Thermal desorption spectroscopy study of native and electron irradiated glycine overlayers on graphite(0001)

Abstract

The adsorption of glycine deposited at 95 K on the graphite basal (0001) plane was studied from submonolayer to multilayer coverages by thermal desorption spectroscopy (TDS) and high- resolution electron loss spectroscopy (HREELS). At all coverages glycine adsorbs in three-dimensional (3D) clusters (no distinct monolayer formation was observed). Glycine molecules desorb around 300 K in a single desorption peak (α peak) with a binding energy of 0.85 eV, representing the cohesive energy of glycine. HREELS analysis of these overlayers is severely hampered by charging of the 3D clusters; only one vibrational feature at 26 meV was observed reproducibly. The glycine adlayers are very sensitive to electron irradiation. This effect was studied in some detail by TDS. Electrons of ∼50 eV energy cause: (i) conversion of glycine molecules from the α state to (surprisingly) a more weakly bound β state (desorption temperature ∼250 K), interpreted as 2D adsorbates, and (ii) dissociation of glycine into fragments that either desorb spontaneously or adsorb on the bare graphite surface, and desorb upon heating (γ peak) at 170 K, i.e., earlier than both the α and β peaks. The cracking pattern of the γ peak suggests that the electrons break the C–C bond in glycine, into COOH and H2N–CH2 fragments.