  The rotational spectrum of the molecular ion HCNH+ is revisited using
double-resonance spectroscopy in an ion trap apparatus, with six transitions
measured between 74 and 445 GHz. Due to the cryogenic temperature of the trap,
the hyperfine splittings caused by the 14N quadrupolar nucleus were resolved
for transitions up to J = 4-3, allowing for a refinement of the spectroscopic
parameters previously reported, especially the quadrupole coupling constant
eQq.
