The EPID exhibits an over-response to low energy incident photons and this nonlinearity is incorporated into the dose calibration procedure. A distance (spectral) dependent dose rate calibration procedure has been developed. The difference between measured and planned dose is less than 2% for 98.0% of pixels in a two-dimensional plane at an SDD of 100 mm. Our application of EPID dosimetry to HDR brachytherapy provides a quality assurance measure of the geometrical distribution of the delivered dose as well as the source positions, which is not possible with any current HDR brachytherapy verification system.

Isomer effects in the double-to-single photoionization ratio of aromatic hydrocarbons

We have measured the ratios of doubly to singly charged molecular parent ions of azulene and phenanthrene, which are isomers of naphthalene and anthracene, respectively. The photon-energy dependences of the ratio curves have been analyzed and compared to each other from threshold to the carbon $K$ edge. Qualitatively, the ratio curves for azulene and naphthalene as well as phenanthrene and anthracene are very similar, but we have found a distinct quantitative difference in both cases for the two isomers.

Photon energy dependence of graphitization threshold for diamond irradiated with an intense XUV FEL pulse

We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultraviolet laser (XUV) pulse of 24–275 eV photon energy provided by free-electron lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify the subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions.Received 8 March 2013DOI:https://doi.org/10.1103/PhysRevB.88.060101©2013 American Physical Society

Electronic relaxation and coherent phonon dynamics in semiconducting single-walled carbon nanotubes with several chiralities

Coherent phonon (CP) dynamics and electronic relaxation in single-walled carbon nanotubes (SWNTs) are investigated in femtosecond pump-probe experiments. Using a sensitive multichannel lock-in amplifier, chirality-specific electronic relaxation and vibrational dynamics are resolved in SWNT ensembles composed of several chiral systems without the need for selective isolation of the different species by purification. The dynamics of vibrational wave packets are studied based on oscillatory changes in the absorbance of the systems. Modulations corresponding to the radial breathing mode (RBM), observed in the time traces of the absorbance change for the four chiral systems (6,4), (6,5), (7,5), and (8,3), have been analyzed in detail. The vibrational modes of the CP spectra are identified from the two-dimensional distribution of probe photon energy versus Fourier frequency. Resonance conditions and mode frequencies lead to definite chirality assignments. Coherent RBM phonon generation is analyzed using the probe photon energy-dependent amplitude profiles as a result of the spectral shift induced by wave-packet motion on the potential surface. The present study clarifies that the observed probe photon energy dependence is due to both the imaginary and real parts of the third-order susceptibility, corresponding to Raman (and Raman-like) gain and loss processes and to molecular phase modulation, respectively. The imaginary part is the dominant contribution to the modulation in the difference absorbance. It shows probe photon energy dependence in the form of a difference in absorbed photon energy between the spectra that are shifted and unshifted with vibrational frequency. The size of the Huang-Rhys factors from the difference fitting to the (6,4), (6,5), and (8,3) systems are 0.26, 0.32, and 0.75, respectively. The trend of the factors originates in the stiffness differences of the SWNT structures. The real part depends on the derivative of the absorbed photon energy spectrum due to cross-phase modulation, resulting from the change in refractive index during the molecular vibrations. This process induces a probe spectral change, as evidenced by first-derivative fitting using a small number of data points of probe photon energies. The effective nonlinear refractive index for each chiral system is determined to range from 0.2 to $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ cm${}^{2}$/W.

Photo-double-ionization mechanisms in aromatic hydrocarbons

We have measured the ratios of doubly to singly charged molecular parent ions of benzene (partially deuterated), naphthalene, anthracene, pentacene, pyrrole, furan, selenophene, and coronene for photon energies ranging from threshold to the carbon $K$ shell. The photon-energy dependence of the ratio curves has been analyzed and compared to each other for the above molecules. We conclude that two---and in some cases three---different photo-double-ionization mechanisms exist for aromatic hydrocarbons. One of the mechanisms is the formation of a two-electron pseudoparticle. This finding may be useful in the quest for understanding high-temperature superconductivity.

