THERMAL BIMOLECULAR REACTIONS OF SIZE SELECTED TRANSITION-METAL CLUSTER IONS - NBN+ + O2, N = 1-6

Niobium cluster ions, Nb(n)+, are generated using a new laser desorption source. This source, floated at 5 kV, is coupled to a high resolution reverse geometry mass spectrometer. Mass selected cluster ions exiting the mass spectrometer are decelerated to several electronvolts and spatially focused on a small hole in the entrance plate to a high pressure (0-7 Torr), variable temperature (80-600 K) drift reactor. The cluster ions gently drift through this 4 cm long cell under the influence of small electric fields. A helium buffer gas is used to thermalize the ions. Trace amounts of O2 are added for the reaction studies. Ions exiting the drift reactor are analyzed using a quadrupole mass filter and ion counting techniques. The quadrupole mass filter has an upper mass range of about 600 dalton, limiting the current maximum cluster size to n = 6. All reactions of Nb(n)+ with O2 are found to be fast, proceeding at 40 +/- 10% of the collision rate. For n = 2-6, the major (90%) product results from loss of NbO from the Nb(n)+.O2 collision complex. For n = 3-6, other (small) competing channels include loss of NbO2 and Nb atoms from the collision complex. Fast sequential reactions of the reaction products with O2 are observed. An analysis of the arrival time distribution of Nb+ at the detector indicates laser desorption forms almost-equal-to 80% ground state (4d4, 5D) and 20% excited state (4d(3)5s1, 5F). Collisional deactivation by He occurs at a rate constant << 1 x 10(-15) cm3 s-1.