Infrared laser-induced photofragmentation of the positive and negative ions of size-selected SF6 (NO)n clusters

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Abstract
A detailed study has been undertaken on the infrared photofragmentation of SF6(NO)±n cluster ions. A line-tuneable CO2 laser has been used to excite the ν3 vibrational mode of the SF6 molecule which is followed by the observation of three separate fragmentation channels: –SF6, –NO, and –2NO. The relative intensities of the fragments are found to be sensitive to the sign of the charge on the ion, the cluster size, and whether n is either odd or even. Within clusters of the same charge, the most marked transitions in fragmentation pattern are found between odd- and even-sized cluster ions, with the decay channels favoring those processes which lead to the formation of even electron ions. There are also large differences in fragmentation pattern between the negatively and positively charged ions. A summation of fragment ion intensities as a function of laser wavelength is used to determine infrared absorption profiles and their shapes confirm a pronounced difference in behavior between SF6 (NO)−n and SF6 (NO)+n. The results for the positively charged ions are interpreted in terms of a central (NO)+3 core which serves to fix the position of the SF6 molecule via an attractive ion-induced dipole interaction. In contrast, observations on the negatively charged ions, suggest that the interaction between the excess electron and the SF6 is predominantly repulsive and also sufficiently diffuse as to keep the molecule comparatively mobile even in large cluster ions. It is suggested that the excess electron in SF6 (NO)−n may occupy a surface state and that the state has associated with it approximately 15 NO molecules.