In many cases fragmentation of molecules upon inner-shell ionization is very unspecific with respect to the initially localized ionization site. Often this finding is interpreted in terms of an equilibration of internal energy into vibrational degrees of freedom after Auger decay. We investigate the X-ray photofragmentation of ethyl trifluoroacetate upon core electron ionization at environmentally distinct carbon sites using photoelectron-photoion-photoion coincidence measurements and ab initio electronic structure calculations. For all four carbon ionization sites, the Auger decay weakens the same bonds and transfers the two charges to opposite ends of the molecule, which leads to a rapid dissociation into three fragments, followed by further fragmentation steps. The lack of site specificity is attributed to the character of the dicationic electronic states after Auger decay instead of a fast equilibration of internal energy.

In this experimental note, we consider the centrifugal instability of a laminar shear layer, generated by the impulsive start of the rotation of a circular solid cylinder about its vertical axis immersed in a linearly stratified fluid. The flow is determined by the Reynolds number, Re, based on the cylinder rotation rate and the cylinder radius, and the Froude number, F r , represented by the ratio of the rotation frequency Ω over the buoyancy frequency N. The onset of the instability starts when the boundary layer reaches a certain thickness. We show for this boundary layer that there is a transition from the centrifugally unstable regime to a wave-like regime at Fr ≈ 1 and a stable flow below a critical Reynolds number. We focus on the centrifugally unstable regime F r 1, for which the onset time and wavelength are predicted by scaling laws that depend on the Reynolds number. Agreement with the theoretical prediction of Kim and Choi ["The onset of instability in the flow induced by an impulsively started rotating cylinder," Chem. Eng. Sci. 60, 599-608 (2005)] in a homogeneous fluid confirms that the instability of this boundary layer is not modified by the presence of stratification. These results therefore show that the centrifugal instability of the spin-up boundary is dominated by inertial motions, suggesting that close lateral boundaries, as in thin-gap stratified Taylor-Couette flow, increase the effects of buoyancy on the instability and wavelength.

In this research work, we report on the numerical predictions and analysis of stable, stationary and closed burner-stabilized reacting fronts under terrestrial-gravity conditions for ultra-lean hydrogen-methaneair premixed mixtures with a 40% hydrogen (H 2 ) and 60% methane (CH 4 ) fuel composition, specified on a molar basis. The transition from a cap-like to ball-like flame shape with decreasing inlet equivalence ratio is predicted in agreement with experimental observations. The predicted flames are compared to both flames that were studied in experiments and numerical solutions of perfectly-spherical flame balls in the absence of gravity and convection. The comparison includes flame size, lean limits, and when pertinent, standoff distances, all for two different reaction mechanisms. The absolute molar consumption rates of both H 2 and CH 4 for the limit flame attain maximum values that are significantly larger than those of the corresponding gravity-free flame ball. The fuel supply mechanism of the normal-gravity limit flame is similar to the fuel supply of flame balls in that it is driven by diffusion even away from the flame front. Heat conduction to the tube wall of the burner and convective heat loss are the dominant forms of heat loss. Furthermore, simulations with inclusion of multicomponent transport and Soret and Dufour effects show that the flame size increases for both flame balls and the burner-stabilized flames. For the latter, a slight modification in the stabilization position is found owing to the intensification of the consumption rates of both H 2 and CH 4 when these effects are accounted for. In summary, the present work considers a new configuration that allows the study of stable and stationary ball-like flames at ultra-lean and nearlimit conditions, and advances the understanding of such flames via detailed numerical computations.

A predictive understanding of soft x-ray near-edge absorption spectra of small molecules is an enduring theoretical challenge and of current interest for x-ray probes of molecular dynamics. We report the experimental absorption spectrum for the ESCA molecule (ethyl trifluoroacetate) near the carbon 1s absorption edge between 285-300 eV. The ESCA molecule with four chemically distinct carbon sites has previously served as a theoretical benchmark for photoelectron spectra and now for photoabsorption spectra. We report a simple edge-specific approach for systematically expanding standard basis sets to properly describe diffuse Rydberg orbitals and the importance of triple excitations in equation-of-motion coupled-cluster calculations of the energy interval between valence and Rydberg excitations.

We report on the design and performance of a velocity map imaging (VMI) spectrometer optimized for experiments using high-intensity extreme ultraviolet (XUV) sources such as laser-driven high-order harmonic generation (HHG) sources and free-electron lasers (FELs). Typically exhibiting low repetition rates and high single-shot count rates, such experiments do not easily lend themselves to coincident detection of photo-electrons and -ions. In order to obtain molecular frame or reaction channel-specific information, one has to rely on other correlation techniques, such as covariant detection schemes. Our device allows for combining different photo-electron and -ion detection modes for covariance analysis. We present the expected performance in the different detection modes and present the first results using an intense high-order harmonic generation (HHG) source. of their impact coordinates on the detector, as well as from the flight time, acquired by means of, e.g., delay-line detectors. REMI gives access to complete, correlated, 3D velocity information of all particles detected in coincidence, as long as the count rates are sufficiently low (<1 event/shot) to avoid detecting fragments from more than one target atom or molecule on the same shot.With high-intensity sources, the repetition rates are typically low, while the single-shot count rates can be very high, which in many cases makes it difficult to use techniques relying on detecting coincidences. Another approach is to use the so-called velocity map imaging (VMI) technique [21], which uses an extraction field configuration that makes the impact coordinates on the detector independent of the location of the ionization event within the interaction volume, as well as of the momentum along the detector axis. This allows for the use of a micro-channel plate (MCP) and a phosphor screen where the impact of a large number of particles can be accumulated on every shot. Under the condition of cylindrical symmetry of the ionization process, the initial three-dimensional momentum distribution of the particles can be recovered from the measured two-dimensional projection using numerical inversion procedures [22,23].There are several demonstrations of using VMI, under low count rate conditions, for photoelectron-photoion coincidence spectroscopy (PEPICO) [24], for example using electron VMI combined with ion time-of-flight (TOF), ion VMI together with electron spectroscopy and electron VMI with ion VMI [25][26][27][28][29].An obvious drawback of VMI in high count rate conditions is that one cannot rely on coincidence detection for extracting information about different electrons or ions coming from the same target molecule. An elegant way to overcome this lack of correlated information in VMI, without sacrificing the high count rates, is to use covariance mapping [30]. Briefly, for any two variables, X ], sampled synchronously in a repetitive measurement, one can calculate the covariance, which is a measure of how well correlated the variations of the two variables are. C...