| The adsorption and photochemistry of 1,3-butadiene (C 4H6) on HOPG(0001) at <85 K was studied using temperature-programmed desorption (TPD) and electron energy loss spectroscopy (EELS). Butadiene weakly adsorbs on HOPG, the monolayer (ML) exhibiting first order desorption with a maximum desorption rate at 129 K and a corresponding desorption energy of 32 kJ/mol. Multilayer butadiene showed zero order desorption with a maximum desorption rate at 105 K and a desorption energy of 28.5 kJ/mol. Irradiation of the C4H6/HOPG(0001) surface (1 and 3 monolayers of C4H6) with UV photons (193 nm, 248 nm, and 351 nm) at 100 mJ/pulse resulted in molecular photodesorption of the adsorbed butadiene layers. The cross section for photodesorption was calculated at each wavelength for both coverages. For 1 monolayer coverage, the cross-sections were 3.3 x 10-20 cm2, 2.4 x 10 -20 cm2, and 1.2 x 10-20 cm2 for 193 nm, 248 nm, and 351 nm, respectively. The 3 monolayer cross-sections were 1.1 x 10-17 cm2, 6.3 x 10-18 cm2, and 1.8 x 10-18 cm2 for 193 nm, 248 nm, and 351 nm, respectively. It was found the photodesorption mechanism could be described by a simple thermal process. The photo-processes occurring in both 1 and 3 ML C4H6(ad)/HOPG were also investigated at lower photon fluences of ≤10 mJ/pulse. Under these conditions, there was no detectable photodesorption or condensed phase photochemistry. |
