Mechanism of the reaction, CH4+O(D-1(2))-> CH3+OH, studied by ultrafast and state-resolved photolysis/probe spectroscopy of the CH4 center dot O-3 van der Waals complex

The mechanism of the reaction CH4+O(D-1(2))--> CH3+OH was investigated by u ltrafast, time-resolved and state-resolved experiments. In the ultrafast ex periments, short ultraviolet pulses photolyzed ozone in the CH4.O-3 van der Waals complex to produce O(D-1(2)). The ensuing reaction with CH4 was moni tored by measuring the appearance rate of OH(v=0,1;J,Omega,Lambda) by laser -induced fluorescence, through the OH A <--X transition, using short probe pulses. These spectrally broad pulses, centered between 307 and 316 nm, pro be many different OH rovibrational states simultaneously. At each probe wav elength, both a fast and a slow rise time were evident in the fluorescence signal, and the ratio of the fast-to-slow signal varied with probe waveleng th. The distribution of OH(v,J,Omega,Lambda) states, P-obs(v,J,Omega,Lambda ), was determined by laser-induced fluorescence using a high-resolution, tu nable dye laser. The P-obs(v,J,Omega,Lambda) data and the time-resolved dat a were analyzed under the assumption that different formation times represe nt different reaction mechanisms and that each mechanism produces a charact eristic rovibrational distribution. The state-resolved and the time-resolve d data can be fit independently using a two-mechanism model: P-obs(v,J,Omeg a,Lambda) can be decomposed into two components, and the appearance of OH c an be fit by two exponential rise times. However, these independent analyse s are not mutually consistent. The time-resolved and state-resolved data ca n be consistently fit using a three-mechanism model. The OH appearance sign als, at all probe wavelengths, were fit with times tau (fast)approximate to 0.2 ps, tau (inter)approximate to0.5 ps and tau (slow)approximate to5.4 ps. The slowest of these three is the rate for dissociation of a vibrationally excited methanol intermediate (CH3OH*) predicted by statistical theory aft er complete intramolecular energy redistribution following insertion of O(D -1(2)) into CH4. The P-obs(v,J,Omega,Lambda) was decomposed into three comp onents, each with a linear surprisal, under the assumption that the mechani sm producing OH at a statistical rate would be characterized by a statistic al prior. Dissociation of a CH4O* intermediate before complete energy rando mization was identified as producing OH at the intermediate rate and was as sociated with a population distribution with more rovibrational energy than the slow mechanism. The third mechanism produces OH promptly with a cold r ovibrational distribution, indicative of a collinear abstraction mechanism. After these identifications were made, it was possible to predict the frac tion of signal associated with each mechanism at different probe wavelength s in the ultrafast experiment, and the predictions proved consistent with m easured appearance signals. This model also reconciles data from a variety of previous experiments. While this model is the simplest that is consisten t with the data, it is not definitive for several reasons. First, the appea rance signals measured in these experiments probe simultaneously many OH(v, J,Omega,Lambda) states, which would tend to obfuscate differences in the ap pearance rate of specific rovibrational states. Second, only about half of the OH(v,J,Omega,Lambda) states populated by this reaction could be probed by laser-induced fluorescence through the OH A <--X band with our apparatus . Third, the cluster environment might influence the dynamics compared to t he free bimolecular reaction.