  We observe light-induced frequency shifts in one-color photoassociative
spectra of magnetically trapped $^4$He$^*$ atoms in the metastable $2^3S_1$
state. A pair of ultracold spin-polarized $2^3S_1$ helium atoms is excited into
a molecular bound state in the purely long range $0_u^+$ potential connected to
the $2^3S_1 - 2^3P_0$ asymptote. The shift arises from the optical coupling of
the molecular excited bound state with the scattering states and the bound
states of two colliding $2^3S_1$ atoms. We measure the frequency-shifts for
several ro-vibrational levels in the $0^+_u$ potential and find a linear
dependence on the photoassociation laser intensity. Comparison with a
theoretical analysis provides a good indication for the s-wave scattering
length $a$ of the quintet ($^5\Sigma_g^+$) potential, $a=7.2\pm 0.6$ nm, which
is significantly lower than most previous results obtained by non-spectroscopic
methods.
