Ultrafast dissociation of the ethyl radical

Alkyl radicals constitute a class of unstable hydrocarbon species with central importance in combustion processes. Especially the dynamics of the H atom loss and the building of a double bond is of major interest. Amaral et al. investigated the photodissociation of ethyl measuring the H atom energy-dependent angular distribution after UV excitation of the molecule and concluded that at least two channels for the dissociation exist [1]. A �slow� channel, which is isotropic in character for H atom release and therefore is expected to be comparable (or longer) in lifetime to the rotational period (~ ps) and a fast anisotropic channel with high kinetic energy release, suggesting a prompt and direct dissociation from the excited ²A�(3s) state via its C₂v configuration ²A₁(3s).

Figure: Fast and slow dissociation channels of the ethyl radical.

By the application of fs pump - multiphoton-probe spectroscopy on ethyl iodine we observed evidences for the two dissociation channels of the ethyl radical [2]. The fitted life time of the 3s-Rydberg state is 233 fs at 265 nm excitation wavelength. A second time constant could be fitted to the detected ion signal to ~ 6-7 ps. This is comparable to the rotational periods of the molecules. The results confirm the presence of a fast dissociation channel with a dissociation rate in the order of ~3*10¹² s^-1 and a �slower� process of ~2*10¹¹ s^-1 [3,4].

[1] G. Amaral, K. Xu, and J. Zhang, J. Chem. Phys., 114 (2001) 5164.
[2] G. Knopp , P. Beaud, P. Radi, M. Tulej, and T. Gerber, Femtochemistry and Femtobiology: ultrafast dynamics in molecular science, world scientific,116, (2002)
[3] W. Hase, B. Schlegel, V. Balbyshev, M. Page, J. Chem. Phys., 100 (1996) 5354.
[4] A. Zyubin, A. Mebel and S.Lin, Chem. Phys. Lett., 323 (2000) 441.

Figure: Excitation scheme for fs pump multiphoton probe ionization of ethyl iodide. The ion signals content a fast and a slow decay.

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