The gas phase reactivity and theoretical structures of germanium ions generated by direct laser vaporisation

The ion series Ge-x(+/-),x=1-7, have been generated using direct laser vaporisation and detected using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Optimised non-local spin density approximation-density functional theory (NLSD) structures for these ions and their corresponding neutrals are presented. These results are used to interpret the relative intensity variation in positive-ion and negative-ion mass spectra and deduce the most prevalent neutral species in the laser plume. Ion-molecule reaction studies confirm the transitions to closed cyclic structures, as predicted by the NLSD calculations, at x = 3 for the anions and x = 4 for the cations. The ions most reactive towards CH3OH, CH3CH2OH, H2S and C-C3H6, were found to be the small cations with terminal, electron-rich (radical) Ge centres, while all the anions, with the exception of Ge-2(-), were essentially unreactive. The NLSD results reveal that there is little structural change upon either electron attachment or ionisation of the neutral structures. Small energy separations are predicted between the a(3) Pi(u) and X-3 Sigma(g)(-) states of Ge-2 (6.2 kcal mol(-1)), the a(3)B(1) and X(1)A(1) spin states of Ge-3 (4.6 kcal mol(-1)) and the singlet and tripler spin stares of Ge-5 (about 4 kcal mol(-1) vertical separation). A Rydberg-like ''square'' quartet state of Ge-4(-) was also discovered, 30 kcal mol- higher in energy than the lowest energy D-2h doublet structure. NLSD did not locate minima corresponding to the anticipated (2) Pi(u) ground states of the mixed valence dimers Ge-2(+) and Ge-2(-), instead locating the states (4) Sigma(g)(+)Ge(2)(+) and 2 Sigma(g)(+)Ge(2)(-) both of which have electron vacancies below the HOMO. Collision induced dissociation (CID) results from FTICR-MS studies, and the recent literature laser photoelectron-zero electron kinetic energy (LPES-ZEKE) results for the anions Ge-2(-) through to Ge-2(-) were interpreted using the theoretical results from this study. (C) 1997 Elsevier Science B.V.