Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold

Abstract

Ketene (CH₂CO) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH₂ (ã ¹A₁). CH₂ was detected by laser‐induced fluorescence (LIF). The appearance threshold and yield curve of individual ¹CH₂ rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency. The dissociation occurs on the ground electronic state potential energy surface. The threshold for CH₂CO→¹CH₂+CO is found to be 30 116.2±0.4 cm⁻¹. By varying the delay between the photolysis and probe pulses, a lower bound of 7×10⁷ s⁻¹ was set for the dissociation rate on the triplet surface at the singlet energy threshold. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. The experimental data provide a rigorous test of theoretical models of photofragment dynamics. The data clearly show that nuclear spin is conserved through the photodissociation. PHOFEX curves calculated from phase space theory (PST) are in excellent agreement with the experiment and show that there is no barrier along the reaction coordinate. The singlet/triplet branching ratio as a function of photolysis laser frequency is inferred from PST fits to the PHOFEX data. Comparisons with the statistical adiabatic channel model (SACM) are also presented. The data show that the only dynamical constraints on product state rotational energy distributions are conservation of energy, angular momentum, and nuclear spin.