TIME-RESOLVED PHOTODISSOCIATION OF GAS-PHASE NICKELOCENE CATION - DETERMINATION OF BOND STRENGTH AND RADIATIVE RELAXATION RATE

Time-resolved photodissociation (TRPD) rate measurements on nickelocene cation were performed at two different UV wavelengths in the ion cyclotron resonance (ICR) mass spectrometer. For completely thermalized ions, one-photon absorption at 266 nm gave a dissociation rate of 168 s(-1), and two-photon absorption at 355 nm gave a dissociation rate of (1.7 +/- 0.3) x 10(6) s(-1), which is the fastest dissociation rate yet measured in our lab. The TRPD results were modeled by RRKM calculations. The critical energy and the activation entropy at 1000 K were determined to be E(0) = 3.24 eV and Delta S-double dagger (1000 K) = 6.34 eu. From the RRKM fit the CpNi(+)-Cp bond strength was assigned as 313 kJ/mol, which is significantly weaker than for ferrocene ion (357 kJ/mol). The dissociation rate was used as a thermometric probe to measure the internal energies and cooling rates of initially hot ions. The radiative cooling rate in the internal energy range 1.0-0.3 eV was 0.48 s(-1). The radiative cooling rate constant has the same order of magnitude as previously found for comparable ions. It is about twice that of ferrocene cation (0.28 s(-1)).