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

The dynamics of the intracluster reaction Ba . . . FCH3 --> BaF + CH3 in th e van der Waals complex of barium and monofluoromethane (CH3F or CD3F) init iated by resonant excitation to its electronic (A) over tilde state has bee n 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 reso nant wavelength of 618 nm. The measured decay times of the parent ion signa ls are tau (IC) = 270 +/- 30 fs for Ba . . . FCH3 and tau (IC) = 430 +/- 20 fs for Ba . . . FCD3. The detailed analysis is based on the ab initio calc ulated potential energy surfaces of the parent complex. It is found that th e reaction proceeds via the next lower electronic (A) over tilde' state of the complex which is populated after the initial (A) over tilde state excit ation by internal conversion with the time constant tau (IC). The subsequen t BaF formation out of the intermediate (A) over tilde' state is restricted to very short times of about 50 fs for both parent clusters due to the com peting dissociation of the complex (Ba . . . FCH3 --> Ba + CH3F). Thus, the effective formation times for the BaF reaction products exceed only slight ly the obtained time constants tau (IC) for internal conversion.