G. K. Ivanov

Temperature dependence of cryochemical H-tunneling reactions

The temperature dependence of tunneling transition of the atomic particle in solids is studied near absolute zero. The different mechanisms of the temperature dependence are considered. They are the medium reorganization, the potential barrier parameters modulation, and the under-barrier friction The nonadiabatic effects are also considered. The rate constant K is described by formula ln K=ln K0+C4T4+C5T5+C6T6+C8T8 at low temperatures. It was conducted through the comparison of theory with the experimental data from the article of Kumada et al. [Chem. Phys. Lett. 261, 463 (1996)]. It turned out that good agreement takes place if one takes into account the quantum properties of the hydrogen crystal with the assumption of the dominated role of medium reorganization.

Low-temperature dissociative recombination of electrons with and molecular ions

The reaction of dissociative recombination is investigated taking into account the electron and rotation - vibrational coupling. Basic equations are formulated within the framework of the multichannel quantum-defect theory for the collision T-matrix. Combining the rigorous account of nonadiabatic coupling in the closed channels with respect to small vibrational excitation with the contribution of far channels along the energy (considered in the adiabatic approximation), we develop a flexible and fairly simple calculation procedure. A coupling among the closed and open Rydberg channels via a dissociative continuum is studied consistently. Isotopic behaviour of the partial and total cross sections, and rate constants of the reaction were analysed in detail. Comparison with experiment shows a high susceptibility of results to the basic parameter of the theory.

l mixing and dissociation of Rydberg molecules accompanying slow collisions with inert-gas atoms

Processes involving a change of orbital angular momentum l and the dissociation of Rydberg molecules when they collide with inert-gas atoms are considered, using a method based on analyzing the terms of the interacting systems. The proposed method makes it possible to take into account the perturbations in a large group of Rydberg states that weakly penetrate into the ion core and to go beyond the limits of the two-level approximations [R. F. Stebbings and F. B. Dunning, eds., Rydberg States of Atoms and Molecules (Cambridge University Press, Cambridge, 1983; Mir, Moscow, 1985)] widely used in the theory of l mixing with the participation of Rydberg atoms. Using the Na**(nd)+Xe reaction as an example, it is demonstrated that this method gives good results in a wide range of variation of the principal quantum number n of the Rydberg electron (n≈10–40). Features of the l mixing and dissociation of Rydberg molecules are studied for the H2**+Xe system. It is shown that collisional dissociation occurs by the formation of an intermediate Rydberg state of the molecules, followed by self-decay into a dissociative continuum.

Multiphoton ionization of molecules under the conditions of strong field-induced perturbation of Rydberg states

Multiphonon ionization of the H2 molecule under the action of a weak (probe) field, which provides the initial population of the low-lying (working) level, and intense monochromatic linearly polarized radiation is studied. The multiphoton ionization process occurs under the conditions of strong field perturbation of two intermediate Rydberg series, np0(1Σu+ and np2(1Πu), of the optical R(0)branch which have different ionization potentials. The series are occupied simultaneously as a result of single-photon absorption by an excited H2* molecule in the working state 4sσH′1Σg+ (v=0). As a result of the irregularity in the arrangement of the intermediate levels from a large group of states that are combined in the multiphoton ionization process a sharp and irregular change occurs in the dependence of the shifts and widths Γn of the levels on the intensity f of the strong field in a transition from one level to another. It is shown that for field intensities f such that the level widths remain much less than the splitting between the levels (Γn≪/n3) the stabilizing effect (i.e., the field-induced narrowing of the levels as f→∞) in the form Γn ∝ 1/f2 (as happens in atoms with a structureless core) is not observed in molecular systems.

Rydberg Atom A** in a Field of Neutral Atom B

The potential energy curves and wave functions of the Rydberg atom A**‐atom B system are considered in the framework of the finite radius potential (FRP) method with taking into account the short‐range and long‐range interactions of Rydberg electron with perturbing particle. This theoretical approach is applied to the Na**‐He system.

Temperature dependence of the rate constants of cryochemical reactions

The temperature dependence of the reaction rate constant for tunneling transfer of an atomic particle in solid near absolute zero was studied. Different mechanisms describing the temperature dependence were considered: reorganization of the medium, modulation of parameters of the potential barrier, and under-barrier friction. It was established that for the rate constant (K) at low temperatures the equation InK=InK0+C4T4+C5T5+C6T6+C8T8 is valid. Experimental data were compared with the theory. A good agreement is achieved when the quantum nature of the hydrogen crystal is applied under the assumption of a predominant role of reorganization of the medium.

Multilevel rotational transitions in the intermediate stage of three-photon ionization of molecules

Ionization and dissociation of diatomic molecules induced by a weak field (after preliminarily populating an intermediate level) and by intense, linearly polarized monochromatic radiation have been studied. Field-induced mixing of rotational components of various electronic-vibrational states of molecules (such as CO, NO, etc.) at field strength f∼10−4–10−5 atomic units can lead to migration among states with different angular momenta J. Therefore, ions with rotational momenta J+ much higher than those prescribed by selection rules for three-photon absorption can be formed from molecules in the ground state. The possibility of selective formation of ions with J+≫1 and zero projection of the angular momentum on the polarization vector of the external electromagnetic radiation has been investigated.

Excitation of Rydberg States of Atoms and Diatomic Molecules by Short Laser Impulses

Excitation of multilevel Rydberg states (atoms and diatomic molecules) is studied under an intense time-periodic perturbation that is instantaneously turned on and off. For a one-quantum excitation from a lower lying state, general expressions for the Laplace images of the population amplitudes of Rydberg states are obtained with regard to their decay characteristics. It is shown that the problem considered is reduced to the determination of the positions and widths of the energy levels of a quantum system in the field of monochromatic laser radiation of the same frequency and intensity as that in a pulse mode. To determine these quantities, an integral formulation is proposed for the eigenvalue problem for energies that is relatively simply solved with regard to the effect of the ionic core and its complex vibration-rotation structure on diatomic molecules. The specific features of the excitation of Rydberg states and the behavior of Rydberg wave packets are studied depending on the intensity and duration of laser radiation. A quantum phenomenon of rotational orientation of electron-excited diatomic molecules is considered.

Theory of temperature dependence of hydrogen tunneling reactions

Abstract The different possible mechanisms of the temperature dependence of the tunnelling transition of an atomic particle in solids near absolute zero and the non-adiabatic effects are determined and considered. The general expression for the rate constant is obtained, taking into account the quantum properties of hydrogenous crystal. Comparison of the theoretical formula with experimental data [3] has permitted the leading mechanism, i.e. medium reorganization, of temperature dependence to be established. Good agreement has been achieved between theory and the discussed experimental data at reasonable values of the system parameters.

Dissociative recombination of electrons and molecular ions

The existing techniques for the calculation of the dissociative recombination (DR) of electrons and molecular ions were compared. The advantages of the method of multichannel quantum defect (MQD), in which equations are formulated directly for the T-matrix of collisions and the unitarity of the scattering S-matrix is thus ensured, were demonstrated. The effect of molecular rotation and of the nonadiabatic electron-rotation coupling on the e− + H2+ →, H* + H reaction was investigated. A procedure was suggested based on the use of the adiabatic approximation (with respect to the nuclear rotation) in the near-threshold area while taking into account the contributions of the excited vibronic states of the Rydberg complex formed in an intermediate stage of the reaction. It is notable that the partial rate constants (and the corresponding cross-sections) arc very sensitive to the initial rotation excitation. However, the temperature-averaged rate constants under equilibrium conditions are only slightly affected by rotation.