Ultrafast spectroscopic interrogation and simulation of excited states and reactive surfaces

Making a movie of a chemical reaction requires intricate knowledge of both the chemical actors and the environment. Furthermore, the subtleties of the reactive surfaces and the effect of external forces such as solvent electrostatics and molecular collisions on these surfaces often play a major role in the reaction outcome. Many of these external effects begin influencing the reaction within tens of femtoseconds, requiring sophisticated experimental techniques in order to observe dynamics on these timescales.;Ultrafast pump-probe spectroscopy is employed to study the benchmark ICN A photodissociation reaction. This prototypical triatomic system is known to produce a highly non-equilibrium distribution of rotationally excited cyano radicals. The generation of these rotors comes from a non-adiabatic transition which applies extra torque to an already rapidly bending I-CN bond and leads to two product states corresponding to the I*( 2P1/2) + cold CN and I(2P3/2) + hot CN channel. Simulations show that this rotationally excited CN pushes solvent molecules out of the way to create a cavity where free rotation takes place for several picoseconds. Our hypothesis is that this spinning CN resembles a gas phase molecule, and thus should have a gas phase spectral signature.;Although the relaxation of the hot CN takes place on relatively long time scales for ultrafast spectroscopists, the initial curve crossing event takes place much earlier (<30 fs) and not much is known about the influence of the solvent on this process. Using sub 40 fs DUV pulses compressed by a novel Gires-Tournois pulse compression system and multiplexed broadband probing, we can interrogate this reaction at much higher levels of detail than previously possible. A clear signature from the gas-phase-like CN B←X transition is observed in both water and ethanol. The spectral contributions of I(2P3/2) and I*(2P1/2) can be removed from the transient spectrum, because of new and independent experiments also reported here that capture these spectra by photodetachment of iodide in ethanol. This analysis allows us to watch the evolution of the CN band and estimate first pass curve-crossing probabilities for the first time in solution.;Semiclassical molecular dynamics simulations of ICN bond breaking in water including the non-adiabatic transition, allow us to microscopically break down the energy flow which takes place immediately after dissociation. Using a spectroscopic sorting criteria which follows only freely dissociated trajectories, we can compare the simulation directly to our experimental observables for the first time.;Future work is needed to measure the equilibrium spectrum of the CN radical in the absence of iodine transients as a last reference, but, with our current time resolution and the ability to simultaneously image the absorption spectrum of multiple transient species as a function of time, we are much closer to extracting all pertinent information from a movie of this benchmark solution phase reaction occurring in real time.