Understanding Unimolecular Dissociations with Loose Transition States: Photofragmentation Dynamics of Ketene at the Singlet Threshold

Abstract

AbstractPhotofragment excitation (PHOFEX) spectra of ketene (CH2CO) were taken by probing rovibronic transitions of the singlet methylene fragment while scanning the photolysis laser frequency. CH2(ã1A1) was detected by laser‐induced fluorescence in a supersonic free jet. The dissociation threshold of the reaction, CH2CO → 1CH2 + CO was determined to be 30116.2 ± 0.4 cm1. The nuclear spin is conserved during the dissociation. Sharp rises in the 1CH2 PHOFEX spectra are observed whenever a new CO rotational state becomes energetically accessible. Phase Space Theory (PST) is in excellent agreement with the experimental data. The Statistical Adiabatic Channel Model (SACM) and the related SACM‐PST theory fail to fit the experimental PHOFEX curves except in the limit where they become indistinguishable from PST (i.e. α/β > 0.7). The data show that there are no barriers to dissociation and that essentially every accessible product quantum state is equally likely to be formed. The implications of these findings for the theory of unimolecular dissociation are discussed.