Fast fragmentation in core-excited molecules

Symmetry breaking is an important phenomenon connected to the excitation of core electrons in molecules. The fundamental question of whether inner-shell electronic-state orbitals are delocalized, and what the underlying mechanism for breaking the symmetry of a molecule upon dissociation are the subject of much debate, especially for the case of diatomic molecules. One method for studying molecular fragmentation, which is based upon a symmetry picture, is multicoincidence ion spectroscopy. Here we give a brief presentation of some of the applications of this technique, where symmetry, nuclear motion and the temporal signature are of importance.     

Subthreshold tunable OPO: a source of nonclassical light for atomic physics experiments

Received: 11 March 1998/Revised version: 1 April 1998     

Dynamical violation of the Zweig rule and the energy dependence of π−p→ ϕn

The large magnitude and rapid decrease with energy of σ(π^?p→?n) in the momentum range < 6 GeV/c indicate a dynamical breaking of the Zweig rule. We provide an explicit box diagram amplitude for this process. Zweig's rule is obeyed exactly at all vertices. The size and rapid fall of the cross section are explained in terms of s channel thresholds and duality cancellations in the intermediate states of the box.     

Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models. II. Extended Results for Square-Lattice Chromatic Polynomial

We study the chromatic polynomials for m×n square-lattice strips, of width 9m13 (with periodic boundary conditions) and arbitrary length n (with free boundary conditions). We have used a transfer matrix approach that allowed us also to extract the limiting curves when n. In this limit we have also obtained the isolated limiting points for these square-lattice strips and checked some conjectures related to the Beraha numbers.     

Unusual Corrections to Scaling in the 3-State Potts Antiferromagnet on a Square Lattice

At zero temperature, the 3-state antiferromagnetic Potts model on a square lattice maps exactly onto a point of the 6-vertex model whose long-distance behavior is equivalent to that of a free scalar boson. We point out that at nonzero temperature there are two distinct types of excitation: vortices, which are relevant with renormalization-group eigenvalue 1/2 and non-vortex unsatisfied bonds, which are strictly marginal and serve only to renormalize the stiffness coefficient of the underlying free boson. Together these excitations lead to an unusual form for the corrections to scaling: for example, the correlation length diverges as J/kT according to Ae ² (1+be ^– +···), where b is a nonuniversal constant that may nevertheless be determined independently. A similar result holds for the staggered susceptibility. These results are shown to be consistent with the anomalous behavior found in the Monte Carlo simulations of Ferreira and Sokal.     

Observation of a continuum-continuum interference hole in ultrafast dissociating core-excited molecules

The femtosecond dissociation of HCl after core excitation has been studied through the resonant Auger decay. The spectra contain contributions from decay occurring at both "molecular" and "atomic" internuclear distances. We have observed a new interference mechanism in these spectra: An atomic spectral line develops into a negative spectral contribution, a "hole," when detuning the excitation energy from the maximum of the Cl2p(-1)sigma(*) resonance. Resonant x-ray scattering theory quantitatively explains this behavior as due to a novel destructive continuum-continuum interference between molecular and atomic contributions to the Auger decay.     

Doppler splitting of In-flight auger decay of dissociating oxygen molecules: the localization of delocalized core holes

By exploiting the core-excitation-induced dissociation of O2, we find that the Auger emission exhibits a Doppler-like energy shift. We show this to be a manifestation of localization of the core hole and propose that the problem of core-hole localization versus delocalization in core-hole spectroscopies may be resolved by considering the nature of the measurement.     

Clean TROSY: compensation for relaxation-induced artifacts

TROSY pulse sequences for recording, e.g., (1)H-(15)N chemical shift correlation spectra of proteins are designed to select only one of four two-dimensional multiplet components. However, all of the variants published so far are prone to relaxation-induced artifacts at the positions of two of the other multiplet components. This article introduces modifications to the two spin-state-selective coherence transfer building blocks of the TROSY mixing sequence resulting in a clean TROSY spectrum with the artifacts largely suppressed. It works by having the new mixing sequence generate peaks of opposite phase at the positions of the relaxation artifacts. The clean TROSY pulse sequence is marginally shorter than the original one and contains the same pulses. Experimental demonstration is presented for the (15)N-labeled proteins RAP 17-97 (N-terminal domain of alpha(2)-macroglobulin receptor associated protein) and EQT, equinatoxin II, from the Mediterranean anemone Actinia equina.     

The propagation of ultrasound in an austenitic weld

The propagation of ultrasound through an austenitic weld is investigated experimentally as well as in a numerical simulation. The weld is insonified at normal incidence to the fusion line with a longitudinal contact transducer. In order to experimentally trace the ultrasound through the weld, slices of different thicknesses from the original weld have been fabricated. Through-transmission A-scans have then been produced for each weld slice and compared with the corresponding numerical simulation. A comparison of the direction of ultrasound propagation through the weld for the two approaches shows quite good agreement. However, attenuation due to scattering at grain boundaries in the weld is poorly modelled in the simulation. In order to improve this, a better model of the weld is needed.     

Eigenvalue amplitudes of the Potts model on a torus

We consider the Q-state Potts model in the random-cluster formulation, defined on finite   two-dimensional lattices of size L×N$L\times N$ with toroidal boundary conditions. Due to the non-locality of the clusters, the partition function Z(L,N)$Z(L,N)$ cannot be written simply as a trace of the transfer matrix TL${\mathrm{T}}_L$. Using a combinatorial method, we establish the decomposition Z(L,N)=_∑l,Dkb(l,Dk)K_l,Dk$Z(L,N)={\sum }_{l,D_k}{b}^{(l,D_k)}{K}_{l,D_k}$, where the characters K_l,Dk=i_∑N(λi)$K_{l,D_k}={\sum }_i{({\lambda }_i)}^N$ are simple traces. In this decomposition, the amplitudes b(l,Dk)$b^{(l,D_k)}$ of the eigenvalues λi${\lambda }_i$ of TL${\mathrm{T}}_L$ are labelled by the number l=0,1,…,L$l=0,1,\dots ,L$ of clusters which are non-contractible with respect to the transfer (N  ) direction, and a representation Dk$D_k$ of the cyclic group Cl$C_l$. We obtain rigorously a general expression for b(l,Dk)$b^{(l,D_k)}$ in terms of the characters of Cl$C_l$, and, using number theoretic results, show that it coincides with an expression previously obtained in the continuum limit by Read and Saleur.