68 research outputs found
Space-charge effects in high-energy photoemission
Pump-and-probe photoelectron spectroscopy (PES) with femtosecond pulsed
sources opens new perspectives in the investigation of the ultrafast dynamics
of physical and chemical processes at the surfaces and interfaces of solids.
Nevertheless, for very intense photon pulses a large number of photoelectrons
are simultaneously emitted and their mutual Coulomb repulsion is sufficiently
strong to significantly modify their trajectory and kinetic energy. This
phenomenon, referred as space-charge effect, determines a broadening and shift
in energy for the typical PES structures and a dramatic loss of energy
resolution. In this article we examine the effects of space charge in PES with
a particular focus on time-resolved hard X-ray (~10 keV) experiments. The
trajectory of the electrons photoemitted from pure Cu in a hard X-ray PES
experiment has been reproduced through -body simulations and the broadening
of the photoemission core-level peaks has been monitored as a function of
various parameters (photons per pulse, linear dimension of the photon spot,
photon energy). The energy broadening results directly proportional to the
number of electrons emitted per pulse (mainly represented by secondary
electrons) and inversely proportional to the linear dimension of the photon
spot on the sample surface, in agreement with the literature data about
ultraviolet and soft X-ray experiments. The evolution in time of the energy
broadening during the flight of the photoelectrons is also studied. Despite its
detrimental consequences on the energy spectra, we found that space charge has
negligible effects on the momentum distribution of photoelectrons and a
momentum broadening is not expected to affect angle-resolved experiments.
Strategy to reduce the energy broadening and the feasibility of hard X-ray PES
experiments at the new free-electron laser facilities are discussed.Comment: 15 pages, 2 tables, 8 figure
Insight on Hole-Hole Interaction and Magnetic Order from Dichroic Auger-Photoelectron Coincidence Spectra
The absence of sharp structures in the core-valence-valence Auger line shapes
of partially filled bands has severely limited the use of electron spectroscopy
in magnetic crystals and other correlated materials. Here by a novel interplay
of experimental and theoretical techniques we achieve a combined understanding
of the Photoelectron, Auger % and Auger-Photoelectron
Coincidence Spectra (APECS) of CoO. This is a prototype antiferromagnetic
material in which the recently discovered Dichroic Effect in Angle Resolved
(DEAR) APECS reveals a complex pattern in the strongly correlated Auger line
shape. A calculation of the \textit{unrelaxed} spectral features explains the
pattern in detail, labeling the final states by the total spin. The present
theoretical analysis shows that the dichroic effect arises from a
spin-dependence of the angular distribution of the photoelectron-Auger electron
pair detected in coincidence, and from the selective power of the dichroic
technique in assigning different weights to the various spin components. Since
the spin-dependence of the angular distribution exists in the antiferromagnetic
state but vanishes at the N\'eel temperature, the DEAR-APECS technique detects
the phase transition from its local effects, thus providing a unique tool to
observe and understand magnetic correlations in such circumstances, where the
usual methods (neutron diffraction, specific heat measurements) are not
applicable.Comment: Accepted by: Physical Review Letter
Single center experience with laparoscopic adrenalectomy on a large clinical series
BACKGROUND: Laparoscopic adrenalectomy is considered the gold standard technique for the treatment of benign small and medium size adrenal masses (<6 cm), due to low morbidity rate, short hospitalization and patient rapid recovery. The aim of our study is to analyse the feasibility and efficiency of this surgical approach in a broad spectrum of adrenal gland pathologies.
METHODS: Pre-operative, intra-operative and post-operative data from 126 patients undergone laparoscopic adrenalectomy between January 2003 and December 2015 were retrospectively collected and reviewed. Diagnosis was obtained on the basis of clinical examination, laboratory values and imaging techniques. Doxazosin was preoperatively administered in case of pheochromocytoma while spironolactone and potassium were employed to treat Conn's disease. Laparoscopic adrenalectomies were all performed by the same surgeon (CG). First 30 procedures were considered as learning curve adrenalectomies.
