Exafs like oscillations in X-ray excited autoionization spectra assisted by compton process

Oscillating features have been observed in the background of the inelastic electrons excited by high energy monochromatic X-ray photons at kinetic energy above the Cr M_2,3VV Auger transition. These structures are absent when X-ray photons of are used. We suggest that these extended features are due to an autoionization process experienced by the 3p core electron assisted by a Compton scattering with the X-ray photons. Through the inelastic Compton process the incoming photons excite the core electrons towards empty states above the Fermi level in a continuous manner up to a maximum energy, which strongly depends on the incident photon energy. These excited states are modulated in energy by the crystalline structure as it has been shown by means of the EXAFS (Extended X-ray Absorption Fine Structure) spectroscopy. These oscillating structures are compared with those obtained through electron excitation for the same Auger transition in the Extended Fine Auger Structure (EXFAS) spectroscopy.     

Measuring fine molecular structures with luminescence signal from an alternating current scanning tunneling microscope

In scanning tunneling microscopy-induced luminescence (STML), the photon count is measured to reflect single-molecule properties, e.g., the first molecular excited state. The energy of the first excited state is typically shown by a rise of the photon count as a function of the bias voltage between the tip and the substrate. It remains a challenge to determine the precise rise position of the current due to possible experimental noise. In this work, we propose an alternating current version of STML to resolve the fine structures in the photon count measurement. The measured photon count and the current at the long-time limit show a sinusoidal oscillation. The zero-frequency component of the current shows knee points at the precise voltage as the fraction of the detuning between the molecular gap and the DC component of the bias voltage. We propose to measure the energy level with discontinuity of the first derivative of such a zero-frequency component. The current method will extend the application of STML in terms of measuring molecular properties.     

Determination of the radiation cross sections of low-energy transitions of isomeric nuclei from observation of laser-induced γ fluorescence

Theoretical estimates are given along with the first experimental results on the observation of resonance fluorescence in nuclei of rubidium isomer under conditions of laser plasma X-ray pumping of the contiguous transition with an energy of 3.4 keV. The laser plasma is prepared by irradiating a silver target by a powerful radiation of a Nd laser with a pulse duration of 600 ps. It is demonstrated how one can use the recorded number of emitted γ quanta to determine the probability of low-energy nuclear transition excited by laser plasma X rays.     

苝四甲酸二酐薄膜电子结构的同步辐射共振光电子能谱研究

利用基于同步辐射的近边X射线吸收精细结构谱(NEXAFS)和共振光电子谱(RPES)研究了苝四甲酸二酐分子(PTCDA)薄膜的电子结构.碳K边NEXAFS谱中能量小于290 eV的四个峰对应于PTCDA分子不同化学环境碳原子1s电子到未占据分子轨道的共振跃迁.RPES谱中观察到共振光电子发射和共振俄歇电子发射导致的共振峰结构,以及二次谐波激发的碳1s信号.根据电子动能对入射光能量的依赖性分别对三类峰结构进行了归属.同时,发现PTCDA分子轨道共振光电子峰的强度具有光子能量依赖性.这种能量选择性共振增强效应是由于PTCDA分子轨道空间分布差异导致的.共振俄歇峰主要源于高结合能(4.1 eV)分子轨道能级电子参与的退激发过程.明确RPES实验谱图中各个峰结构的起源有助于准确利用基于RPES的芯能级空穴时钟谱技术定量估算有机分子/电极异质界面处电子从分子未占据轨道到电极导带的超快转移时间.     

