SPECTROSCOPY AND DYNAMICS OF THE LASER INDUCED INTRACLUSTER (Ba··FCH3)* -> BaF* + CH3 AND Ba* + FCH3 REACTION

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An overview of the spectroscopy and dynamics of the intracluster Ba··FCH3 + hv → BaF* (Ba*) + CH3 (CH3F) bimolecular reaction is presented. The Ba··FCH3 weakly bound complex is produced by laser vaporisation of a solid Ba sample followed by supersonic expansion of the Ba vapour with the solvent gas. Typical mass spectra obtained by the laser ionisation technique and time-of-flight mass spectrometry are presented. The Ba··FCH3+ ionisation potential was found to be 4·55 ± 0·03 eV. Laser induced photodepletion of the Ba··FCH3 complex was observed at distinct wavelengths of the excitation laser, together with two open channels for complex photofragmentation: a reactive channel, giving BaF as a product, and a non-reactive channel where Ba is formed. The photodepletion spectrum was measured in the 545–630 nm region, displaying two distinct regions that were interpreted as two different electronic states of the complex. The lifetime of the excited A-state was estimated by applying the inverse Fourier transform to the measured spectrum, giving a lifetime of about 250 fs. This result was confirmed in a pump and probe femtosecond experiment, where a lifetime of about 270 ± 30 fs was measured. In addition to the photodepletion spectrum, product action spectra were measured. This allowed the direct determination of the reaction probability as a function of excitation energy. In the A-state, the BaF action spectrum has been measured over the 16065–16340 cm-1 energy range. The reaction probability, PR (E), displays a peak structure with an energy spacing of 10·9 cm-1 that may be related to the internal motion of the transition state of the reaction. For the B-state, product action spectra for both the reactive and non-reactive channel are presented for the 17795–18250 cm-1 energy range. Here, the reaction probabilities show an oscillatory behaviour with opposite phase, which could be due to quantal interferences between the two photofragmentation channels. In addition, the energy spacing of the oscillations can be related to the internal motions of the transition state. All these features are discussed taking into account the interplay between the spectroscopy and dynamics associated with each electronically excited state of the complex that governs the photoinitiated harpooning reaction.