Excited Ns Research Articles

Theoretical transition probabilities, radiative lifetimes and Stark broadening parameters of singly ionized magnesium

ABSTRACTThe presence of spectral lines of singly ionized magnesium (Mg ii) in stellar atmospheres has been reported in different stars. Recently, the low-resolution spectrum obtained from Supernova 2014 J in M82, in which Mg ii absorption lines centred on 4400 Å as well as 7600 Å stand out, has been analysed. This is the motive for the atomic data calculations in this work, which are of much interest in the astrophysical area. In this article, ab initio relativistic Hartree–Fock calculations in an intermediate coupling formalism using Cowan’s code allowed us to obtain the required transition probabilities to calculate the theoretical radiative lifetimes for excited nS−, nP−, nD− and nF− states of singly ionized magnesium. An asymptotic dependence of lifetime (τnl) on the effective principal quantum number (n*) has been determined. Also, the Griem semi-empirical approach was used to obtain the theoretical Stark parameters (width and shift) of spectral lines; these data are displayed for an electron density of 1017 cm−3 and temperatures T = 10–100 (×103 K). We have compared the results of lifetimes for 16 levels and Stark parameters for seven spectral lines with previously reported experiments available in the literature. Finally, we discuss the behaviour of the Stark parameters versus temperature for three relevant spectral lines (2802.70, 2797.99 and 7868.04 Å).

Measurement of delayed fluorescence in plastic scintillator from 1 to 10 μs

The time dependence of the relative light emission of Eljen Technology EJ-200 polyvinyltoluene-based plastic scintillator was measured between 1 and 10 μ s after the passage of a particle shower, a singly charged particle (atmospheric muon), and with a UV LED exciting the fluor. This was compared in magnitude to the integrated response for the prompt light (within 500 ns of excitation). A model with a time-dependent yield consisting of three exponentially decaying components (fast, medium, and slow) was developed to fit the data. Note that the exact time structure of early (< 1 μ s ) light emission was not measured for individual components, only for all three components together This model assumes all three components share the same rise time. The decay time constants of the fast, medium and slow components are, respectively, 7.8 ns, 490 ns, and 2370 ns. The relative total normalized yields for each component are: fast 95.8%, medium 2.2%, and slow 2.0%.

State transfer of nS ultracold Rydberg atoms in external electric fields

The state transfer from cesium nS (n = 45−53) states to Stark (product) states induced by a weak electric field (WEF) pulse was investigated using the state-selective field ionization method in a standard magneto-optic trap (MOT). The WEF pulse shifts the nS Rydberg states that anticross with the (n − 4) hydrogen-like manifolds, causing state transitions from the initial excited nS state to the Stark states. The mechanism of transfer was investigated by changing the rising and falling time of the WEF pulse and the switching off time of the external field pulse had an important role during the evolution process of product states. The population of the product states is also measured as a function of the principal quantum number and Rydberg densities.

The two-electron atomic systems. S-states

Abstract A simple Mathematica program for computing the S -state energies and wave functions of two-electron (helium-like) atoms (ions) is presented. The well-known method of projecting the Schrodinger equation onto the finite subspace of basis functions was applied. The basis functions are composed of the exponentials combined with integer powers of the simplest perimetric coordinates. No special subroutines were used, only built-in objects supported by Mathematica. The accuracy of results and computation time depend on the basis size. The precise energy values of 7–8 significant figures along with the corresponding wave functions can be computed on a single processor within a few minutes. The resultant wave functions have a simple analytical form consisting of elementary functions, that enables one to calculate the expectation values of arbitrary physical operators without any difficulties. Program summary Program title: TwoElAtom-S Catalogue identifier: AEFK_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEFK_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 10 185 No. of bytes in distributed program, including test data, etc.: 495 164 Distribution format: tar.gz Programming language: Mathematica 6.0; 7.0 Computer: Any PC Operating system: Any which supports Mathematica; tested under Microsoft Windows XP and Linux SUSE 11.0 RAM: ⩾ 10 9 bytes Classification: 2.1, 2.2, 2.7, 2.9 Nature of problem: The Schrodinger equation for atoms (ions) with more than one electron has not been solved analytically. Approximate methods must be applied in order to obtain the wave functions or other physical attributes from quantum mechanical calculations. Solution method: The S-wave function is expanded into a triple basis set in three perimetric coordinates. Method of projecting the two-electron Schrodinger equation (for atoms/ions) onto a subspace of the basis functions enables one to obtain the set of homogeneous linear equations F . C = 0 for the coefficients C of the above expansion. The roots of equation det ( F ) = 0 yield the bound energies. Restrictions: First, the too large length of expansion (basis size) takes the too large computation time giving no perceptible improvement in accuracy. Second, the order of polynomial Ω (input parameter) in the wave function expansion enables one to calculate the excited nS -states up to n = Ω + 1 inclusive. Additional comments: The CPC Program Library includes “A program to calculate the eigenfunctions of the random phase approximation for two electron systems” (AAJD). It should be emphasized that this fortran code realizes a very rough approximation describing only the averaged electron density of the two electron systems. It does not characterize the properties of the individual electrons and has a number of input parameters including the Roothaan orbitals. Running time: ∼10 minutes (depends on basis size and computer speed)