Electron-Impact Induced Molecular Fluoresence Spectroscopy

Abstract

By the bombardment of gaseous molecules with monoenergetic electrons the molecules can be electronically excited and/or fragmented, depending upon the energy supplied by the electrons. Any fragments which may be produced also have the possibility of being produced in electronically (as well as vibrationally and rotationally) excited states. The electronically excited molecules and fragments both possess the possibility of radiative decay (fluorescence). The measurement of this fluorescence as a function of both electron energy and photon wavelength is termed “electron-impact induced fluorescence spectroscopy.” A plot of fluorescence intensity as a function of the electron energy at a given photon wavelength is generally referred to as an “excitation function,” which is exactly analogous to the excitation spectrum which can be obtained in conventional (optically excited) fluorescence spectrometry. A plot of the fluorescence intensity as a function of photon wavelength at a given electron energy is termed the “fluorescence spectrum.” A three dimensional plot of the fluorescence intensity as a function of both the photon wavelength and the electron energy may also be constructed, and is referred to as an “excitation contour plot.”     

Remarks concerning the 58 keV line from Her X1 interpreted as a synchrotron emission line

We assume the interpretation given by Trumper et al. regarding the prominent X-ray line of Her X1 to be valid, viz., that the 58 keV line of Her X1 stems from an electron in a magnetized plasma making a transition from the first quantized state (associated with a uniform magnetic field) to the ground state. Ignoring gravitational red shifts the width of the line is calculated from considerations of (1) the transition rate associated with an electron in the first excited state in a uniform magnetic field of the proper magnitude and (2) level broadening due to recooil of the electron due to photon emission and absorption. The resulting theoretical line width is then compared to the line width of ≈12 keV reported by Trumper et al. The importance of taking into account recoil effects is stressed.     

Resonant Photon Drag of Dipolar Excitons

The theory of the photon drag of dipolar excitons in double-quantum-well nanostructures is presented. It is shown that the exciton-drag flux density features a resonant behavior if the photon frequency is close to some transition frequency in the discrete exciton spectrum. When the structure is irradiated with polarized light, the resonant enhancement of the drag current occurs when the photon energy coincides with the energy of an excited level of the exciton internal motion and the components of the angular momentum of internal motion in the initial and final states differ by one. The proposed effect can be used to control exciton transport in nanostructures based on a two-dimensional exciton gas.     

Photoluminescence and photoinduced magnetic phase transition in an organic radical TTTA crystal studied by two-photon absorption

Photoluminescence as well as photoinduced diamagnetic to paramagnetic phase transition in a strongly correlated electron system, an organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal, was investigated under two-photon excitation with different photon densities. Below the threshold photon density to drive the photoinduced phase transition (PIPT), the diamagnetic phase shows a broad luminescence band with a large Stokes shift, whose intensity obeys almost second power law of the excitation photon density. Above the threshold photon density, on the other hand, the diamagnetic to paramagnetic phase transition effectively takes place with a large conversion yield and a steep response instead of an occurrence of the photoluminescence, indicating that the phase transition is optically induced by two-photon absorption. As far as we know, this is the first observation of the PIPT phenomena mediated by two-photon absorption.     

Intersubband optical absorption of electrons in double parabolic quantum wells of Al_xGa_(1-x)As/Al_yGa_(1-y)As

Some realizable structures of double parabolic quantum wells(DPQWs) consisting of Al_xGa_(1-x)As/Al_yGa_(1-y)As are constructed to discuss theoretically the optical absorption due to the intersubband transition of electrons for both symmetric and asymmetric cases with three energy levels of conduction bands. The electronic states in these structures are obtained using a finite element difference method. Based on a compact density matrix approach, the optical absorption induced by intersubband transition of electrons at room temperature is discussed. The results reveal that the peak positions and heights of intersubband optical absorption coefficients(IOACs) of DPQWs are sensitive to the barrier thickness, depending on Al component. Furthermore, external electric fields result in the decrease of peak, and play an important role in the blue shifts of absorption spectra due to electrons excited from ground state to the first and second excited states. It is found that the peaks of IOACs are smaller in asymmetric DPQWs than in symmetric ones. The results also indicate that the adjustable extent of incident photon energy for DPQW is larger than for a square one of a similar size. Our results are helpful in experiments and device fabrication.     

