Abstract: Many consequential chemical processes take place on ultrafast timescales, including molecular vibrations and bond breaking. Measurements that follow ultrafast molecular dynamics in real time are changing our understanding of these processes. We are designing new tools to study ultrafast molecular dynamics and quantum mechanics with the sensitivity enough to study the molecules in molecular beams and the spectral resolution sufficient for vibrational and rotational resolution. These new tools are based upon frequency comb lasers, which have the unique feature of both precision frequencies and ultrafast pulse durations.
Specifically, home-built ytterbium fiber-laser frequency combs and amplifier systems provide stable sources of ultrafast frequency comb pulses. External enhancement cavities increase the laser power interacting with the sample and the sample interaction length, thus improving the signal by several orders of magnitude. Dual comb detection, using a new home-built comb based upon multiple electro-optic modulators, improves the spectral resolution of the measurement. These frequency-comb techniques are applied to both ultrafast transient absorption spectroscopy and two-dimensional spectroscopy. The proof-of-concept cavity-enhanced transient absorption spectrometer increased the detection sensitivity by 4-orders-of-magnitude over previous best instrumentation. By combining these techniques with a supersonically-cooled molecular beam, we will obtain a new view on molecular dynamics and molecular quantum mechanics.