  We describe two-photon photoassociation (PA) experiments operated in an
ultracold gas of metastable $^4$He$^*$ atoms in the $2^3S_1$ state.
Atom-molecule dark resonances as well as Raman signals provide information on
the exotic molecule in the least bound vibrational J=2, $v=14$ state of the
$^5\Sigma_g^+$ interaction potential of two spin-polarized metastable atoms.
The physical origin of the various two-photon PA signals is first discussed.
Their linewidths are interpreted in term of processes limiting the lifetime
$\tau$ of the exotic molecule. A value of $\tau=1.4\pm0.3 \mu$s is found, which
is discussed in view of two recent calculations of the Penning ionization rate
induced by spin-dipole coupling and of the atom-molecule inelastic collisions.
