Adsorption and Laser-Induced Thermal Desorption of 1,3-Butadiene on HOPG(0001)

Abstract

The adsorption and UV photochemistry of 1,3-butadiene (CH 2 =CHCH=CH 2 ) on highly oriented pyrolytic graphite (HOPG(0001)) was studied using temperature-programmed desorption (TPD) and electron energy loss spectroscopy (EELS). Butadiene physisorbs on HOPG with the monolayer exhibiting first-order desorption kinetics and a corresponding desorption energy of 32 kJ/mol. The multilayer exhibits zero order desorption kinetics with a desorption energy of 28.5 kJ/mol. Irradiation of the C 4 H 6 /HOPG(0001) adlayer (one and three monolayers (ML) of C 4 H 6 ) with UV photons at 193, 248, and 351 nm and fluences of up to 100 mJ/cm 2 _ pulse resulted in molecular photodesorption. Photodesorption was described by a laser-induced thermal desorption process as supported by simple calculations of the surface temperature rise. The photoprocesses occurring in both 1 and 3 ML C 4 H 6 adlayers on HOPG(0001) were also investigated at incident powers of ≤ 10 mJ/cm 2 _pulse. Under these conditions, there was no detectable photodesorption or condensed phase photochemistry at any of the wavelengths investigated.