SU‐E‐T‐266: New Improved Methods to Read Output Factors for Small Fields

Linear Accelerators represent big challenges in measuring output factors for small fields. Output factors are crucial for dose calculations in intensity modulated radiation therapy (IMRT) planning. The challenge comes from charged particle non‐equilibrium conditions and high dose gradients, type, and the detectors used. We analyze all available detectors and the output factors obtained with them. Based on this we come with recommendations in a protocol for accurately measuring the output factors for commissioning any linac that will perform IMRT. Advances in therapy equipment implicitly encourage the use of smaller treatment field sizes on equipment originally designed for treatments based on conventional, broad photon fields.Challenges at small fields: penumbra size vs. detector size, lack of lateral electronic equilibrium (LEE), photon energy and spectrum dependence on detector response, finite detector volume, detector density effect, detectors' polarity effects, physics at small fields: field perturbation, fluence. Method: Initial data gathered have led to necessity of using small detectors for small fields. We propose to use different detectors to measure the output factors for microMLC linacs using three detectors: a diode detector and two types of micro ionization chambers (for corroborative measurements and consistency). We compared the performance of the Edge detector, PTW pinpoint and Exradin A1 ionization chamber. The output factors are taken at three different depths: (dmax, 2cm, and 5.0cm) in SAD setup. Comparison of these data lead to precise measurement of the correction factor to provide 1%–2% variation in calculated doses for IMRT with the Treatment Planning System. DISCUSSION: From TG‐155 recommendations, dosimetric measurements should be taken with more than one detector system. Small detectors should be used that has minimum energy, dose and dose rate dependence. If field size is small compared to the detector, measurements should be taken at greater source to surface distance with proper correction. Advanced Radiation Physics Inc

Study of γ d → π + π − d Reaction in an Energy Region of 0.67 ≤ E γ ≤ 1.08 GeV

Double charged pion photoproductions from the deuteron have been studied at Research Center for Electron Photon Science (ELPH), Tohoku University. Tagged photon beams in an energy range of 0.67 ≤ E γ ≤ 1.08 GeV were impinged on the liquid deuteron target. Produced charged particles were observed with the Neutral Kaon Spectrometer2 (NKS2). The γ d → π + π − d process was clearly separated among the three charged tracks. We obtained the photon energy dependence of the normalized yields of this process.

Tunable High Harmonic Generation driven by a Visible Optical Parametric Amplifier

We studied high-harmonic generation (HHG) in Ar, Ne and He gas jets using a broadly tunable, high-energy optical parametric amplifier (OPA) in the visible wavelength range. We optimized the noncollinear OPA to deliver tunable, femtosecond pulses with 200-500 μJ energy at 1-kHz repetition rate with excellent spatiotemporal properties, suitable for HHG experiments. By tuning the central wavelength of the OPA while keeping energy, duration and beam size constant, we experimentally studied the scaling law of conversion efficiency and cut-off energy with the driver wavelength in argon and helium respectively. Our measurements show a λ−5.9±0.9 wavelength dependence of the conversion efficiency and a λ1.7±0.2 dependence of the HHG cut-off photon energy over the full visible range in agreement with previous experiments of near- and mid-IR wavelengths. By tuning the central wavelength of the driver source and changing the gas, the high order harmonic spectra in the extreme ultraviolet cover the full range of photon energy between ~25 eV and ~100 eV. Due to the high coherence intrinsic in HHG, as well as the broad and continuous tunability in the extreme UV range, a high energy, high repetition rate version of this source might be an ideal seed for free electron lasers.

