Laser stimulated fragmentation and desorption from the surface of organic films: Perylene derivatives

Abstract

The mass spectra of laser desorption from the surface of perylene-tetracarboxylic dianhydride (PTCDA) films deposited in vacuum onto indium arsenide (InAs) and gallium arsenide (GaAs) substrates have been studied. The desorption was induced by 10-ns pulses of neodymium laser radiation (quantum energy, 2.34 eV) with the energy density E varied within 0.5–20 mJ/cm2. It is established that laser radiation produces fragmentation of PTCDA molecules and desorption of the molecular fragments. The main fragments observed in the mass spectra are identified. Initially, carboxy and dianhydride groups are detached, decomposed, and desorbed in the form of CO, CO2, and a small amount of atomic oxygen species. An increase in the laser pulse power leads to the desorption of the perylene nucleus (M = 248 amu) and its halves (M = 124 amu). No evidence for laser-stimulated desorption of intact PTCDA molecules was observed. The possible mechanisms of photostimulated fragmentation of PTCDA molecules and the subsequent desorption of fragments are discussed.