UV-photodissociation dynamics of small molecules and free radicals studied by high-n Rydberg hydrogen atom time of flight spectroscopy.

摘要

The power of the H-atom Rydberg Time of Flight Technique (HRTOF) for the study of photodissociation reactions involving the ejection of H-atoms is demonstrated both in closed shell molecules (CH₃I, HONO,CH ₃CH₂I) as well as free radicals (CH₃CH₂).; In the case of HONO, the NO₂ fragments are produced in at least three electronic states: ground X˜ 2A₁ and excited à 2B₂ and B˜ 2B ₁ (and/or C˜ 2A₂) states with an average CM product translational energy is ⟨E_T⟩ = 0.3E_avail. NO₂ fragments are highly vibrationally excited in each of these electronic states, and in particular, a long vibrational progression of NO₂ bending mode in the à 2B₂ state has been observed. Branching ratios of the NO ₂ electronic states are estimated: X˜ 2A₁ :à 2B₂: B˜ 2B ₁/C˜ 2A₂ ≈ 0.13:0.21:0.66. The O-H bond photodissociation of HONO from the second electronically excited singlet state B˜ 1A′ proceeds via multiple dissociation pathways and complex non adiabatic processes account for the formation of all four lowest electronic states of NO₂.; In CH₃I and CH₃CH₂I, a aniostropic H channel with high energy release was detected. Around 81% of the available energy appears as product translational energy in both molecules. In the first case vibrational structure was resolved with excitation in the C-I stretching mode and out-of-plane modes.; Undoubtedly, the most promising results towards the future are the studies on free radicals, in particular the C₂H₅. Our technique renders excellent sensitivity despite the experimental difficulties in working with radicals. A previously undetected aniostropic H channel was observed at 245 mn in accordance to ab initio calculations. Furthermore, isotopic studies revealed that the dissociation occurs in the β position.; One of the most impressive features of our technique is the possibility of detecting different H-channels at the same time and being able to resolve its relative contribution and dynamic behavior. HONO, CH₃X (Cl, Br) at 121.6 mn, C₂H₅I and C₂H₅ are excellent examples of this. Furthermore, in the cases of ethyl radical and ethyl iodide both channels observed correspond to H atoms that differ only on their dynamical origin but actually lead to the formation of the same products in the same electronic states.