Many-body theory for simultaneous photoionization and photoexcitation of helium

Brueckner-Goldstone many-body perturbation theory is applied to study the simultaneous photoionization and photoexcitation (SSP) of helium. The results are used to examine the validity and limitations of other approaches to the problem. The oscillator strengths ($\text{d}\text{?}\text{?}$) for the transition 1s²→(2s ?p)¹P and 1s²→(2p ?s)¹P are compared with the results of Brown. The cross section for SPP at the photoexcitation threshold (65.399 eV) is found to be in agreement with the experimental result of Samson.     

On photoexcitation of baryon antidecuplet

We show that the photoexcitation of the nonexotic members baryon antidecuplet, suggested by the soliton classification of low-lying baryons, is strongly suppressed on the proton target. The process occurs mostly on the neutron target. This qualitative prediction can be useful in identifying the nonexotic members of the antidecuplet in the known baryon spectrum. We also analyze the interrelation between photocouplings of various baryon multiplets in the soliton picture and in the nonrelativistic quark model.Received: 9 April 2003, Published online: 18 November 2003PACS: 13.30.-a Decays of baryons - 13.60.-r Photon and charged-lepton interactions with hadrons - 14.20.-c Baryons (including antiparticles)     

Ultrafast intrachain photoexcitation of polymeric semiconductors

We report on excited state dynamics in isolated poly(9,9-dioctylfluorene) chains obtained by embedding the polymer in an inert plastic matrix. Early events (<300 fs) of intrachain photophysics are detected by pump-probe spectroscopy using tunable UV 25-fs pump pulses and sub-10-fs visible probe pulses. We show that higher-lying optical states, reached by multiphoton transitions, give rise to on-chain charge separation on the ultrafast time scale. The intrachain charge pair decays geminately within 500 fs to the lowest singlet state. Characteristic time scales for internal conversion and intramolecular vibrational redistribution are also determined.     

Polarization dependence of photoexcitation in photofield emission

The mechanisms of one-photon photoexcitation in photofield emission have been studied experimentally by measuring the photocurrent from selected crystallographic planes of a tungsten field emitter as a function of the polarization of the incident light. Two distinct components of the photocurrent were identified. The corresponding excitation mechanisms were deduced from the results of the polarization measurements supplemented by band structure considerations. Surface effect emission dominates at large angles of light incidence, and is observed even from within the geometric shadow of the tip. It depends on the light polarization inside the metal in a characteristic way. Indirect bulk photoemission is prominent at small angles of light incidence and is independent of the polarization inside the metal. It is concluded that bulk and surface excitations in photofield emission can be distinguished by appropriately selecting the direction of polarization and the angle of incidence of the incident light.     

Linear and nonlinear photoexcitation dynamics in pi-conjugated polymers

Linear and nonlinear recombination kinetics with various lifetime distributions were identified for long-lived photoexcitations in a series of pi-conjugated polymer films using modulation frequency and excitation intensity dependencies of the photoinduced absorption. This includes monomolecular, bimolecular, and defect-limited recombination processes that lead to saturation. Using generalized kinetics parameters, we found characteristic plots for all recombination processes. Specifically, the bimolecular recombination process shows superlinear intensity dependence away from the steady state; on the contrary, dispersive bimolecular recombination leads to sublinear dependence.     

Atomic magneto-dipole photoionization

The special features of magneto-dipole photoionization of atomic s states are analyzed and the possibilities of experimental observation of this effect are discussed. It has been shown that despite the smallness of total cross sections for magnetic processes as compared with electric-dipole ones, the experimental observation of magnetic effects is possible, in principle, if photoelectrons are detected in the directions perpendicular to both the polarization vector of photon and its momentum; i.e. in the directions where the differential cross section for electric-dipole ionization of atomic s states is close to zero. The capabilities of the derived general formulas for the magneto-dipole cross sections are illustrated by numerical calculations for s subshells of He and Be atoms.     

碱金属原子的光激发与光电离

本文研究了碱金属原子在三步激光脉冲作用下的光激发和光电离过程的动力学特性, 重点关注和比较了锂和铯原子的异同. 针对多种激发模式, 本文不但建立了其原子布居数在各个跃迁态的速率方程组, 还给出了各相关态的光激发和光电离过程的解析解. 通过精心设计并选择了特殊情况, 显著简化了解析解的数学表达式, 从而凸显和讨论了其物理内涵. 通过自行编程, 系统地计算和观察了各种激发模式对锂原子的光激发和光电离过程的可能影响, 研究和讨论了电离率随激光参数的变化规律. 在相同激发模式下, 比较和分析了采用两种不同激发路径所导致的各态原子布居率的变化, 凸显了改变原子参数所产生的作用. 探讨了锂和铯原子在类似的激发条件下在电离率方面的差别. 最后, 基于本文的研究结果, 本文指出了优化电离率的多种途径.     