Effects of electric fields and collisions on highly excited rubidium atoms

The effects of static and pulsed electric fields on the multiphoton ionization (MPI) of rubidium atoms at both low (atomic beam) and high (heat pipe) densities are studied using tunable OPO lasers. Two-photon excitation of np states is induced by the external electric field at both low and high densities. In addition, np signal is also seen at very low electric fields in the heat pipe, providing evidence for collision mixing as well as field mixing. At low Rb densities strong resonance features are observed in the energy region between the zero field limit (IP) and the field ionization limit. In addition, collisional detachment and charge transfer between excited ns and nd Rb Rydberg states and nozzle-jet cooled polar molecules (acetonitrile and acetone) are studied under crossed-beam conditions. The formation of dipole bound anions for acetone is only seen under nozzle jet expansion conditions and the maximum in the Rydberg electron transfer (RET) rate versusn depends upon the expansion gas (\(n_{\rm max}\) increases in the order H2, He, Ne, Ar, Xe). For acetone (low dipole moment and large \(n_{\rm max}\)), collisional detachment dominates the charge transfer, whereas for acetonitrile (high dipole moment and low \(n_{\rm max}\)), charge transfer is seen to dominate the creation of Rb+.

The effect of excited atomic states on the harmonic generation in a photoionized plasma

Owing to the conservation by the electrons of the memory of their intra-atomic states, a photoionized plasma arising in the mode of suppression of the ionization barrier exhibits a pronounced dependence of the efficiency of generation of the third and fifth harmonics of the pumping field on such states of electrons. For the maximal values of generation efficiency, this dependence is directly proportional to the tenth power of the principal quantum number of excited ns states of electrons.

Mechanism of the photoinduced addition of methanol to the double bond of 2,2,4,6-tetramethyl- and 1,2,2,4,6-pentamethyl-1,2-dihydroquinolines.

The mechanism of the photoinduced Markovnikov addition of methanol to the double bond of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) was studied by the flash photolysis technique over a wide range of acetic acid and KOH concentrations. The successive formation of two transients was observed in neutral solutions in the case of compound 1. These transients have lifetimes on ms time-scales and absorption spectra with maxima at 420 and 480 nm for the first and the second transient, respectively. The decay rate constants of these transients (k1 and k2) were measured over the temperature range from 10 to 45 degrees C and the activation energies E1 approximately 0 and E2 (31.4 +/- 3.1) kJ mol(-1) were determined. In MeOH acidified with acetic acid only the second transient (lambda(max) = 480 nm) was observed, and it is proposed that this is due to a carbocation. The deuterium kinetic isotope effects determined in MeOD are 2 and 1.3 for k1 and k2, respectively, indicating that the first reaction is proton transfer proceeding via the A-SE2 mechanism and the second reaction is the nucleophilic addition of the solvent to the carbocation. In alkaline media, only the first transient (lambda(max) = 420 nm) was observed with a lifetime of 250 ms at [KOH] > 2 x 10(-3) mol dm(-3). In the case of compound 2, the formation of a carbocation with lambda(max) = 490 mn and a lifetime of several ms is observed within 10 ns of excitation in neutral, acidic, and alkaline solutions. The carbocation is quenched with KOH with a rate constant of 7 x 10(6) mol(-1) dm(3) s(-1). The quantum yields of the reaction and of the fluorescence were measured and possible mechanisms discussed.

