2s2p 1P Research Articles

Experimental study on the interference of autoionizing states of He

The interference of the low-lying doubly excited autoionizing states of helium (2s2(1S),2s2p(3P),2p2(1D) and 2s2p(1P)) is studied. In this paper we concentrate on the interference of the direct and indirect ionization (He+1s-1) in the neighborhood of the critical energy (93.15eV) of the possible state-to-state interference of the 2s2(1S) and 2p2(1D) resonances. The spectra of autoionizing and scattered electrons measured in three angular ranges at around 87–97eV primary energy are shown with the computer fitted parameters of the peaks.

Collision induced two-pulsed photon echo at the transition 0–1 in a weak longitudinal magnetic field

It was shown experimentally in ytterbium vapour at the transition (6s2) 1S0 (6s6p) 3P1 (type ), that the weak longitudinal magnetic field destroys the collision induced two-pulsed photon echo generated in a gas mixture with heavy atomic buffer; in agreement with theoretical prediction.

Relativistically prolonged lifetime of the 2s2p 3P0 level of zero nuclear-spin beryllium-like ions

The E1M1 transition rate of the line in beryllium-like ions has been calculated within the framework of relativistic second-order perturbation theory. Both multiconfiguration and quantum-electrodynamical computations have been carried out independently to better understand and test for all major electron–electron correlation contributions in the representation of the initial, intermediate and final states. By comparing the results from these methods, which agree well for all ions along the beryllium isoelectronic sequence, the lifetime of the metastable 2s2p 3P0 level is found to be longer by about 2–3 orders of magnitude for all medium and heavy elements than was estimated previously. This makes the 3P0 level of beryllium-like ions to one of the longest living (low-lying) electronic excitations of a tightly bound system with potential applications for atomic clocks and in astro physics and plasma physics.

State-Selective Differential Cross Section Measurements for the One-Electron Capture Process in C3+–He and C3+–Ne Systems at Elab = 33 eV

Using a crossed-beam apparatus, we measured the relative state-selective differential cross sections (DCSs) for the following reactions: C3+ (1s2 2s 2S) + He (1s2 1S) → C2+ (1s2 2s2p 1P) + He+ (1s 2S) + 10.6 eV and C3+ (1s2 2s 2S) + Ne (2p6 1S) → C2+ (1s2 2p2 1D) + Ne+ (2p5 2P) + 8.2 eV. The scattering angle studied in the laboratory frame, θlab, was from −3.0 to 24°, and the laboratory collision energy Elab was 33 eV. In both systems, the DCSs for the reaction are zero at the center-of-mass angle θcm = 0, and show a peak at a certain angle and a broad hump at larger angles. A classical trajectory analysis within the two-state approximation based on the ab initio potentials for (CHe)3+ revealed that these structures observed are ascribed to the reactions that occur in different trajectories. The peak corresponds to the reactions occurring in the outgoing part of the trajectory, and the hump is associated with the reactions occurring mainly in the incoming part of the trajectory.

Spectrum and energy levels of quadruply-ionized molybdenum (Mo V)

The spectrum of quadruply-ionized molybdenum Mo V was observed from 200 to 4700 Å with sliding spark discharges on 10.7 m normal- and grazing-incidence spectrographs. The existing analyses of this spectrum (Tauheed et al 1985 Phys. Scr. 31 369; Cabeza et al 1986 Phys. Scr. 34 223) were extended to include the 5s2, 5p2, 5s5d, 5s6s, 4d5f, and 4d5g configurations as well as the missing 3H6 level of 4d4f and about 75 levels of the core-excited configuration 4p54d3. The values of the 4d5d 1S0, 5s5p 1P1, and 4d6p 3P0 levels were revised. There are now about 900 lines classified as transitions between 66 even parity and 191 odd parity energy levels. Of these, about 600 lines and 130 levels are new. From the optimized energy level values, Ritz-type wavelengths were determined for about 380 lines, with uncertainties varying from 0.0003 to 0.002 Å. The observed configurations were theoretically interpreted by means of Hartree–Fock calculations and least-squares fits of the energy parameters to the observed levels. The fitted parameters were used to calculate oscillator strengths for all classified lines. A few unclassified lines and undesignated levels are also given. An improved value for the ionization energy was obtained by combining the observed energy of the 4d5g configuration with an ab initio calculation of its term value. The adopted value is 438 900 ± 150 cm−1 (54.417 ± 0.019 eV).

