Photodissociation Dynamics in Molecular Systems

Photodissociation dynamics describes the process by which a molecule absorbs one or more photons, promoting it from the ground electronic state into an excited state manifold and thereby cleaving one or more chemical bonds. The energy absorbed is redistributed among translational, rotational and vibrational degrees of freedom of the photofragments, often mediated by non-adiabatic transitions at conical intersections or avoided crossings on coupled potential energy surfaces. Advances in ultrafast spectroscopy, free-electron laser sources and velocity-map imaging have enabled time-resolved, state-specific characterisation of bond fission pathways in gas-phase molecules. Studies range from simple triatomics, where central-atom elimination or roaming mechanisms prevail, to more complex polyatomics exhibiting site-selective fragmentation and charge-transfer phenomena. Photodissociation underpins processes of global importance: it drives the composition of planetary atmospheres, influences interstellar and circumstellar chemistry, and contributes to prebiotic pathways for molecular oxygen generation. The ability to predict quantum yields and fragment state distributions informs atmospheric modelling, astrochemical networks and the design of photochemical control strategies in synthetic and biological systems.

Research from Nature Portfolio

High-resolution translational spectroscopy of hydrogen sulphide has revealed that the photofragmentation branching ratios and internal state populations of SH(X), SH(A), S(3P) and H₂ co-products depend critically on the parent quantum level of a Rydberg 1B₁ state. Nuclear-spin and rotational predissociation pathways lead to inverted diatomic internal populations, illuminating the role of vibronic coupling in cometary H₂S photolysis. Experimental studies of vacuum ultraviolet three-body dissociation of water have demonstrated that H₂O under intense VUV excitation predominantly yields two hydrogen atoms and an oxygen atom in both 1D and 3P states. This pathway offers a prebiotic source of molecular oxygen in primitive atmospheres and calls for revision of interstellar photochemical models. Time-resolved extreme-ultraviolet spectroscopy combined with electron-ion partial covariance imaging has provided the first site-specific, femtosecond-scale view of photodissociation in a prototypical chiral molecule. By tracking iodine 4d core-to-valence transitions, researchers have disentangled competing fragmentation channels and mapped the ultrafast evolution of molecular binding energies, offering a blueprint for future site-selective studies at free-electron laser facilities.

Photodissociation Dynamics in Molecular Systems publication trend

The graph below shows the total number of articles in photodissociation dynamics in molecular systems across all publications each year (not limited to Nature Index journals).

Technical terms

Photodissociation: The absorption-induced breaking of one or more chemical bonds in a molecule.

Potential energy surface: A multidimensional surface representing the energy of a molecular system as a function of nuclear coordinates.

Conical intersection: A region where two electronic potential energy surfaces become degenerate, enabling ultrafast non-adiabatic transitions.

Non-adiabatic transition: A radiationless process in which nuclear motion induces a change in electronic state.

Vibronic coupling: Interaction between electronic and vibrational motions that facilitates state mixing and dissociation.

Velocity-map imaging: An experimental technique that records the velocity distribution of photofragments to infer kinetic energy and angular anisotropy.

Free-electron laser (FEL): A tunable, high-intensity light source capable of delivering ultrashort pulses in the vacuum ultraviolet or XUV regions.

References

1. Rotational and nuclear-spin level dependent photodissociation dynamics of H2S. Nature Communications (2021).
2. Three body photodissociation of the water molecule and its implications for prebiotic oxygen production. Nature Communications (2021).
3. A localized view on molecular dissociation via electron-ion partial covariance. Communications Chemistry (2022).
4. Exploring the vacuum ultraviolet photochemistry of astrochemically important triatomic molecules. National Science Review (2023).
5. The vibronic state dependent predissociation of H 2 S: determination of all fragmentation processes. Chemical Science (2023).
6. Dynamics of the A-band ultraviolet photodissociation of methyl iodide and ethyl iodide via velocity-map imaging with ‘universal’ detection. Physical Chemistry Chemical Physics (2015).