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We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped $\chem{^4{He}^*}$ atoms in the metastable 23S1 state. A pair of ultracold spin-polarized 23S1 helium atoms is excited into a molecular bound state in the purely long-range 0u+ potential connected to the 23S1-23P0 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 23S1 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, $a=7.2\pm 0.6\un{nm}$, which is significantly lower than most previous results obtained by non-spectroscopic methods

Frequency shifts of photoassociative spectra of ultracold metastable helium atoms: A new measurement of the s -wave scattering length