Cascade Transitions Research Articles

Radiative lifetimes of the lowest ${^3{\rm P}}_1$ metastable states of Ca and Sr

The lifetimes of the lowest 3P1 states of Ca and Sr have been measured in atomic beam experiments. These measurements have been made by observing the exponential decay of the fluorescence emitted from excited states populated either by a discharge or a dye-laser pumping. In both cases, the velocity distributions of the radiating atoms were also measured by a time-of-flight technique. Our results show that the lifetime measurements using the discharge excitation are hampered by a systematic error introduced probably by cascade transitions that repopulate the upper energy levels of the transitions of interest. The radiative lifetimes of the 4s4p 3P1 state of Ca and 5s5p 3P1 state of Sr are determined to be 0.34 ± 0.02 ms and 21.3 ± 0.5 μs, respectively.

Synchrotron x-ray-diffraction study of α-cristobalite at high pressure and high temperature

Energy-dispersive x-ray diffraction using synchrotron radiation was carried out on α-cristobalite to 3 GPa and 350°C in a cubic anvil press. A cascading structural phase transition occurred beyond 0.61 GPa at room temperature. The transition was accompanied by a splitting of most of the a-cristobalite reflections: the (111) reflection at 0.61 GPa through the (211) reflections at 2.13 GPa, with many other lines between. The pressure of this transition decreased with increasing temperature.

Double electron capture in low-energy Fe17++He collisions.

Double electron transfer processes for the system ${\mathrm{Fe}}^{17+}$+He have been studied utilizing high-resolution Auger electron spectroscopy. A thorough analysis of the experimental conditions (single collision condition and metastable fraction content at the incident ion beam and their influence) has been performed, leading to the following conclusions. The double capture leads to the preferential population of 2${\mathit{p}}^{5}$5l8${\mathit{l}}^{\ensuremath{'}}$ configurations sharing their decay between Auger processes and radiative cascade transitions. The observed sodiumlike ``quartet'' and ``doublet'' states result from cascade contributions from higher-lying levels. Tentative mechanisms for the production of the quartet states are suggested using a spin-flip model. \textcopyright{} 1996 The American Physical Society.

Lifetime measurements of some 2p53p and 2p53d levels in Ne-like Cr XV and Fe XVII

Experimental lifetimes of two 2p53p and two 2p53d levels of neonlike Cr XV and Fe XVII have been obtained using the beam-foil technique. Decay curves of the 3s–3p transitions were analyzed by the ANDC (Arbitrary Normalized Decay Curve) method taking into account cascading transitions from the 3d levels. Those of the 3p-3d transition were done by multi-exponential fitting. The lifetimes obtained in ns for the 3d 3F3 and 3d 3F4 levels are 0.136 and 0.117 for Cr XV and 0.083 and 0.097 for Fe XVII, and those of 3p 3D2 and 3p 3D3 are 0.208 and 0.171 for Cr XV and 0.176 and 0.173 for Fe XVII, respectively. The results are compared with theoretical values and the Cr XV results are also compared with previous experimental work. Recent theoretical values agree well with the experimental values presented here.

New regime of conical emissions in multi-level optically-pumped atomic vapors

We present experimental results of spectral and spatial properties of the laser-pumped cascade emissions from sodium vapor when the 3 S 1 2 4 P 1 2 , 3 2 transitions of sodium are excited with resonant 330 nm narrowband radiation. We observe a pronounced hollow-conical emission of the 3P3S cascade transition only. The angle and frequency of the hollow-conical radiation depend on the sodium number density. A qualitative theoretical approach that explains the observed effects is outlined.

