The dominance of Auger-type electron emission in ion-molecule collisions

C. A. TACHINO; M. E. GALASSI; R. D. RIVAROLA

París (Francia)

Congreso; International conference on many particle spectroscopy of atoms, molecules, clusters and surfaces; 2008

     As a consequence of electron ionization of amolecular target in a collision process, vacancies in different molecular orbitals are created,leading the target in a highly excited state. After a time delay, once the projectile is far away,the vacancies in the inner orbitals are filled byresidual target electrons, and the excess of energy generated in this process is employed torelease x–ray photons (radiative transitions) orto produce ionization of other bound molecular electrons (Auger–type emission)[1–3].     The knowledge of autoionization and radiative probabilities have great importance indifferent scientific areas, such as astrophysicsand biological physics (see, for example, refs.[4, 5]).     The existing experimental data for autoionization rates and probabilities were obtained mainly for photon impact on noblegases (see, for example, ref. [6]). To ourknowledge experimental or theoretical information on q–fold autoionization probabilities(q> 1) for atomic targets such as C, N and O,or for molecular targets is scarce. Based onexperimental photoionization probabilities [6]and on the work of Spranger and Kirchner[7], we estimate the importance of Auger–typeemission on total multiple ionization cross sections for impact of fast proton beams on diatomic molecules. To this end, we employthe independent electron approximation andbinomial distributions to calculate cross sections for q–ionization degree. The diatomicmolecule is described as an ensemble of twoindependent atoms separated by the corresponding molecular equilibrium distance. Impact parameter probabilities for single ionization are obtained using different approximations based on the Continuum DistortedWave–Eikonal Initial State model.     By comparison with experiments it isshown that, for CO targets, Auger type emission dominates the multiple electron ionizationreaction at high enough collision energies, as itwas previously shown to happen with atomictargets [7–9].References [1] P. Auger, Compt. Rend. 177, 169 (1923). [2] P. Auger, Compt. Rend. 180, 65 (1925). [3] P. Auger, J. Phys. Radium 6, 205 (1925). [4] K. Kobayashi et al., Nucl. Instrum. Meth-     ods Phys. Res. B 199, 348 (2003). [5] N. Tmneanu et al., Chem. Phys. 299, 277     (2004). [6] T. Carlson et al., Phys. Rev. 151, 41     (1966). [7] T. Spranger and T. Kirchner, J. Phys. B     37, 4159 (2004). [8] M. Galassi et al., Phys. Rev. A 75, 052708     (2007). [9] C. Archubi et al., J. Phys. B 40, 943     (2007).[10] E. Wells et al., Phys. Rev. A 72, 022726     (2005).