  High-resolution infrared studies of laser ablation products from
carbon-selenium targets have revealed a new vibrational band at 2057 cm$^{-1}$
that is identified as the vibrational fundamental of the SeC$_3$Se cluster.
Because of the rich isotopic composition of selenium and the heavy nuclear
masses involved, the vibrational band shows a relatively compact and complex
structure despite the simple linear geometric arrangement. Overall,
rotational-vibrational lines of six isotopologues could be assigned and fitted
permitting the derivation of an accurate selenium-carbon bond length.
  Spectroscopic analysis has been greatly supported by high-level
quantum-chemical calculations of the molecular structure and the harmonic and
anharmonic force fields performed at the CCSD(T) level of theory.
Scalar-relativistic effects on the molecular structure were also considered but
found of little importance.
