Intermolecular energy transfer in two-channel unimolecular reactions: the pyrolysis of 1-iodopropane

Pressure-dependent unimolecular reaction rate coefficients have been obtained for the two channels of decomposition of 1-iodopropane (dilute in Ne), using very low-pressure pyrolysis (VLPP). The interpretation, taking finite diffusion rates into account, gives convincing evidence for “weak” gas/gas collisions and “strong” gas/wall collisions.     

“Atomic Bremsstrahlung”: Retrospectives,current status and perspectives

We describe here the “Atomic bremsstrahlung” (AB)—emission of continuous spectrum electromagnetic radiation, which is generated in collisions of particles that have internal deformable structure that includes positively and negatively charged constituents. The deformation of one or both colliding partners induces multiple, mainly dipole, time-dependent electrical moments that become a source of radiation. The history of AB invention is presented and its unusual in comparison to ordinary bremsstrahlung (OB) properties, are discussed. As examples, fast electron atom, non-relativistic and relativistic collisions are considered. Attention is given to ion–atom and atom–atom collisions. Specifics of “elastic” and “inelastic” (i.e. radiation accompanied by destruction of collision partners) AB will be mentioned. Attention will be given to possible manifestation of AB in nature and in some exotic systems, for instance scattering of electrons upon muonic hydrogen. Some cooperative effects connected to AB will be considered. New classical schemes similar to AB will be presented.     

“Efficiency” of collisions in a microwave infrared four-level system in ammonia

The increase of microwave lines of NH₃ which do not share a common level with the infrared pumped transition has been observed and interpreted as due to “β type” collisions.The authors give an explanation based upon the “truly resonant collisions” hypothesis to explain the breadth of the monitored microwave signal.     

Trajectory calculation results on the orientation of a diatomic molecule in collision with an atom

Orientational effects in collisions between a diatomic molecule and a noble gas atom are studied by classical trajectory calculations. A preferable ”perpendicular orientation“ of the molecule is found at the moment of closest approach of the atom. This orientational effect is more pronounced in collisions with heavy atoms.     

Electronic structure of molecules using one-component wave functions and relativistic effective core potentials

A one-component approach to molecular electronic structure is discussed that includes the dominant relativistic effects on valence electrons and yet allows the use of the traditional quantum-chemistry techniques. The approach starts with one-component Cowan–Griffin relativistic orbitals that successfully incorporate the effects of the mass-velocity and Darwin terms present in more complicated wave functions such as the Dirac–Hartree–Fock. The approach then constructs “relativistic” effective core potentials (RECPS ) from these orbitals, and uses these to bring the relativistic effects into the molecular electronic calculations. The use of effective one-electron spin-orbit operators in conjunction with these one-component wave functions to include the effects of spin-orbit coupling is discussed. Applications to molecular systems involving heavy atoms and comparisons with available spectroscopic data on molecular geometries and excitation energies are presented. Finally, a new approach to the construction of RECPS encompassing the Hamiltonian and shapeconsistent approach is presented together with a novel analysis of the long-range behavior of the RECPS .     

Concept of the “stability” of the quantum mechanical state

A new concept of the “stability” of the quantum mechanical state is presented. This concept is closely related to the hydrodynamical theory of quantum mechanics. For charged particles, the “stability” against the application of the external electromagnetic field is examined. Under a nonlinear interaction, a new type of the solitonlike phenomenon is shown as a novel, illustrative example of the “stable” state. An extension for the relativistic treatment is also examined.     

A new diabatic molecular representation for triplet-triplet transitions in He+-He collisions

A “frozen-orbital” diabatic basis has been constructed for an impact parameter treatment of collisions of He⁺ with metastable He(2³S) at 1000 eV laboratory ion energy. Except for the two highest states used, the diabatic states correlate very well with the separated-atom energies, and the Σ-Π rotational couplings deviate little from the proper asymptotic behaviour (≈R^?2). Cross sections and transition probabilities are presented for some elastic and inelastic channels.     