Intensity ratio of EUV emission lines in Fe XV studied with electron beam ion traps

We present emission spectra of highly charged Fe ions in the extreme ultraviolet range obtained with two electron beam ion traps. The intensity ratio between the 3s3p3P2 - 3s3d 3D3 (233.9 Å) and 3s3p 1P1 - 3s3d 1D2 (243.8 Å) transitions in Fe XV is given for various electron beam parameters. The experimental ratios are compared with theoretical model calculation as well as our previous experiment, where significant discrepancy with theoretical model was found. The present result, which has been obtained with a higher resolution spectrometer to investigate possible line blending, confirms that the discrepancy is not due to line blending.

Precision frequency measurement of 1S0–3P1 intercombination lines of Sr isotopes**Project supported by the National Natural Science Foundation of China (Grant No. 61127901) and the Key Project of the Chinese Academy of Sciences (Grant No. KJZD-EW-W02).

We report on frequency measurement of the intercombination (5s2)1S0–(5s5p)3P1 transition of the four natural isotopes of strontium, including 88Sr (82.58%), 87Sr (7.0%), 86Sr (9.86%), and 84Sr (0.56%). A narrow-linewidth laser that is locked to an ultra-low expansion (ULE) optical cavity with a finesse of 12000 is evaluated at a linewidth of 200 Hz with a fractional frequency drift of 2.8×10−13 at an integration time of 1 s. The fluorescence collector and detector are specially designed, based on a thermal atomic beam. Using a double-pass acousto-optic modulator (AOM) combined with a fiber and laser power stabilization configuration to detune the laser frequency enables high signal-to-noise ratios and precision saturated spectra to be obtained for the six transition lines, which allows us to determine the transition frequency precisely. The optical frequency is measured using an optical frequency synthesizer referenced to an H maser. Both the statistical values and the final values, including the corrections and uncertainties, are derived for a comparison with the values given in other works.

Inelastic cross sections for low-energy Mg + H collisions

Quantum calculations of cross sections for the inelastic processes in Mg + H collisions are improved. It is shown that the largest cross section among the endothermic processes with the value of approximately 80 Å2 corresponds to the process of the ion-pair formation: Mg(3s4s1 S)+H → Mg+ + H−. The mechanism of the process is based on nonadiabatic transitions between the MgH(2Σ+) molecular states, which provide the main mechanism for inelastic processes in Mg + H collisions. On the other hand, nonadiabatic transitions between MgH(2II) states affect some cross sections rather significantly. For example, transitions between the MgH(2II) states increase the cross section for the excitation process Mg(3s3p 1P)+H → Mg(3s3d 1D)+H almost by an order of magnitude as compared to the cross section obtained within the MgH(2Σ+) symmetry.

Collision-induced photon echo in ytterbium vapour at the transition 0–1: magnetic field effect

Collision-induced photon echo generated in ytterbium vapour at the transition (6s2) 1S0 ↔(6s6p) 3P1 (transition of the type 0 ↔ 1) was investigated in the presence of the longitudinal magnetic field strength between zero and 5.6 G. For the weak magnetic fields, not over 0.27 G, the collision-induced echo signal decreases with magnetic field increase. Further increase of the magnetic field strength reveals oscillatory behavior of the echo signal. Experimental results are in qualitative agreement with theoretical predictions.

Absolute measurements of electron impact excitation cross-sections of atoms using cavity ringdown spectroscopy

A new method of using cavity ringdown spectroscopy (CRDS) technique to measure apparent electron impact excitation cross-sections (EIECS) for different energy levels of an atom is described. The method involves measurements of absolute population densities of the energy levels in concern using the high sensitivity CRDS, leading to an analytical expression of EIECS as a function of the population densities. Determination of EIECS in this method does not require an external calibration. The proposed method is validated by determining EIECS of the metastable levels of Hg (6s6p 3P0) and Ne (2p53s 3P2), and the results are in good agreement with the cross-sections reported in the literature. The population densities in both cases were extracted from the published data. This method exhibits promising potential for absolute measurements of EIECS of metastable energy levels.

A strontium lattice clock with inaccuracy and its frequency

We have measured the absolute frequency of the optical lattice clock based on 87Sr at PTB with an uncertainty of using two caesium fountain clocks. This is close to the accuracy of todayʼs best realizations of the SI second. The absolute frequency of the 5 s2 1S0 – 5s5p 3P0 transition in 87Sr is .13(17) Hz. Our result is in excellent agreement with recent measurements performed in different laboratories worldwide. We improved the total systematic uncertainty of our Sr frequency standard by a factor of five and reach , opening new prospects for frequency ratio measurements between optical clocks for fundamental research, geodesy or optical clock evaluation.

