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We present 75 near-infrared (NIR; 0.8$-$2.5 $\mu$m) spectra of 34stripped-envelope core-collapse supernovae (SESNe) obtained by the CarnegieSupernova Project-II (CSP-II), encompassing optical spectroscopic Types IIb,Ib, Ic, and Ic-BL. The spectra range in phase from pre-maximum to 80 days pastmaximum. This unique data set constitutes the largest NIR spectroscopic sampleof SESNe to date. NIR spectroscopy provides observables with additionalinformation that is not available in the optical. Specifically, the NIRcontains the resonance lines of He I and allows a more detailed look at whetherType Ic supernovae are completely stripped of their outer He layer. The NIRspectra of SESNe have broad similarities, but closer examination throughstatistical means reveals a strong dichotomy between NIR "He-rich" and"He-poor" SNe. These NIR subgroups correspond almost perfectly to the opticalIIb/Ib and Ic/Ic-BL types, respectively. The largest difference between the twogroups is observed in the 2 $\mu$m region, near the He I $\lambda$2.0581 $\mu$mline. The division between the two groups is not an arbitrary one along acontinuous sequence. Early spectra of He-rich SESNe show much stronger He I$\lambda$2.0581 $\mu$m absorption compared to the He-poor group, but with awide range of profile shapes. The same line also provides evidence for traceamounts of He in half of our SNe in the He-poor group.

Carnegie Supernova Project-II: Near-infrared Spectroscopy of Stripped-Envelope Core-Collapse Supernovae