  We present measurements of the infrared response of the quasi-one-dimensional
organic conductor (TMTSF)2$SO3 along (E||a) and perpendicular (E||b') to the
stacking axis as a function of temperature. Above the metal-insulator
transition related to the anion ordering the optical conductivity spectra show
a Drude-like response. Below the transition an energy gap of about 1500 cm-1
(185 meV) opens, leading to the corresponding charge transfer band in the
optical conductivity spectra. The analysis of the infrared-active vibrations
gives evidence for the long-range crystal structure modulation below the
transition temperature and for the short-range order fluctuations of the
lattice modulation above the transition temperature. Also we report about a new
infrared mode at around 710 cm-1 with a peculiar temperature behavior, which
has so far not been observed in any other (TMTSF)2X salt showing a
metal-insulator transition. A qualitative model based on the coupling between
the TMTSF molecule vibration and the reorientation of electrical dipole moment
of the FSO3 anion is proposed, in order to explain the anomalous behavior of
the new mode.