Sympathetic cooling of trapped negative ions by self-cooled electrons in a fourier transform ion cyclotron resonance mass spectrometer

Hot electrons confined in a Penning trap at 3 tesla self-cool to near room temperature in a few seconds by emission of cyclotron radiation. Here, we show that such cold electrons can “sympathetically” cool, in ～10 s, laser desorbed/ionized translationally hot Au^? or C₇₀ ^? ions confined simultaneously in the same Penning trap. Unlike “buffer gas” cooling by collisions between ions and neutral gas molecules, sympathetic cooling by electrons is mediated by the mutual long-range Coulomb interaction between electrons and ions, so that translationally hot ions can be cooled without internal excitation and fragmentation. It is proposed that electrosprayed multiply charged macromolecular ions can be cooled sympathetically, in the absence of ion-neutral collisions, by self-cooled electrons in a Penning trap.     

On “field” atomic collisions

Some features of atomic collisions in the presence of an external electric field are discussed. It is shown that near the field points of pseudo-intersections of “terms” the probabilities of nonadiabatic transitions increase rapidly. The possibility of field stabilization of Feshbach type resonances is established. The possibility of determining electron—atom parameters by the “field” oscillations of a photodetachment cross section is noted.     

Core/valence partition and relativistic effects in effective potentials for transition metals

Two different versions, relativistic and nonrelativistic, of the “shape-consistent” effective potential (EP ) formalism are analyzed in ab initio calculations for transition metals. The influence of alternative core/valence partitions on the reliability of the procedure is discussed. The accuracy of EP results for transition metals is shown to depend on adequate choices of the valence subspace and proper inclusion of relativistic effects.     

Vibrattonal excitation of molecules by collisions with “hot” hydrogen atoms from laser photolysis

Excimer laser (ArF) photolysis of diatomic and triatomic hydrides produces hydrogen atoms with translational energies in excess of 15000 cm^?1 per atom. Subsequent collisions of these “hot” atoms with CO₂ and N₂O produces vibrationally excited molecules which can be detected by their characteristic infrared emission.     

Core polarization and relativistic effects competition in the first ionization potentials for some systems in the Cu,Ag, and Au isoelectronic sequences

Single-configuration relativistic Hartree–Fock values of the first ionization potentials for Cu through Kr⁷⁺, Ag through I⁶⁺, and Au through Pb³⁺ are computed in “frozen” and “relaxed core” approximations with and without allowance for core polarization. Effects of polarization of the atomic core by the valence electron are included by introducing a polarization potential in the one-electron Hamiltonian of the valence electron. The core polarization potential depends on two parameters, the static dipole polarizability of the core α and the cut-off radius r₀, which are chosen independently of the ionization potential data. It is demonstrated that by including the core polarization potential with α and r₀ parameters, which are simply chosen instead of being empirically fitted, it is still possible to account, on the average, for at least 70% of the discrepancy between the single-configuration relativistic Hartree–Fock ionization potentials and the experiment, a discrepancy usually ascribed to the contribution of valence-core electron correlations, and to bring the theoretical ionization potentials to an average agreement with experiment of around 1%. It can be concluded from this study that for low and medium Z elements the core polarization dominates for neutral systems or systems in low ionization stages, whereas for highly ionized systems the relativistic effects prevail. For heavy elements, however, the core polarization influence is comparable to the relativistic one only for neutral systems, whereas for ions the relativistic effects are overwhelmingly predominant.     

Relativistic effects in transitions of highly ionized heavy atoms

Wavelenghts, oscillator strengths, and lifetimes of KL shell heavy ions such as Fe²⁰⁺, Br³⁰⁺, etc. are predicted, including both relativistic and correlation effects. In such ions the distinction between “normal” electric dipole (El) and spin—orbit allowed electric dipole transitions (SOAEl) disappears and the Z-dependence of oscillator strengths is very different from the predictions of non-relativistic orbital theories. The feasibility of observing these ions in heavy ion accelerators is indicated. The results have implications for relativistic many-electron theory, ESCA, X-ray spectra, and molecular total energies.     

