Recent Publications

Jorge Kohanoff

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  1. Title: Electronic Stopping Power in Gold: The Role of d Electrons and the H/He Anomaly

    Author(s): Zeb M., Kohanoff J.J., Sánchez-Portal D Arnau A, Juaristi J.I., Artacho E.,

    Physical Review Letters, 108, No. 22, pp. 225504-225508 (31 May 2012)

    doi: 10.1103/PhysRevLett.108.225504

    The electronic stopping power of H and He moving through gold is obtained to high accuracy using time-evolving density-functional theory, thereby bringing usual first principles accuracies into this kind of strongly coupled, continuum nonadiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: (i) The nonlinear behavior of stopping power versus velocity is a gradual crossover as excitations tail into the d-electron spectrum; and (ii) the low-velocity H/He anomaly (the relative stopping powers are contrary to established theory) is explained by the substantial involvement of the d electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by s-like electron-hole pair formation only.

  2. Title: Nonadiabatic Forces in Ion-Solid Interactions: The Initial Stages of Radiation Damage

    Author(s): Correa A.A., Kohanoff J.J., Artacho E., Sánchez-Portal D, Caro A.,

    Physical Review Letters, 108, No. 21, pp. 213201-213204 (21 May 2012)

    doi: 10.1103/PhysRevLett.108.213201

    The Born-Oppenheimer approximation is the keystone for molecular dynamics simulations of radiation damage processes; however, actual materials response involves nonadiabatic energy exchange between nuclei and electrons. In this work, time dependent density functional theory is used to calculate the electronic excitations produced by energetic protons in Al. We study the influence of these electronic excitations on the interatomic forces and find that they differ substantially from the adiabatic case, revealing a nontrivial connection between electronic and nuclear stopping that is absent in the adiabatic case. These results unveil new effects in the early stages of radiation damage cascades.

  3. Title: Excess Electron Interactions with Solvated DNA Nucleotides: Strand Breaks Possible at Room Temperature

    Author(s): Smyth M., Kohanoff J.J.,

    Journal of the American Chemical Society, 134, pp. 9122-9125 (18 May 2012)

    doi: 10.1021/ja303776r

    When biological matter is subjected to ionizing radiation, a wealth of secondary low-energy (<20 eV) electrons are produced. These electrons propagate inelastically, losing energy to the medium until they reach energies low enough to localize in regions of high electron affinity. We have recently shown that in fully solvated DNA fragments, nucleobases are particularly attractive for such excess electrons. The next question is what is their longer-term effect on DNA. It has been advocated that they can lead to strand breaks by cleavage of the phosphodiester C3′–O3′ bond. Here we present a first-principles study of free energy barriers for the cleavage of this bond in fully solvated nucleotides. We have found that except for dAMP, the barriers are on the order of 6 kcal/mol, suggesting that bond cleavage is a regular feature at 300 K. Such low barriers are possible only as a result of solvent and thermal fluctuations. These findings support the notion that low-energy electrons can indeed lead to strand breaks in DNA.