Laser-induced breakdown and subsequent plasma are produced in ambient air in the proximity of an aqueous surface using a Nd:YAG Q-switched laser at 1064 nm with a pulse width of 9 ns and a delivered focused input energy of 170 mJ. The distance between the focal point of a 10 cm convex lens and the aqueous surface is 4 mm with laser propagation perpendicular to the surface. Using an intensified CCD camera attached to a 1-m spectrometer, spatial and wavelength-resolved plasma emission data are obtained for delay times after breakdown ranging from 50 ns to 10 μs with a gate window typically 5 ns. Plasma electron density is determined by applying Lorentzian fitting and FWHM extraction to three Stark-broadened spectral lines: N II 3P-3Do multiplet (593.85 nm), Hα (656.27 nm), and the Na D doublet (589.00 and 589.59 nm). One-dimensional spatially resolved measurements of the total emission intensity and electron density are obtained by binning the camera image along the laser axis in intervals of 250 μm and are reported as a function of time from 50 ns to 10 μs. Two plasmas are ignited from a single laser pulse; one from laser breakdown at the water surface and the other a few nanoseconds later from laser-induced air breakdown at the focal point of 4 mm above the water surface. Comparisons between the evolution of the air plasma near and far from the water surface are presented along with data for the water surface plasma itself.

We have experimentally investigated the influence of repeated laser pulses on laser-induced breakdown (LIB) in water following a focused 1064 nm laser pulse, for repetition rates between 0.5 and 20 Hz and up to 100 pulses. LIB plasma image data are collected as a function of repetition rate and laser pulse number, using an intensified CCD camera with sub-nanosecond camera-laser timing. Each pulse induces multiple breakdowns, shockwaves, and macroscopic cavitation bubbles. These effects disappear by the time the next pulse arrives. Even so, we find that the water target retains effects from previous pulses that persist up to roughly a second and that modify subsequent LIB properties including plasma location, spatial extension, and total emission intensity. We quantify the effects by tracking the first moment of the plasma emission intensity, which we call the “center of intensity” (CoI). Three distinct reproducible repetition-rate-dependent phases are identified with repeated pulses. In particular, (1) with initial pulses, emission intensity decreases and the CoI broadens and propagates downstream from the laser focus reaching a maximum axial distance away from the focal point, (2) with additional pulses, the CoI narrows and travels back upstream toward the laser, and (3) with continued pulses, the plasma gains back a portion of the lost intensity and the CoI reaches a steady state position, not at the focal point. Two possible causal effects are highlighted; particle inclusion concentration changes and microbubbles in the laser path.

A spectrum of field-ionized triplet Rydberg states of gerade symmetry H2 has been measured, excited from the υ″ = 0, N″ = 1-3 rovibrational levels of the metastable c3Πu-2pπ state in a 6 keV fast molecular beam. The field-ionized spectrum is kinetic energy labeled in order to separate it from the well-studied υ+ ≥ 1 autoionization spectrum. The spectrum consists of both ns and nd Rydberg series with n between 10 and 28 converging to the υ+ = 0, N+ = 1-3 levels of the X+ 2Σg+ ground state of H2+. Transitions with changes in vibration and/or rotation are also identified. The spectral positions of 59 transitions in the field ionization spectrum are identified and assigned quantum numbers using the predictions of multichannel quantum-defect theory (MQDT). The transition energies and subsequent effective quantum defects are compared between the experiment and theory. Most of the observed transitions, within the experimental uncertainty of 0.2 cm-1, agree with the energies predicted by MQDT.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. ELECTRON TRANSFER AND K-SHELL EXCITATION IN SINGLE COLLISIONS OF 47-365 MeV Ca17+ WITH Ne, Ar, AND Kr M. Clark, J. Tanis, K. Berkner, E. Bernstein, R. Dubois, W. Graham, R. Mcfarland, T. Morgan, D. Mueller, A. Schlachter, et al.

We have used optical Rayleigh and Thomson scattering to investigate the expansion dynamics of laser induced plasma in atmospheric helium and to map its electron parameters both in time and space. The plasma is created using 9 ns duration, 140 mJ pulses from a Nd:YAG laser operating at 1064 nm, focused with a 10 cm focal length lens, and probed with 7 ns, 80 mJ, and 532 nm Nd:YAG laser pulses. Between 0.4 μs and 22.5 μs after breakdown, the electron density decreases from 3.3 × 1017 cm−3 to 9 × 1013 cm−3, while the temperature drops from 3.2 eV to 0.1 eV. Spatially resolved Thomson scattering data recorded up to 17.5 μs reveal that during this time the laser induced plasma expands at a rate given by R ∼ t0.4 consistent with a non-radiative spherical blast wave. This data also indicate the development of a toroidal structure in the lateral profile of both electron temperature and density. Rayleigh scattering data show that the gas density decreases in the center of the expanding plasma with a central scattering peak reemerging after about 12 μs. We have utilized a zero dimensional kinetic global model to identify the dominant particle species versus delay time and this indicates that metastable helium and the He2+ molecular ion play an important role.

