  A phase-stabilized femtosecond laser comb is directly used for
high-resolution spectroscopy and absolute optical frequency measurements of
one- and two-photon transitions in laser-cooled \rb atoms. Absolute atomic
transition frequencies, such as the 5S$_{1/2}$ F=2 \ra 7S$_{1/2}$ F"=2
two-photon resonance measured at 788 794 768 921(44) kHz, are determined
without \textit{a priori} knowledge about their values. Detailed dynamics of
population transfer driven by a sequence of pulses are uncovered and taken into
account for the measurement of the 5P states via resonantly enhanced two-photon
transitions.
