Solvent-induced photostability of acetylene molecules in clusters probed by multiphoton dissociation.

We have studied the multiphoton photodissociation of (C(2)H(2))(n) and (C(2)H(2))(n) x Ar(m) clusters in molecular beams. The clusters were prepared in supersonic expansions under various conditions, and the corresponding mean cluster sizes were determined, for which the photodissociation at 193 nm was studied. The measured kinetic energy distributions (KEDs) of the H fragment from acetylene in clusters peak around 0.2 eV, in agreement with the KED from an isolated C(2)H(2) molecule. However, the KEDs from the clusters extend to kinetic energies of over 2 eV, significantly higher than the maximum fragment energies from an isolated molecule of about 1 eV. The photofragment acceleration upon solvation is a rather unusual phenomenon. The analysis based on ab initio calculations suggests the following scenario: (i) At 193 nm, photodissociation of acetylene occurs mostly in the singlet manifold. (ii) The solvent stabilizes the acetylene molecule, preventing it from undergoing hydrogen dissociation and funneling the population into a vibrationally hot ground state. (iii) The excited C(2)H(2) absorbs the next photon and eventually dissociates, yielding the H fragment with a higher kinetic energy corresponding to the first C(2)H(2) excitation. Thus, the H-fragment KED extending to higher energies is a fingerprint of the cage effect and the multiphoton nature of the observed processes. The photon-flux dependence of the KEDs reflects the rate of the vibrational energy flow from the hot ground state of acetylene to the neighboring solvent molecules.