Electronic absorption, resonance Raman and excited-state resonance Raman spectroscopy of rhenium(I) and copper(I) complexes, with substituted dipyrido[3,2-a : 2′,3′-c]phenazine ligands, and their electron reduced products

The electronic absorption and resonance Raman spectra of a series of rhenium(I) and copper(I) complexes with substituted dipyrido[3,2-a : 2′,3′-c]phenazine (dppz) ligands were investigated. The ligands were ben[i]dipyrido[3,2-a : 2′,3′-c]phenazine, 11,12-dimethyldipyrido[3,2-a : 2′,3′-c]phenazine, 10-methyldipy- rido[3,2-a : 2′,3′-c]phenazine and 11-methyoxydipyrido[3,2-a : 2′,3′-c]phenazine. The spectroelectrochemistry of the reduced complexes and the emission and resonance Raman spectra of the excited states are reported. Vibrational wavenumber calculations of the unsubstituted ligand suggest the presence of normal modes that are localized to various sections of the ligand structure; the resonance Raman spectra of the complexes are interpreted with reference to these calculations. The analysis of the spectra revealed that the Franck–Condon state initially formed by visible photoexcitation (450 nm) is metal-to-ligand charge-transfer in nature. Spectroelectrochemical resonance Raman and electronic absorption measurements revealed the spectral signatures for the radical anions of each of the ligands used in this study. These spectral features were used to assign the excited states formed by the complexes. Most of the complexes studied show spectral features in their excited-states that suggest that the predominant state formed within 5 ns of excitation is ligand-centred. Copyright © 2000 John Wiley & Sons, Ltd.

Experimentally derived model for the locomotor pattern generator in the Xenopus embryo.

1. Simulations of Xenopus embryo spinal neurons were endowed with Hodgkin-Huxley-style models of voltage-dependent Na+, Ca2+, slow K+ and fast K+ currents together with a Na(+)-dependent K+ current. The parameters describing the activation, inactivation and relaxation of these currents were derived from previous voltage-clamp studies of Xenopus embryo spinal neurons. Each of the currents was present at realistic densities. 2. The model neurons fired repetitively in response to current injection. The Ca2+ current was essential for repetitive firing in response to current injection. The fast K+ current appeared mainly to control spike width, whereas the slow K+ current exerted a powerful influence on the reptitive firing properties of the neurons without markedly affecting spike width. 3. The properties of the model neurons could be made more consistent with those previously reported for Xenopus embryo neurons during intracellular recordings in vivo, if the shunting effect of the sharp microelectrode was incorporated into the model. 4. The model neurons were then used to create a simplified version of the spinal network that controls swimming in the frog embryo. This model network could generate the motor pattern for swimming: the activity between the left and right sides alternated with a cycle period that varied from 50 to 120 ms. This is very similar to the range of cycle periods observed in the real embryo. The shunting effect of the microelectrode was once again taken into account. 5. Reductions of the K+ currents perturbed the motor pattern and gave three forms of aberrant motor activity very similar to those previously seen during the application of K+ channel blockers to the real embryo. The ability to generate the correct motor pattern for swimming in the model depended on the balance between the K+ currents and the inward Na+ and Ca2+ currents rather than their absolute values. 6. The model network could generate a motor pattern for swimming over a very wide range of excitatory (2-10 nS) and inhibitory (2-400 nS) synaptic strengths. Rough estimates of the physiological synaptic strengths in the real circuit (around 20-60 nS for inhibition and 2-5 nS for excitation) fall within the range of synaptic strengths that gave simulation of the swimming motor pattern in the model. 7. The cycle period of the motor activity in the model shortened either as the excitatory synapses were strengthened or as the inhibitory synapses were weakened. 8. The prediction that the strength of the mid-cycle inhibition determines cycle period has been tested by using low levels of strychnine to reduce glycinergic reciprocal inhibition in a graded manner in the real embryo. As the inhibition was reduced, the cycle period of fictive swimming in the embryo shortened by amounts very close to those predicted by the model. 9. This new experimentally derived model can replicate many of the known features of fictive swimming in the real embryo and may be of value as an analytical tool in attempting to understand how the spinal circuitry of the Xenopus embryo and related amphibian embryos control a variety of motor behaviours.

