Non-Born–Oppenheimer effects in molecular photochemistry: an experimental perspective

Abstract

Non-adiabatic couplings between Born–Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating ‘reactive’ outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H 2 O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born–Oppenheimer approximation'.

Molecular photodissociation dynamics revealed by Coulomb explosion imaging

“…CEI also reveals that Z = 3 parent ions decay by CQC bond fission and by two triple dissociation processes. One, a near-concerted process (7), releases two Cl + fragments with clearly isomer specific kinematics. The other is attributable to secondary decay of internally excited H 2 C 2 Cl 2+ fragments formed via process (6).…”

“…Methods of calculating nuclear dynamics on such PESs are also advancing impressively, enabling increasingly rigorous and fruitful points of comparison between experiment and theory. 4–7…”

“…Methods of calculating nuclear dynamics on such PESs are also advancing impressively, enabling increasingly rigorous and fruitful points of comparison between experiment and theory. [4][5][6][7] Until quite recently, experimental studies of molecular photofragmentation processes in the gas phase have tended to group into one of two classes. 8 Both start with the 'pump' stepphotoexcitation, typically with a short duration pulse of reasonably monochromatic light from a laser.…”

Chemistry without the Born–Oppenheimer approximation

“…The opinion pieces opening this theme issue illustrate the importance of chemistry without, and beyond, the Born–Oppenheimer approximation. Ashfold & Kim [ 4 ] offer an experimental perspective on the breakdown of the Born–Oppenheimer approximation and present examples highlighting the response of small isolated and solvated molecules to ultraviolet radiation. They focus more specifically on understanding the photostability and photoreactivity of these molecular systems by a concerted experimental and theoretical effort.…”

Decoherence and Its Role in Electronically Nonadiabatic Dynamics

“…This is often called the Born–Oppenheimer approximation, and, for many problems, such as the vibrational spectroscopy of most closed-shell molecules or the reaction kinetics of most thermally activated chemical reactions, it is an excellent approximation. However, there are also many processes where we must consider two or more electronic states; − this includes photochemical reactions promoted by visible light (vis) or ultraviolet (UV) photons, many electron transfer reactions, quenching and chemical reaction of electronically excited species, electronic energy transfer, Penning ionization, dissociative attachment, excitation of electron–hole pairs, and many other processes. These processes are called electronically nonadiabatic processes or non-Born–Oppenheimer processes, and they are the subject field of this article.…”