RESULTS: One hundred twenty-six patients were included in the study. Functioning tumors were diagnosed in 84 patients, 27 patients were affected by pheochromocytomas, 29 by Conn's disease, 28 by Cushing disease. Surgery mean operative time was 137.33 min (range 100-180) during the learning curve adrenalectomies and 96.5 min (range 75-110) in subsequent procedures. Mean blood loss was respectively 160.2 ml (range 60-280) and 90.5 ml (range 50-200) in the first 30 procedures and the subsequent ones. Only one conversion to open surgery occurred. No post-operative major complications were observed, while minor complications occurred in 8 patients (0,79%). In 83 out of 84 functioning neoplasms, laparoscopic adrenalectomy was effective in normalization of endocrine profile.
CONCLUSIONS: Laparoscopic adrenalectomy is a safe and feasible procedure, even for functioning masses and pheochromocytomas. A multidisciplinary team including endocrinologists, endocrine surgeons and anaesthesiologists, is recommended in the management of adrenal pathology, and adrenal surgery should be performed in referral high volume centers. A thirty-procedures learning curve is recommended to improve surgical outcomes
Photoemission from the gas phase using soft x-ray fs pulses: An investigation of the space-charge effects
An experimental and computational investigation of the space-charge effects
occurring in ultrafast photoelectron spectroscopy from the gas phase is
presented. The target sample CFI is excited by ultrashort (100 fs)
far-ultraviolet radiation pulses produced by a free-electron laser. The
modification of the energy distribution of the photoelectrons, i.e. the shift
and broadening of the spectral structures, is monitored as a function of the
pulse intensity. A novel computational approach is presented in which a survey
spectrum acquired at low radiation fluence is used to determine the initial
energy distribution of the electrons after the photoemission event. The
spectrum modified by the space-charge effects is then reproduced by -body
calculations that simulate the dynamics of the photoelectrons subject to the
mutual Coulomb repulsion and to the attractive force of the positive ions. The
employed numerical method allows to reproduce the complete photoelectron
spectrum and not just a specific photoemission structure. The simulations also
provide information on the time evolution of the space-charge effects on the
picosecond scale. Differences with the case of photoemission from solid samples
are highlighted and discussed. The presented simulation procedure constitutes
an effective tool to predict and account for space-charge effect in
time-resolved photoemission experiments with high-intensity pulsed sources.Comment: 18 pages, 4 figures, 1 tabl
Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO 1 − x F x and Sr(Fe 1 − x Co x ) 2 As 2
A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3 core level photoemission spectra in the parent and optimally doped CeFeAsO{}_{1\ensuremath{-}x}F ( 0, 0.11) and Sr(Fe{}_{1\ensuremath{-}x}Co)As ( 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10{}^{\ensuremath{-}15} s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3{\ensuremath{\mu}}_{B} in CeFeAsO and 2.1{\ensuremath{\mu}}_{B} in SrFeAs. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9{\ensuremath{\mu}}_{B} in CeFeAsOF and 1.3{\ensuremath{\mu}}_{B} in Sr(FeCo)As. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity
Colorectal Cancer Stage at Diagnosis Before vs During the COVID-19 Pandemic in Italy
IMPORTANCE Delays in screening programs and the reluctance of patients to seek medical
attention because of the outbreak of SARS-CoV-2 could be associated with the risk of more advanced
colorectal cancers at diagnosis.
OBJECTIVE To evaluate whether the SARS-CoV-2 pandemic was associated with more advanced
oncologic stage and change in clinical presentation for patients with colorectal cancer.
DESIGN, SETTING, AND PARTICIPANTS This retrospective, multicenter cohort study included all
17 938 adult patients who underwent surgery for colorectal cancer from March 1, 2020, to December
31, 2021 (pandemic period), and from January 1, 2018, to February 29, 2020 (prepandemic period),
in 81 participating centers in Italy, including tertiary centers and community hospitals. Follow-up was
30 days from surgery.
