  In this letter, we study the scattering of light by a single subwavelength
slit in a metal screen. In contrast to previous theoretical works, we provide a
microscopic description of the scattering process by emphasizing the generation
of surface plasmons at the slit apertures. The analysis is supported by a
rigorous formalism based on a normal-mode-decomposition technique and by a
semi-analytical model which provides accurate formulae for the plasmonic
generation strengths. The generation is shown to be fairly efficient for metals
with a low conductivity, like gold in the visible regime. Verification of the
theory is also shown by comparison with recent experimental data [H.F. Schouten
et al., Phys. Rev. Lett. 94, 053901 (2005)].
