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

Abstract

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 φsens, 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, φsens=(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 φsens=(0±0.15)×10−4.