The recent measurements by D. Srdoč, B. Obelić, and I. Krajcar Bronić (J. Phys. B, 20, 4473-4484, 1987) and by I. Krajcar Bronić, D. Srdoč, and B. Obelić (Radiat. Res., 115, 213-222, 1988) revealed nonsmooth, oscillatory variation of the W value for low-energy electrons as a function of the number of carbon atoms in normal alkanes. This behavior is contrary to what has been expected generally and prompted us to undertake the present study. Calculations were carried out using the continuous-slowing-down approximation for the degradation spectrum, the binary-encounter theory for cross sections, and relevant molecular properties, to explain the behavior of the W value. The main contributor to the oscillatory variation is the branching ratio for neutral dissociation to ionization of super-excited states. We also present an interpretation of the trend of the W value in the series C2 H2, C2 H4, and|${\rm C}_{2}{\rm H}_{6}$|.

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