DETECTION OF LARGE MOLECULAR-IONS BY SECONDARY-ION AND SECONDARY-ELECTRON EMISSION

Secondary ion and secondary electron emission has been investigated in a tandem time-of-flight mass spectrometer using incident molecular ions produced by laser desorption as primary ions. Measurements of the ratio of secondary electrons to secondary ions from a stainless steel target were made with a range of projectile masses and energies so that the separate dependence on velocity and mass could be determined. The projectiles ranged in mass from about 300 to 6000 u, and in energy from 300 eV to 30 keV. Secondary accelerating voltages down to 20 V were used to reduce spurious contributions to the electron intensity caused by fragmentation of the projectiles or by grid effects. The electron-to-ion ratio is determined almost entirely by the velocity of the projectile; it is largely insensitive to the mass (for a given velocity). The ratio decreases rapidly at first with decreasing velocity but appears to approach a constant value of about 0.1 below a velocity almost-equal-to 1.4 x 10(6) CM S-1 (almost-equal-to 1 eV u-1). A strong increase in the absolute electron yield and in the electron-to-ion ratio was observed for a fresh CsI target compared to a stainless steel target.