Laser-induced control of (multichannel) intracluster reactions

Abstract

An experimental and theoretical study on the excited Ba ··· FCH3(A) photodissociation yield as a function of the excitation laser fluence is reported. Experimentally, it was found that the two- photodissociation channel yields, i.e. the reactive BaF and non-reactive Ba ∗ products, increased exhibiting a similar behaviour, as the laser fluence changed from 0.2 up to ca. 4 mJ/cm 2 . Beyond this value the BaF yield levels off and the Ba ∗ decreases over the 4-7 mJ/cm 2 range. The theoretical simulation of the excited state electron-ion dynamics within the time-dependent density functional theory revealed that the reactive channel dominated the photofragmentation dynamics as it occurs within a femtosecond time scale and became accelerated as the photodissociation laser fluence increased. By contrast, the non-reactive channel only manifested for low laser fluences at the nano/picosecond time regime resulted inactive as the laser fluence increased. A simple scheme to control the dynamics of the intracluster multichannel reaction is suggested in which the slowest the channel the easiest to close it as the excitation laser power increases. PACS. 36.40.Jn Reactivity of clusters - 36.40.Qv Stability and fragmentation of clusters - 34.50.Rk Laser-modified scattering and reactions Laser control of chemical reactions is currently attracting a considerable attention from both theoretical and exper- imental point of view (1-3). Among several schemes to implement such an objective one of the most promising is based on quantum interference. For unimolecular reac- tions, in which a single reactant molecule is transformed to several product molecules, two types of control have been applied insofar. Namely, the "two-beam interference" (4-6) and the "two-pulse time delay" control (7,8). While these two control schemes are based on a limited number of op- timisation parameters (i.e., the phase difference between the two lasers and the time delay between the pump and dump laser pulses, respectively) a new powerful method has been developed (9) based on the adaptative shaping of femtosecond laser pulses which works satisfactorily even for complex systems. Although, in many cases, the elec- tron/ion dynamics of the system is not fully understood, there are examples in which the dynamics of a unimolec- ular reaction induced by an optimal femtosecond pulse, generated from adaptative learning algorithms, has been deciphered (10).