We report the detailed analysis of translationally cold rubidium molecule formation through photoassociation. Cold molecules are formed after spontaneous decay of photoexcited molecules from a laser cooled atomic sample, and are detected by selective mass spectroscopy after two-photon ionization into Rb ₂ ⁺ ions. A spectroscopic study of the 0 _g ^- (5 S + 5 P _3/2 ) pure long-range state of ⁸⁷Rb₂ is performed by detecting the ion yield as a function of the photoassociation laser frequency; the spectral data are theoretically analyzed within the semiclassical RKR approach. Molecular ionization is resonantly enhanced through either the 2 ³ Π _g or the 2 ³ Σ ⁺ _g intermediate molecular states. Some vibrational levels of the latter electronic state are observed and assigned here for the first time. Finally, cold molecules formation rates are calculated and compared to the experimentally measured ones, and the vibrational distribution of the formed molecules in the a ³ Σ ⁺ _u ground triplet state is discussed.