Magnetic field induced transition rates in Ne- and Be-like ions for plasma diagnostics and E1M1 two-photon decay rate determination

We report on theoretical results of magnetic field induced transitions (MITs) in Ne- and Be-like ions without nuclear spin for two applications. Firstly, MITs are promising candidates in the determination of magnetic fields in plasmas. In our work on Ne-like ions we present accurate theoretical MIT rates for 2p6 1So – 2p53s 3Po,2 [1]. Furthermore, for Be-like ions, it has been proposed to extract the rate of the E1M1 two-photon transition 2s2 1S0 – 2s2p 3P0 by measuring the lifetime of the 3P0 state using a storage ring, which involves an external magnetic field. The MIT rates are carefully evaluated and shown to be of the same order as the E1M1 rates [2].

Storage-ring measurement of the hyperfine induced 2s2p 3P0 → 2s2 1S0 transition rate in berylliumlike sulfur

The hyperfine induced 2s2p 3P0 → 2s2 1S0 transition rate in Be-like 33S12+ has been measured employing electron-ion recombination spectroscopy at a heavy-ion storage ring. The measured value of 0.094(4) s−1 is in excellent agreement with the results of the most recent theoretical calculations.

Photoionization cross sections of the 3s3p 3P state of Mg: the discrepancy between theory and experiment

The photoionization cross sections of the excited levels (3s3p 3P0,1,2) of atomic Mg are calculated theoretically using both the nonrelativistic and fully relativistic R-matrix methods. The big discrepancies (a factor of about 3), which exists between theoretical and former experimental determinations, urges us to make additional measurements of the photoionization cross sections for the excited states of Mg.

Polarization of the stimulated photon echo in ytterbium vapour at the transition 0 ↔ 1

Experimental results were obtained at the inter-combination transition (6s2) 1S0 ↔ (6s6p) 3P1 (type 0 ↔ 1) of 174Y b vapour for several combinations of linear and circular polarizations of three exciting pulses. A peculiar case appears when the first pulse has linear polarization crossed with the polarizations of the second and third exciting pulses: the stimulated photon echo (SPE) does not appear in pure ytterbium; dilution of ytterbium by Xe implies a collision induced SPE with non-monotonic amplitude dependence on buffer pressure, and with polarization along the first exciting pulse. For circular polarizations, when the first exciting pulse has polarization opposite to the circular polarizations of the second and third pulses, the SPE does not appear either in pure ytterbium vapour or in its mixture with a heavy atomic buffer. SPE polarization dependence on the third exciting pulse area was demonstrated. This result may be of interest for optical data processing. All the results agree with theoretical predictions.

X-ray observations of Ca19 +, Ca18 + and satellites from Alcator C-Mod tokamak plasmas

X-ray spectra of n = 2 to 1 transitions in hydrogen-like Ca19 +, helium-like Ca18 + and nearby satellites have been obtained from Alcator C-Mod tokamak plasmas using a spatially imaging high resolution x-ray spectrometer system. For Ca19 +, the intensity ratio of Lyα2 (1s 1S1/2–2p 2P1/2) to Lyα1 (1s 1S1/2–2p 2P3/2) was found to be ∼0.531 ± 0.005 over a range of plasma parameters, which is somewhat greater than the ratio of the statistical weights of the upper n = 2 levels, 1/2. This difference is mainly due to interaction with the 2S1/2 fine structure sub-level. Experimental results are compared to calculations from COLRAD, a collisional-radiative modelling code, and good agreement is shown. For Ca18 +, the intensity ratio of the dielectronic satellite k (1s22p 2P1/2–1s2p2 2D3/2) to the resonance line w (1s2 1S0–1s2p 1P1) is sensitive only to the electron temperature. The observed brightness ratio scaling with Te is in good agreement with the calculated ratio of the respective dielectronic recombination to the collisional excitation rates.

Quantum chemistry and its “ages”