Scaling‐law for the energy dependence of anatomic power spectrum in dedicated breast CT

To determine the x-ray photon energy dependence of the anatomic power spectrum of the breast when imaged with dedicated breast computed tomography (CT). A theoretical framework for scaling the empirically determined anatomic power spectrum at one x-ray photon energy to that at any given x-ray photon energy when imaged with dedicated breast CT was developed. Theory predicted that when the anatomic power spectrum is fitted with a power curve of the form k f(-β), where k and β are fit coefficients and f is spatial frequency, the exponent β would be independent of x-ray photon energy (E), and the amplitude k scales with the square of the difference in energy-dependent linear attenuation coefficients of fibroglandular and adipose tissues. Twenty mastectomy specimens based numerical phantoms that were previously imaged with a benchtop flat-panel cone-beam CT system were converted to 3D distribution of glandular weight fraction (f(g)) and were used to verify the theoretical findings. The 3D power spectrum was computed in terms of f(g) and after converting to linear attenuation coefficients at monoenergetic x-ray photon energies of 20-80 keV in 5 keV intervals. The 1D power spectra along the axes were extracted and fitted with a power curve of the form k f(-β). The energy dependence of k and β were analyzed. For the 20 mastectomy specimen based numerical phantoms used in the study, the exponent β was found to be in the range of 2.34-2.42, depending on the axis of measurement. Numerical simulations agreed with the theoretical predictions that for a power-law anatomic spectrum of the form k f(-β), β was independent of E and k(E) = k(1)[μ(g)(E) - μ(a)(E)](2), where k(1) is a constant, and μ(g)(E) and μ(a)(E) represent the energy-dependent linear attenuation coefficients of fibroglandular and adipose tissues, respectively. Numerical simulations confirmed the theoretical predictions that in dedicated breast CT, the spatial frequency dependence of the anatomic power spectrum will be independent of x-ray photon energy, and the amplitude of the anatomic power spectrum scales by the square of difference in linear attenuation coefficients of fibroglandular and adipose tissues.

Strong quadrupole interaction between the krypton 4p photoionization and 3d resonant excitation channels

The angular distribution of Kr 4p photoelectrons was investigated experimentally in the photon energy range of the 3d−1 → np resonant excitations. The experimental dipole and non-dipole anisotropy parameters were determined for the spin-orbit components of the Kr 4p shell. An interference effect was observed between the direct photoionization and the resonant excitation participator Auger decay processes in the photon energy dependence of the experimental anisotropy parameters.

Cut-off scaling of high-harmonic generation driven by a femtosecond visible optical parametric amplifier

We studied high-harmonic generation (HHG) in Ar, Ne and He gas jets using a broadly tunable, high-energy optical parametric amplifier (OPA) in the visible wavelength range. We optimized the noncollinear OPA to deliver tunable, femtosecond pulses with 200–500 µJ energy at the 1 kHz repetition rate with excellent spatiotemporal properties, suitable for HHG experiments. By tuning the central wavelength of the OPA while keeping other parameters (energy, duration and beam size) constant, we experimentally studied the scaling law of cut-off energy with the driver wavelength in helium. Our measurements show a λ1.7 + 0.2 dependence of the HHG cut-off photon energy over the full visible range in agreement with previous experiments of near- and mid-IR wavelengths. By tuning the central wavelength of the driver source, the high-order harmonic spectra in the extreme ultraviolet cover the full range of photon energy between ∼25 and ∼100 eV. Due to the high coherence intrinsic in HHG, as well as the broad and continuous tunability in the extreme UV range, a high energy, high repetition rate version of this source might be an ideal seed for free electron lasers.

Lateral electrical transport and photocurrent in single and multilayers of two-dimensional arrays of Si nanocrystals

We have investigated electrical transport and photocurrent in single and multilayers of two-dimensional arrays of silicon nanocrystals (SiNCs) suitable for photovoltaic applications. The films were grown on quartz by low pressure chemical vapor deposition of Si and subsequent thermal oxidation steps. We found that at high voltages, electrical transport is governed by space charge limited currents due to the presence of traps. At low voltages, electrical transport is ohmic. Carrier mobility, carrier concentration, and trap density in the films were extracted from the electrical measurements. Combining photocurrent and absorption measurements for the films with different SiNC sizes, we found a remarkable similarity in the photon energy dependence of the photocurrent and of the absorbed light from the SiNCs, confirming a proportionality relation between the two quantities. Also, from the combined study of electrical transport and photocurrent, minority carrier lifetimes were extracted.

