Dynamics of Ion-Pair States in Van der Waals Complexes

The study of ion-pair states in weakly bound van der Waals complexes has emerged as a vital frontier in molecular physics and physical chemistry. These ion-pair states—characterised by a cation and anion residing within an overall neutral assembly—act as gateways to non-adiabatic processes such as predissociation, charge transfer and photodissociation. In van der Waals complexes, long-range electrostatic and induction forces stabilise configurations that facilitate population of ion-pair manifolds upon electronic excitation. Femtosecond and picosecond time-resolved spectroscopies have revealed that the formation, coupling and decay of these states govern energy disposal pathways and influence reaction yields in environments ranging from atmospheric aerosols to interstellar clouds. Computational models employing ab initio potential energy surfaces and mixed quantum–classical dynamics have advanced our understanding of the lifetimes, branching ratios and angular distributions associated with ion-pair mediated channels. Together, experimental and theoretical insights shed light on the global impact of ion-pair dynamics for fields as diverse as laser control of chemical reactivity, design of novel molecular materials and astrochemical modelling.

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Technical terms

Van der Waals complex: A weakly bound assembly of molecules or atoms held together by dispersion and induction forces rather than covalent bonds.

Ion-pair state: An electronic state of a neutral complex in which one fragment is cationic and the other anionic, stabilised within the host assembly.

Predissociation: A non-radiative decay process in which an excited state crosses into a dissociative state, leading to bond cleavage.

Rydberg state: A high-lying electronic state of a molecule in which an electron occupies an orbital of large principal quantum number, resembling those of isolated atoms.

Potential energy surface (PES): A multidimensional surface representing the potential energy of a system as a function of nuclear coordinates, essential for modelling molecular dynamics.

Trajectory surface hopping: A computational method combining classical nuclear motion with stochastic transitions between quantum electronic states to simulate non-adiabatic dynamics.

References

1. Role of ion-pair states in the predissociation dynamics of Rydberg states of molecular iodine. Physical Chemistry Chemical Physics (2016).
2. Spectroscopy of weakly-bound complexes in highly excited electronic states: the He-I2(E3?g) ion-pair state. Journal of Physics Conference Series (2014).
3. Study of the Vibrational Predissociation of the NeBr2 Complex by Computational Simulation Using the Trajectory Surface Hopping Method. Mathematics (2020).