The isotopes (122),(124),Cd-126 were studied in a "safe" Coulomb-excitation experiment at the radioactive ion-beam facility REX-ISOLDE at CERN. The reduced transition probabilities B(E2; 0(g. s)(vertical bar) -> 2(1)(+)) and limits for the quadrupole moments of the first 2(+) excited states in the three isotopes were determined. The onset of collectivity in the vicinity of the Z = 50 and N = 82 shell closures is discussed by comparison with shell model and beyond mean-field calculations.

The completely unknown spectrum of excited states of the odd-odd nucleus Pa-230 was studied employing the one-neutron transfer reaction Pa-231(d, t)Pa-230 at a beam energy of 22 MeV. The excitation energy and the cross section were measured for, in total, 81 states below 1.4 MeV. Level assignments of these states are based on a semiempirical model and comparison with theoretical predictions, based on distorted-wave Born approximation (DWBA) calculations for the cross sections. For 12 rotational bands the band-head energy and the rotational parameter are determined. The K quantum numbers and the Nilsson configurations are established. Empirical values for the Gallagher-Moszkowski splittings and for Newby shifts are obtained. DOI: 10.1103/PhysRevC.87.044322

Excited states of the isotope U-234 were studied employing the one-neutron transfer reaction U-235(d, t)U-234 at a beam energy of 11 MeV. The reaction channel was selected by an identification of the outgoing tritons in a Delta E-E measurement. In coincidence with the tritons, gamma rays were also detected with the highly efficient MINIBALL spectrometer. Based on the analysis of gamma gamma coincidence data, the level scheme of U-234 was extended. The 4(+) state at an energy of 1886.7 keV provides evidence for a possible two-phonon gamma gamma excitation, which is based on three gamma transitions from this state to the gamma vibrational band. However, this state is populated with a high cross section via the one-neutron transfer reaction, which is not expected for a highly collective excitation. The situation is compared to other known cases from the rare-earth region and to the neighboring isotone Th-232, which so far is the only known case for a two-phonon excitation in the actinide region.

Excited states of the odd-odd isotope Pa-230 have been identified by means of the one-neutron transfer reaction Pa-231((d) over right arrow ,t)Pa-230 at a beam energy of 22 MeV. The ground state of Pa-230 is assigned to be a 2(-) state. A sequence of four rotational states is measured on top of the ground state up to spin values of 6h. The band head of the first excited rotational band has an excitation energy of 48 key, the corresponding rotational band is observed up to a spin value of 5h. Based on the distribution of the strength among the low-lying states, the Nilsson configuration 1/2[530](p) + 3/2[631](n) is considered the most probable configuration of the ground state, leaving 1/2[530](p) - 5/2[633](n) for the second excited band. The energy difference between the band heads of these two bands is compared to similar states in other N = 139 isotones, especially the directly neighboring odd-mass isotone Th-229. Here the two neutron orbitals 5/2[633](n) and 3/2[631](n) cause an extremely low-lying isomeric state. (C) 2012 Elsevier B.V. All rights reserved.

D-2 and H-2 Stark recurrence spectra have been measured at scaled energies of epsilon = -2.3 and epsilon = -3.2. An isotope shift in the scaled-action locations of recurrence peaks is found and explained by the increased density of states associated with perturbing rotational series. This reinforces the insight gained in previous work on the correspondence between the structures of a quantum spectrum and its classical periodic orbits.

D${}_{2}$ and H${}_{2}$ Stark recurrence spectra have been measured at scaled energies of $\ensuremath{\epsilon}=\ensuremath{-}2.3$ and $\ensuremath{\epsilon}=\ensuremath{-}3.2$. An isotope shift in the scaled-action locations of recurrence peaks is found and explained by the increased density of states associated with perturbing rotational series. This reinforces the insight gained in previous work on the correspondence between the structures of a quantum spectrum and its classical periodic orbits.

We have carried out optical Thomson scattering measurements from a laser induced breakdown in He at 1 atmosphere. The breakdown was created with a Nd:YAG laser with 9ns pulse duration and 400mJ pulse energy focused into a chamber filled with He. A second harmonic Nd: YAG laser with 9ns pulses and up to 80mJ energy was used to obtain temporally and spatially resolved data on the electron density and temperature. In parallel experiments, we measured the emission of the 447.1nm line from He I. Initial results suggest good agreement between densities inferred but full Abel inversion is needed for conclusive results.

