CH2I2 molecules adsorbed on Al2O 3 and Ag surfaces are used as model systems for studying the photofragmentation and desorption processes on solid surfaces involving electronic excitation of the adsorbate. The molecules are activated into an antibonding state by a ultraviolet (UV) light pulse and the desorption behavior is monitored by time-of-flight mass spectrometry. The surface coverage and chemical changes due to laser irradiation are determined by x-ray photoemission and thermal desorption spectroscopies. The desorption yields as well as the mass and the translational energy distributions are measured as a function of the surface coverage, the laser fluence, and the number of laser pulses. The UV radiation effect on a mixture of CH2I2 and NH3 as coadsorbates on Al2O3 is also examined. The results reveal the characteristic electronic, thermal, and "explosive" desorption effects depending on both the gaseous exposure and the laser irradiation conditions. The quantum yields, the molecular selectivities, the energetics of dissociation and the desorption mechanisms are discussed based on the experimental observations. For clarity, the results and analyses are presented in two parts: CH2I2/Al2O3 in paper I and CH2I2/Ag in paper II of this series. © 1989 American Institute of Physics.