Dynamics of the Cage Effect for Molecular Photodissociation in Solids

Abstract

The photodissociation of small guest molecules in rare-gas crystals is studied by Molecular Dynamics simulations, and the results are compared to recent experiments. The findings throw light upon the physical mechanisms on the detailed atomic level whereby the cage effects occurs in solid-state reactions. Several types of cage effects are found and characterized in terms of the dynamical mechanisms involved, and their consequences on the yield of cage-exit by the product, and on the timescale of the mutual separation of the photofragments. The following main results are discussed:

• (1) The photolysis of Cl₂ in Ar exhibits a delayed photofragment separation for low excitation energies (hv≤ 8eV), with a pronounced change-over to a direct cage exit mechanism for higher energies.

• (2) In the photolysis of F2 in Kr, anomalously high yields are found at low temperature for the F atom cage exit. Long distance migration (λ > 40 A) is seen for some of the F atoms following photolysis.

• (3) Cage exit of H atoms from photolysis of HCl in Ar occurs as two successive pulses in time — a pulsating cage effect.

• (4) It is found that in photolysis of ICN in Ar, there is no cage exit of the CN photofragment, even at fairly high temperatures. This study suggests that the complete caging of molecular photogragments is rather general for reactions in solids, and contrasts with the corresponding behavior for liquids.

Detailed understanding emerges from the simulations as to the dynamical origins of these effects, and general insights are gained on the nature of the cage effect in solids.