Temperature-modulated photoluminescence induced by photoexcitation in semiconductors

A new technique of temperature-modulated photoluminescence is reported. The periodic heating is induced by metastable optical radiation. An analysis is made of phase shifts. The technique is applied to GaP : N. The results are compared with those obtained with an external heater.     

Two-center resonant photoionization

Photoionization of an atom A, in the presence of a neighboring atom B, can proceed both directly and via resonant excitation of B with subsequent energy transfer to A through two-center electron-electron correlation. We show that in such a case the photoionization process can be very strongly enhanced and acquire interesting characteristic features, both in its time development and the electron spectrum.     

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.     

Absorption spectra induced by photoexcitation and electrochemical doping in polythiophene

Changes of absorption spectra in a polythiophene film caused by photo- excitation and electrochemical doping are studied. The photoexcitation and the slight doping reveal that the formation of a shallow polaron is responsible to the carrier transport. The doping also induces a deep polaron which can explain the change of spectra upon doping.     

Strong-field approximation in photoionization

Abstract

The definition and characteristics of the strong-field environment for an atom in a laser field are specified in terms of the relevant intensity parameters. The limits of perturbation theory are set, and it is emphasized that this must be done in terms of laser field energy, not electric field strength. The formal basis and special features of the SFA (strong-field approximation) are reviewed, and it is pointed out that the three methods encompassed in the so-called KFR (Keldysh-Faisal-Reiss) technique are actually quite different. Validity conditions and some applications of the SFA are given.     

Decoherence in attosecond photoionization

The creation of superpositions of hole states via single-photon ionization using attosecond extreme-ultraviolet pulses is studied with the time-dependent configuration-interaction singles (TDCIS) method. Specifically, the degree of coherence between hole states in atomic xenon is investigated. We find that interchannel coupling not only affects the hole populations, but it also enhances the entanglement between the photoelectron and the remaining ion, thereby reducing the coherence within the ion. As a consequence, even if the spectral bandwidth of the ionizing pulse exceeds the energy splittings among the hole states involved, perfectly coherent hole wave packets cannot be formed. For sufficiently large spectral bandwidth, the coherence can only be increased by increasing the mean photon energy.     

Xenon: Oscillator strengths and photoionization

Electron energy loss spectra of Xe have been analyzed by means of Fano and Lu's collision theory of spectra. A new and improved set of empirical parameters has been determined,eigenquantum defects, orthogonal transformation matrix, anddipole matrix elements. These data have been used to calculate the autoionized profile and the asymmetry parameterβ of the angular distribution of photoelectrons. The autoionization profile is in agreement with the measurements of Huffman, Tanaka, and Larrabee. In addition, the theory predicts a distinct interference effect betweens andd excitation. The energetic shifting between the measured and calculated asymmetry parameterβ observed previously is removed. Nevertheless, deviations concerning the magnitude ofβ nears state excitation are still left unexplained. They are probably due to insufficient resolution of the photoelectron measurement.     

Electron-ion recombination and photoionization of Fe XXI

Results for electron-ion recombination and photoionization of , with emphasis in high-temperature region, are presented from ab initio unified method. The unified method, based on close coupling (CC) approximation and R-matrix method, (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR), (ii) enables self-consistent sets of photoionization and recombination cross sections from using an identical wavefunction for both the processes, and (iii) provides state-specific recombination rates of a large number of bound states. A large CC wavefunction expansion, which includes the ground and 28 core excitations of n=2 and 3 complexes and span a wide energy range, has been used. Compared to Δn=2-2, Δn=2-3 core excitations are found to introduce strong resonant structures and enhance the background photoionization cross sections (σ_PI) in the high-energy region. These features along with prominent photoexcitation-of-core (PEC) resonances at n=3 core thresholds have increased the unified total recombination rate coefficients (α_R(T)) at temperatures , region of maximum abundance of the ion in collisional equilibrium, by a factor of 1.6 over previous calculations. State-specific recombination rate coefficients α_R(nLS), which include both the RR and DR, are presented for the first time for 685 bound states with n?10 and l?9. The unified total recombination rate with photoelectron energy α_R(E) is presented and the role of low-energy near-threshold fine structure resonances is illustrated. The present results should provide a reasonably complete self-consistent set of recombination rates and photoionization cross sections for astrophysical modelings of high-temperature plasmas from optical to far-ultraviolet wavelength regions.