Dissociation dynamics of acetylene Rydberg states as a function of excited state lifetime

Abstract

The state selective photodissociation of acetylene, C2H2/C2D2, was studied in the wavelength range 121.2–132.2 nm by high resolution Rydberg atom time-of-flight measurements on the atomic fragment, H/D. In the wavelength region studied members of all four Rydberg series and the highly excited Ẽ valence state were state selectively excited using tunable vacuum-ultraviolet laser radiation. The lifetime of the excited states which were studied varied from 58 fs to more than 2 ps. Formation of the ethynyl radical in its X̃ electronic ground state and its first electronically excited à state is observed with practically no indication of B̃ state fragments. Two decay channels with different dissociation dynamics were also observed. In both channels the observed decay dynamics depended strongly on the excited state of the parent molecule. Further there are major differences between these two dissociation pathways with respect to the measured internal energy and angular distributions. In one channel the dissociation is dominated by dynamical effects and the C2H fragments are formed with a high degree of vibrational excitation. In contrast to this in the second channel a smooth internal energy distribution is observed indicating that the fragment quantum state distribution is spread over a considerable range of the available phase space. Moreover, this second channel can be fit with a phase space model constrained only by conservation of energy and angular momentum. This is further evidence for the randomization of internal energy during the dissociation process.

The predissociation of highly excited states in acetylene by time-resolved photoelectron spectroscopy

“…The photofragment action spectra of the C 2 D radical does not show an angular dependency as strong as the ones observed in C 2 H, while still exhibiting two dissociation channels. 6 This is in line with the similarity that we observe between the decay times observed for the ion and angleselected photoelectron signal.…”

“…6 were recorded after excitation to the vibrational 2 0 1 and 4-5 0 2 levels, respectively, leading to similar conclusions. 6 The angular emission patterns for the states measured in this study might provide conclusive information about this matter.…”

“…6 were recorded after excitation to the vibrational 2 0 1 and 4-5 0 2 levels, respectively, leading to similar conclusions. 6 The angular emission patterns for the states measured in this study might provide conclusive information about this matter. A study of the photoelectron spectra from these states using an imaging detector would be an advantage, 34 as recently achieved in other femtosecond timeresolved experiments on photodissociation dynamics 35 and intersystem crossing.…”

“…1 ⌸ u (3d␦ g ) Rydberg state lying ϳ6000 cm Ϫ1 above the F state. 6,33 These spectra indicate two open dissociation channels: ͑i͒ a very fast dissociation ͑femtosecond time scale͒ in a quasi-linear geometry leading to fast H fragments and to C 2 H(A 2 ⌸) fragments with low vibrational excitation in the stretching modes; and ͑ii͒ a slower dissociation ͑picosecond time scale͒ taking place in nonlinear geometry and leading to slow H fragments and C 2 H(A 2 ⌸) fragments with a broad, structureless statistic-like energy and isotropic angular distributions. 6,33 This broad internal energy distribution in the C 2 H(A 2 ⌸) fragments corresponds to highly excited bending levels, as was recently shown by the analysis of the C 2 H fragment emission spectra, 11 and is compatible with a slow dissociation process leaving time to the nuclei for large amplitude bending motion.…”

“…These bands have been explored via photofragment action spectroscopy where lifetimes ranging from 60 up to 150 fs were reported. 3,6 Here, we investigate the dynamics from the F 4 0 2 state and the E 4-5 0 2 state. The excitation scheme is shown in Fig.…”