NANOSECOND LASER EXCITATION OF BENZENE-CHROMIUM-TRICARBONYL AND DIBENZENE-CHROMIUM AT 351-NM, 248-NM AND 193-NM - WAVELENGTH-DEPENDENT COMPETITION BETWEEN IONIZATION AND DISSOCIATION

Laser ionization of benzene-chromium-tricarbonyl C6H6Cr(CO)3 and dibenzene-chromium (C6H6)2Cr is reported for excitation at 193, 248, and 351 nm. In the case Of C6H6Cr(CO)3 at 193 nm, molecular ions are not observed. The ion with the highest mass is C6H6Cr(CO)+. This ion and C6H6Cr+, Cr(CO)2+, Cr(CO)+ and Cr+ ions are formed after absorption of two photons. At 248 nm the spectra are dominated by Cr+, and to a lesser extent by C6H6Cr(CO)+ and C6H6+ ions. The Cr+ ions are formed after absorption of three photons, the C6H6Cr(CO)+ ions after two photons. At 351 nm only Cr+ ions are seen, formed after absorption of four photons. In the case of (C6H6)2Cr, molecular ion formation is dominant at 193 nm at low laser intensities after absorption of one photon. C6H6Cr+ and Cr+ ions are formed at higher laser intensities after absorption of two photons. At 248 nm the mass spectra are dominated at low laser intensities by C6H6Cr+ ions (formed after absorption of two photons) and at high laser intensities by Cr+ ions (formed after absorption of three photons). At 351 nm the mass spectra are dominated by Cr+ ions, formed after absorption of three photons. Additionally, molecular ions and C6H6Cr+ ions are seen at low laser intensities. The absorbed photon energies are corroborated by published electron impact ionization and appearance energies. The multiphoton ionization/dissociation mechanism is deduced from metastable ion analysis. At 193 nm metal ions are formed without intermediate dissociation to metal atoms. For both compounds ionization precedes dissociation at 193 nm. At 248 and 351 nm dissociation to Cr atoms is effectively competing with ionization after biphotonic excitation.