Three Dimensionality and Orbital Characters of the Fermi Surface in(Tl,Rb)yFe2−xSe2

We report a comprehensive angle-resolved photoemission spectroscopy study of the tridimensional electronic bands in the recently discovered Fe selenide superconductor ((Tl,Rb)(y)Fe(2-x)Se2 (T(c)=32 K). We determined the orbital characters and the k(z) dependence of the low energy electronic structure by tuning the polarization and the energy of the incident photons. We observed a small 3D electron Fermi surface pocket near the Brillouin zone center and a 2D like electron Fermi surface pocket near the zone boundary. The photon energy dependence, the polarization analysis and the local-density approximation calculations suggest a significant contribution from the Se 4p(z) and Fe 3d(xy) orbitals to the small electron pocket. We argue that the emergence of Se 4p(z) states might be the cause of the different magnetic properties between Fe chalcogenides and Fe pnictides.

An environmental BeO-OSL dosimeter for emergency response

A conceptual design is presented to use measurements of localized absorbed dose in inner cities for production of high resolution maps of the radioactive contamination following a nuclear emergency or radiological attack. The doses are derived from luminescent detectors pre-fixed at places of high importance (e.g. public squares). For such an environmental dosimeter, BeO is used, which can be read out using optically stimulated luminescence (OSL). A suitable casing of black Perspex has been developed to give a sufficiently accurate estimate of the air kerma value at the detector position. The dosimeter is characterized according to light tightness, dose response and angular photon energy dependence. A short overview of the approach for map production is also given. ► An inexpensive, environmentally stable BeO based OSL dosimeter has been developed for emergency response. ► The detector enables fast readouts and shows highly favorable dosimetric properties. ► A conceptual design is described to produce maps of radioactive contamination from localized dose measurements in urban areas.

Lifetime of the unpaired electron species in calf thymus DNA thin films induced by nitrogen and oxygen K-shell photoabsorption

Purpose: To study the lifetime of the short-lived unpaired electron species induced in a calf thymus DNA film by soft X-rays in the energy regions around nitrogen and oxygen K-edge.Materials and methods: Dry calf thymus DNA films were measured by electron paramagnetic resonance (EPR) spectrometer during the monochromatic soft X-ray irradiation provided from a synchrotron accelerator (SPring-8, Super Photon ring-8 GeV). Two operation modes of SPring-8 with different electron bunch structures were used to explore dependence of EPR intensity on pulse interval of soft X-rays.Results: Observed EPR spectra did not show significant difference, in the spectral shape, intensity, g-factor and photon energy dependence, between the two bunch modes.Conclusions: The total yield of the unpaired electron species were estimated by assuming the exponential relaxation of the unpaired electrons with two different bunch modes of the synchrotron. The lifetime of the unpaired electron species is estimated to be in the range of a few tens to several hundreds of μs.

Background-dependent Lorentz violation: natural solutions to the theoretical challenges of the superluminal neutrino propagation

To explain both the OPERA experiment and all the known phenomenological constraints/observations on Lorentz violation, the Background Dependent Lorentz Violation (BDLV) has been proposed. We study the BDLV in a model independent way, and conjecture that there may exist a "Dream Special Relativity Theory", where all the Standard Model (SM) particles can be subluminal due to the background effects. Assuming that the Lorentz violation on the Earth is much larger than those on the interstellar scale, we automatically escape all the astrophysical constraints on Lorentz violation. For the BDLV from the effective field theory, we present a simple model and discuss the possible solutions to the theoretical challenges of the OPERA experiment such as the Bremsstrahlung effects for muon neutrinos and the pion decays. Also, we address the Lorentz violation constraints from the LEP and KamLAMD experiments. For the BDLV from the Type IIB string theory with D3-branes and D7-branes, we point out that the D3-branes are flavour blind, and all the SM particles are the conventional particles as in the traditional SM when they do not interact with the D3-branes. Thus, we not only can naturally avoid all the known phenomenological constraints on Lorentz violation, but also can naturally explain all the theoretical challenges. Interestingly, the energy dependent photon velocities may be tested at the experiments.