The performance of the hollow-cathode discharge continuous-wave multicolour He-Cd+ laser

We report on the performance of a hollow-cathode discharge continuous wave He-Cd+ laser module, which is capable of simultaneously delivering stable, milliwatt output power at the three primary spectral lines: blue ( lambda =441.6 nm), green ( lambda =533.7 and 537.8 nm), and red ( lambda =635.5 and 636.0 nm). In particular, the laser output characteristics for single- and multi-line operation and the power interaction between the laser lines are presented. The results show that the most efficient simultaneous laser oscillations at the red, green and blue lines occur for the He pressure (8 mbar at an inner diameter of the hollow cathode of 4 mm) at which the hollow-cathode effect is strongest. We find strong power interaction between the green and red lines, showing experimentally that, apart from the already known increase in the He-Cd+ laser output power in the green owing to the radiative cascade transitions in the red, also the red lines gain in their output power under simultaneous operation with the green lines, which de-populate the lower laser levels of the red transitions. Under some conditions the de-population of the lower laser levels of the red transitions by the green lines' oscillations can be the dominant mechanism for the build-up of the population inversion between the upper and lower levels of the red transitions.

Evidence for the I?=8?, K=8 isomeric state in184Hf

Using an 11.4MeV/u136Xe beam of the UNILAC accelerator of GSI and anatW target, A=184 isobars were produced and separated by using an on-line mass separator. A new isomeric state was found in184Hf. This state decays with a half-life of 48(10) s and emits three γ-rays of 73, 482, and 555 keV as well as lower-lying cascade transitions. The hindrance of the 73 keV-E1 transition with ΔK=8 is compared with those of other even-even hafnium isotopes. The half-life of184Lu was estimated to be between 15 and 25s.

X-rays from radiative electron capture of highly-charged heavy ions in an electron cooler

The x-ray spectra induced by spontaneous capture of free cooling electrons into bare and hydrogenlike heavy ions have been observed. Au78+, Au79+ and U91+ ions were stored in the storage ring ESR at velocities between 60% and 65% of the velocity of light and the x rays were measured in coincidence with stored particles having captured one electron in the electron cooler. The lines observed can be attributed to the direct transition of cooling electrons into theK shell of the projectile and to the characteristicL →K transitions. The latter are mainly induced by capture into higher shells and subsequent cascade transitions. The measurements are compared to detailed calculations within the framework of presently available theories. Although the relative structure of the measured spectra is in fair agreement with the theoretical prediction, the total charge-changing rate in the electron cooler is about a factor of three smaller than expected.

Cascade inversionless and inversion lasers

It is shown that simultaneous lasing on two cascade transitions may be realized in an ensemble of incoherently pumped three- level atoms confined in a doubly resonant cavity. Depending on the ratio of the cavity decay rates for the two laser fields, the cascade system may exhibit both lasing without inversion and with inversion. That is, one lasing field is generated from light amplification by coherence while the other is by stimulated emission from population inversion, and vise versa.

Polarization of Fe XXV 1s2-1s2l lines: Collisional resonances and radiative cascade contributions to 1s2l magnetic-sublevel excitation rates.

The effects of collisional resonances and radiative cascades on the linear polarization of He-like iron (Fe xxv lines from 1s2l to 1${\mathit{s}}^{2}$ levels have been investigated. Detailed calculations have been carried out for the 1s3l3l' resonance contributions to electron-impact excitation rates from the 1${\mathit{s}}^{2}$ ground level to the individual magnetic sublevels of 1s2l configurations. Excitation collision strengths from 1${\mathit{s}}^{2}$ to 1s3l sublevels as well as radiative cascade transitions from 1s3l to 1s2l' sublevels have also been computed. The autoionization transition-matrix elements (for the resonance effects) and the collison strengths (for the radiative cascades) have been computed in the distorted-wave approximation using intermediate coupling with fine-structure mixing multiconfiguration bound wave functions. The results indicate that the collisional resonance contributions, when averaged over a small energy range just covering them, have a somewhat significant depolarizing effect on the (1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$--1s2p $^{1}$${\mathit{P}}_{1}$, electric dipole) w line, the (1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$--1s2p $^{3}$${\mathit{P}}_{2}$, magnetic quadrupole) x line, and the (1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$--1s2p $^{3}$${\mathit{P}}_{1}$, fine-structure electric dipole) y line. However, the averaged polarization degree over some single resonances can reach high values.Now for the (1${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}_{0}$--1s2s $^{3}$${\mathit{S}}_{1}$, relativistic magnetic dipole) z line the individual resonances are found to induce a polarization degree less than 15% in absolute value but after averaging over all 1s3l3l' resonances z remains practically unpolarized. Concerning radiative cascades, it is shown that they can create a relative small degree of polarization on the z line, the highest value being close to 14%. For w, x, and y lines, the cascades have a weak depolarizing effect, except for x at large incident energy. It is expected that the contributions from 1snl cascades as well as 1s3lnl' resonances, with n\ensuremath{\ge}4, would not greatly change the results already obtained.