Reparameterization invariance of NRQED self-energy corrections and improved theory for excited D states in hydrogenlike systems

Canonically, the quantum electrodynamic radiative corrections in bound systems have been evaluated in photon energy regularization, i.e., using a noncovariant overlapping parameter that separates the high-energy relativistic scales of the virtual quanta from the nonrelativistic domain. Here, we calculate the higher-order corrections to the one-photon self-energy calculation with three different overlapping parameters (photon energy, photon mass and dimensional regularization) and demonstrate the reparameterization invariance of nonrelativistic quantum electrodynamics (NRQED) using this particular example. We also present new techniques for the calculation of the low-energy part of this correction, which lead to results for the Lamb shift of highly excited states that are important for high-precision spectroscopy.     

The shape of doppler-broadened spectral lines produced by light atom reflection from metals: I. Theory

A model for predicting the shapes of Doppler-broadened spectral lines due to de-excitation by photon emission of excited neutral atoms backscattered from solids is presented. The distribution of backscattered particles is calculated on the basis of single Rutherford collisions within the solid accompanied by energy loss due to electronic stopping during passage of the particle through the solid. The effect of the energy dependence of the stopping power, radiationless de-excitation of the escaping neutral atom and the probability of the particle leaving the surface in the desired excited state are incorporated into the calculation. The broadening of the spectral line is determined by combining the Doppler shift formula with the energy- and angular distribution of the backscattered particles.     

反铁磁粒子的势助量子隧穿和宏观量子效应

用周期瞬子方法研究了反铁磁粒子中激发态量子隧道效应.给出了各种低激发态下的隧穿幅、能级分裂以及前置因子.用统计平均得到给定温度下的隧穿率.理论计算的隧穿率随温度的变化关系与实验结果相符. 关键词：     

Simple cases of spontaneous emission from an atom-photon cluster localized in a microcavity

The spontaneous emission from an atomic ensemble localized in a microcavity with the participation of microcavity photons and an external broadband quantized electromagnetic field at the Raman resonance of photons with an optically forbidden (two-photon) atomic transition has been studied. The average spontaneous decay intensity has been calculated for simple cases. It is shown that the dynamics of spontaneous emission from this atomic ensemble differs generally from the conventional superradiance (spontaneous emission of an atomic ensemble at a one-photon optically allowed transition from excited to the ground state. When the atomic ensemble is strongly excited, the delay times and the emission pulse shape differ significantly. The parameter ranges where the spontaneous emission from the atomic ensemble under consideration at a two-photon Raman transition can be described as conventional superradiance with renormalized parameters are found. In the case of single excitation the photon emission probability depends on the number of photons and atoms in the microcavity.     

Multiphoton Spectroscopy

Abstract

Absorption and cmission spectroscopies, which are widely utilized by chemists, can be classified as one–photon spectroscopy. A molecule is excited from its ground state of energy E₀ to thc excited state of E_e by the absorption of one photon of frequency given by the Bohr's condition (hω=E_e-E₀). The reverse transition is accompanied by the emission of one photon of the same frcquency. Therefore, the molecular state energies can be directly obtained by the photon encrgies absorbed or emitted. Infrared, microwave, electronic absorption, and emission spectroscopies belong to this category of one–photon spectroscopy.     

有机化合物电子光谱中的助色基及其作用机理探讨(Ⅰ)

本文就电子光谱中助色基的作用进行了讨论 ,认为不能一概地认为助色基能使与之相连的生色基最大吸收波长统统发生红移 ,而应具体问题具体分析 :对于n→π 来说 ,一方面助色基的p轨道和生色基的最低π 反键空轨道相互作用 ,使π 反键轨道能量上升 ,另一方面助色基的p轨道和生色基的n轨道正交 ,n轨道能量基本保持不变 ,因此n→π 跃迁能增加 ,吸收波长发生蓝移。对于π→π 跃迁来说 ,助色基的p轨道和生色基的最高π成键轨道相互作用生成的新的最高π成键轨道 ,能量虽然有所升高 ,但升高的程度与π 反键轨道升高的程度相比 ,在一些化合物中π轨道升高程度较大 ,因而π→π 跃迁能减小 ,吸收波长发生红移 ;在另一些化合物中π轨道升高程度较小 ,因而π→π 跃迁能增加 ,吸收波长发生蓝移。     

