Photooxidation and metal-ligand photolysis reactions of bis(cyclopentadienyl)chromium, chromocene, in the range 24 390-15 630 cm -1 are studied in the gas phase by using time-of-flight mass spectroscopic detection. Photooxidation of the intact chromocene molecule unexpectedly dominates in the range 23 530-24 000 cm -1 . The relative importance of photooxidation compared to photofragmentation is strongly wavelength dependent. A prominent species at all wavelengths is the chromium ion, but in a wavelength region corresponding to the lowest energy ligand to metal charge transfer excited electronic state absorption, the strongest peak is from the chromocene ion. The excitation spectra are reported for three selected species: chromocene ion, mono-(cyclopentadienyl)chromium ion, and the chromium ion. The spectrum obtained by monitoring the metal ion contains sharp peaks that are assigned to neutral chromium atom resonances. Sharp losses of intensities in the molecular ion spectra are observed at these wavelengths. The wavelength dependencies of the photoreactions are interpreted and explained in terms of the identity of the initially populated excited electronic state and the ionization energy of the molecule. When the initially populated excited electronic state is the ligand to metal charge transfer state, the first photon causes minimal bond weakening and the second photon excites the intact chromocene above the ionization energy, resulting in efficient ionization of the parent molecule. When the initially populated state is a lower energy ligand field excited electronic state, bond weakening occurs and absorption of a second photon results in significant photodissociation producing intense fragment peaks dominated by the metal ion peak.
Gas-phase photolysis of metal-containing molecules readily causes photofragmentation to produce metal atoms and ions. [1][2][3][4] More recently it has been shown by luminescence spectroscopy that photolysis of these molecules also readily produces diatomic metal-containing molecules in the gas phase. [5][6][7][8][9][10][11] These discoveries have assisted in the elucidation of the photolytic decomposition pathways relevant to laser-assisted chemical vapor deposition (LCVD). Both desired fragments (e.g., TiN in the LCVD of TiN films) 11 and undesirable fragments containing heteroatoms that contaminate the final films (e.g., CuF, CrF, NiF, and PtC in the laser CVD of the respective metal films) 6-10 have been identified. The fluorinated acetylacetonate ligands (1,1,1,5,5,5-hexafluoro-2-4-pentanedionate ) hfac) that were used in the latter studies provide good precursor volatility and allow for facile transport in the gas phase, but they may also result in fluorine and/or carbon contamination of the final deposit. [12][13][14] The formation of diatomic metal fluorides can be easily eliminated by removing the fluorine substituents from the precursor, but the carbon that leads to metal carbide molecules cannot be so easily eliminated. A least-motion mechanism that was proposed to explain PtC formation is based on a change in the excited state from M-O bonding to η 1 bonding of the hfac ligand to the metal by its central carbon. 6,10 This type of bonding is known for some similar compounds in the ground state. 15 MF formation obviously must follow a different mechanism.During the course of our studies of the gas-phase photolysis of Pd(hfac) 2 , we discovered that PdC + is a common photoproduct over a wide range of irradiation wavelengths, but that the ratio of Pd + to PdC + is very sensitive to the wavelength. In the detailed studies reported in this communication, we show that changing the wavelength by a few angstroms can cause the formation of PdC + to disappear below the detection limit. The photoproducts are monitored by using time-of-flight mass spectroscopy. We demonstrate the selectivity over the 21 700-23 800cm -1 excitation region and explain the phenomenon in terms of the electronic excited states.The TOF mass spectrometer was constructed on the basis of a design in the literature. 16 Pd(hfac) 2 is admitted to the high-vacuum
Rydberg states of calcium monofluoride in the n* = 1720 region have been observed by ionization-detected opticaloptical double-resonance spectroscopy via the D2Σ+ v = 1 intermediate state. All members of the six core-penetrating Rydberg series in the n* = 1720 region and several components of the 17f and 17g core-nonpenetrating Rydberg states have been assigned. While the assignment of core-penetrating Rydberg states is straightforward without use of an effective Hamiltonian model, "spectrum-only" assignment of core-nonpenetrating states is complicated because strong l-uncoupling causes the core-nonpenetrating states to evolve rapidly from Hund's case (b) to Hund's case (d) coupling. We describe "spectrum-only" assignment procedures, developed in the spirit of Gerhard Herzberg, that can be used to assign opticaloptical double-resonance spectra of core-penetrating and core-nonpenetrating Rydberg states using only information contained in the spectrum rather than predictions derived from an effective Hamiltonian model. The ambiguities that arise in the assignment of each class of states are discussed in detail.Key words: CaF, electric quadrupole moment, Rydberg states, laser spectroscopy.
Gas-phase photoreactions and photoproducts of the mixed-ligand compound (eta(4)-cycloocta-1,5-diene)(eta(5)-cyclopentadienyl)cobalt are reported. Significant amounts of the monoligated complexes CoCOD and CoCp are produced, and the relative amounts are wavelength dependent. The COD ligand (with the weakest metal-ligand bonds) is always preferentially labilized as expected, but the relative amounts of the CoCOD and CoCp fragments change by 1 order of magnitude as the excitation wavelength is changed. The gas-phase photoreactions exhibit other surprising features that are uncommon in the photoreactions of organometallic compounds in the gas phase. Significant amounts of the intact cation are formed, in contrast to most reported reactions where fragmentation of the weak metal-ligand bonds precedes ionization. Most surprisingly, fragmentation of the ligands occurs while the ligands are still coordinated. The resulting metal complexes contain ligand fragments that remain coordinated to the metal. Breaking several carbon-carbon bonds with retention of weaker metal-ligand bonds is unexpected. For example, C(5)H(5) undergoes fragmentation while still coordinated to the cobalt, yielding smaller compounds such as Co(CH)(+), Co(C(2)H(2))(+), Co(C(3)H(3))(+), and Co(C(4)H(6))(+). Correspondingly, coordinated COD yields Co(C(6)H(6))(+), Co(C(5)H(5))(+), Co(C(4)H(6))(+), Co(C(3)H(3))(+), Co(C(2)H(2))(+), and Co(CH)(+). The wavelength dependence of the ligand labilization is examined by utilizing mass-selected resonance enhanced multiphoton ionization spectroscopy. Both broad bands and sharp lines are observed in the mass-selected excitation spectra. The action spectra obtained in the gas phase while monitoring the cobalt ion follow the absorption onset found in solution. The changes in fragmentation pathways are interpreted in terms of the initially accessed excited state.
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