  The properties of the least-bound vibrational level (v=14) of the quintet
sigma state formed during the ultracold collision of two spin-polarized
metastable helium atoms are crucial to studies of photoassociation spectroscopy
of metastable helium. We report a calculation of the autoionization lifetime of
this state induced by spin-dipole coupling of the quintet sigma state to the
singlet sigma state from which Penning and associative ionization processes are
highly probable. We find a lifetime of about 150 microseconds, significantly
larger than the recent experimental estimates of (4-5) microseconds.
