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.     

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.     

Dynamic model for vibronic excitation in low energy atom-molecule collisions

Product state distributions from low energy He⁺N₂ and N⁺₂He collisions are presented. These data, obtained from collision-produced emission spectra, show severe deviations from Franck-Condon behavior which cannot be solely the result of molecular distortion arising from electrodynamic effects. A model based on consideration of the short-range interaction between the collision partners is proposed.     

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.     

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.     

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.     

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.     

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.     

Intensity and polarization of light emitted in slow ion-atom collisions

We study the intensity and polarization of light emitted during slow ion-atom collisions. We describe the nuclei as moving along classical trajectories while the electronic rearrangement is treated using time-dependent molecular orbitals. The intensity of emitted light is calculated from the diatomic time-dependent dipole. We evaluate the diatomic dipole matrix elements involving 1s, 2s, and 2p traveling atomic orbitals suitable for time-dependent collision studies. We calculate the intensity and the polarization of light emitted in p + H(1s) collisions at kinetic energies from 10 to 1000 eV, for several impact parameters, changing over time. The emitted intensity goes through a maximum as the collision energy increases and lasts between 10 and 1 fs; the polarized light components parallel and perpendicular to the incoming beam direction show pronounced dependences on impact parameters and time. © 1996 John Wiley & Sons, Inc.     

The theory of direct atom-molecule reactions

A theory is presented for atom-molecule reactions that occur by direct interaction of the incident particle (atom or ion) with atoms in the molecule.     

Resonance structures in polarization bremsstrahlung from electron–atom collisions

Generalized results of the experimental and theoretical investigations on the resonance structure of polarization bremsstrahlung in the collisions of high-energy (nonrelativistic) and intermediate-energy electrons with atoms have been presented. The features of the gas-jet method of generating electromagnetic radiation used in the experimental investigation of polarization bremsstrahlung are considered. The features and regularities of the resonance structure in the differential spectra of X-ray bremsstrahlung have been analyzed as a function of the incoming electron energy.     

Classical dependence of polarization on potential energy surface in rotationally inelastic collisions

Quasiclassicol trajectory calculations have been performed for model potential energy surfaces to investigate polarization in (j,m_j) → (j',m_j) integral cross sections For j = 0, it was found that the occurrence of polarization requires an attractive well, and that it be in the collinear configuration Results for j≠ 0 are also presented.     

Communication: partial polarization transfer for single-scan spectroscopy and imaging

A method is presented to partially transfer nuclear spin polarization from one isotope S to another isotope I by the way of heteronuclear spin couplings, while minimizing the loss of spin order to other degrees of freedom. The desired I spin polarization to be detected is a design parameter, while the sequence of pulses at the two Larmor frequencies is optimized to store the greatest unused S spin longitudinal polarization for subsequent use. The unitary evolution for the case of I(N)S spin systems illustrates the potentially ideal efficiency of this strategy, which is of particular interest when the spin-lattice relaxation time of S greatly exceeds that of I. Explicit timing and pulses are tabulated for the cases for which M ≤ 10 partial transfers each result in equal final polarization of 1/M or more compared to the final I polarization expected in a single transfer for N = 1, 2, or 3 I spins. Advantages for the ratiometric study of reacting molecules and hyperpolarized initial conditions are outlined.     

Theory of direct atom-molecule reactions. Part II

Processes that may be considered as sequences of pair collisions giving rise to the stable molecule AB are discussed on the basis of a general method of representing the cross-section of A+BC AB+ + in the impulse approximation via a mean over the initial state of BC. It is shown that such processes resemble the one-step processes considered in part I in not being purely classical in character (contrary to the assumptions of [2]) and in being substantially dependent on the mode of overlap of the wave functions_BC and _AB even at high collision energies. The reactions T+H₂HT+H, T+D₂ DT+D are considered on the basis of the general method.     

Body-fixed equations for atom-molecule scattering: Exact and centrifugal decoupling methods

We demonstrate an exact method for solving the close-coupled equations for rotationally inelastic scattering in the body-fixed (BF) coordinate representation, using a slight modification of the exponential method which allows application of the known boundary conditions for the space-fixed (SF) coordinate frame. The nature of the coupling in the BF frame suggests approximations which reduce the problem to a set of problems each of a smaller dimensionality. Using an atom-rigid rotor model potential, we have compared the decoupling approximation suggested by Pack and by McGuire and Kouri to the fully coupled problem. We find the centrifugal decoupling (CD) approximation to work best at low total angular momenta J, and for low rotor states j. Opacity functions for inelastic transitions are generally better than ones for elastic transitions, due to errors in the phases of elastic S-matrix elements. However, at high J, CD often severely underestimates the inelastic transition probabilities.     

Proton magnetic resonance imaging with para-hydrogen induced polarization

A major challenge in imaging is the detection of small amounts of molecules of interest. In the case of magnetic resonance imaging (MRI) their signals are typically concealed by the large background signal of e.g. the body. This problem can be tackled by hyperpolarization which increases the NMR signals up to several orders of magnitude. However, this strategy is limited for (1)H, the most widely used nucleus in NMR and MRI, because the enormous number of protons in the body screens the small amount of hyperpolarized ones. Here, we describe a method giving rise to high (1)H MRI contrast for hyperpolarized molecules against a large background signal. The contrast is based on the J-coupling induced rephasing of the NMR signal of molecules hyperpolarized via PHIP and it can easily be implemented in common pulse sequences. We discuss several scenarios with different or equal dephasing times T(2)* for the hyperpolarized and thermally polarized compounds and verify our approach by experiments. This method may open up unprecedented opportunities to use the standard MRI nucleus (1)H for e.g. metabolic imaging in the future.     

Quantum stereodynamics of Li + HF reactive collisions: the role of reactants polarization on the differential cross section

A complete quantum study for the state-to-state Li + HF(v,j,m) → LiF(v',j',Ω') + H reactive collisions has been performed using a wave packet method, for different initial rotational states and helicity states of the reactants. The state-to-state differential cross section has been simulated, and the polarization of products extracted. It is found that the reactivity is enhanced for nearly collinear collisions, which produces a vibrational excitation of HF, needed to overcome the late barrier. It is also found that LiF(v' = 0) products are preferentially forward scattered, while vibrationally excited LiF(v' = 1 and 2) are backward scattered. These results are interpreted with a simple reaction mechanism, based on the late character and bent geometry of the transition state, originating from a covalent/ionic crossing, which consists of two steps: the arrival at the transition state and the dissociation. In the first step, in order to get to the saddle point some HF vibrational excitation is required, which favors head-on collisions and therefore low values of m. In the second step a fast dissociation of H atom takes place, which is explained by the ionic Li(+)F(-)H character of the bent transition state: the FH(-) is repulsive making that H depart rapidly leaving a highly rotating LiF molecule. For the higher energy analyzed, where resonances slightly contribute, the orientation and alignment of product rotational states, referred to as reactants frame (with the z-axis parallel to k), are approximately constant with the scattering angle. The alignment is close to -1, showing that j' is perpendicular to k, while starting from initial states with well defined rotational orientation, as states with pure m values, the final rotational are also oriented. It is also found that when using products frame (with the z'-axis parallel to k') the rotational alignment and orientation of products varies a lot with the scattering angle just because the z' axis changes from being parallel to anti-parallel to k when varying from θ = 0 to π.