Photodetachment and photofragmentation pathways in the [(CO ₂) ₂(H ₂O) _m] ^- cluster anions

The mass-selected [(CO ₂) ₂(H ₂O) _m] ^- cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO ₂ ^-(H ₂O) _mCO ₂ and (O ₂CCO ₂) ^-(H ₂O) _m parent cluster structures, particularly for m=2 and 3, only the CO ₂ ^- based clusters are both required and sufficient to explain all fragmentation pathways for m≥3. Three types of anionic photofragments are observed: CO ₂ ^-(H ₂O) _k, O ^-(H ₂O) _k, and CO ₃ ^-(H ₂O) _k, k≤m, with their yields varying depending on the parent cluster size. Of these, only CO ₂ ^-(H ₂O) _k can potentially result from (O ₂CCO ₂) ^-(H ₂O) _m parent structures, although an alternative mechanism, involving the dissociation and recombination of the CO ₂ ^- cluster core, is possible as well. The O ^-(H ₂O) _k and CO ₃ ^- (H ₂O) ^k channels are believed to be triggered by the dissociation of the CO ₂ ^- cluster core. In the CO ₃ ^-(H ₂O) _k channel, seen only in the range of m=3-6, the CO ₂ ^- core dissociation is followed by an intracluster association of nascent O ^- with the solvent CO ₂. This channel's absence in larger clusters (m > 6) is attributed to hindrance from the H ₂O molecules.