The electronic structure of a shallow acceptor and its bound exciton confined in GaAs/AlGaAs quantum wells

The exciton bound (BE) at the shallow Be acceptor confined in a narrow GaAs/AlGaAs quantum well (QW) has been investigated by means of selective photoluminescence (SPL) and PL excitation (PLE) experiments. For the shallow acceptor, several excited states have been observed as two-hole transition (THT) satellites of the BE. The heavy hole (hh)-like excited nS(Γ 6 ) acceptor states dominate the satellite spectrum, but also the light hole (lh)-like excited 2S(Γ 7 ) and the parity forbidden transition to the 2P 3/2 excited state have been monitored in SPL spectra. When detecting a THT satellite, the 1S(Γ 7 ) hh-like acceptor ground state has been spectrally resolved from the 1S(Γ 6 ) hh-like state in PLE spectra. Several BE states have been theoretically predicted and observed in PLE spectra with the J=5/2 state at lowest energy, like in bulk GaAs. The proposed interpretation of the acceptor hh- and 1h-states is confirmed by SPL and PLE experiments in the presence of an applied magnetic field and in polarized PLE measurements. An effective g-value for the acceptor BE recombination in varying degree of confinement is derived from these Zeeman measurements

Excited states of shallow acceptors confined in GaAs/AlxGa1-xAs quantum wells.

An optical study of the excited states of shallow acceptors confined in narrow GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells (QW's) is presented. The excited states have been investigated via the two-hole transitions (THT's) of the bound exciton recombination observed in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. Several heavy-hole (hh) -like excited nS(${\mathrm{\ensuremath{\Gamma}}}_{6}$) acceptor states dominate the satellite spectrum, but also the light-hole (lh) -like excited 2S(${\mathrm{\ensuremath{\Gamma}}}_{7}$) acceptor state has been observed in these SPL spectra. In addition, the THT corresponding to the parity-forbidden transition to the 2${\mathit{P}}_{3/2}$ excited state has been observed. When detecting any of the THT satellites corresponding to the 2S acceptor state, the 1S(${\mathrm{\ensuremath{\Gamma}}}_{6}$) hh-like acceptor ground state has been spectrally resolved from the 1S(${\mathrm{\ensuremath{\Gamma}}}_{7}$) lh-like acceptor state in PLE spectra. Together with the previously published results for the p-like excited states, these data complete the electronic structure of shallow acceptors in a QW.

Photodissociation dynamics of S4N4 at 222 and 248 nm

Emission from several electronically excited states of NS is observed when the energetic molecule S4N4 is photolyzed with radiation from an excimer laser. Photolysis at 248 nm generates fluorescence from the B 2Π1/2,3/2, H 2Π1/2, G 2Σ−, and I 2Σ+ states of NS. NS(B 2Π1/2,3/2) and NS(C 2Σ+) fluorescence is observed when the photolysis wavelength is changed to 222 nm. The NS(H) and NS(C) spectra are postulated to arise from a resonant interaction between the KrF and KrCl excimer photons, respectively, and vibrationally hot ground state NS. LIF excitation scans on the NS X 2Π1/2,3/2 → B 2Π1/2,3/2 system confirm the production of rotationally and vibrationally excited NS(X) up to v″=4. A mechanism, based on the experimental data (i.e., spectral composition, laser fluence studies, excited state time histories), calculated heats of formation, and Gaussian molecular orbital calculations, is proposed to account for the observed emissions. For photolysis at 248 nm it is hypothesized that a two photon absorption promotes the ground singlet state of S4N4 to an upper repulsive singlet state, which rapidly dissociates (τ≪30 ns), producing an acyclic S3N3 fragment and vibrationally excited monomeric NS(X). The photofragments can interact further with the excimer radiation to produce NS(B) and NS(H), respectively. A similar mechanism is proposed to account for the presence of the NS(B) and NS(C) excited states for the 222 nm photolysis.

Fluorescence Lifetime and Quenching Rates for N,N,N′,N′-Tetramethyl-P-Phenylenediamine in the Vapor Phase

The fluorescence lifetime of N,N,N′,N′-tetramethyl- p-phenyIenediamine (TMPD) in the vapor phase has been determined to be 3.2 ± 0.3 ns for excitation at 337 nm. The rate constants for quenching of electronically excited TMPD by ground-state TMPD, O2, and CO2 have been determined to be &lt;1 × 10−10 cm3/s, (9.9 ± 1.0) × 10−10 cm3/s, and &lt;4 × 10−13 cm3/s, respectively. The rate for TMPD implies that self-quenching is negligible up to pressures of at least 10 Torr. The quenching rate by oxygen is sufficiently high to ensure that use of TMPD as a quantitative marker for fuel evaporation in exciplex-based vapor/liquid visualization systems is probably not possible if significant quantities of oxygen—as would be the case in combustion environments—are present.