The popular methods of computational quantum chemistry (CQC) have acquired the status of “mainstream” quantum chemistry (QC), with countless useful applications to molecular science, especially for properties, potential energy surfaces, and reactions of the ground states. In earlier publications, four “ages of QC” have been defined, the classification having been based exclusively on the progress of QC in the direction of CQC.The core of the present article has a dual character: On the one hand, it is a commentary on the nature of QC, whereby it is argued that modern QC, while keeping its focus on the many‐electron problem, (MEP), and on the consequences of Electron Correlations on observable quantities, has a domain and a scope that are larger than those determined, more or less, by CQC. Hence, additional “ages” are singled out and proposed, which are connected to “time‐independent” and “time‐dependent” theoretical “many‐electron” formulations and calculations in which the “continuous spectrum” is involved explicitly. They are connected to experimental directions, which challenge theory to provide not only phenomenology but also quantitative answers and predictions for properties and phenomena involving various types of “unstable (nonstationary) states,” which decay irreversibly into the continuous spectrum either via interactions within the atomic (molecular) Hamiltonian (e.g., autoionizing states) or via the interaction with external electromagnetic fields.On the other hand, the article presents, in the form of brief reviews and retrospective accounts, a gleaning from our contributions to the theory and to methods of calculation for the quantitative treatment of such MEPs. Specifically, in the context of the previous paragraphs, I comment briefly on basic concepts, I point to early theoretical work, and, in support of the arguments, I refer to practical theoretical constructions and sample results regarding the following two themes: “resonances in many‐electron systems” and “theoretical time‐resolved many‐electron physics.”Both the themes are connected to the recent developments on the experimental front of the interaction of atoms and molecules with ultrashort radiation pulses (weak or strong) in the femtosecond and attosecond regimes. The first theme is treatable within energy‐dependent or time‐dependent frameworks. The second one requires the computation of a physically transparent Ψ(t), which solves to a very good approximation the many‐electron time‐dependent Schrödinger equation (METDSE) for given characteristics of the pulses.Our approaches to related problems have been developed and implemented within state‐ and property‐specific formulations. These are constructed in terms of energy‐dependent Hermitian and non‐Hermitian formalisms, as well as in terms of time‐dependent ones, whose essential elements are explained here. In either case, in line with the main tenet of QC, the focus is on the understanding and efficient solution of the corresponding MEPs, especially in terms of nonperturbative methods. For time‐dependent problems involving the interaction with strong fields, the solution of the relevant METDSE is carried out by applying the “state‐specific expansion approach.” This type of calculation entails the solution of many thousands of coupled equations, where the input consists of bound–bound, bound–free, and free–free coupling matrix elements, on‐ and off‐resonance with respect to the frequency of the pulse.As an example of a solution of a time‐resolved many‐electron process, I present snapshots of the time‐dependent formation of the asymmetric profile of the He 2s2p 1P0 resonance state excited by a femtosecond pulse. This profile is formed within about 180 fs and is the same as the one which is well‐known from theory and experiment on the energy axis.The domain and horizon of modern Quantum Chemistry are broader than those of its original component, namely, Computational Quantum Chemistry of the ground state. Hamiltonians can create energy‐ or time‐dependent unstable states whose effects are measured inside the continuous spectrum. The understanding and practical solution of the corresponding many‐electron problems can be carried out reliably, using state‐specific, nonperturbative methodologies that solve the Schrödinger equation efficiently as a function of real or complex energies, or of time. © 2014 Wiley Periodicals, Inc.

Spectroscopy and frequency measurement of the 87Sr clock transition by laser linewidth transfer using an optical frequency comb

We performed spectroscopic observations of the 698-nm clock transition in 87Sr confined in an optical lattice using a laser linewidth transfer technique. A narrow-linewidth laser interrogating the clock transition was prepared by transferring the linewidth of a master laser (1064 nm) to that of a slave laser (698 nm) with a high-speed controllable fiber-based frequency comb. The Fourier-limited spectrum was then observed for an 80-ms interrogating pulse. We determined that the absolute frequency of the 5s2 1S0–5s5p 3P0 clock transition in 87Sr is 429 228 004 229 872.0 (1.6) Hz referenced to the SI second.

Autoionization resonances in the partial photoionization cross sections of Al V

The photoionization cross sections for 2p or 2s sub-shell ionization of Al V are calculated using the R-matrix method in LS-coupling approximation. In the present work, the distorted-wave method is employed to calculate the orbital wave-functions, and the R-matrix method is employed to calculate the photoionization cross sections from ground state 2s22p5 2P and excited state 2s2p6 2S. The 12 series of autoionization resonances between the 2s22p4 3P and 1s22p6 1S threshold are identified.

Plasma effect on the Kα group emission of He-like molybdenum

The intercombination line (1s2p(3P1)−1s2) and the resonance line (1s2p(1P1)−1s2) of He-like Mo, which are the two electronic dipole transitions in the Kα group emissions, are studied under dense plasma conditions by using the ion–sphere model combined with the multi-configuration Dirac–Fock model. The results show that the two electronic dipole transitions red shift nearly linearly with the increase in plasma electron density. Moreover, the red-shift energy becomes more sensitive to the plasma electron density, when the plasma electron temperature is lower. It is worth to note that the energy discrepancy between these two spectral lines is equal to the exchange energy of state 1s2p (3P1). This result may be useful to dense plasma diagnosis in connection with variation of the relative position and the spectral fine structure splitting in high-density plasmas.