Molecular analysis by ionization of laser-desorbed neutral species

A powerful molecular surface analysis technique for the analysis of complex materials, such as polymer/additive systems, consists of laser desorption of surface molecules and subsequent ionization of these gas-phase molecules with resonant or nonresonant laser ionization. These molecular ions are subsequently detected by Fourier-transform mass spectrometry or time-of-flight mass spectrometry. We show that different wavelengths for the postionization step permit selectivity that provides important additional information on the chemical makeup of these complex materials. Near-UV wavelengthsselectively ionize aromatic polymer additives, far-UV wavelengths photoionize other nonaromatic species; and vacuum-UV wavelengths provide access to all the desorbed species. In addition to these applied results, we study many fundamental issues of laser desorption, such as desorption thresholds, velocity distributions, postionization wavelength selectivity, etc. The Fourier-transform mass spectrometer and time-of-flight mass spectrometer are discussed in detail.