Attosecond two-photon interferometry for doubly excited states of helium

We show that the correlation dynamics in coherently excited doubly excited resonances of helium can be followed in real time by two-photon interferometry. This approach promises to map the evolution of the two-electron wave packet onto experimentally easily accessible noncoincident single-electron spectra. We analyze the interferometric signal in terms of a semianalytical model which is validated by a numerical solution of the time-dependent two-electron Schr?dinger equation in its full dimensionality.     

Geometric quantities of lower doubly excited bound states of helium

Expectation values of single electron and interelectronic geometric quantities such as $\langle r\rangle$, $\langle r_{12}\rangle$, $\langle r_<\rangle$, $\langle r_>\rangle$, $\langle \cos\theta_{12}\rangle$ and $\langle \theta_{12}\rangle$ are calculated for doubly excited $2{\rm p}n{\rm p}\,{}^1P^{\,\rm e}\,(3\leq n\leq5),\, 2{\rm p}n{\rm p}\,{}^3\!P^{\,\rm e}\,(2\leq n\leq5)$ and $2{\rm p}n{\rm d}\,{}^{1,3}D^{\,\rm o}\,(3\leq n\leq5)$ states of helium using Hylleraas-$B$-spline basis set. The energy levels converge to at least 10 significant digits in our calculations. The extrapolated values of geometric quantities except for $\langle \theta_{12}\rangle$ reach 10 significant digits as well; $\langle \theta_{12}\rangle$ reaches at least 7 significant digits using a multipole expansion approach. Our results provide a precise reference for future research.     

Observation of triplet doubly excited states in single photon excitation from ground state helium

We have observed, for the first time, LS-forbidden triplet doubly excited states, in single photon excitation of ground state helium, below the second ionization threshold. These states are identified as (3)D(o) and (3)P(o) and their excitation is due to spin-orbit interaction that mixes them with the optically allowed (1)P(o) states. This observation is possible due to the very high efficiency in detecting metastable atoms created after the fluorescence decay of the doubly excited states, and the new capabilities of third generation synchrotron vacuum ultraviolet sources with high resolution beam lines.     

氦原子单激发和双激发态里德伯系列的相对论能量计算

采用Rayleigh-Ritz变分方法和组态相互作用方法，并进一步考虑相对论修正和质量极化效应，研究了氦原子单激发和双激发态里德伯系列的相对论能量，计算结果与其他理论和实验符合得很好.此外，还计算了该系统之间的振子强度、辐射跃迁率及跃迁波长，振子强度三个规范的计算结果显示出很好的一致性. 关键词： 氦原子 变分计算 振子强度 辐射跃迁率     

Radiative decay of doubly excited states in helium below the He+ (N = 2) ionization threshold

The radiative decay of (1)P(o) doubly excited states in helium has been investigated using a novel apparatus in which metastable atoms and vacuum ultraviolet photons are detected. The intensity ratio of the energetically narrow (sp,2n-)(1)P(o) and (2p,nd)(1)P(o) series to the broader (sp,2n+)(1)P(o) series is strikingly enhanced in comparison with conventional photoabsorption, photoion, or photoelectron measurements using synchrotron radiation. The experimental approach is a new way forward for the study of energetically narrow doubly excited states.     

Detection of the 1pe series of doubly excited helium states below N=2 via the stark effect

The Stark effect on the doubly excited states of helium below the N=2 threshold has been studied by vacuum ultraviolet fluorescence yield spectroscopy. Two new series of states are observed at moderate fields (<10 kV/cm), and assigned to the previously unobserved even 1pe series, and a group of 1De series. The 1Se states are observed indirectly via their mixing with nearby 1 po states. The observations at moderate field contradict theoretical predictions that field strengths about an order of magnitude greater are necessary to observe the Stark effect on He doubly excited states at low quantum numbers.     

类氦铀离子的双电子激发态辐射衰变特性

讨论了类氢铀离子与H2分子碰撞中,经KLL共振转移和激发过程形成类氦铀离子双电子激发态的机制,分析了实验测量到的X射线能谱和各种竞争过程对X射线能谱的贡献.对由于纯电子-电子相互作用形成的双激发态的Kα辐射衰变的超级卫星线的角分布测量结果表明,角分布呈各向异性,电子态布居存在很强的定向排列.     

Formation of excited states in high-Z helium like systems

High-Z helium like ions represent the simplest multi electron systems for studying the interplay between electron–electron correlations, relativistic as well as quantum electrodynamical effects in strong fields. In contrast to the adjacent lithium like ions, however, almost no experimental information is available about the excited states in the high-Z domain of the helium sequence. Here, we present a theoretical analysis of the X ray production and decay dynamics of the excited states in helium like uranium. Emphasis has been paid particularly to the formation of the ³P₀ and ³P₂ levels by using electron capture into hydrogen like U⁹¹⁺. Both states are of interest for precise measurements on high-Z helium like ions in the future. This revised version was published online in August 2006 with corrections to the Cover Date.     

