  Broadband ultrafast optical spectroscopy methods, such as transient
absorption spectroscopy and 2D spectroscopy, are widely used to study molecular
dynamics. However, these techniques are typically restricted to optically thick
samples, such as solids and liquid solutions. In this article we discuss a
cavity-enhanced ultrafast transient absorption spectrometer covering almost the
entire visible range with a detection limit of $\Delta$OD $ < 1 \times
10^{-9}$, extending broadband all-optical ultrafast spectroscopy techniques to
dilute beams of gas-phase molecules and clusters. We describe the technical
innovations behind the spectrometer and present transient absorption data on
two archetypical molecular systems for excited-state intramolecular proton
transfer, 1'-hydroxy-2'-acetonapthone and salicylideneaniline, under jet-cooled
and Ar cluster conditions.
