Dissociation, relaxation, and oxidation of highly vibrationally excited gas phase metal carbonyl and cluster anions.

Abstract

Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) was used to study processes occurring in highly vibrationally excited metal-containing anions. The low-pressure environment of the FT-ICR trapping cell is optimal for the intentional study of nonthermal species. Vibrationally excited anions were prepared with well-defined excess internal energies by the absorption of a single 1064 nm photon into any species requiring the absorption of two such photons to dissociate. The effects of vibrational excitation on the oxidation reactions of Cr(CO)₅-, Al₁₆-, and Al₁₈- were systematically investigated. All three reactions slowed down with increasing anion vibrational energy, due to the large increase in the back-dissociation rate constants with excess internal energy. The branching fractions for the oxidation of Cr(CO)₅- and Al₁₆- also were substantially altered by excess vibrational energy. These data, along with the translationally-excited data of other workers, were used to gain insight into the mechanisms of these quite different oxidation reactions. Radiative relaxation rate constants were also measured for Cr(CO)₅- and Al₁₆- using the oxidation branching ratio as an ion thermometric probe. A complementary value was determined for Cr(CO)₅- with a two-pulse photodissociation experiment, and showed that the radiative relaxation rate constant for this ion is strongly energy dependent. Also, radiative relaxation of Cr(CO)₅- is about an order of magnitude faster than Al₁₆-. All these observations were attributed to the presence of high-frequency CO stretching modes in Cr(CO)₅-, and a detailed model was developed to support this interpretation. Finally, the photodissociation and photodetachment behavior of the M₂(CO)(n)- (where M = Cr, Mn, Fe, and Co; and 4 ≤ n ≤ 9, depending on the metal) and Al(n)- (n = 3 to 23) was investigated at 1064 nm and, for the dinuclear complexes, from 575 to 630 nm. Apparent metal atom loss from highly coordinatively unsaturated dinuclear carbonyl anions was instead ascribed to electron detachment and subsequent scavenging by the neutral metal carbonyl background.