Mechanism of the laser initiated ultrafast intracluster reaction in Ba···FCH3 and Ba···FCD3

The dynamics of the intracluster reaction Ba···FCH3 → BaF + CH3 in the van der Waals complex of barium and monofluoromethane (CH3F or CD3F) initiated by resonant excitation to its electronic à state has been analyzed on the femtosecond timescale. To elucidate the mechanism we have compared the results of pump–probe experiments for the two complexes Ba···FCH3 and Ba···FCD3, both excited by 120 fs laser pulses at the resonant wavelength of 618 nm. The measured decay times of the parent ion signals are τIC = 270 ± 30 fs for Ba···FCH3 and τIC = 430 ± 20 fs for Ba···FCD3. The detailed analysis is based on the ab initio calculated potential energy surfaces of the parent complex. It is found that the reaction proceeds ia the next lower electronic Ã′ state of the complex which is populated after the initial à state excitation by internal conversion with the time constant τIC. The subsequent BaF formation out of the intermediate Ã′ state is restricted to very short times of about 50 fs for both parent clusters due to the competing dissociation of the complex (Ba···FCH3 → Ba + CH3F). Thus, the effective formation times for the BaF reaction products exceed only slightly the obtained time constants τIC for internal conversion.