Observation and interpretation of a time-delayed mechanism in the hydrogen exchange reaction.

摘要: Extensive theoretical and experimental studies have shown the hydrogen exchange reaction H+H2 --> H2+H to occur predominantly through a 'direct recoil' mechanism: H--H bonds break form concertedly while system passes straight over collinear transition state, with recoil from collision causing H2 product molecules scatter backward. Theoretical predictions agree well observations of this scattering process. Indirect mechanisms involving H3 intermediates been suggested as well, but these are difficult test because bimolecular reactions cannot be studied by femtosecond spectroscopies used monitor unimolecular reactions. Moreover, full quantum simulations time evolution not performed. For isotopic variant reaction, H+D2 HD+D, forward features observed in angular distribution attributed possible resonances associated quasibound complex. Here we extend measurements wide range energies interpret results using time-dependent simulation thus showing that two different modulate measured features. One is direct leads backward scattering, other indirect after delay about 25 femtoseconds.