Ultraviolet photodissociation dynamics of formyl fluoride Part 1 Competition between the decay channels for the à state

Abstract

Excitation spectra were recorded over the range 39500 cm ⁻¹ to 40300 cm ⁻¹ for laser induced fluorescence (LIF) of jet-cooled formyl fluoride excited to its first excited singlet state (à ¹ A″), and between 39700 cm ⁻¹ and 43800 cm ⁻¹ for the 2 + 1 resonance-enhanced multiphoton ionisation (REMPI) of H atoms arising from its predissociation. H atoms were not detected below a threshold energy ca. 2200 cm ⁻¹ above the origin of the à state (37492 cm ⁻¹ ), even though Doppler measurements of the H atom recoil energy indicate that the dissociation limit D ₀ (H–CFO) lies approximately 5000 cm ⁻¹ below this threshold. A detailed investigation was made of the branching between the LIF and H atom yield channels as a function of rotational state for excitation within eight vibronic bands. At threshold, this branching is energetically controlled. A few hundred cm ⁻¹ higher accidental resonances caused random variations in the branching ratio, over more than an order of magnitude, between near-lying rotational states. The variations in LIF and H atom yields were completely complementary. This made possible the determination of absolute quantum yields for dissociation, which approached unity for many states. By 1000 cm ⁻¹ above the threshold, these effects had become systematic and indicated an enhancement of the dissociation rate through b-axis Coriolis coupling. LIF became vanishingly weak above 40300 cm ⁻¹ , by which energy the dissociation yield was close to 1 for all states. These results are discussed in the light of earlier studies and correlation diagrams between the electronic states of HFCO and its dissociation products. The threshold to H + FCO is attributed to a barrier on the lowest triplet state surface arising from a conical intersection of two triplet surfaces for planar HFCO. Intersystem crossing mediates this dissociation above its threshold energy. Below this energy internal conversion to the continuum of the ground state facilitates competition to LIF by dissociation to the molecular products HF + CO. Measurements of the recoil anisotropy of the H atoms for selected excitation transitions show that the recoil vector is strongly out of the FCO plane, which accords with a theoretical prediction for the triplet-state mechanism. The theoretical analysis presented in this paper should find application to the photodissociation dynamics of many other small parent molecules of the type RXCO.