A series of alkaline earth metal cation-alkyl halide molecule complexes including Mg
+-(FCH
3)
n, n=l-4; Mg
+-ClCH
3; Mg
+-C
6H
5X (X = H, F, Cl, Br); Mg
+-C
2H
3Cl, and Ca
+-C
6H
6 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
+-XCH
3 (X = F, Cl). Photodissociation of Mg
+-FCH
3 produces exclusively MgF and CH
3+, whereas in Mg
+-ClCH
3, 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 CH
3+. The different reaction patterns of Mg
+-FCH
3 and Mg
+-ClCH
3 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
+-(FCH
3)
n at n ≥ 2. While photo-reaction of the singly solvated complex Mg
+-FCH
3 yields exclusively CH
3+, excitation of larger complexes Mg
+(FCH
3)
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
+-C
6H
5X (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
+-C
6H
5F 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 C
6H
5F. 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.
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