Photodissociation studies of CH3I and CF3I in fluid 4Helium nanodroplets

A novel experimental apparatus to study the dynamics of photodissociation and photoionization processes in the interior or at the surface of helium droplets by means of imaging techniques has been designed, built and characterized. This molecular beam machine combines a versatile cryogenic cluster source with a cross beam scattering stage to introduce impurities into the droplets and a velocity map imaging instrument as the principal analysis tool. In a first series of experiments we have used this apparatus to examine the 266 nm A band photolysis of CH3I and CF3I embedded in liquid 4He clusters with selected mean sizes in the range from ≈ 2000 to ≈ 20000 atoms. The in-situ creation of photofragments in the droplet interior constitutes an original approach to studying the translational dynamics of neutral microscopic particles in the quantum liquid, which until now were poorly characterized. For all mean droplet sizes under investigation we observe that fragments of every studied species can escape from the droplets. A predominantly thermally driven fragment release clearly is not compatible with the three-dimensional velocity distributions of the products. Accordingly, the vast majority of departing photofragments is thought to escape from the clusters simply by pushing away helium atoms, an escape process which we designate "direct". It is found that the translational and angular relaxation of the escaping products generally increases both with decreasing fragment mass and increasing mean droplet size. We furthermore show that droplet angular momenta can at most have a minor effect on the measured angular product distributions which therefore are thought to mainly reveal deflections of the fragment trajectories inside the droplets. Accompanying classical Monte Carlo simulations based on independent pairwise hard-sphere scattering can reproduce qualitative and quantitative properties of the observed speed and angular distributions and suggest strongly that binary fragment–helium collisions are at the origin of the translational and angular relaxation of the escaping products. By state-specifically detecting methyl fragments in the vibrational ground and in the first umbrella mode (ν2) excited state we show that vibrational cooling in the course of the fragment escape is not complete and that the mean relative kinetic energy loss of departing molecular fragments does not depend significantly on the level of vibrational excitation. Departing iodine and CH3 fragments are found to leave the helium droplets as fragment–Hen complexes with sizes n of up to 15 and more helium atoms. These partially solvated fragments exhibit characteristic droplet frame speeds that strongly correlate with the complex size n: Larger structures escape with lower speeds from the helium clusters. The velocity map images of size-selected IHen products show furthermore that the characteristic complex speeds depend little on the initial kinetic energy of the nascent iodine fragments or on the helium droplet size. Instead, the variation of the latter parameters is shown to profoundly affect the size distribution of the escaping IHen products. We present evidence that these complexes build up already in the droplet interior and argue that their formation should be regarded as the dynamical development of helium solvation shells around the translationally relaxing radicals. It is proposed that both the speed of a moving fragment relative to the helium bath and the strength of the fragment–He interaction determine the instantaneous size of a forming complex structure to a good degree of approximation. By velocity mapping bare parent molecules we furthermore demonstrate the occurence of CH3–I fragment recombination in helium droplets with mean sizes of more than ≈ 3000 atoms. The (CH3I)+ ion signal appears with a time constant of ≈ 5 ns and shows a finite kinetic energy release which is thought to arise from a complete evaporation of the solvent atoms as a result of the internal cooling of the recombined molecules. For CF3I no recombination signal is observed. This effect is attributed to the different fragment masses and the dissimilar partitioning of the total kinetic energy in the CF3I photolysis reaction.