Ultrafast investigations on the initial steps of soot formation

The origin of the soot production,which is of major importance to combustion, is still not fully understood. Femtosecond spectroscopy is used to follow the fast nuclear rearrangements of C₆H₆ species preliminary to the formation of the first benzene ring. 1,5-hexadiyne is an important intermediates in the ring closure following the fast reaction path of the propargyl ´self-reaction´.

[1] J. A. Miller and J. Klippenstein, J. Phys. Chem. A , 107 (2003) 7783.

Figure: The benzene formation mechanism via the loose transition states of 1,5-hexadiyne, 1,2-hexadiene-5-yne and 1,2,4,5-hexatetraene is schematically presented.

It is believed that chemically activated intermediates may form aromatic rings faster than bimolecular collisions, and therefore fundamental dissociation, isomerization or H atom loss reactions gain importance. The time resolved transformation between the trans to the gauche conformer of 1,5-hexadiyne is one of the goals of this project. Initial experiments on a mixture of 1,5-hexadiyne in 50% pentane applying a dispersed off-resonant TG-method are performed. Compared to 100% pentane, the mixture shows mainly a stronger coherent spike at zero delay. However, in the frequency domain a difference in the spectra has been observed at -0.7 ps delay time, which may be used to characterize the underlaying processes.

[2] G. Knopp , A. M. Walser, M. Meisinger, P. P. Radi, and T. Gerber (in preparation).

Figure: The frequency dispersed early time response from a 800 nm off-resonant DFWM experiment of 1,5-hexadiyne in 50% pentane compared to 100% pentane shows a rather similar trace in the time domain. However, in the frequency domain at ~ 0.7 ps delay time the spectra show different characteristics.

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