Results are presented of a bubble-chamber study of the interaction of negative kaons with helium yielding $\Lambda {\pi }^{-}$ or ${\Sigma }^0{\pi }^{-}$ in the final state. Both low-momentum kaons and kaons at rest were used. In the at-rest sample, the two-step process $K^{-}N\to \Sigma \pi$ followed by $\Sigma N\to \Lambda N$ accounts for (18±2)% of the $\Lambda {\pi }^{-}{\mathrm{He}}^3$ final state. Removing these $\Sigma -\Lambda$ conversion events, on the basis of the pion kinetic energy, gives spectra that are compared with an impulse model. A good fit to the data is obtained by using a Hulthén form factor, a mixture of atomic-orbital-capture states with (71±9)% $s$ state and (29±9)% $p$ state. For this fit, the $S$-wave $\overline{K}N$ amplitude determined by Kim is used, and the $P$-wave amplitude is assumed to be dominated by the $Y^{*}\mathrm{}\left(1385\right)\mathrm{}$ with the coupling to $\overline{K}N$ given by $\mathrm{SU}\left(3\right)\mathrm{}$ symmetry. The same final state initiated by kaons in flight shows a good agreement with the predictions of the impulse model. No evidence is seen for a recently reported enhancement in the $\Lambda {\pi }^{-}$ system at 1440 MeV. The $\Lambda {\pi }^{-}\mathrm{pd}$ final state is found to be dominated by the $\Sigma -\Lambda$ conversion process. Stopping-$K^{-}$-reaction branching ratios and cross sections are calculated and compared with impulse-model predictions. No evidence is found for a $\Lambda n$ bound state, and an upper limit for its production is 5×${10}^{-4\mathrm{}}$ per stopping $K^{-}$.

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