Collisional Quenching Research Articles

Flash Photolytic Production, Reactive Lifetime, and Collisional Quenching of O2(b 1Σg+, υ′ = 0)

Collisional quenching of O2(b 1Σg+, υ′ = 0) at room temperature by O2 and a variety of foreign gases has been investigated with a pulsed lifetime measurement technique. O2(b 1Σg+, υ′ = 0) has been produced in a pulsed mode through flash photolysis of O2 in the vacuum uv and has been detected through the emission of the (0, 0) band at 7620 Å of the forbidden O2(b 1Σg+ → X 3Σg− transition. The (0, 0) band intensity has been measured as a function of time after the photolysis flash and as a function of the O2 and foreign gas pressures. Quenching rate constants are derived from the reactive lifetimes. The photolytic production of O2(b 1Σg+, υ′ = 0) from O2 and the quenching by O2 has been studied at O2 pressures from 0.02–100 torr. The observations at low O2 pressures from 0.02 to about 1 torr are consistent with the previously established fast O2(b 1Σg+) production mechanism, O2 + hν → O(1D) + O(3P), O(1D) + O2(X 3Σg−) → O(3P) + O2(b 1Σg+), in the Schumann–Runge continuum region. No emission of the (1, 1) and (2, 2) atmospheric bands has been observed indicating that under the conditions employed, O2(b 1Σg+, υ′ > 0) is either initially formed only to a relatively small degree in this mechanism or that O2(b 1Σg+, υ′ > 0) is relaxed or quenched by O2 within less than 103 collisions. From the (0, 0) band fluorescence decay rates measured at O2 pressures from 0.02 to about 0.5 torr a quenching rate constant of 4.5 × 10−16 cm3 molecule−1·sec−1 is derived. At higher pressures, the decay rate deviates from a linear dependence on the O2 pressure indicating that the reactive lifetime is influenced by some secondary process at these pressures. The decay rate measured for example at 20 torr, O2 would correspond to a quenching coefficient of 4.8×10−17 cm3 molecule−1·sec−1. Quenching of O2(b 1Σg+, υ′ = 0) by He, Ne, Ar, Kr, Xe, H2, N2, CO, CO2, SF6, NH3, H2O, CH4, C2H6, C2H4, NO, NO2, and N2O has been investigated by measuring reactive lifetimes at constant O2 pressures as a function of the added foreign gas pressures. Quenching rate constants are reported and compared with previous results.

Collisional Quenching of Competitive Unimolecular Reactions. Vibrational Energy Transfer

Two competitive–consecutive unimolecular reaction systems were investigated. An H+olefin chemical activation technique was used to produce highly vibrationally excited polyatomic alkyl radicals (Rp*) in a narrow, nonequilibrium distribution of energy states centered near 45 kcal mole−1. The radicals studied, 3,3-dimethylhexyl-2·* and 3-methylhexyl-2·*, each decompose unimolecularly by several competitive C–C rupture reaction channels to stable olefins and smaller radical fragments; also, isomerization by H-atom transfer to secondary radicals, 4,4-dimethylhexyl-2·* and 4-methylhexyl-2·*, respectively, competes with C–C rupture. The isomerized species retain the vibrational excitation and undergo further consecutive unimolecular reactions including C–C rupture and H-atom isomerization. The spectrum of competitive reactions of Rp* via various paths, each characterized by a different energy threshold, provides a type of competitive chemical reaction “spectroscopy” wherein the amounts of the various decomposition events associated with each threshold act as transducers of the steady-state energy level populations of the reactive species. The competition between collisional stabilization and reaction via the several reaction paths of Rp* was measured for H2 and CF4 heat-bath molecules. The experimental results are compared with calculations made on a stochastic basis. Various collisional transition probability models characterized by different distributions of vibrational energy jump sizes were examined. Based on a step ladder model of transition probabilities, H2 transfers ∼ 400 cm−1 per collision and CF4 >̃ 1600 cm−1 per collision; for an exponential distribution of transition probabilities, a larger amount of vibration energy, e.g., ∼ 500 cm−1 for H2, is appropriate. The conclusions obtained by this technique are independent of information on the magnitudes of the collision cross sections. Some of the more important aspects of the radical chemistry are considered and several disproportionation–recombination ratios were obtained.