Photodissociation of mass-selected cationic complexes and clusters

Abstract

A series of alkaline earth metal cation-alkyl halide molecule complexes including Mg⁺-(FCH₃)_n, n=l-4; Mg⁺-ClCH₃; Mg⁺-C₆H₅X (X = H, F, Cl, Br); Mg⁺-C₂H₃Cl, and Ca⁺-C₆H₆ have been produced in a laser-ablation pick-up source or nozzle expansion source. The target complexes are mass selected and excited by a tunable dye laser. The photodissociation action spectra are obtained by recording the fragment signals as a function of the excitation laser wavelength in a broad spectral region. The absorption bands in the action spectra are mainly assigned to the transitions ²P [lefe arrow] ²S centered on the Mg⁺ and Ca⁺ ions but perturbed by the presence of the solvent molecules. It is found that the photo-induced reaction channels of these cation-molecule complexes are strongly dependent on the type and size of the solvent molecules in the complexes.

Different photo-reaction patterns are identified with different halide substitution in the singly solvated cationic complexes Mg⁺-XCH₃ (X = F, Cl). Photodissociation of Mg⁺-FCH₃ produces exclusively MgF and CH₃⁺, whereas in Mg⁺-ClCH₃, non-reactive quenching product Mg⁺ is also observed besides the reactive channel mentioned above. A fast charge transfer mechanism is suggested for the energetically unfavorable reaction channel which leads to the formation of CH₃⁺. The different reaction patterns of Mg⁺-FCH₃ and Mg⁺-ClCH₃ are attributed to the different ground state structures of the complexes, which impose the initial configurations for the photo-induced reactions.

When more solvent molecules are introduced around the metal cation center, an abrupt change is discovered in the photo-induced reaction patterns for the complexes Mg⁺-(FCH₃)_n at n ≥ 2. While photo-reaction of the singly solvated complex Mg⁺-FCH₃ yields exclusively CH₃⁺, excitation of larger complexes Mg⁺(FCH₃)_2-4 produce predominantly bare and solvated MgF⁺. Photo-induced evaporation of the large complexes is also observed although with much lower yields. A solvation-enhanced photo-induced "harpooning reaction" mechanism is suggested to interpret this abrupt change in reaction patterns with increasing complex size.

For all the aromatic complexes Mg⁺-C₆H₅X (X = H, F, Cl, Br), the formation of Mg⁺ is found to be the predominant dissociation pathway. The similar photodissociation patterns and action spectra of these complexes with different halide substitutions suggest that all these complexes share a similar structure with Mg⁺ lying atop the benzene ring. Reaction fragment MgF⁺ is also observed in the photodissociation of Mg⁺-C₆H₅F with a different action spectrum. We believe that the MgF⁺ channel is from a different isomer, in which Mg⁺ is attached to the F end of C₆H₅F. All these experimental results are discussed with the help of quantum ab initio calculations.

Systematic photodissociation studies of small cluster cation Se_n⁺ (n = 3 - 8) allowed the appearance potentials of all the observed cluster fragment ions to be estimated from their yield curves as a function of the laser wavelength. In general, the odd-numbered cluster cations have much larger dissociation thresholds than those of the even-numbered cluster cations. In addition, the dissociation thresholds of the odd-numbered cations decrease with the increasing cluster size, while those of the even-numbered clusters increase with the increasing cluster size. The neutral dimer evaporation is found to be the lowest energy photodissociation channel for all the cluster cations.

Date of Award	2000
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology