PHOTOCHEMISTRY OF TRANS-STILBENE ADSORBED ON AL2O3(0001)

Temperature-programmed desorption (TPD) was used to study the desorpti on and photochemistry of trans-stilbene on Al2O3(0001) in ultrahigh va cuum. At low coverages, the TPD traces of cis-stilbene and trans-stilb ene both show only one peak, attributed to monolayer desorption, at 27 5 and 315 K, respectively. As the coverage is increased, a second, mul tilayer peak is seen at 230 K for cis-stilbene and at 260 K for trans- stilbene. The relative intensities of the monolayer and multilayer pea ks were observed to fit a statistical adlayer sticking model similar t o those of other hydrocarbons and described in previous work. When tra ns-stilbene is exposed to light at 313 nm, TPD traces show a new peak at 380 K which is attributed to the formation of the stilbene dimer, 1 ,2,3,4-tetraphenylcyclobutane (or isodistilbene). The photodimerizatio n is coverage and temperature dependent with little or no photochemist ry occurring at coverages below 0.10 ML or temperatures below 100 K. T he data are consistent with the dimerization occurring through an exci mer intermediate. Little or no photoisomerization was observed at any of the coverages studied.