EXPOSURES Any type of surgical procedure for colorectal cancer, including explorative surgery,
palliative procedures, and atypical or segmental resections.
MAIN OUTCOMES AND MEASURES The primary outcome was advanced stage of colorectal cancer
at diagnosis. Secondary outcomes were distant metastasis, T4 stage, aggressive biology (defined as
cancer with at least 1 of the following characteristics: signet ring cells, mucinous tumor, budding,
lymphovascular invasion, perineural invasion, and lymphangitis), stenotic lesion, emergency surgery,
and palliative surgery. The independent association between the pandemic period and the outcomes
was assessed using multivariate random-effects logistic regression, with hospital as the cluster
variable.
RESULTS A total of 17 938 patients (10 007 men [55.8%]; mean [SD] age, 70.6 [12.2] years)
underwent surgery for colorectal cancer: 7796 (43.5%) during the pandemic period and 10 142
(56.5%) during the prepandemic period. Logistic regression indicated that the pandemic period was
significantly associated with an increased rate of advanced-stage colorectal cancer (odds ratio [OR],
1.07; 95%CI, 1.01-1.13; P = .03), aggressive biology (OR, 1.32; 95%CI, 1.15-1.53; P < .001), and stenotic
lesions (OR, 1.15; 95%CI, 1.01-1.31; P = .03).
CONCLUSIONS AND RELEVANCE This cohort study suggests a significant association between the
SARS-CoV-2 pandemic and the risk of a more advanced oncologic stage at diagnosis among patients
undergoing surgery for colorectal cancer and might indicate a potential reduction of survival for
these patients
Structural and magnetic properties of FexMn1-x thin films on Cu(001) and on Co/Cu(001)
The structural and magnetic properties of FexMn1-x alloy thin films deposited on Cu(001) and on Co/Cu(001) are investigated. A layer-by-layer growth mode of the alloy films in a large range of concentration is deduced from the presence of regular oscillations in medium-energy electron-diffraction intensity. Low-energy electron-diffraction experiments reveal that Fe50Mn50 films keep the fcc structure when growing on Cu(001) and on Co/Cu(001). Magneto-optic Kerr effect is used to study the magnetic properties of Co/FeMn bilayers. A temperature-dependent increase in the coercive field of the Co film, when the thickness of the adjacent FeMn film exceeds a certain value, is related to the antiferromagnetic state of the FexMn1-x alloy
Check the Lambert-Beer-Bouguer law: a simple trick to boost the confidence of students toward both exponential laws and the discrete approach to experimental physics
Exponential decay is a prototypical functional behaviour for many physical phenomena, and therefore it deserves great attention in physics courses at an academic level. The absorption of the electromagnetic radiation that propagates in a dissipative medium provides an example of the decay of light intensity, as stated by the law of Lambert-Beer-Bourguer. We devised a very simple experiment to check this law. The experimental setup, its realization, and the data analysis of the experiment are definitely simple. Our main goal was to create an experiment that is accessible to all students, including those in their first year of academic courses and those with poorly equipped laboratories. As illustrated in this paper, our proposal allowed us to develop a deep discussion about some general mathematical and numerical features of exponential decay. Furthermore, the special setup of the absorbing medium (sliced in finite thickness slabs) and the experimental outcomes allow students to understand the transition from the discrete to the continuum approach in experimental physics
A teaser made simple: a didactic measurement of the spectral answer of a human eye calibrated luxmeter
A simple didactic experiment has been designed and realized, in order to
illustrate to undergraduate students in scientific faculties some basic concepts
lying behind the fundamentals of geometrical optics. The spectral response of a
human-eye-calibrated lux meterwas measured using a very trivial experimental
arrangement. The white light of a halogen lamp was decomposed into its
spectral components through a diffraction grating, so that collecting the
radiation at different dispersion angles allowed one to measure the intensity as a
function of wavelength. The experiment can be used to effectively illustrate the
concepts of spectral distribution, the radiometry versus photometry conversion
and photopic response, and the famous historical experience by Herschel on
the ‘temperature of colours’
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