Helium cluster isolation spectroscopy is a recently developed spectroscopic method that involves the formation of a beam of large helium clusters (10(4) atoms pel-cluster), the capture by the clusters of the atoms or molecules of interest in a law-pressure pick-up cell, and the spectroscopic study of the isolated species. Here we exploit the unique feature of this method of allowing the selective preparation of high-spin molecular species (e.g., triplet dimers) over their low-spin (singlet) counterparts to measure the spectra of several alkali dimers in their triplet manifold. By probing via laser-induced fluorescence their lowest triplet-to-triplet transitions, Li-2, Na-2, K-2, and NaK are found to reside on the surface of the helium clusters. Since the spectroscopic shifts induced by the helium cluster are minimal, vibrational analysis of the electronic transitions produces transition frequencies that can be compared to previous ab initio and experimental values. Both bound-bound and bound-free transitions have been observed. Emission spectra reveal the presence of vibrational relaxation and nonadiabatic intersystem crossings of the excited dimers that result from the proximity of the helium cluster surface. Through this study we improve our understanding of triplet alkali dimer potential energy curves, we test an efficient analytical model to represent them, and we provide input information for the study of nonadditive effects present in quartet (spin-polarized) alkali trimers which can be formed using the same method.