State-resolved distribution of OH X Π2 products arising from electronic quenching of OH A Σ2+ by N2

Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections

Nonadiabatic effects are ubiquitous in photophysics and photochemistry, and therefore, many theoretical developments have been made to properly describe them. Conical intersections are central in nonadiabatic processes, as they promote efficient and ultrafast nonadiabatic transitions between electronic states. A proper theoretical description requires developments in electronic structure and specifically in methods that describe conical intersections between states and nonadiabatic coupling terms. This review focuses on the electronic structure aspects of nonadiabatic processes. We discuss the requirements of electronic structure methods to describe conical intersections and nonadiabatic couplings, how the most common excited state methods perform in describing these effects, and what the recent developments are in expanding the methodology and implementing nonadiabatic couplings.

Dynamical Outcomes of Quenching: Reflections on a Conical Intersection

This review focuses on experimental studies of the dynamical outcomes following collisional quenching of electronically excited OH A2Σ+ radicals by molecular partners. The experimental observables include the branching between reactive and nonreactive decay channels, kinetic energy release, and quantum state distributions of the products. Complementary theoretical investigations reveal regions of strong nonadiabatic coupling, known as conical intersections, which facilitate the quenching process. The dynamical outcomes observed experimentally are connected to the local forces and geometric properties of the nuclei in the conical intersection region. Dynamical calculations for the benchmark OH-H2 system are in good accord with experimental observations, demonstrating that the outcomes reflect the strong coupling in the conical intersection region as the system evolves from the excited electronic state to quenched products.

Products of the quenching of NO A 2Σ+ (v = 0) by N2O and CO2

Collisional quenching of NO A (2)Σ(+) (v = 0) by N(2)O and CO(2) has been studied through measurements of vibrationally excited products by time resolved Fourier transform infrared emission. In both cases vibrationally excited NO X (2)Π (v) is seen and quantified in levels v≥ 2 with distributions which are close to statistical. However the quantum yields to produce these levels are markedly different for the two quenchers. For CO(2) such quenching accounts for only ca. 26% of the total: for N(2)O it is ca. 85%. Far more energy is seen in the internal modes of the CO(2) product than those of N(2)O. The results are rationalised in terms of cleavage of the N(2)-O bond being dominant in the latter case, with either a similar O atom production or a specific channel producing almost exclusively NO in low vibrational levels (v = 0,1) for quenching by CO(2). Minor reactive channels yielding NO(2) are seen in both cases, and O((1)D) is observed with low quantum yield in the reaction with N(2)O. The results are discussed in terms of previous models of the quenching processes, and are consistent with the very high yield of NO X (2)Π (v = 0) previously observed by laser induced fluorescence for quenching of NO A (2)Σ(+) (v = 0) by CO(2).