Signals of the femtosecond photon echo in inorganic films and their recording

We report the results of the first experiment on recording of photon echo and self-diffraction signals in a zinc oxide semiconductor film of nanoscale thickness, which are excited by two-photon absorption of femtosecond laser pulses with wavelengths of 800 to 840nm for the exciton transition at room temperature.     

First room temperature lasing from the fundamental state of V-grooved quantum wire lasers

Lasing from the ground state electron and heavy-hole-like transition of quantum wire (QWR) is demonstrated for the first time at room temperature, with an oxide-isolated V-grooved GaAs/AlGaAs triple QWR laser grown by flow-rate modulation epitaxy (FME). The lasing peaks at all temperatures (4–300 K) are in reasonably good agreement with both the photon energies of the peaks of the photoluminescence curves and the numerical calculation of the electronic sub-band energy states of the corresponding QWR structure. These results are considered to be responsible for the reduced heterointerface inhomogeneities (the Stokes shift 0.3 meV) of the FME grown QWR, giving a low-loss wave guide in the QWR laser.     

有机化合物电子光谱中的助色基及其作用机理探讨(Ⅱ)

助色基的未成键电子对与生色基的最低π~*反键轨道和最高π成键轨道线性组合成三个新的分子轨道。其中能量最高的π~*是反键轨道,它的能量一定高于原生色基的π~*轨道;最高占据π轨道的能级与原生色基的π轨道相比,孰高孰低将随生色基和助色基的不同而异;原生色基的n轨道与助色基的未共用电子对所占轨道相互正交,故两者间的作用可忽略不计,n轨道能级基本保持不变。因此助色基使生色基的n→π~*跃迁吸收波长发生蓝移,而π→π~*跃迁吸收波长是红移还是蓝移,则随生色基及助色基的不同而异。     

Angle- and energy-dependent core-level photoelectron energy loss studies in Al and In

Electron energy loss structures of Al and In core-level photoemission spectra, in particular surface and bulk plasmon losses, have been investigated as functions of photon energy (i.e., photoelectron kinetic energy). These studies utilized synchrotron radiation to provide a variable photon source in the ultra-soft X-ray region, thus allowing these loss processes to be studied at photoelectron kinetic energies for which the mean free path of the electrons is minimal. The Al plasmon loss structure was also studied with soft X-ray radiation in an angle-resolved mode, allowing the variation of effective photoelectron sampling depth with different electron take-off (collection) angles. These results for the relative intensity of the bulk and surface plasmons as a function of electron kinetic energy and electron exit angle are in qualitative agreement with the predictions of ?unji? and ?ok?evi?. The core-level binding energies of surface atoms have also been studied with the result that no significant shift has been observed with respect to bulk-atom core levels.     

Electron spin pumping of Rb atoms on He nanodroplets via nondestructive optical excitation

We measured laser-induced-fluorescence (LIF) and beam-depletion (BD) spectra of rubidium atoms (5S-5P transition) on the surface of superfluid helium nanodroplets (M-He_{N} with M=Rb). It is known that when M is a lighter alkali atom electronic excitation always leads to detachment of the excited atom (M;{*}). The dissociation energy, few tens cm;{-1}, comes either as photon excess energy or from the barrierless formation of a M;{*}-He exciplex. We observe that this picture does not hold when M=Rb and the photon excess energy is small: we are able to excite atoms without detaching them from the droplet, thanks to a barrier preventing formation of the exciplex. This system is ideally suited for optical spin pumping in a He nanodroplet, whose achievement we explicitly demonstrate in a pump-probe magnetic circular dichroism experiment.