Photodissociation of Alkyl Iodides in Helium Nanodroplets II: Solvation Dynamics

The solvation dynamics of non-thermal species in liquid helium has been investigated by photolyzing alkyl iodide molecules, CH3I, C2H5I and CF3I, embedded in helium nanodroplets. The iodine and CH3 fragments are found to leave the droplets solvated by a finite number of helium atoms, this in contrast to the C2H5 and CF3 fragments. The speed distributions of the IHeN and CH3HeN complexes show a prominent correlation with the degree of solvation N. It is argued that this correlation is caused by a dynamical adjustment of the solvation structure size to the relative speed of the traveling fragments as they pass through the helium bath. The absence of C2H5HeN and CF3HeN complexes is attributed to the large internal energy of these alkyl fragments and the existence of an efficient non-radiative relaxation pathway which leads to a rapid destruction of any possibly formed complexes.