A quantum-mechanical explanation of vibronic phenomena in atom-molecule collisions

The quantum-mechanical equivalent of a classically vibrating molecular ion during an ion-pair formation collision is presented. The classical vibration in the quantal representation is explained as an interference between partial waves which evolve along neighbouring vibronic states during the collision.     

Rates of vibrational and dissociative excitation of diatomic molecules in atom-molecule collisions in a hot gas

Vibrational relaxation and dissociation in O₂-Ar at low O₂ contents are considered. The populations of the vibrational levels are found as functions of time. The vibrational relaxation time and the dissociation rate constant at 3000 to 20 000 K are calculated. The relaxation equation for the vibrational energy per unit volume in the presence of dissociation is considered.     

A time-dependent quantal analysis of vibronic excitation via charge transfer in ion-molecule collisions

A semi-classical multiple state model describing charge transfer in ion-molecule system is presented. Analytical expressions for the state-to-state transition probabilities are given for both the weak-coupling and the high-velocity limits. The expressions are compared to a two-state model describing charge transfer in ion-atom systems. Some numerical calculations are presented to illustrate the various phenomena which can occur in multiple state charge transfer processes. These numerical calculations will be based upon a simple system derived from the system (Ar + N₂)⁺.     

The use of rotationally and orbitally adiabatic basis functions to calculate rotational excitation cross sections for atom-molecule collisions

Four schemes for constructing rotational-orbital basis sets for rotational close coupling calculations for atom-molecule. collisions are considered and are applied to calculate 0 → 1, 0 → 2, and 0 → 3 rotational excitation cross sections for He+ HF at energies 0.05 and 0.017 eV. Adiabatic basis sets are shown to converge faster than conventional basis sets in all cases; in some cases the difference is very dramatic. Further, l-dominant bases are shown to be useful for the 0 → 2 and 0 → 3 cross sections.     

Spin exchange in low energy electron-ion collisions

The elastic scattering of an electron by a hydrogenic ion of charge (Z-1) is investigated. For scattering energiesE _kin in the eV region, where the Bohr-Sommerfeld parametery is large compared to one, the cross section for spin exchange of the electrons is found to be proportional toZ ^?2 andE ^?1 _kin. Spin exchange is dominated by radiative electron capture forZ>25.     

Electron transfer-induced fragmentation of thymine and uracil in atom-molecule collisions

Ion-pair formation has been studied in hyperthermal (30-100 eV) neutral potassium collisions with gas phase thymine (C(5)H(6)N(2)O(2)) and uracil (C(4)H(4)N(2)O(2)). Negative ions formed by electron transfer from the alkali atom to the target molecule were analysed by time-of-flight (TOF) mass spectrometry. The most abundant product anions are assigned to CNO(-) and (U-H)(-)/(T-H)(-) and the associated electron transfer mechanisms are discussed. Special emphasis is given to the enhancement of ring breaking pathways in the present experiments, notably CNO(-) formation, compared with free electron attachment measurements.     

Classical trajectory study of rotational excitation in low energy HeCO and HeH2 collisions

Classical trajectory calculations for the rotational excitation of CO and H₂ by collision with He have been carried out and compared to the accurate quantum mechanical calculations of other workers. The agreement is reasonably encouranging, although some inherent limitations of this strictly classical approach are observed and discussed.     

Adiabatic and diabatic representations for atom-molecule collisions: Treatment of the collinear arrangement

In this work some aspects of the atom-molecule interactions are extended to include electronic transitions. The main emphasis is directed towards the close relationship between the adiabatic and the diabatic representations. We show how one may transform from the adiabatic scheme to the diabatic one without losing physical information and with minimal amount of numerical efforts. The case of two surfaces (or two electronic states) is treated in particular detail. The main outcome of this study is that, although the electronic information regarding the atom-molecule interaction is given in the adiabatic scheme, one should transform to the diabatic scheme when treating the nuclear interactions.     

A semiclassical model for the vibrational excitation of spherical-top molecules in collisions with ions

A simple theory is presented to explain the previously observed single-mode vibrational excitation of the spherical-top molecules CH₄, CF₄ and SF₆ in collisions with H⁺ and Li⁺. The theory is based on a three-dimensional forced-oscillator model which has been modified to take account of many independent harmonic oscillators. For small-angle collisions the linear driving forces are the dipole-, polarizability- and quadrupole-derivatives taken from IR, Raman spectroscopy and simple estimates, respectively. To explain the results at larger angles near and beyond the rainbow it has been necessary to introduce short-range repulsive forces between the ions and the outer atoms of the molecule. For small angles both the predicted first moment of the energy transfer and the time-of-flight spectra agree quantitatively with the experimental results. At large angles, for which only the first moment of the energy is available, good qualitative agreement is obtained after a slight adjustment of the potential parameters. The energy transfer as a function of time is calculated and shows a different oscillatory behavior for the proton and Li⁺-ion systems. Also the effect of intra-mode coupling is investigated and shown to have only a small effect on the overall energy transfer. The paper closes with a discussion of the implications of these experiments and the possible role of rotational excitation. The field strengths in these ion scattering experiments are shown to be greater than in the strongest focused Q-switched laser pulses.     

