Ultrafast spectroscopy using pulses of light between 10^-9 and $10^-18 seconds has become a broad field studying atomic physics, structural biology, and of course, chemical reactions and dynamics. Laser oscillators and amplifiers have worked in the past decades to drop the temporal resolutions of instruments down to 10’s of femtoseconds for commercially available Ti:Sap systems, and within 100’s of attoseconds for cutting edge research. Stabilizing these ultrafast pulses into a frequency comb has become a popular method of keeping shot-to-shot reproducability while also allowing for cavity enhancement of ultrafast pulses. The work presented in this dissertation builds upon previous designs for an ultrafast Yb:fiber laser system to create a frequency comb that will be used for cavity enhanced transient absorption spectroscopy (CETAS) on gas phase systems. A home-built frequency comb and cavity enhancement system within a differentially pumped vacuum chamber is built and quantified for molecular beam measurements within a supersonic expansion. This system uses off a frequency comb from a Yb:fiber laser and chirped pulse amplifier to produce 100 fs pulses at an 85 MHz repetition rate. Pound-Drever-Hall locking of this comb to the enhancement cavities stabilizes the system and allows for an increase of the signal proportional to the cavity finesse. Expansion of the molecular beam into the differentially pumped chamber ensures cavity optics stay in a low-noise environment free from molecular contamination that would reduce cavity finesse. Noise reduction including lock-in detection and autobalanced noise subtraction are performed to decrease the noise floor of the instrument to -95 dBc/Hz while increasing the signal by three orders of magnitude. The signal increase from this technique allow for gas phase studies into chemical systems that previously were unattainable due to low number densities. Gas phase measurements also give an clear view into molecular dynamics unimpeded by solvent interactions, leading to extremely important comparisons to theory. Spectroscopy of singlet fission precursors are discussed.