Bradley R. Arnold

Spectroscopy of Cyclobutadiene

The dark age of cyclobutadiene (1), when any information about this elusive species was mostly speculative, lasted almost one century(1) and ended with the report of Pettit and co-workers(2, 3) that the molecule had been generated as a reactive intermediate and trapped. The correctness of this claim was later confirmed beyond reasonable doubt(4, 7)

Repopulation of nitrogen excited triplet state following laser-induced filamentation.

Laser-induced filamentation was used to study the dynamics of excited molecular nitrogen decay processes. It is well-known that upper excited nitrogen triplet states can be repopulated at time delays far longer than their fluorescence lifetimes. Examination of the time-resolved emission from several different species indicates that there are two major mechanisms acting to repopulate the N2(C(3)Πu) excited state. The results implicate dissociative electron recombination with the nitrogen cation dimer, N4(+), and energy pooling between two N2(A(3)Σu(+)) triplet states as the main pathways to repopulate the emissive upper triplet state. The densities of N2(A(3)Σu(+)) and free electrons produced during filamentation were measured under atmospheric pressures in nitrogen and estimated to be [N2(A(3)Σu(+))]0 = 3 × 10(15) cm(–3) and [e(–)]0 = 3 × 10(13) cm(–3). The methods outlined in this report could find significant utility in measuring the concentration profiles of these important reactive intermediates within laser-induced filaments produced under different conditions.