A CROSSED LASER-MOLECULAR BEAM STUDY OF THE PHOTODISSOCIATION DYNAMICS OF ZN(C2H5)2 AND (ZN(C2H5)2)2 AT 248-NM AND 193-NM

A molecular beam of zinc-diethyl (ZnEt2) is photodissociated at 248 and 193 nm and the velocity distributions of the photofragments are measured by time-of-flight techniques. One and two photon processes are observed. The dominant one photon process at both wavelengths is the dissociation of (ZnEt2)2 to form two ZnEt2 monomers. The absence of secondary dissociation of the ZnEt2 photofragments at both excitation wavelengths and the small fraction of the available energy partitioned to product translation implicates dissociation to an excited electronic potential energy surface correlating to one electronically excited ZnEt2* monomer. The mass spectrum of the ZnEt2 photofragments is the same as measured for "cold" ZnEt2 monomers in the molecular beam, suggesting that the electronically excited ZnEt2* monomers have fluoresced prior to ionization in the mass spectrometer. A small photodissociation signal of uncomplexed ZnEt2 is observed only at low expansion pressures. The sensitive dependence of this monomeric photodissociation signal to the Ar pressure of the adiabatic expansion suggests that ground state vibrational excitation is required for monomeric photodissociation at 248 nm. In contrast to the dimer single photon photodissocation channel, when ZnEt2 monomers are photodissociated, a significant fraction of the available energy appears as product translation. A qualitative molecular orbital analysis can explain the observed fast photoproduct velocity if dissociation occurs via a repulsive triplet state which correlates to electronic ground state products. The two photon process observed is assigned to single photon photodissociation of the electronically excited ZnEt2* monomers produced in the dimer photodissociation step. The photofragment velocity distributions for the two photon channel can be quantitatively modeled by sequential ethyl eliminations on the ground state ZnEt2 and ZnEt potential energy surfaces. The product velocity distributions are consistent with a microcanonical energy distribution for both ethyl eliminations. Approximately 50% of the ethyl photofragments are created with sufficient vibrational energy to break the weak ethyl C-H bond (36 kcal/mol) forming ethylene. Implications of the ZnEt2 photodissociation mechanism for Zn film deposition using 248 and 193 nm excimer radiation are discussed. © 1990 American Institute of Physics.