ADSORPTION AND PHOTOCHEMISTRY OF ETHENE ON NACL CRYSTALLITES

IR spectra, thermodynamics and photochemistry of ethene, C2H4, physiso rbed onto NaCl(100) cuboids of a sublimed polycrystalline film have be en examined. The heat of adsorption of ethene onto the NaCl film range s -10+/-3<Delta(ads)H<-17+/-3 kJ mol(-1) over a coverage range of 0.5< Theta<1.0. The vibrational spectrum of the adsorbed ethene resembles t hat of condensed phase and matrix isolated ethene where intensities an d frequencies are only slightly modified from the gas phase. The ethen e, adsorbed at less than a full monolayer on the crystallites, is then subjected to 184.9 nn light whereupon it photodissociates almost excl usively into acetylene and H-2. Further, it is not the ethene molecule s, but the NaCl substrate which appears to be the main antenna of phot on capture. We present evidence that suggests that the excitation is s tored as self-trapped excitons (STEs) in the NaCl. This energy is then passed to the adsorbed ethene via triplet-triplet energy transfer. It is proposed that the acetylene is produced either through a triplet e thylidene or vinylidene intermediate.