The reaction U-234((d) over right arrow, t)U-233 was measured at the Munich Q3D magnetic spectrometer with a polarized deuteron beam. The beam energy was 22 MeV, and a vector polarization of the deuterons of 80% was achieved. Angular distributions of the reaction cross section and analyzing power at seven angles between 5 degrees and 35 degrees were analyzed. Spin and parity assignments for 33 states were determined by comparison with results from distorted wave Born approximation (DWBA) calculations. Based on these assignments and energy systematics, the observed states were sorted into rotational bands. The Nilsson configurations of the bands are identified by examining the population strengths within each band. Two rotational bands with Nilsson configurations 1/2[501] and 3/2[501] could be identified for the first time.

The temporal evolution of excitation temperature in a laser-produced atmospheric pressure helium plasma is measured and its electron density is explored using a number of different methods. These are the Stark broadening of the He lines at 4026, 4471 and 5015 angstrom, peak separation and peak intensity ratio of the allowed and forbidden components of the 4471 angstrom line and Stark broadening of the H-beta line in hydrogen-helium mixtures. The various results obtained from the helium lines agree within roughly an average factor of 2 at all times. Values obtained from peak separation values differ slightly from the Stark measurements by amounts that depend on the analysis method used. At early delay times, electron density values obtained from the H-beta line are considerably lower than those from helium lines. Electron density versus temperature curves and temporal evolution of the line intensities indicate that the laser-produced plasma is not in complete thermal equilibrium. Measured electron densities range from similar to 10(17) to slightly below 10(15) cm(-3). These are electron densities now being generated in electrically produced microplasmas where the use of spectroscopic techniques is also an important diagnostic tool.

Thomson scattering from laser-induced plasma in atmospheric helium was used to obtain temporally and spatially resolved electron temperature and density profiles. Electron density measurements at 5 μs after breakdown are compared with those derived from the separation of the allowed and forbidden components of the 447.1 nm He I line. Plasma is created using 9 ns, 140 mJ pulses from Nd:YAG laser at 1064 nm. Electron densities of ∼5 × 1016 cm−3 are in good agreement with Thomson scattering measurements, benchmarking this emission line as a useful diagnostic for high density plasmas.

Using scaled-energy Stark spectroscopy, we report the observation of recurrences due to closed orbits, both geometric and diffractive, in the nu = 0, R = 1, nd Rydberg series of H-2 (16 < n < 26) interacting with the nu = 0, R = 3 series (13 < n < 15). The data support the molecular closed-orbit theory prediction of diffractive trajectories due to inelastic scattering of the excited electron on the molecular core. We have made similar measurements in He, and a comparison between the recurrence properties of H-2 and its united atom equivalent is given.

We use the 4s[3/2](2)(o) metastable state of atomic argon created by charge-transfer collisions between a 5 keV Ar+ beam and K vapor to perform laser scaled-energy Stark photoabsorption and recurrence spectroscopy of even-parity Rydberg states, detected by field ionization and forced autoionization, in a region of the spectrum that contains two distinct perturbations. We apply a uniform electric field that changes with the frequency of the laser to maintain a constant scaled energy relative to the first ionization limit of the atom, associated with the ion core in a P-2(3/2) configuration. Local perturbations to the Rydberg series (15 <= n <= 28) occur due to n(')=8 and 10 Stark states that belong to the series converging to the second ionization threshold with the ion core in a P-2(1/2) configuration. The resulting absorption spectra show multielectron effects due to configuration interaction and angular momentum coupling. These effects have dynamical implications in the recurrence spectrum. In particular, the variations in the Stark structure of the absorption spectrum result in a recurrence spectrum that shows two prominent multielectron features. One is the presence of recurrence peaks with scaled actions less than that of the hydrogenic primitive orbit. We attribute this to excitation in which both the ion core and the Rydberg electron absorb energy during photoexcitation. The second multielectron effect observed is recurrence peaks that occur at the sum of scaled actions of the primitive orbit (and its repetitions) associated with the pair of perturbations and a hydrogenic closed classical orbit. We attribute these peaks to core-changing inelastic scattering of the Rydberg electron with the ionic core due to angular momentum coupling. For comparison, we measured the regular autoionizing Rydberg series of argon between the first and second ionization thresholds, where no perturbing resonances exist.

Using the 4s'[1/2](0) and 4s[3/2](2) metastable states of argon created in a plasma discharge we perform photoionization spectroscopy of even-parity autoionizing Ar(np(')) Rydberg states between the first and second ionization limits. Fitting the data to a sum of Fano-type or Shore-type profiles allows us to extract the widths and energies of the three overlapping resonance states [1/2](1), [3/2](1), and [3/2](2) in the 11p' manifold. We compare the experimental analysis of the 11p' multiplet to theoretical calculations which are based on the configuration interaction Pauli-Fock approach with core polarization. Agreement between the experimental and theoretical resonance parameters is good.