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Abstract. A Monte Carlo model has been constructed to study the energy degradation of electrons in SO2 gas. We have reviewed all the electron-SO2 collision cross sections and present an assembled set of cross sections along with their analytical representations. The Monte Carlo simulations are carried out in the 1-200 eV energy range to obtain the "yield spectra." The yield spectra contain the nonspatial information about the electron degradation processes and can be employed to calculate the "yield" (or population) for any inelastic process at the required energy. The numerical yield spectra have been represented by an analytical form, generating the analytical yield spectra (AYS). The AYS can be easily applied to atmospheric and laboratory problems dealing with the electron degradation in SO2. The AYS is applied to calculate the efficiencies for various inelastic processes. The energy distribution of the secondary electrons produced per incident electron is presented at a few sample incident energies. The mean energy per ion pair for SO2 is found to be 23 eV at 200 eV. Ionization is the dominant loss channel at energies > 30 eV. More than 50% of the electron energy goes into this loss channel at energies > 50 eV. Excitation of SO2 bands is the major loss process in the 6-20 eV energy range, while below 5 eV all the electron energy goes to the electron attachment process. Only about 1-2 % of the electron energy goes in the production of line emissions of neutral and ionized oxygen and sulfur atoms at energies greater than --100 eV and is much less at lower energies. These are the first results reported on the apportionment of the electron energy among various loss channels in SO2 gas and will be useful in the interpretation and understanding of the processes on Io from the Galileo and Hubble Space Telescope data.

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