Photoassociation Spectroscopy of ColdHe(23S)Atoms

We observe vibrational states by photoassociation spectroscopy of cold $\mathrm{He}\left(2{}_{}{}^3{}_{}{}^{}S\right)$ atoms. Photoassociation resonances are detected as peaks in the Penning ionization rate over a frequency range of 20 GHz below the atomic $2{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}-2{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ transition frequency. We have observed three vibrational series, of which two can be identified. A possible mechanism to explain the observed increase of the Penning ionization rate is discussed.