REACTIONS OF SODIUM CLUSTERS WITH OXYGEN MOLECULES

In a crossed molecular beam experiment, the reactive scattering of sod ium clusters Na-n (n less than or equal to 33) with molecular oxygen O -2 is investigated. By measuring the angular and the velocity distribu tions of the scattered reaction products, direct information on the re action mechanisms is obtained. The sodium clusters are generated in a supersonic expansion of sodium vapor from an oven with a refilling sys tem with argon carrier gas. The products are detected by photoionizati on at a wavelength of 308 nm and mass analyzed in a time-of-flight mas s spectrometer (TOF-MS). In addition, a fast chopper with a pseudorand om sequence modulating the sodium cluster beam allows us to determine at the same time the velocity distributions of the products. The scatt ering of Na clusters with O-2 shows two series of reaction products. T he monoxides NanO are centered around Na3O with 2 less than or equal t o n less than or equal to 5, while the metal-rich dioxides NanO2 are d etected for 3 less than or equal to n less than or equal to 32 with va rying intensities and a maximum at Na5O2. For all products, the measur ed angular and velocity distributions exhibit forward scattering with a large fraction of the reaction energy deposited into internal excita tion of the products. This is in agreement with the classical harpooni ng model. The general result is explained by the energetic stability o f the product complexes.