  Phase-modulated wave-packet interferometry is combined with mass-resolved
photoion detection to investigate rubidium atoms attached to helium
nanodroplets in a molecular beam experiment. The spectra of atomic Rb
electronic states show a vastly enhanced sensitivity and spectral resolution
when compared to conventional pump-probe wave-packet interferometry.
Furthermore, the formation of Rb*He exciplex molecules is probed and for the
first time a fully resolved vibrational spectrum for transitions between the
lowest excited $5\Pi_{3/2}$ and the high-lying electronic states $2^2\Pi$,
$4^2\Delta$, $6^2\Sigma$ is obtained and compared to theory. The feasibility of
applying coherent multidimensional spectroscopy to dilute cold gas phase
samples is demonstrated in these experiments.
