INFLUENCE OF SURFACE-CHARGE AND SURFACE-STRUCTURE ON THE MOLECULAR AND IONIC SUBLIMATION OF KCL SINGLE-CRYSTAL

A mass spectrometric method was used to study a kinetics of potassium chloride single crystal sublimation in the form of neutral molecules a nd positive and negative ions. In electron-impact-ionization mass spec trum K+, KCl+ and K2Cl+ ions originated from KCl and K2Cl2 molecular p recursors were detected in the temperature range 780-900 K. The curvat ure exposed by the temperature dependencies of ion currents, In IiT-1/ T, was interpreted as a result of surface charge influence on the rate of sublimation. Theoretical and experimental retardation factors were analyzed for free sublimation of KCl and NaCl crystals [J.E. Lester, G.A. Somorjai, J. Chem. Phys. 49 (1968) 2940]. In thermal ionization m ass spectrum K+, K-2(+), K2Cl+, K3Cl2+ positive ions and Cl-, Cl-2(-), and KCl2- negative inns were identified in the ranges 710-1020 K and 950-1020 K, respectively; the negative ion currents were much weaker t han positive ones. During an induction period, a gradual decrease of i sothermal ion currents was observed. The temperature dependence, In I( K+)-1/T, measured at successive stages of vaporization revealed (i) th e existence of knee temperature. above and below which the ion emissio n is characterized by different activation energies for ion desorption , E+, (ii) the values of E+ were found to be dependent on the time ela psed from the vaporization start, i.e., on the developing surface morp hology. The comparison of mass spectra of both types obtained in this work with those measured under equilibrium conditions showed the simil arity of the electron-impact-ionization mass spectra and the significa nt difference between the thermal ionization ones. All the results are discussed in light of terrace-ledge-kink model of vaporization and su rface charge concept that an a corollary of defect theory. (C) 1998 El sevier Science B.V.