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Femtosecond Dynamics of the tert-Butyl Radical, t-C4H9
Bastian Noller, Raman Maksimenka, Ingo Fischer, Mario Armone, Bernd Engels, Christian Alcaraz, Lionel Poisson page, Jean-Michel Mestdaghpage

The excited state dynamics of the tert-butyl radical, t-C4H9, was investigated by femtosecond time-resolved photoionization and photoelectron spectroscopy. The experiments were supported by ab initio calculations. Tert-butyl radicals, generated by flash pyrolysis of azotert-butane, were excited into the A 2A1 (3s) state between 347 and 307 nm and the 3p band at 274 and 268 nm and ionized by 810 nm radiation, in a [1+2’] or [1+3’] process. Electronic structure calculations confirm that the two states are of s- and p-Rydberg character, respectively. The carbon framework becomes planar and thus ion-like in both states. 

 
Main orbitals involved in the dynamics

Main orbitals involved in the dynamics

The photoelectron spectra are broad and seem to be mediated by accidental intermediate resonances in the probe step. All time-resolved photoelectron spectra can be described by a single decay time. For the A 2A1 state lifetimes between 180 and 69 fs were measured. Surprisingly a much longer lifetime of around 2 ps was found for the 3p state. In order to understand the decay dynamics, the potential energy was computed as a function of several important nuclear coordinates. A 1,2 H-atom shift to the iso-butyl radical seems not to be important for the excited state dynamic. Qualitative considerations indicate curve crossings between the ground state, the 3s state and a valence-state along the asymmetric C-C stretch coordinate that correlates to the dimethylcarbene + methyl product channel. The implications of the present study for earlier work on the nanosecond time scale are discussed.

 
Time-resolved photoelectron signal

Time-resolved photoelectron signal

Femtosecond Dynamics of Isolated Phenylcarbenes
Last Update: 12 January 2011