  Homo- and heteronuclear alkali quartet trimers of the type K_{3-n}Rb_{n}
(n=0,1,2,3) formed on helium nanodroplets are probed by one-color femtosecond
photoionization spectroscopy. The obtained requencies are assigned to
vibrations in different electronic states by comparison to high level ab initio
calculations of the involved potentials including pronounced Jahn-Teller and
spin-orbit couplings. Despite the fact that the resulting complex vibronic
structure of the heavy alkali molecules complicates the comparison of
experiment and theory we find good agreement for many of the observed lines for
all species.
