Probing NO$_{2}$ close to the $\tilde {A}{ }^2B_2 /\tilde {X}{ }^2A_1 $ conical intersection by time-resolved imaging spectroscopy

Time-resolved imaging spectroscopy (TRIS) is emerging as a versatile technique with which to study the non-adiabatic coupling of vibrational and electronic degrees of freedom in molecules. The electronic predissociation of NO$_{2}$ in the near UV proceeds by internal conversion between the $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states and is a benchmark example of such barrierless reactions. We have applied time-resolved imaging to measure the time-evolution, angular and kinetic energy distributions of NO$^{+}$, NO$_{2}^{+}$ and photo electrons produced in pump-probe experiments using harmonics from a regeneratively amplified self-mode locked Ti:sapphire laser. Oscillations in the slow NO$^{+}$ and photoelectron signals are observed and are interpreted as measuring the energy level density of the coupled $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states close to the conical intersection. By using an optical pulse shaper we are able to manipulate the spectrum of the $\sim $400 nm excitation to create pulse sequences with which we can exert partial control over the coupling between the $\tilde {A}{ }^2B_2 \mbox{ and }\tilde {X}{ }^2A_1 $ states.