Observation and analysis of the Xe−I collisional-pair photoassociation spectrum

The interpretation of the photoassociation spectrum arising in collisions of Xe and I atoms is refined with allowance for new data concerning the interaction potential of Xe and I collisional pairs. Spectroscopic constants for the XeI(B) state are determined:R′ _e =3.264961 Å,D′ _e =33,289.05 cm^?1,w′ _e =113.867826 cm^?1, andw′ _e x′ _e =0.238304 cm^?1.     

Phonon density of states in epitaxial FE/CR(0 0 1) superlattices

Incoherent nuclear resonant absorption of synchrotron radiation at the 14.413 keV nuclear resonance of ⁵⁷Fe was employed to measure directly the Fe-projected (partial) phonon density of states (DOS) in epitaxial FeCr(0 0 1) superlattices and in an ⁵⁷Fe_0.03Cr_0.97(0 0 1) alloy film MBE-grown on MgO(0 0 1). The measurements were performed at 300 K with 2.3 meV energy resolution around 14.413 keV. At the interfaces, longitudinal vibrations of Fe atoms are suppressed, and a strong resonance phonon mode appears near 23 meV. This revised version was published online in August 2006 with corrections to the Cover Date.     

Sound velocities of compressed Fe3C from simultaneous synchrotron X‐ray diffraction and nuclear resonant scattering measurements

The applications of nuclear resonant scattering in laser‐heated diamond anvil cells have provided an important probe for the magnetic and vibrational properties of ⁵⁷Fe‐bearing materials under high pressure and high temperature. Synchrotron X‐ray diffraction is one of the most powerful tools for studying phase stability and equation of state over a wide range of pressure and temperature conditions. Recently an experimental capability has been developed for simultaneous nuclear resonant scattering and X‐ray diffraction measurements using synchrotron radiation. Here the application of this method to determine the sound velocities of compressed Fe₃C is shown. The X‐ray diffraction measurements allow detection of microscale impurities, phase transitions and chemical reactions upon compression or heating. They also provide information on sample pressure, grain size distribution and unit cell volume. By combining the Debye velocity extracted from the nuclear resonant inelastic X‐ray scattering measurements and the structure, density and elasticity data from the X‐ray diffraction measurements simultaneously obtained, more accurate sound velocity data can be derived. Our results on few‐crystal and powder samples indicate strong anisotropy in the sound velocities of Fe₃C under ambient conditions.     

Integrating modes of policy analysis and strategic management practice: requisite elements and dilemmas

There is a need to bring methods to bear on public problems that are inclusive, analytic, and quick. This paper describes the efforts of three pairs of academics working from three different though complementary theoretical foundations and intervention backgrounds (ie ways of working) who set out together to meet this challenge. Each of the three pairs had conducted dozens of interventions that had been regarded as successful or very successful by the client groups in dealing with complex policy and strategic problems. One approach focused on leadership issues and stakeholders, another on negotiating competitive strategic intent with attention to stakeholder responses, and the third on analysis of feedback ramifications in developing policies. This paper describes the 10-year longitudinal research project designed to address the above challenge. The important outcomes are reported: the requisite elements of a general integrated approach and the enduring puzzles and tensions that arose from seeking to design a wide-ranging multi-method approach.     

How nitrogenase shakes--initial information about P-cluster and FeMo-cofactor normal modes from nuclear resonance vibrational spectroscopy (NRVS)

Nitrogenase catalyzes a reaction critical for life, the reduction of N(2) to 2NH(3), yet we still know relatively little about its catalytic mechanism. We have used the synchrotron technique of (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to study the dynamics of the Fe-S clusters in this enzyme. The catalytic site FeMo-cofactor exhibits a strong signal near 190 cm(-)(1), where conventional Fe-S clusters have weak NRVS. This intensity is ascribed to cluster breathing modes whose frequency is raised by an interstitial atom. A variety of Fe-S stretching modes are also observed between 250 and 400 cm(-)(1). This work is the first spectroscopic information about the vibrational modes of the intact nitrogenase FeMo-cofactor and P-cluster.     

Nuclear resonance vibrational spectra of five-coordinate imidazole-ligated iron(II) porphyrinates

Nuclear resonance vibrational spectra have been obtained for six five-coordinate imidazole-ligated iron(II) porphyrinates, [Fe(Por)(L)] (Por = tetraphenylporphyrinate, octaethylporphyrinate, tetratolylporphyrinate, or protoporphyrinate IX and L = 2-methylimidazole or 1,2-dimethylimidazole). Measurements have been made on both powder and oriented crystal samples. The spectra are dominated by strong signals around 200-300 cm(-1). Although the in-plane and out-of-plane vibrations are seriously overlapped, oriented crystal spectra allow their deconvolution. Thus, oriented crystal experimental data, along with density functional theory (DFT) calculations, enable the assignment of key vibrations in the spectra. Molecular dynamics are also discussed. The nature of the Fe-N(Im) vibrations has been elaborated further than was possible from resonance Raman studies. Our study suggests that the Fe motions are coupled with the porphyrin core and peripheral groups motions. Both peripheral groups and their conformations have significant influence on the vibrational spectra (position and shape).     

Photoassociation Spectroscopy: Determination of Parameters of Potentials of Diatomic Molecules

Laser-induced fluorescence studied with the help of a tunable dye laser is used to record the photoassociation spectrum of equilibrium mercury atoms in the range of 34 700–37 300 cm^–1. The so-called Franck-Condon structure, which represents periodic variations of absorption intensity on a continuous background, is observed. The structure makes it possible to determine spectroscopic parameters of the upper bound potential and the lower repulsive potential, including the potential with a shallow Van der Waals well. The Numerov-Cooley procedure is used to find the numerical solution of the boundary problem, namely, eigenfunctions and eigenvalues of the Schrödinger equation for the one-dimensional motion in a potential field, matrix elements of transitions and partial waves, and the absorption spectrum of collisional pairs of atoms are calculated. The absorption spectrum of mercury dimers was simulated taking into account 100 vibrational and 200 rotational levels. The comparison of theoretical and experimental spectra according to Tellinghuisen made it possible to determine the lower part of the potential curve for the dimer excited state.     

Van der Waals molecules and collision pairs of noble-gas atoms ahead of a shock wave front

The effect of creation of an excess concentration of free electrons in an anomalously thick layer (≈5 cm) ahead of an explosively driven shock wave in noble gases is discussed and interpreted. This effect is the ionization of excited 1_u-state molecules produced due to the absorption of a small intensity flux (as compared to the ionization one) of photons (with energies substantially lower than the atom ionization threshold) by unexcited colliding complexes and van der Waals molecules. A model is proposed which explains the excitation of xenon molecules ahead of the radiationless shock wave of an open discharge. The absorption spectra of colliding complexes and van der Waals molecules adjacent to each other near the atomic absorption lines can be resolved into two spectra, and these spectra can be changed by an increase in gas temperature. As a result, radiation capable of exciting van der Waals molecules penetrates through the shock wave of an open discharge and excites xenon molecules there. The present work develops further the knowledge concerning the radiation energy transport in the shock wave front. It also proves that in front of an explosively driven shock wave a great number of excited molecules of noble gases are actually formed, and this means considerable progress toward a VUV laser with optical pumping. Translated from Preprint No. 56 of the P. N. Lebedev Physical Institute, Moscow, 1993.