Why are some ML2 molecules (M = Ca, Sr, Ba; L = H, F, Cl, Br) bent while others are linear? Implications of the pseudo Jahn-Teller effect

The unexpected bent geometries of some alkaline earth dihalides and dihydrides, ML(2) (M = Ca, Sr, Ba; L = H, F, Cl, Br) have been explained in the literature using various models that attribute the effect to different phenomena like covalency, metal core polarization, sd-hybridization, and electron pair repulsion. We employ (based on first principles) the pseudo Jahn-Teller effect, as the only source of instability of high-symmetry configurations in nondegenerate states, to analyze the origin of the geometry of these systems and show that this approach explains all of their main structural features, including the topology of the Laplacian of the electron density and the vibrational frequencies. The main contribution to the distortion of the linear configuration is due to the pseudo Jahn-Teller mixing by bending of the sigma(u) HOMO formed by the ligand orbitals with the unoccupied pig orbitals of the metal (with main d(xz) and d(yz) character), resulting in new covalency which stabilizes the bent configuration. We show that the model approaches to the problem, mentioned above, are either restricted particular cases of the pseudo Jahn-Teller interaction, or they yield very small contributions to the instability that do not explain the origin of the bending. All of our conclusions are supported by high-quality ab initio calculations.     

Pseudo Jahn-Teller origin of cis-trans and other conformational changes. The role of double bonds

Based on the pseudo Jahn-Teller effect (PJTE) theory, an approach is developed to rationalize and predict the conformations and conformational changes in molecular systems with a common pattern, a double bond. It is shown that starting with the high-symmetry geometry of the environment (in many cases D(2d)), the double bond descends from an e(2) electronic configuration (e is a twofold degenerate MO) which produces a variety of PJT distortions, the main of which is the rotational (b(1)) transformation D(2d) → D(2h) accompanied by the formation of the double bond. Further PJT interactions with higher energy E-states may trigger additional distortions which in D(2h) symmetry are classified as in-plane (e(i)) cis and trans, and out-of-plane (e(o)) chair and boat. The realization of these conformations depends on the positions of the excited E-states and the PJTE parameter values. The two emerging PJTE problems, ((3)A(2) + (3)E(1) + (3)E(2)) ? (e(i) + e(o)) and ((1)A(1) + (1)B(1) + (1)B(2) + (1)E(1) + (1)E(2)) ? (b(1) + e(i) + e(o)), are formulated in the matrix form and provide a general picture of the ground and excited adiabatic potential energy surfaces. Following this scheme in combination with ab initio calculations, the possible conformations and conformational transitions are analyzed for several specific systems including (in increasing complexity) N(2)H(2), C(2)H(4), N(2)(NH(2))(2) and N(2)(C(6)H(5))(2) (azobenzene). The family of molecular systems with a double bond is vast, but the importance of the PJT approach developed here is also in its general validity as it can be applied to any other systems.     

Low emittance, high brilliance relativistic electron beams from a laser-plasma accelerator

Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5×101? A m?1 rad?1 makes it suitable for compact XUV FELs.     

Lost topological (Berry) phase factor in electronic structure calculations. Example: the ozone molecule

It is shown that standard computations of electronic structures of polyatomic systems that yield the global minimum configuration and vibrational frequencies may be faulty if the symmetry of this configuration is lower than the highest possible one and the origin of this distortion, which is always due to the Jahn-Teller effect, is neglected; this may lead, in particular, to the loss of the Berry phase factor that changes the vibronic energy level spectrum and which we show to be present even when there are no apparent conical intersections. The general case and the ozone molecule are analyzed.