Collisional activation of cyclobutene by hexafluorobenzene: A chemical probe for highly energetic collision in reactive systems

Article abstract of DOI:10.1063/1.456118

A new technique for studying reactive collisions is reported.A vibrationally hot donor molecule excites a cold reactant molecule via a single supercollision, transporting it above its threshold energy for reaction.In addition, this technique provides a means by which the functional form of transition probabilites can be found experimentally.As a specific example, vibrationally excited (110 kcal mol^-1) electronic ground state hexafluorobenzene, produced by UV excitation followed by internal conversion, is the donor of vibrational energy in the collisional excitation of two acceptors, cyclobutene and cyclopropane.For cyclobutene φ_sens, the quantum yield of 1,3-butadiene formation in terms of light absorbed by the donor, increased markedly as the total pressure decreased.At the lowest pressures employed, 20 mTorr, φ_sens=(9.1+/-0.6)*10^-4.The data are discussed in terms of highly energetic (strong) collisions which transfer enough energy to the cyclobutene to bring molecules above the critical energy for reaction (E₀=32.4 kcal mol^-1).In the case of cyclopropane (E₀=65 kcal mol^-1) no sensitized isomerization to propene was detected and φ_sens=(0+/-0.15)*10^-4.