Spectroscopy and Dynamics of Excited Harpooning Reactions:  The Photodepletion Action Spectrum of the Ba···FCH₃ Complex

Abstract

A laser time-of-flight mass spectrometer setup used to investigate the spectroscopy and dynamics of weakly bound complexes is described. Complexes were produced by laser vaporization of metals into a supersonic expansion of helium and a solvent. The first results consisting of laser ionization and photodepletion spectra of Ba‚‚‚(FCH 3 ) n for 1 e n e 3 are reported. The measurement of the Ba‚‚‚FCH 3 + signal as a function of the ionization photon energy allowed estimation of the complex ionization potential, which was found to be 4.5( 0.1 eV. The photodepletion spectrum of the Ba‚‚‚FCH 3 complex displayed two distinct regions. At longer wavelengths it showed a high cross-section (60-70 Å 2 ) and a well-defined vibrational structure, while a much smaller cross-section (2-4 Å 2 ), with a more diffuse structure, appeared at shorter wavelengths. In addition, clear resonances were observed at longer wavelengths, indicating a significant coupling between the bound excited potential of the complex and the (open channel) ionic potential leading to BaF * + CH 3 reaction products. All of 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.

Transition State Spectroscopy of the Photoinduced Ca + CH3F Reaction. 2. Experimental and Ab Initio Studies of the Free Ca···FCH3 Complex

“…The first one implies a significant alteration of the calcium electronic structure that brings a level of calcium within the excitation region. This can be due either to a substantial electron transfer between Ca and CH 3 F in the excited state, or to the promotion of the excited electron to a Rydberg orbital, stabilized by the interaction with CH 3 F. The second tentative explanation was invoked due to the observation of structures in the action spectrum, an observation that makes the present Ca/CH 3 F system very different from the apparently similar Ba/CH 3 F system (Ca and Ba are both alkaline earth atoms) studied in the groups of Ureña [17,18,19,20,21,22] and Radloff [23,24,25,26,27]. The structures observed in the Ca/CH 3 F system were tentatively assigned to the C-F stretch in the CH 3 F molecule, suggesting that this stretch plays an important role in the dynamics of turning on the reaction.…”

“…The Na/HF [9], Li, Na/HF, CH 3 F [10,11,5], Na 2 /CH 3 Cl [12], Ca/HX [13,14,15,16] and Ba/CH 3 F [17,18,19,20,21,22] systems were explored using the latter technique. Realtime studies have also been performed on the Ba/CH 3 F system [23,24,25,26,27].…”

Rotationally Resolved REMPI Spectra of CaH in a Molecular Beam

“…A detailed description of the experimental set-up employed in this work has been given elsewhere (9,10): atomic Ca is produced by laser vaporization of a rotating Ca disk, using the second harmonic output of a Nd-YAG laser with an energy of 1 mJ. The CaH molecules are formed in the subsequent supersonic expansion of the Ca vapour in a gas pulse of a mixture of He and H 2 (∼20%).…”

Dynamical and stereodynamical studies of alkaline-earth atom–molecule reactions

“…. FCH 3 (circles, data from [128]), but with a maximum photo-depletion cross-section of about 35 Å 2 . The energy dependence of the reaction probability may be obtained from the ratio of the product action spectrum of the channel under consideration, and the total photo-depletion spectrum of the parent molecule, as it was mentioned above.…”

“…Notice the double-peak structure with an energy spacing of $150 cm À1 . This figure is reproduced with permission from[128].338K.-C. Lin and A. Gonza´lez Uren˜aA closer look at figure 32 reveals an increase in intensity of the Ba and BaF species, suggesting two open channels for the complex fragmentation, namely:…”