Quantum-mechanical theory of atom-molecule and molecular collisions in a magnetic field: spin depolarization

A theory for quantum-mechanical calculations of cross sections for atom-molecule and molecular collisions in a magnetic field is presented. The formalism is based on the representation of the wave function as an expansion in a fully uncoupled space-fixed basis. The systems considered include 1S-atom-2Sigma-molecule, 1S-atom-3Sigma-molecule, 2Sigma-molecule-2Sigma-molecule, and 3Sigma-molecule-3Sigma-molecule. The theory is used to elucidate the mechanisms for collisionally induced spin depolarization.     

Semicalassical vibronic model for the Creutz-Taube ion

Institute of Chemistry, Academy of Sciences of the Moldavian SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No 1, pp. 44–48, January–February, 1991.     

A vibronic model for exchange-coupled mixed-valence dimers

A quantum-mechanical calculation of the spin—vibronic states of an exchange-coupled mixed-valence dimer is presented. It is shown that the temperatur     

Partial differential cross sections for inelastic He++Ne collisions at low energy

We report differential cross section measurements with high angular resolution for different channels of the inelastic processes He⁺+Ne→He⁺+Ne* and He⁺+Ne→He*+Ne⁺, for collision energies between 100 and 200 eV. For the Ne states (2p ⁵3s)^1,3 P ₁, which decay optically, we determined the fraction with the alignment at right angles to the scattering plane. The results are used to discuss the mechanism of the processes and the influence of the spin-orbit interaction upon the collision.     

The nature of the low energy band of the Fenna-Matthews-Olson complex: vibronic signatures

Based entirely upon actual experimental observations on electron-phonon coupling, we develop a theoretical framework to show that the lowest energy band of the Fenna-Matthews-Olson complex exhibits observable features due to the quantum nature of the vibrational manifolds present in its chromophores. The study of linear spectra provides us with the basis to understand the dynamical features arising from the vibronic structure in nonlinear spectra in a progressive fashion, starting from a microscopic model to finally performing an inhomogeneous average. We show that the discreteness of the vibronic structure can be witnessed by probing the diagonal peaks of the nonlinear spectra by means of a relative phase shift in the waiting time resolved signal. Moreover, we demonstrate that the photon-echo and non-rephasing paths are sensitive to different harmonics in the vibrational manifold when static disorder is taken into account. Supported by analytical and numerical calculations, we show that non-diagonal resonances in the 2D spectra in the waiting time, further capture the discreteness of vibrations through a modulation of the amplitude without any effect in the signal intrinsic frequency. This fact generates a signal that is highly sensitive to correlations in the static disorder of the excitonic energy albeit protected against dephasing due to inhomogeneities of the vibrational ensemble.     

Electronic excitation in potassium alkyl iodide collisions

The production of highly excited states of the alkyl iodides by collision with fast potassium atoms has been studied by a time of flight molecular beam technique. A simple electron capture and recapture mechanism is advanced to account for the observations.     

Photoluminescence studies of oligothiophene self-assembled monolayers at low excitation energy

Photoluminescence spectroscopy studies have been performed on self-assembled monolayers (SAMs) on Au(111) of thiophene oligomers with the number of thiophen rings N=3 and N=4. Photoluminescence spectra of SAMs reveal excitonic behavior with different band resolution and temperature dependence. These differences are attributed to different SAMs structure (degree of ordering).     

Nonadiabatic transitions in the exit channel of atom-molecule collisions: fine-structure branching in Na+N2

We study Na+N(2) collisions by laser excitation of the collision complex in a differential scattering experiment. The measured relative population of the Na(3p) fine-structure levels reflects the nonadiabatic transitions occurring in the exit channel of the collision. Theoretical results obtained with a classical-path formalism and accurate quantum chemical data for NaN(2) are found to be in good agreement. The presence of a conical intersection for the T-shaped geometry has a profound influence on the observed fine-structure branching.     

Integral cross sections for electronic excitation in KHg collisions

The energy dependence of the integral cross section for the electronic excitation in collisions of K and Hg is investigated for energies between 50 eV and 1500 eV. By the measurement of the spectra of the emitted light the 4²P_{$\text{3}\text{2}$} and the 4²P_{$\text{1}\text{2}$} states of potassium are found to be dominant. For these the energy dependence of the cross sections is studied in detail. By the measurement of the polarization the contributions to the 4²P_{$\text{3}\text{2}$} state are differentiated with respect to |m_j|.