High Energy Dynamics of Atmospherically Important Molecules Observed with Femtosecond Pump-Probe Spectroscopy

Abstract:

Concern over the ever-increasing concentration of CO₂ in our environment has sparked a renewed interest in the photodynamics of atmospheric molecules. Gases like Carbon Monoxide (CO) and Carbon dioxide (CO₂) are ubiquitous in our atmosphere and play important roles in the life cycle and the greenhouse effect. I will present our recent laboratory results showing the excited state dynamics of these molecules through femtosecond (fs) pump-probe spectroscopy. Photodissociation of CO₂ proceeds via multiple pathways depending on available energy, but commonly into CO and O. Here, strong-field excitation 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. We have also applied fs laser pulses to drive the Coulomb explosion of gas-phase formic acid dimers and observed the multiply charged ions with mass spectrometry. Potential energy curves are also calculated and confirm the existence of a metastable states for COⁿ⁺, n ≤ 3. This combined experimental and theoretical effort shows the existence of a metastable state in the trication of carbon monoxide for the first time. Experimental measurements of the kinetic energy release of the ions are consistent with molecular dynamics simulations of the Coulomb explosion of a formic acid dimer and suggest that almost no movement occurs during the ionization mechanism.