Sputtered neutrals mass spectrometry of organic molecules using multiphoton postionization

Abstract

Multiphoton postionization of sputtered neutral molecules has been investigated by high-resolution time-of-flight mass spectrometry. The desorption of the molecular species is induced by a pulsed 10 keV Ar+-ion beam with primary-ion dose densities below 1013 PI/cm2. For postionization an excimer laser at 193 nm wavelength or a tunable dye laser operating between 240 and 300 nm are used. In systematic investigations fragment patterns and useful yields of the commonly occurring amino acids were investigated. With λ=193 nm nearly all of them could successfully be identified by their (M-45) fragment which corresponds to the loss of the carboxyl group. The photoion yield varied over nearly two orders of magnitude for different amino acids. The aromatic ones could also be postionized at longer wavelengths (260–300 nm). Ion yields and fragment patterns critically depend on energy deposition of both the sputter process and the multiphoton absorption. This can be seen by comparison with gas-phase ionization of evaporated molecules and by the dependence of the ionization signal on the laser power density. In contrast to the postionization of sputtered atoms the ionization of sputtered molecules is not characterized by strong spectral features and only a poor degree of ionization selectivity can be achieved. In general, the results demonstrate that the combination of ion-beam sputtering and multiphoton postionization offers great potential not only in element analysis, but also in surface analysis of organic materials.