PHOTOCHEMICAL DESORPTION OF MONOATOMIC AL AND NA IONS FROM NA-Y ZEOLITES BY NEAR-THRESHOLD PULSED EASES ABLATION STUDIED WITH THE MASS-SPECTROMETRIC TIME-OF-FLIGHT TECHNIQUE

The pulsed laser desorption of monoatomic Al and Na ions was studied a t 266 nm with the mass spectrometric time-of-flight (TOF) method at la ser fluences between 60 and 85 mJ cm(-1). This range is below the lase r ablation threshold, and fragmentation of the zeolite framework has n ot been observed. Near the desorption threshold (similar to 60 mJ cm(- 1)) the kinetic energy (KE) distribution of the translating Al species fits a Gaussian profile, which can be interpreted on the basis of a p hotochemical model with surface relaxation processes taken into accoun t. Laser desorption of Na species induced the pink-colored center asso ciated with Na clusters. It is proposed that the Na clusters are forme d by migration and should be less reactive to laser desorption than ot her laser-desorbed Na species in zeolites. As laser fluence increased, the KEs of laser-desorbed Al and Na species had shifted Maxwell-Boltz mann (SMB) distributions, which are due to thermalization of the speci es in flux by collision. Ionic and neutral species are observed in the collisional flux when the KE at TOF peak maximum (KE(p)) is above and below the ionization potential of the desorbed species, respectively. KEs of translating ionic species (> 6.6 eV) are higher than the 266 n m photon energy (4.66 eV), which is explained by a multiphoton excitat ion process. The origin of laser-desorbed Al species is considered. It is suggested that the 266 nm laser TOF method may be very sensitive t o the mobile Na and Al species in zeolites.