Transient phenomena in the photochemistry of trans-azocumene

Laser flash photolysis work demonstrates that the dominant process at room temperature in the photolysis of trans-azocumene is its isomerization to the cis isomer, cumyl radicals are also produced within 20 ns of excitation.

Two-photon excitation spectroscopy and excited state decay kinetics of isolated diphenylbutadiene

We report the two-photon excitation spectrum and decay kinetics of the two-photon prepared states for diphenylbutadiene in a helium free jet. The spectrum is that of a symmetry allowed two-photon transition and is assigned using 17 harmonic fundamentals. A reexamination of the previously reported one-photon excitation spectrum allows the assignment of several additional u symmetry fundamentals of diphenylbutadiene’s excited 1Ag state. Curves of intensity vs time for light detected at two times the wave number frequency of the laser minus a Stoke’s shift of between 1000 and 9000 cm−1 show two components. One, due to two-photon resonance enhanced hyper-Raman scattering, reproduces the temporal profile of the exciting laser. The other, due to fluorescence from the excited 1Ag state, decays exponentially. The fluorescence lifetime decreases with increasing excitation frequency from 100 ns for 0–0 band excitation to 13 ns for excitation at 1235 cm−1 above the origin, consistent with the onset of a nonradiative decay channel previously observed for diphenylbutadiene in one-photon studies. Above this energy only hyper-Raman scattering is detected.

Fluorescence Lifetime and Oxygen Quenching Rate of Gas-Phase Fluoranthene Excited at 337.1 nm

We report that the fluorescence lifetime is 38 ± 2 ns and that the oxygen quenching rate constant is 2.49 × 109 L mol−1·s−1 for gas-phase fluoranthene excited at 337.1 nm. A fluorescence lifetime of 24 ± 3 ns for excitation at 351 nm has been reported previously by Ehrlich and Wilson [J. Chem. Phys. 67,5391 (1977)]. According to the current spectroscopic assignments, the 337.1-nm and the 351-nm lasers excite fluoranthene molecules to the same S2 electronic state with excess vibrational energies of 1900 cm−1 and 710 cm−1, respectively. Therefore, our measured lifetime reveals an anomalous trend of the fluorescence lifetime if both the current spectroscopic assignment and the value of 24 ns for the lifetime excited at 351 nm are accepted. Our measured oxygen quenching rate constant indicates that the oxygen quenching rate for fluoranthene is about 100 times slower than that of anthracene and pyrene. The unusually slow oxygen quenching rate in the gas phase may be related to the previously reported unusual inertness of excited fluoranthene molecules in other environments. Possible explanations of these abnormal behaviors of fluoranthene are discussed.

PHOTOACTIVITY OF TRANSIENT INTERMEDIATES IN THE PATHWAY FROM THE RED‐ABSORBING TO THE FAR‐RED‐ABSORBING FORM OF Avena PHYTOCHROME AS OBSERVED BY A DOUBLE‐FLASH TRANSIENT‐SPECTRUM ANALYZER

Abstract— Phototransformation from the red‐absorbing form of phytochrome (Pr) to the far‐red‐absorbing form occurs via at least two reaction stages. We report here on the photoactivity of the intermediates present during these two stages, as detected with 114/118‐kdalton Avena phytochrome by a transient spectrum analyzer with two actinic lasers of microsecond duration. Flash activation of intermediates present during the first stage results in their photoconversion back to Pr within the time resolution provided by the analyzer, which is 10 μs. Since these intermediates are present within 60 ns of excitation of Pr (Cordonnier et al., 1981), a single flash of ms duration as used here should yield a photostationary equilibrium between them and Pr. The proportion of Pr converted to intermediates by a single saturating flash is estimated to be about 30%. Flash activation of intermediates present during the second stage converts them to another transient intermediate stage, which decays thermally to Pr within 2 ms.

Quenching collisions of rubidium in the nS (32⩽n⩽45) Rydberg levels with helium

Reports measurements of the cross sections for collisional quenching of highly excited rubidium nS (32<or=n<or=45) states by helium. A time-resolved selective field ionisation technique is used for this study. The experimental results are compared with the values calculated using an approximate scaling formula.

Thermal collisions between highly excited and ground-state alkali atoms

Measurements have been made of the cross sections for collisional quenching of highly excited rubidium nS (12<or=n<or=18) and nD3/2(9<or=n<or=13) states by ground-state rubidium. The experimental values are shown to be close to the geometrical cross section. The large magnitude of the quenching cross section mainly reflects the long range of the Rydberg electron-alkali interaction.