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Continuum photoabsorption of lithium chloride (LiCl) was investigated using a coupled-channel time-dependent wave packet approach. Photodissociation cross sections for the production of ground-state Li and Cl atoms were computed up to temperatures of 1500 K for a thermal distribution of rotational levels. At such temperatures, LiCl is believed to be the primary Li-bearing gas in cool stellar atmospheres. Narrow Rydberg resonances in the total absorption spectrum are found to dominate the thermally averaged cross section due to the large density of Rydberg states in the predissociation gap. Comparison with measured photoabsorption cross sections, where available, is made. [ABSTRACT FROM AUTHOR]

Quasiclassical trajectory and quantum-mechanical scattering calculations are reported for the O(³P)+H2(X ¹Σg + ; υ=1-3, j=0)→?OH(X ²II)+H(²S) reaction at energies close to the reaction threshold. The dynamics of the reaction have been investigated for zero total angular momentum using the lowest ³A" potential-energy surface developed by Rogers et al. [J. Phys. Chem. A 104, 2308 (2000)] and its recent extensions by Brandão et al. [J. Chem. Phys. 121, 8861 (2004)] which provide an improved description of the van der Waals interaction. Good agreement is observed for this system between quasiclassical and quantal results for incident kinetic energies above the tunneling regime. Quantum-mechanical calculations also confirm recent theoretical predictions of a strong collision-energy dependence of the OH(υ'=0)/OH(υ'=1) product branching ratio in the O(³P)+H2(υ=1) reaction, which explains the differences observed in OH vibrational populations between experiments using different O(³P) sources. [ABSTRACT FROM AUTHOR]

The role of van der Waals forces in O(3P)+H2(υ=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial 3A″ potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandão et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures. [ABSTRACT FROM AUTHOR]

The H+LiF(X 1Σ+,υ=0-2,j=0)→HF(X 1Σ+,υ′,j′)+Li(2S) bimolecular process is investigated by means of quantum scattering calculations on the chemically accurate X 2A′ LiHF potential energy surface of Aguado et al. [A. Aguado, M. Paniagua, C. Sanz, and J. Roncero, J. Chem. Phys. 119, 10088 (2003)]. Calculations have been performed for zero total angular momentum for translational energies from 10-7 to 10-1 eV. Initial-state selected reaction probabilities and cross sections are characterized by resonances originating from the decay of metastable states of the H...F-Li and Li...F-H van der Waals complexes. Extensive assignment of the resonances has been carried out by performing quasibound states calculations in the entrance and exit channel wells. Chemical reactivity is found to be significantly enhanced by vibrational excitation at low temperatures, although reactivity appears much less favorable than nonreactive processes due to the inefficient tunneling of the relatively heavy fluorine atom strongly bound in van der Waals complexes. [ABSTRACT FROM AUTHOR]

Quantum-mechanical calculations are reported for the Li+HF(v=0,1,j=0)→H+LiF(v′,j′) bimolecular scattering process at low and ultralow temperatures. Calculations have been performed for zero total angular momentum using a recent high-accuracy potential-energy surface for the X2A′ electronic ground state. For Li+HF(v=0,j=0), the reaction is dominated by resonances due to the decay of metastable states of the Li...F–H van der Waals complex. Assignment of these resonances has been carried out by calculating the eigenenergies of the quasibound states. We also find that while chemical reactivity is greatly enhanced by vibrational excitation, the resonances get mostly washed out in the reaction of vibrationally excited HF with Li atoms. In addition, we find that at low energies, the reaction is significantly suppressed due to the less-efficient tunneling of the relatively heavy fluorine atom. [ABSTRACT FROM AUTHOR]

Ab initio configuration interaction calculations have been performed for the X [sup 1]Σ[sup +] and B [sup 1]Σ[sup +] electronic states of LiCl. Potential energy curves, dipole moment functions, and dipole transition moments have been computed for internuclear distances between R=2.5a[sub 0] and 50a[sub 0]. Single- and double-excitation configuration interaction wave functions were constructed using molecular orbitals obtained from a two-state averaged multiconfiguration self-consistent-field calculation. This procedure yielded an accurate energy splitting between the covalent and ionic separated-atom limits. The calculated avoided crossing of the X and B state curves occurs at R=16.2a[sub 0], in close agreement with previous calculations using a semiempirical covalent–ionic resonance model. X [sup 1]Σ[sup +] state spectroscopic constants are in excellent agreement with experimental values.© 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Comprehensive absorption line lists for [SUP40]CaH have been calculated for electronic transitions from the X [SUP2]∑[SUP+] ground state to the A [SUP2]Π, B/B' [SUP2]∑[SUP+], and E [SUP2]Π low-lying excited states. The lists include transition energies and oscillator strengths for all possible allowed transitions and were computed using the most recent set of theoretical potential energy curves and dipole transition moment functions, with adjustments to account for experimental dissociation energies and asymptotic limits. Good agreement with previous calculations and available experimental data has been obtained. Oscillator strengths for the transition from the X [SUP2]∑[SUP+] state to the D [SUP2]∑[SUP+] state are also given, but due to the large uncertainty of the available dipole transition moment function the line list is highly uncertain. For the C [SUP2]∑[SUP+]← X [SUP2]∑[SUP+] transition, a Franck--Condon approximation has been used. [ABSTRACT FROM AUTHOR]