Doubly excited states in the beam-foil Li spectrum

Beam-foil excited spectra of Li atoms and ions have been investigated in the wavelength range 2000–6000 Å. Line widths and lifetimes are given for the lines at 3155 Å, 3144 Å, and 5539 Å, the two latter representing transitions between doubly excited states.     

The effects of doubly excited states on ionization balance

The effects of highly doubly excited states on ionization balance are investigated. In the calculation, A Collisional-Radiative model in Detailed-Conflguration-Accounting （DCA） is applied to population calculations for NLTE pl＆smas. Configuration-averaged rate coefficients that needed in the rate equations are obtained based onthe first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate theelectron wave functions. The mean ionization stage of high-Z plasma Lu is presented. The comparison shows thatthe mean ionization stage increases more than 3 stages when doubly excited states 5l6l＇ and 5l5l＇ are not included in the population calculations.     

The effects of doubly excited states on ionization balance

The effects of highly doubly excited states on ionization balance are investigated. In the calculation, A Collisional-Radiative model in Detailed-Configuration-Accounting (DCA) is applied to population calculations for NLTE plasmas. Configuration-averaged rate coefficients that needed in the rate equations are obtained based on the first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate the electron wave functions. The mean ionization stage of high-Z plasma Lu is presented. The comparison shows that the mean ionization stage increases more than 3 stages when doubly excited states 5l6l' and 5l5l' are not included in the population calculations.     

Laser excited fluorescence spectrum of nitrogen dioxide

The fluorescence spectrum of NO₂ excited by a single mode argon-ion laser was studied at a spectral resolution of 0.1 cm^?1. In particular the Stokes and the anti-Stokes ν₂ band excited by the 5145 Å line of an argon-ion laser were examined in detail. Some of the vibrational and rotational parameters of the ground electronic state and the rotational constants of the B₂ vibronic state were obtained.     

Complex-scaling treatment for high-lying doubly excited resonances for screened Coulomb helium atom

This work presents an investigation on the doubly excited ¹S ^e autoionizing states of screened helium atom lying below the n = 4 threshold of the He⁺ ion. The potential generated in this system is represented by a Yukawa type potential. We have employed complex-coordinate rotation method, as it is a powerful scheme to study high lying resonances. Hylleraas type wave function is used to consider the correlation effect between all the charged particles. Our resonance parameters for the resonances lying below the He⁺ (n = 2) threshold agree well with those of the existing calculations by using the stabilization method. Resonances associated with higher thresholds are new calculations. All the present results are well converged with basis length N = 444.     

Microwave spectrum of 3-bromopropene in the excited torsional states

The microwave spectra in the excited states of the CC torsion for the ⁷⁹Br and ⁸¹Br isotopic species of 3-bromopropene were measured in the frequency region 15.3–23.7 GHz. The a-type R-branch and b-type Q-branch rotational transitions in the first and second excited states of one conformer, skew, have been assigned and analyzed. Analysis of the spectrum yields the rotational constants and the nuclear quadrupole coupling constants. From relative intensity measurements the energy differences associated with the CC torsion, between the ground and first excited state, the ground and second excited state have been found to lie 109 and 206 cm^?1, respectively.     

Radiative and relativistic effects in the decay of highly excited states in helium

A recent experimental study [J.-E. Rubensson et al., Phys. Rev. Lett. 83, 947 (1999)] measured a significant fluorescence yield of the He( 2lnl(')) photoexcited resonances, showing major qualitative differences from nonrelativistic predictions. We present a further theoretical study of these states, and perform R-matrix multichannel quantum defect theory calculations to extract fluorescence and ionization cross sections. These theoretical results are in excellent agreement with newer, higher-resolution measurements. Radiative and spin-orbit effects are quantified and shown to play an important role in the overall characterization of highly excited states.     

The microwave spectrum of trifluoroacetonitrile in excited vibrational states

The microwave transitions J → J + 1 have been observed in the symmetric top molecule trifluoroacetonitrile, CF₃CN, for molecules excited into several of the low-lying vibrational states. Measurements made in the degenerate states v₇ = 1 v₈ = 1 have been fitted to the formula derived by Grenier-Besson and Amat (1) for transitions J → J + 1 in degenerate vibrational states of molecules with C_3v symmetry. The measurements for the state v₈ = 1 have been extended to 150 GHz where it is shown that the above formula becomes inadequate to describe accurately the observed spectrum.