ALBERT

Notes: We have formulated a law for state-to-state rotational transfer (RT) in diatomic molecules based on the angular momentum (AM) theory proposed by McCaffery et al. [J. Chem. Phys. 98, 4586 (1993)]. In this, the probability of angular momentum change in the rotor is calculated by assuming the dominant process to be the conversion of linear to angular momentum at the repulsive wall of the intermolecular potential. The result is a very simple expression containing three variable parameters, each of which has physical significance in the context of the model. Fits to known RT data are very good and suggest strongly that linear to angular momentum change is indeed the controlling process in RT. The parameters of the fit are sufficiently available to give the model predictive power. Using this formulation, RT probabilities may be calculated for an unknown system with little more than the atomic masses, bond length, and velocity distribution. We feel that this represents an important step in the development of a simple physical picture of the RT process.

Notes: The fully vibrationally resolved participator Auger spectra originating from the decay of the C 1s(2σ)−12π1 resonance in CO are presented. The C 1s(2σ)−12π1 v'=0 resonance has been excited with a 75 meV monochromator bandpass, i.e., in Auger resonant Raman conditions, and the participator Auger spectrum observed. The C 1s(2σ)−12π1 v'=1 resonance is also excited and the corresponding participator Auger spectrum observed with a monochromator bandpass slightly larger than the inherent width. The results are compared to theoretical simulations using coherent lifetime-vibrational interference theory which accounts for the details of the spectrum. We have observed an interference shift on the transitions to different vibrational sublevels in the final state. A high resolution C 1s photoelectron spectrum of CO is also presented. The lifetime width of the C 1s core–hole state is determined to be 97(10) meV, whereas the C 1s(2σ)−12π1 resonance is measured to have a width of 86(10) meV. © 1995 American Institute of Physics.

Notes: ELMing (edge-localized) H-mode discharges with densities as high as 40% above the Greenwald density and good energy confinement, HITER-89P=2, were obtained with D2 gas puffing on DIII-D [Chan et al., Proceedings of the 16th IAEA Conference, Montreal (International Atomic Energy Agency, Vienna, 1996), Vol. 1, p. 95]. These discharges have performance comparable to the best pellet fueled DIII-D discharges. Spontaneous peaking of the density profile was an important factor in obtaining high energy confinement. Without density profile peaking, the energy confinement at high density degraded with reduction in the H-mode pedestal pressure under the stiff temperature profile conditions observed at high density on DIII-D. Reduction in the pedestal pressure was associated with loss of access to the second stable regime for ideal ballooning modes at the edge, and change in the edge-localized mode (ELM) instability from a low to high toroidal mode number. Gyrokinetic stability calculations indicate that the core of the high-density discharges is dominated by ion temperature gradient mode turbulence. A turbulent transport simulation with the GLF23 [Waltz et al., Phys. Plasmas 4, 2482 (1997)] code produced stiff temperature profiles in agreement with the experiment and did not indicate the formation of an internal transport barrier. Helium transport studies showed an anomalous inward particle pinch at high density. The highest density discharges were terminated by onset of a magnetohydromagnetic instability, which is consistent with destabilization of neoclassical tearing modes through peaking of the pressure profile. © 2001 American Institute of Physics.

Notes: Many sensors have been applied to the problem of measuring neutral atomic oxygen fluxes in low Earth orbit. The techniques used to date tend to suffer from several key disadvantages, variously: large mass and power budgets, large size, high cost, the ability to make only one measurement and poor time resolution. In this article preliminary results from ground-based testing of a novel atomic oxygen sensor based on a semiconducting metal oxide are reported. Such sensors are simple and relatively cheap while also requiring small power and mass budgets and, most importantly, are reusable. The sensors have been used in laboratory experiments to investigate the axial variation of atomic oxygen flux in a pulsed laser atomic oxygen source; the results compare well with readings taken with a carbon-coated quartz crystal microbalance. A small instrument based on these sensors has been designed and built for application on the UK's STRV-1c microsatellite. © 1999 American Institute of Physics.

Notes: We present a laser-target scaling model which permits approximate prediction of the dependence of ablation pressure, mechanical coupling coefficient, and related parameters in vacuum upon single-pulse laser intensity (I), wavelength (λ), and pulse width (τ) over extremely broad ranges. We show that existing data for vacuum mechanical coupling coefficient for metallic and endothermic nonmetallic, surface-absorbing planar targets follows this empirical trend to within a factor of 2 over 7 orders of magnitude in the product (Iλ(τ)1/2). The comparison we present is valid for intensity equal to or greater than the peak-coupling intensity Imax, where denseplasma formation mediates laser-target coupling. Mechanical coupling coefficients studied ranged over two orders of magnitude. The data supporting this trend represent intensities from 3 MW/cm2 to 70 TW/cm2, pulse widths from 1.5 ms to 500 ps, wavelengths from 10.6 μm to 248 nm, and pulse energies from 100 mJ to 10 kJ. With few exceptions, data approximating one-dimensional or planar expansions were selected. Previously, meaningful scaling of ablation pressure parameters with I, λ, τ was not possible because existing data concentrated in a small range of these parameters. Our own data, obtained in the low- and midrange of (Iλ(τ)1/2), completes the experimental picture. Since this new data was derived from five separate experiments with specialized character and purpose, detailed accounts of this work will appear separately. In this paper, we summarize the experimental conditions and selectonly those data which are relevant to the scaling issue. We find that laboratory-scale laser experiments can often give impulse coupling data which agree with results from much higher-energy experiments without much error, and at much lower cost. We review a theory of vacuum laser ablation, specialize it to a quantitative description of mechanical coupling, and show that the resulting model provides a simple physical description which comes quite close to the observed empirical trend. This is accomplished with minor elaborations of the theory as originally presented to account for the temperature dependence of plasma ionization states, while adhering to the premise that a simple and generally applicable treatment of laser impulse production should be available. The theoretical model can quantitatively predict vacuum ablation pressure foropaque targets without adjustable parameters to the factor-of-2 accuracy in which we are interested. Other published scaling models omit one or more of the important variables, lack broad applicability, or deviate more noticeably from the observed trend.

