Dissociation on ground-state potential-energy surfaces

Abstract

Two new techniques allowing for the resolved spectral study of dissociation on ground-state potential-energy surfaces are presented. First, Stark level-crossing spectra of highly vibrationally excited ground-state formaldehyde (S^*₀) characterize the initial rovibrational wavefunctions of dissociative formaldehyde states. Individual S^*₀ D₂CO states are resolved near the barrier to dissociation but broaden and overlap at higher energies. Neighbouring resolvable states possess enough individual vibrational character to vary by over one order of magnitude in lifetime. A potential barrier height of 78.0–81.1 kcal mol^–1 has been determined for H₂CO → H₂+ CO dissociation. Second, photofragment excitation (PHOFEX) spectra probe the production of correlated fragment states from ketene photodissociation. Molecular-beam PHOFEX spectra show that ketene possesses no observable rotational structure and only weak vibrational structure. The PHOFEX spectrum of an individual rotational state of the lowest vibronic state of singlet methylene gives the energetic threshold for formation of that state along with ground-state CO. The threshold for the combination with each successive CO(v″= 0, J″) channel is clearly resolved. A statistical calculation reproduces the main features of the PHOFEX spectra. Analysis of the PHOFEX spectra gives a H₂CCO →¹CH₂+ CO dissociation threshold of 30 102 ± 15 cm^–1. Finally, the effect of a potential barrier in the dissociation coordinate on statistical vs. dynamical control of dissociation is discussed.