M. Čížek

Charge transport through a flexible molecular junction

Vibrationally inelastic electron transport through a flexible molecular junction is investigated. The study is based on a mechanistic model for a biphenyl molecule between two metal electrodes. Employing methods from electron-molecule scattering theory, which allow a numerically exact treatment, we study the effect of vibrational excitation on the transmission probability for different parameter regimes. The current-voltage characteristic is analyzed for different temperatures, based on a Landauer-type formula. Furthermore, the process of electron assisted tunneling between adjacent wells in the torsional potential of the molecule is discussed and the validity of approximate methods to describe the transmission probability is investigated.

Resonances and Threshold Phenomena in Low-Energy Electron Collisions with Hydrogen Halides: New Experimental and Theoretical Results

Low-energy electron collisions with HCl and HBr and the deuterated compounds have been investigated by experimental and theoretical methods. New experimental results have been obtained on relative differential cross-sections for vibrational excitation and dissociative electron attachment. Measurements with high energy resolution for rotationally cooled molecules have revealed, in addition to shape resonance, threshold peaks and Wigner cusps, the existence of surprisingly sharp oscillatory structures in the elastic and v = 0 → 1 vibrational excitation cross-sections in a narrow range below the dissociative attachment threshold. The theoretical analysis is based on an improved nonlocal resonance model which has been constructed on the basis of ab initio fixed-nuclei scattering phase shifts for HCl and HBr and accurate ab initio calculations of the bound part of the HCl− and HBr− potential-energy functions. The high degree of agreement which has been obtained between experiment and theory for all channels indicates that the mechanisms responsible for the rich threshold structures in the collision cross-sections are completely understood.

Interaction of O− and H2 at low temperatures

Reactive collisions between O(-) and H2 have been studied experimentally at temperatures ranging from 10 K to 300 K using a cryogenic radiofrequency 22-pole ion trap. The rate coefficients for associative detachment, leading to H2O + e(-), increase with decreasing temperature and reach a flat maximum of 1.8 × 10(-9) cm(3) s(-1) at temperatures between 20 K and 80 K. There, the overall reaction probability is in good agreement with a capture model indicating efficient non-adiabatic couplings between the entrance potential energy surfaces. Classical trajectory calculations on newly calculated potential energy surfaces as well as the topology of the conical intersection seam leading to the neutral surface corroborate this. The formation of OH(-) + H via hydrogen transfer, although occurring with a probability of a few percent only (about 5 × 10(-11) cm(3) s(-1) at temperatures 10-300 K), indicates that there are reaction paths, where electron detachment is avoided.

Dissociative electron attachment to HBr: A temperature effect

The effects of rovibrational temperature on dissociative electron attachment to hydrogen bromide has been investigated from the experimental and theoretical point of view. Theoretical calculations based on the nonlocal resonance model predict a strong temperature effect on the Br{sup -} fragment ion yield due to population of higher vibrational and rotational states. A crossed beam experimental setup consisting of a temperature controlled effusive molecular beam and a trochoidal electron monochromator has been used to confirm this prediction. The high degree of agreement between experiment and theory indicates the validity of the theoretical model and its underlying physical picture.

Dissociative attachment of low-energy electrons to vibrationally excited hydrogen molecules

Dissociative electron attachment to hot hydrogen molecules is studied in the framework of nonlocal resonance model. The method based on the use of the Bateman approximation, well known in nuclear physics, is adapted for solving the Lippmann-Schwinger integral equation of the nonlocal resonance model and applied to the calculation of cross sections of inelastic resonant electron-molecule collisions. The proposed method is compared with the Schwinger-Lanczos algorithm used extensively for the treatment of these processes. It is shown that the Bateman approximation is very useful and efficient for treating the non-separable nonlocal potentials appearing in the integral kernels of the nonlocal resonance models. The calculated cross sections for the dissociative attachment of electrons to vibrationally excited hydrogen molecules are of importance for astrophysics.

Model of electron tunneling through molecular junction: Numerically exact solution and performance of approximation methods

We propose a model of the electron tunneling through a molecular junction, with torsional vibrational motion of the molecule coupled to the electron. The quantum dynamics for this two dimensional model is solved numerically by expansion of the wave-function in the Fourier series in the vibrational coordinate and the inversion of the system of equations resulting from the integral Lippmann-Schwinger equation. The fast convergence of this spectral method is observed and essentially exact solution is obtained. The resulting transmission functions are discussed in different regimes and the performance of some common approximation techniques (frozen vibrations, Chase (adiabatic) approximation, method of the local complex potential) is tested.

Improved nonlocal resonance model for electron – HCl collisions

We present an improved nonlocal resonance model for electron-HCl collisions. The short-range part of the model is based on ab-initio electron scattering eigenphase sums calculated using the Schwinger Multichannel method and the long-range part on the potential curve of the bound HCli anion. We have calculated cross sections for vibrational excitation and dissociative electron attachment. For all collision processes where quantitative experimental data are available, the new model agrees with experiments better than the previous model.

Long‐lived states of molecular hydrogen anion

The existence of long‐lived states (of the order of microseconds) of the molecular hydrogen anion H2− is discussed both from theoretical and experimental points of view. The history of experimental search for these states is briefly reviewed and a theoretical explanation based on the use of the nonlocal resonance model offered. Final unambiguous confirmation of the existence by means of the accelerator mass spectrometry and mass spectrometry and the measurement of their lifetimes in electrostatic ion‐beam trap is described.

Resonances in low-energy rare-gas atom scattering

Precise values of energies and widths of low-lying resonances in some rare-gas atom systems (Ar-Ar, Ne-Kr, Ne-Xe) are reported.