Vacuum ultraviolet photoionization and photodissociation of ferrocene

Photoionization and photodissociation of ferrocene [Fe(C₅H₅)₂] is examined by using vacuum ultraviolet (VUV) photons from a synchrotron radiation source and a time-of-flight (TOF) photoionization mass spectrometer. VUV absorption by ferrocene results in Fe(C₅H₅)^.+₂, FeC₅H⁺₅, FeC₃H⁺₃, Fe^.+ and C₁₀H^.+_x (x = 8 - 10). The dependency of the product distribution on photon energy indicates sequential elimination of C₅H₅ · ligands as a major dissociation channel, but concerted elimination of two C₅H₅ · ligands also takes place to a lesser degree. Through analysis of TOF peak shape, it is found that Fe(C₅H₅)^.+₂ molecular ion dissociates into FeC₅H⁺₅ and C₅H₅ · via two channels. One is nonstatistical dissociation with a fast rate, and the other is slow unimolecular decay that becomes more discernible at low photon energy. The rate of unimolecular decay, exemplified by a value of k = 2.4 ± 1.0 × 10⁶ s^-1 at photon energy of 14.76 eV, is well in accord with the RRKM rate. (Int J Mass Spectrom 181 (1998) 59-66)