LAMMA analysis of crystalline organic films damaged by 100 keV electron irradiation

Abstract

Electron irradiation of organic material results in a radiation induced damage of ionisation and rupture of chemical bonds. Secondary and tertiary processes produce a loss of mass and a crystal damage resulting in a fading of electron diffraction intensities [1 9]. After a prolonged irradiation the material is transformed into a carbon-rich polymer. Most of the non-carbon atoms are lost through volatile fragment products. The investigation of electron irradiated films of organic molecules by LAMMA is of interest for the following reasons : 1. to get information about the fragmentation and cross-linking processes of the organic molecules as secondary processes of radiation damage, and 2. to demonstrate that an electron pre-irradiation changes the LAMMA spectra and that an observation of organic specimens in a transmission or scanning electron microscope before LAMMA experiments should be avoided. The specimen films (100 nm) were prepared by high vacuum evaporation of organic compounds (e.g. leucine, tetracene, perylene, uracile and phthalocyanine) onto a 15 nm collodion supporting film mounted on a specimen grid. Collodion films have the advantage of contributing only very small characteristic peaks to the LAMMA spectrum. Different squares (235 x 235 ~tm) of the specimen grid were irradiated in a transmission electron microscope with different charge densities with 100keV electrons and subsequently investigated by LAMMA. The results are averaged over 10 different spectra. Typical results shall be discussed for an aliphatic (leucine) and aromatic compound (tetracene). Figure 1 shows a positive ion LAMMA spectrum ofleucine with typical peaks at masses 132 (M+ H), 86 (M-COOH), 44 (C2H4N), 41 (C3Hs) and 30 (CH4N). The intensive peak at 39 is caused by potassium. Figure 2 shows the change of the averaged intensities of the mass peaks as a function of the irradiation dose. All ion signals are normalized to the (M + H) + ion signal at m/e = 132 of the non irradiated sample. The mass peaks at 132 and 86 (Fig. 2b) decrease due to fragmentation processes whereas the fragment ions at 30, 41 and 44 (Fig. 2a) reach a maximum at intermediate charge doses. The peak at mass 90 appears in irradiated specimens only and could be caused by recombination of lower mass irradiation products. For comparison Fig. 4 shows the decrease (fading) of electron diffraction intensity of lattice planes with lattice parameters d = 0.28 nm and d = 0.47 nm. The charge density for a complete fading of d = 0.28 nm is in the order of 2 • 10 .3 Ccm -2. Figure 3 shows the dependence of the signals at mass numbers 228 (M), 226 (M-2) and 50 (C4H2) on the irradiation dose of 100keV electrons. All signals decrease monotonously with the dose. The corresponding fading diagram of Fig. 5 indicates a complete loss of diffraction contrast for 100 keV electrons for a dose of about 0.2 Ccm -2, quite in agreement with the mass spectrometric results. A comparison of the results obtained for leucine with that given for tetracene indicates the higher resistance of aromatic compounds to electron irradiation by more than one order of magnitude as compared to that of aliphatic compounds. The results will be published in detail elsewhere [101.