Ultrafast Dynamics of Atmospheric Molecules Observed with Femtosecond Pump-Probe Spectroscopy

Simple molecules like carbon monoxide (CO) and carbon dioxide (CO₂) is ubiquitous in our atmosphere and plays important roles in the life cycle and the greenhouse effect. I will present our recent results showing excited state dynamics of these molecules through femtosecond (fs) pump-probe spectroscopy. Photoexcitation from a 35 fs pump laser impulsively transfers CO₂ from the X → A state, preparing a bending vibrational wavepacket that influences its dissociation. Using mass spectrometry, we report changes in the fragmentation pattern as a function of time delay between the pump and probe laser pulses that reflect the vibrational motion. At well-defined time delays the dissociation oscillates between observable CO⁺ and O₂⁺ fragments. Strong-field excitation is also applied to drive the Coulomb explosion of small gas-phase molecules such as formic acid, where multiply charged ions are observed. Potential energy curves are calculated and confirm the existence of a metastable states for COⁿ⁺, n ≤ 3. Kinetic energy release measurements for the ions are consistent with our molecular dynamics simulations.