In this work, we report total cross sections for the single electron capture process induced on DNA/RNA bases by high-energy protons. The calculations are performed within both the continuum distorted wave and the continuum distorted wave-eikonal initial state approximations. The biological targets are described within the framework of self-consistent methods based on the complete neglect of differential overlap model whose accuracy has first been checked for simpler bio-molecules such as water vapour. Furthermore, the multi-electronic problem investigated here is reduced to a mono-electronic one using a version of the independent electron approximation. Finally, the obtained theoretical predictions are confronted with the scarcely available experimental results. [ABSTRACT FROM AUTHOR]

Among the numerous constituents of eukaryotic cells, theDNAmacromolecule is considered as the most important critical target for radiation-induced damages. However, up to now ion-induced collisions on DNA components remain scarcely approached and theoretical support is still lacking for describing the main ionizing processes. In this context, we here report a theoretical description of the proton-induced ionization of the DNA and RNA bases as well as the sugar-phosphate backbone. Two different quantummechanical models are proposed: the first one based on a continuum distorted wave-eikonal initial state treatment and the second perturbative one developed within the first Born approximation with correct boundary conditions (CB1). Besides, the molecular structure information of the biological targets studied here was determined by ab initio calculations with the Gaussian 09 software at the restricted Hartree-Fock level of theory with geometry optimization. Doubly, singly differential and total ionization cross sections also provided by the two models were compared for a large range of incident and ejection energies and a very good agreement was observed for all the configurations investigated. Finally, in comparison with the rare experiment, we have noted a large underestimation of the total ionization cross sections of uracil impacted by 80 keVprotons, whereas a very good agreementwas shown with the recently reported ionization cross sections for protons on adenine, at both the differential and the total scale. [ABSTRACT FROM AUTHOR]

The chemistry of technetium is being explored at the University of Nevada Las Vegas. Our goal is to investigate both the applied and fundamental aspects of technetium chemistry, with a special emphasis on synthesis, separations, and materials science. The synthetic chemistry focuses on metal–metal multiple bonding, oxides and halides. Synthesis and characterizations of (n-Bu4N)2Tc2X8, Tc2(O2CCH3)4X2 ( X = Cl, Br), TcO2, Bi2Tc2O7, Bi3TcO8, TcBr3 and TcBr4 have been performed. The applied chemistry is related to the behavior of Tc in the UREX process. Separation of U/Tc has been conducted using anion exchange resin and metallic Tc waste form synthesized and characterized. [ABSTRACT FROM AUTHOR]

Ab initio calculations of low-lying electronic states of CrH are presented, including potential energies, dipole and transition dipole moment (TDM) functions, and radiative lifetimes for X 6Σ+, A 6Σ+, 3 6Σ+, 1 6Π, 2 6Π, 3 6Π, and 6Δ. Calculation of dynamic correlation effects was performed using the multistate complete active space second-order perturbation method, based on state-averaged complete active space self-consistent-field reference wave functions obtained with seven active electrons in an active space of 16 molecular orbitals. A relativistic atomic natural orbital-type basis set from the MOLCAS library was used for Cr. Good agreement is found between the current calculations and experiment for the lowest two 6Σ+ states, the only states for which spectroscopic data are available. Potential curves for the 3 6Σ+ and 2 6Π states are complicated by avoided crossings with higher states of the same symmetry, thus resulting in double-well structures for these two states. The measured bandhead T0=27 181 cm-1, previously assigned to a 6Π←X 6Σ+ transition, is close to our value of T0=28 434 cm-1 for the 2 6Π state. We tentatively assign the ultraviolet band found experimentally at 30 386 cm-1 to the 3 6Π←X 6Σ+ transition for which the computed value is 29 660 cm-1. The A 6Σ+←X 6Σ+ TDM and A 6Σ+ lifetimes are found to be in reasonable agreement with previous calculations. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ionization and fragmentation of uracil molecules (C4H4N2O2, m = 112 amu) in collisions with fast highly charged C, O and F ions have been investigated using a time-of-flight mass spectrometer. The measurement of total ionization cross sections (TCS) is reported for different charge states (q), such as Fq+ with q = 5-8; Oq+ with q = 5,7; Cq+ with q = 5 and 6. These studies reveal a (q/v)~1.5 dependence of TCS, in contrast, to the well-known q2-dependence in ion-atom collisions. Scaling properties of the TCS with projectile energy and charge states are obtained. The experimental results for TCS measurements are compared with the theoretical calculations performed within classical and quantum mechanical frameworks. The trends in energy dependence of the TCSs is qualitatively well reproduced by the different models and more specifically by the classical description, which provides the best agreement with measurements. [ABSTRACT FROM AUTHOR]

We investigate the (0001) surface of single crystal quartz with a submonolayer of Rb adsorbates. Using Rydberg atom electromagnetically induced transparency, we investigate the electric fields resulting from Rb adsorbed on the quartz surface, and measure the activation energy of the Rb adsorbates. We show that the adsorbed Rb induces negative electron affinity (NEA) on the quartz surface. The NEA surface allows low energy electrons to bind to the surface and cancel the electric field from the Rb adsorbates. Our results will be important for integrating Rydberg atoms into hybrid quantum systems, as fundamental probes of atom-surface interactions, and for studies of 2D electron gases bound to surfaces. [ABSTRACT FROM AUTHOR]