Unimolecular reaction paths of electronically excited species. III. Production of CH+3ions from CH3OH+as an example of isolated state dissociation

摘要

The CH3OH+ion dissociates to CH+3+OH as soon as the energy of stateBtilde;thinsp;2Aprime; is reached. This fragmentation cannot be described by RRKM theory.Ab initiopotential energy surfaces at the CI level have been calculated as a function of the CO bond stretching coordinateRand of the COH angle fgr; for the lowest2Aprime;,2APrime;,4Aprime;, and4APrime; states. Several zones of strong nonadiabatic interaction have been detected. They correspond to avoided crossings and account for the existence of internal conversions among the different states. In particular, there exists an avoided crossing with a small energy gap between statesBtilde;thinsp;2Aprime; andAtilde;thinsp;2Aprime;, which is localized in a very restricted range of internuclear distances (R=1.555 Aring; and fgr;=69.8deg;). This avoided crossing plays a very important role because it is the only lowhyphen;energy dehyphen;excitation channel of stateBtilde;. The nonadiabatic coupling matrix elements lang;Atilde;Verbar;part;/part;qVerbar;Btilde;rang; have been calculated for coordinatesRand fgr;. They are represented by very narrow Lorentzians and they bring about a very high nonadiabatic transition probability. Thus, the population of the upper electronic states (Btilde;thinsp;2Aprime;,Ctilde;thinsp;2APrime;, andDtilde;thinsp;2Aprime;) detected in the photoelectron spectrum in the 14ndash;18 eV energy range easily cascades down to statesAtilde;thinsp;2Aprime; via this mechanism. Since the zone of nonadiabatic interaction is very narrow, the population starts its evolution on the potential energy surface of statesAtilde; with wellhyphen;defined initial conditions. In the present case, internal conversion is a much more specific preparation than optical excitation. The reactive flux then branches into two paths. One of them leads directly to the CH+3+OH asymptote and escapes statistical energy redistribution. The other path leads to the bottom of the potential energy surface and gives rise to the RRKM/QET component of the mass spectrum, i.e., leads to the other fragmentations. lsquo;lsquo;Isolated state decayrsquo;rsquo; thus corresponds to dissociation of the nonrandomized fraction of the population and not to isolation, i.e., to lack of radiationless transition to the lower electronic states. The circumstances which give rise to such a behavior are analyzed.