Fermionic and bosonic coupled channel calculations in momentum space for electron-positron pair creation in relativistic heavyy-ion collisions

Electron-positron pair creation in relativistic heavy ion collisions is non-perturbatively described by coupled channel equations in a coordinate system with equal constant, but opposite velocities of the ions. The basis states are obtained by discretizing the states of the free Dirac equation in the positive and negative continuum. With the standard (fermionic) coupled channel method the free pair production in Pb⁸²⁺ on Pb⁸²⁺ atE _1ab=10 GeV/nucleon is calculated. In order to treat multiple pair production with a reduced number of states we introduce a bosonic formalism for the electron-positron pairs in the framework of coupled channels including the rescattering of the electrons and positrons by the electromagnetic field of the ions. We apply this formalism to collisions of Pb⁸²⁺ on Pb⁸²⁺ atE _1ab=200 GeV/nucleon.     

Why is there a molecular relativistic effect?

The almost exclusive association of the molecular geometry dependence of the relativistic correction with the valence orbital contribution to the mass-velocity and Darwin terms is investigated using SCF and MCSCF wavefunctions. The requirement of orthogonality of the valence orbitals to the core orbitals is confirmed to be the mechanism responsible for the increase in (the absolute value of) relativistic energy upon decrease of the internuclear distance. Certain “fingerprint”-type features of the valence relativistic correction, revealing the identity of the particular core orbital giving rise to it, are identified.     

Intermolecular motion in strong infrared laser fields

The effect of a strong infrared laser field on the collision between two rare gas atoms is examined as a model of intermolecular motion under such conditions. After examination of the classical collision dynamics, three novel classes of collisions are identified, all of which result from interaction with the angular potential well. The signatures of these “collision mechanisms” in the classical deflection function and differential cross-section are determined. The generality of the results and the feasibility of using a modified crossed molecular beam experiment to observe these laser-induced effects are discussed.     

Transfer of state multipoles in excited A 1Σ+u7Li2 following rotationally inelastic collisions with He: Experiment and theory

Circularly polarized dye laser radiation is used to prepare rotational levels j = 1 to j = 20 of the A ¹Σ⁺_u excited state of ⁷Li₂ with well defined values of the state multipoles K = 0, 1 and 2. Inelastic collisions with helium atoms populate other j levels and we have measured the circular polarisation ratio of emission, C, from these levels. C is plotted versus final j′ for each value of Δj from +2 to +18 and a family of curves is obtained which may be used as a critical test of current theories. The results are interpreted in terms of cross sections σ^K for transfer of the state multipoles under isotropic collision conditions. The observation of substantial polarisation following inelastic collision indicates that the σ^K are dominated by certain restricted scattering channels. Relative magnitudes of the multipole cross sections are calculated using the “l-dominant”, “restricted Δm_j channels” nd the Born approximation. These cross sections are then used to calculate C.     

Born-Floquet theory of electron-atom collisions in the presence of a laser field

We present a theory of fast electron-atom collisions in the presence of a strong laser field, which treats the interaction of the laser field with both the projectile electron and the target atom in a fully non-perturbative way. The theory is illustrated by considering the laser-assisted “elastic” scattering of fast electrons by atomic hydrogen, for non-resonant as well as resonant cases.     

Angular momentum decoupling approximations in the quantum dynamics of reactive systems

Two methods for implementing angular momentum decoupling approximations in quantum mechanical reactive scattering examined. Applications of both reactive and nonreactive H + H₂ collisions indicate that for the most intense individual reactive transitions and for all degeneracy-averaged ones, these decoupling methods (especially the “proper” decoupling method) yield results in good agreement with those of fully-coupled calculations. However, for the less intense reactive transitions and all individual nonreactive transitions, very large errors can result from use of these approximate methods.