The field-induced transitions in a novel organic system, (DMETSeF) 2AuI 2

We have measured magnetoresistance and Hall resistance of a novel organic conductor, (DMETSeF) 2-AuI 2. DMETSeF (dimethyl ethylene dithio tetraselena fulvalene) is a newly synthesized donor molecule. The temperature dependence of (DMETSeF) 2AuI 2 salt depends on the batch of the sample preparation. Some crystals are metallic down to 0.5 K, and the others are metallic down to 40K, but the resistivity increases with the log(T) temperature dependence from 15 K down to 0.5 K. In this material, we found a series of cascade transitions due to the magnetic field similar to the well-known FISDW (Field Induced Spin Density Wave) states in (TMTSF) 2X family. The large magnetoresistance and the period of the step-like transitions cannot be understood as the Shubnikovde Haas oscillation. The band calculation indicates 1D like Fermi surface and very similar to those of TMTSF family, therefore FISDW can be one of the candidates to understand the state. But the experimental result is somewhat different from those of TMTSF family. This result is the first field induced state in organic systems other than TMTSF family.

Fractal dimensions of strange attractors obtained from the Taylor-Couette experiment

We present scenarios from the rotational Taylor-Couette flow, which has been implemented as a high precision hydrodynamic experiment. The system shows a rich variety of routes to chaos, e.g. the period doubling cascade, quasiperiodic and intermittent transitions. From a scalar time series, that is the axial velocity component of the flow field measured with Laser-Doppler velocimetry, strange attractors are reconstructed using Takens' delay time coordinates. To obtain an estimate of the fractal dimensions of these structures in phase space we calculate the correlation dimension from the reconstructed attractors. We discuss the fractal dimension as a function of Reynolds number and geometry of the experiment.

The 12C(a,g)16O Cross Section at Low Energies

An R-matrix formula for the cross section for radiative capture reactions is developed and applied to fit recently measured 12C(oc,y)160 data, for both ground-state transitions and cascade transitions through the 6�92 and 7 �12 MeV levels. The correct treatment of the channel contributions is significant for the E2 cascade transitions. Consistent fits of the cascade and ground-state data suggest a value of the channel radius larger than those previously used, and consequently a value of the low-energy astrophysical S-factor appreciably larger than that adopted recently.

Beam-foil lifetime study of Al-like titanium, iron and nickel

The lifetimes of the resonance levels in the Al-like ions Ti X, Fe XIV and Ni XVI have been obtained from beam-foil measurements using a variety of analytical techniques. The spectral analysis and identification of cascade transitions necessary for application of the ANDC procedure have been aided by recent observations of laser-produced plasmas. The lifetime values obtained from ANDC analyses for the 3s 3p2 doublet levels agree well with predictions based on semi-empirical Hartree-Fock calculations. However, the ANDC results for the 3s2 3d levels fall between the values obtained using multi-exponential curve-fitting and from the calculations.

