Spectral Signatures of Inter-System Crossing Mediated by Energetically Distant Doorway Levels: Examples from the Acetylene S₁ State

Abstract

We review recent research on the acetylene S(1) state that illustrates how mechanistic rather than phenomenological information about intersystem crossing (ISC) may be obtained directly from frequency-domain spectra. The focus is on the dynamically rich "doorway-mediated" ISC domain that lies between isolated spectroscopic spin-orbit perturbations and statistical-limit interactions between one singlet "bright state" and a quasi-continuum of triplet "dark states". New and improved experimental and data processing techniques permit the statistical-model curtain to be drawn back to reveal mechanistically explicit pathways via one or more identifiable, hence, manipulatable, doorway states, between a user-selected bright state and the undifferentiated bath of dark states.

Modelling the vibrationally mediated photo-dissociation of acetylene

“…This picture agrees with many ab initio calculations 13,21,22 , which suggest that after intersystem crossing there would have to be a sequential energy transfer along several modes involving large molecular contortions that would make such a mechanism slow 13 . This agrees with the observed µs metastable state lifetimes of the excited acetylene upon irradiation 23,24 and a lower limit for a predissociation lifetime has been suggested of at least tens of picoseconds 25 .…”

Relaxation dynamics of Au25L18 nanoclusters studied by femtosecond time-resolved near infrared transient absorption spectroscopy

“…30,31 In many cases, vibrational coupling significantly influences the rate of ISC. [32][33][34][35] Temperature-dependent absorption measurements indicate electron-phonon coupling is strong for Au 25 nanoclusters, which may help to mediate an intersystem crossing event. 36 ISC is only one possible explanation that can reconcile the current data.…”

Ultracold Molecules under Control!

“…The collision complex of two ultracold reactants is like the highly excited molecular states that are well-studied in chemistry 316 under paradigms such as internal relaxation, 317,318 unimolecular decomposition, 319−321 transition state theory, 322 or semiclassical or chaotic dynamics. 323,324 Although the lowenergy entrance channel is undoubtedly highly quantum mechanical in its long-range collision dynamics, once it is coupled to the high-energy scale of the collision complex the conventional chemical dynamics from these familiar fields takes over.…”