Photodissociation of t-butyl nitrite on Ag(111): rotational and electronic-state distributions of the NO (X2Π) photofragments

The 248 nm photodissociation of t-butyl nitrite adsorbed on Ag(111) leads to ejection of three categories of translationally and rotationally excited NO. The translational and rotational energies are correlated. The slowest NO is characterized by a Boltzmann distribution of low-J states with a temperature close to that of the substrate. The fastest NO is characterized by a narrow non-thermal Gaussian distribution of high-J states, peaking at J=54.5±1. Between these two lies a broad range of intermediate-speed and intermediate-J-state NO molecules. The fastest category is attributed to collisionless ejection of NO, the slowest category to NO that is momentarily captured and fully thermalized, and the intermediate category is NO partially relaxed through collisions with surrounding species. The relative contributions to the three categories vary with coverage, reflecting changes in adsorbate orientation and the composition of the local environment. The intensity of the collisionless component does not reach saturation at 40 monolayers (ML), the intermediate component saturates at 5 ML, and the thermalized component maximizes at 10 ML and then decreases by ∼30% before saturating at 15 ML. The spin–orbit population ratio, n(F1)/n(F2), of NO is close to unity for collisionless trajectories and increases to 3.9 for thermalized trajectories.