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Stimulated Emission Pumping (SEP) spectroscopy has been applied to the study of the ground state vibronic structure of a linear tetra-atomic free radical, HCCS, which has two Renner-Teller active bending modes. The radical was produced in a supersonic jet by a pulsed discharge in a $C2H2$ and $CS2$ mixture diluted in Ar. Two Nd YAG laser pumped dye lasers with linewidths $0.02cm-1$ are used to observe SEP spectra. Eight vibronic bands in the ground electronic states were observed from the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$ origin band excitation, and 4 bands from the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2$ origin band excitation. Three of the observed bands are assigned to the CS stretching mode, and others to overtones and combinations of two bending modes with $\Pi$ vibronic symmetry. The observed vibronic energy levels are successfully analyzed by taking into accunt the Renner-Teller interaction including the cross vibronic terms. The determined parameters agree with previous ab initio $calculationsa$ and mm-wave $spectroscopyb$.