Notes: The amplitude and frequency of modes driven in the edge region of tokamak high mode (H-mode) discharges [type I edge-localized modes (ELMs)] are shown to depend on the discharge shape. The measured pressure gradient threshold for instability and its scaling with discharge shape are compared with predictions from ideal magnetohydrodynamic theory for low toroidal mode number (n) instabilities driven by pressure gradient and current density and good agreement is found. Reductions in mode amplitude are observed in discharge shapes with either high squareness or low triangularity where the stability threshold in the edge pressure gradient is predicted to be reduced and the most unstable mode is expected to have higher values of n. The importance of access to the ballooning mode second stability regime is demonstrated through the changes in the ELM character that occur when second regime access is not available. An edge stability model is presented that predicts that there is a threshold value of n for second regime access and that the most unstable mode has n near this threshold. © 2000 American Institute of Physics.

Notes: The decay of core-excited electronic states in free 1,3 trans butadiene molecules has been studied using high-resolution synchrotron radiation and electron spectrometry. The core-level energy shift between the terminal and central carbon atoms is 0.64 eV making selective excitation of core electrons from these atoms possible. Resonant excitation to the au(π*) valence state leads to autoionizing decay channels which proceed according to the atomic site in the molecule. The radiationless decay is localized, and certain molecular orbitals are excluded from the decay depending upon the site of the core hole. This phenomenon is confirmed by semiempirical INDO calculations based upon the equivalent core approximation. The vibrational structure of the resonances below the carbon K edge has been measured and fit to extract vibrational energies and intensities, chemical shifts, and the lifetimes of the centrally and terminally excited states. The C 1s spectrum is also measured with vibrational resolution and the energies of the normal vibrational modes are extracted. The bond lengths are derived by application of a linear coupling analysis. © 1996 American Institute of Physics.

Notes: Recent progress in the development of high-resolution electron spectrometers in combination with highly monochromatized undulator radiation has allowed observation of the vibrationally resolved gas-phase C 1s photoelectron spectra of methane and ethane. For both molecules, the C–H stretching modes are well resolved and for ethane the active C–C stretching mode has been observed for the first time. The spectra have been measured at low kinetic energies and detailed fittings using post-collision interaction line profiles have been made both, using a free parameter fit and a fit adhering to a linear coupling model. The free parameter fit allows for any anharmonicity in the vibrational energies. The linear coupling model, on the other hand, assumes that the initial and final state potential curves are harmonic and differ only in the normal coordinates. This simple model is used to reduce the number of free parameters in the fit, which greatly simplifies the analysis. An intensity model based on the linear coupling predicts that the intensities of the C–H stretching modes are directly related to the number of C–H bonds around the core ionized atom. The result is verified for ethane and shows a potential for further reduction of free parameters for large molecules and polymers. Ab initio calculations of molecular geometry and vibrational frequencies have also been carried out using the equivalent core (Z+1) approximation. The values predicted for the decrease in bond length have then been compared to those determined empirically by the linear coupling approach. The calculation of ethane indicates that symmetric C–H and C–C stretching modes are important upon core ionization. The corresponding vibrational frequencies have been calculated and agree well with observed values. © 1997 American Institute of Physics.

Notes: High-resolution Auger electron spectra from the decay of the Cl 2p→σ* excitation in HCl and DCl have been measured. The spectra are analyzed, separating molecular and atomic features, which are assigned to transitions to the HCl (5σ2π)4σ* and Cl (3s3p)6 states, respectively. Auger line shapes, as affected by the molecular dissociation, are studied by comparing the experiment with the results of Monte-Carlo computer simulations based on a semiclassical model. © 1996 American Institute of Physics.

Notes: The Doppler profiles of Ba(3P2) atoms desorbed from the surface of argon clusters following the deactivation of Ba(1P1) have been measured. These measurements have been performed for desorption from pure ArN clusters and as a function of a known average number of CH4 molecules deposited on the cluster. Analysis of the profile widths with respect to the kinetic energy release from deactivation indicates that desorption occurs along a single Ba–Ar and Ba–CH4 coordinate in the former and latter cases, respectively. By comparing the kinetic energy distributions in the desorbed barium with the relative kinetic energy available at the temperature of the cluster it is found that the collisions leading to deactivation in both cases are gas kinetic at the temperature of the cluster (35 K). The residual anisotropies in the Doppler profiles reveal the Ba–Ar deactivation to be a relatively inefficient process allowing the barium to undertake a full migration on the cluster surface before desorbing. This results in an essentially isotropic distribution of recoil velocities. In contrast Ba–CH4 deactivation is sufficiently fast to preserve some degree of anisotropy in the desorbed barium velocity distribution. The anisotropy results from the depolarization of the barium orbital due to both the migration of the barium on the cluster surface and axial relaxation of the orbital by collisions with neighboring argon atoms. Calculations of the anisotropies resulting from both reorientating mechanisms show a significant degree of relaxation and migration to occur before the barium is desorbed. © 1997 American Institute of Physics.