Temperature programmed desorption traces for four depositions of toluene on HOPG. The signals correspond to the parent ion of toluene (m/z = 92). The first three traces correspond to toluene deposited on HOPG at 120 K for 120 s (blue), 240 s (dark orange), and 600 s (red). The green trace shows a 360 s toluene deposition with the HOPG held at 150 K. With increasing deposition time, the monolayer peak around 200 K grows, followed by the multilayer peak at 138 K for longer deposition times. The broad peak that appears above 240 K is attributed to desorption of toluene from the sample holder. When toluene is deposited at 150 K, only the monolayer peak is present and shifted to higher temperatures due to re-ordering.