A new method for the detection and visualization of nuclear levels

A new method has been developed to detect the excited levels populated directly in beta decay, provided these primary levels are de-excited by two or more gamma transitions in cascade. If these transitions are not strongly converted, the levels can even be visualized as a level spectrum. This method uses a multidetector summing coincidence spectrometer, which is briefly described. If the spin variation ΔJ between the initial and final state of the beta decay is very low, a spectrometer with only two detectors will be adequate to visualize the levels or prove their existence. Comparison between the summing coincidence spectra obtained with two, three or more detectors will allow identification of the levels which decay by two, three or more transitions in cascade. Furthermore, this method produces information on the spin of the excited states if the spin of the ground state of the final nucleus is known. Several applications of the summing coincidence method are described. A three-detector spectrometer has been used to study the level spectrum of 108Pd (from the decay of 108mAg): it is a single line corresponding to the known 6 + excited state. The level spectrum obtained with a 226Ra source and a two-detector spectrometer consisted of 51 prominent lines corresponding to the excited states of 214Po (three are new excited states of 1015.02 (4 +), 1712.65 (3 +) and 2802.6 keV); many high-energy lines were reinforced in the spectrum displayed by the three-detector spectrometer. Existence of an excited state of 62.69 keV in 214Bi can be deduced from the spectrum displayed by a two-detector spectrometer, at an energy in agreement with the 218At alpha-decay data.

Level Excitation Cross Sections of Atomic Fragments in Low-Energy Electron Impact Dissociation of Group IV Hydrides

The decay curves of atomic emissions observed in the low-energy electron impact excitation of GeH4, SiH4, and CH4 molecules were measured to evaluate the contributions of cascade transitions to the line emissions. By separating the cascade contributions from the fragment emission cross sections, level excitation cross sections were determined for the Ge I 5s 1Po, Si I 4s 1Po, and C I 3s 1Po levels. The measurements were carried out at the electron energies of 100 eV and around 30 eV. In every case, it was found that about half of these emissive fragments were produced by the radiative cascade from higher levels.

TWO-PHOTON PUMPEL SIX-AND FOUR-WAVE MIXING AND STIMULATED RADIATION IN LITHIUM VAPOR

This paper reports the generation of the coherent and stimulated radiation in UV, visible and infrared regions by two-photon pumping. It includes the tunable coherent radiation around 1.03 μm generated by a special six-wave mixing process, the stimulated radiation at 812.6 nm generated by molecular two-photon dissociation or atomic cascade transition, coherent radiation at 323.3 nm and 670.8 nm generated by the four-wave mixings.

Lasers operating on cascade transitions of triatomic molecules

The general conditions necessary for the operation of gas lasers based on cascade vibrational-rotational transitions of molecules in the pulse-periodic regime are formulated. The features of such triatomic-molecule lasers with arbitrary excitation are discussed. A computation model is proposed for determining the energy characteristics (maximum attainable efficiency, average and relative lasing power, specific energy output) of free-flow cascade lasers. Systems are analyzed with various excitation methods: gasdynamic (including those with optical feedback), electric-discharge, and chemical (with the DF-CO2 laser as the example). Practical recommendations with respect to the choice of mixture composition, optimum cavity lengths along the gas stream, and Q-switching frequency are made for the cascade lasers. A molecular gasdynamic laser emitting three wavelengths simultaneously is proposed.

Interaction of a laser-produced plasma with a solid surface: soft X-ray spectroscopy of high-Z ions in a cool dense plasma

Experimental data and theoretical calculations are presented for X-ray spectra from a laser-produced plasma interacting with a solid surface and are compared with those obtained from the hot-burst core of the plasma. Intensities of the spectral lines of the principal series of the H-like and He-like magnesium ions were investigated. A close study of the resonance and intercombination lines was carried out with high spectral resolution in both regions of the intense X-ray emission: near the target and near the obstacle. The plasma near the target has an electron temperature of Te=500-700 eV and an electron density of Ne=1020-1021 cm-3, whereas near the obstacle Te=70-100 eV and Ne=2*1019-3*1019 cm-3. It is shown that the X-ray emission of high-Z ions from the near-obstacle plasma results from the three-body recombination followed by cascade transitions due to collisional and radiative processes.

Optically pumped lasers from Na 2 D 1Π u state

We report on the first observation of infrared stimulated emission at wavelengths around 5.8 μm and 3 μm on D 1Π u→(5) 1Σ + u→(2) 1Σ + u (inner) cascade transitions of Na 2. The D 1Π u state is optically pumped with a pulsed XeCl excimer laser (308 nm) or tunable frequency-doubled dye-laser in the range of 309-289 nm. The infrared emission is accompagnied by ultra-violet radiation near 364 nm. Most of the optically pumped laser lines are identified.