Vacuum–UV fluorescence spectroscopy of CCl4, SiCl4 and GeCl4 in the range 9–25eV

Abstract

The vacuum–ultraviolet (UV) and visible spectroscopy of MCl4 (M=C, Si, Ge) using fluorescence excitation and dispersed emission techniques is reported. Fluorescence excitation spectra have been recorded following photoexcitation with monochromatised synchrotron radiation in the vacuum–UV energy range 9–25 eV with an average resolution of ca. 0.06 eV. All the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. The Rydberg states arise predominantly from electron excitation from one of the Cl 3pπ non-bonding molecular orbitals of MCl4, and quantum defects for the Rydberg states characteristic of a chlorine atom are obtained. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of ca. 4–8 nm at the energies of the peaks in the excitation spectra. Five different decay channels are observed: (a) CCl2 Ã1B1–X̃1A1 fluorescence at ca. 420–600 nm from CCl4 excited in the range 9–12 eV, (b) SiCl2/GeCl2 ã3B1–X̃1A1 and Ã1B1–X̃1A1 fluorescence from SiCl4/GeCl4 excited in the range 9–14 eV, (c) SiCl+4/GeCl+4C̃2T2–X̃2T1 and C̃2T2–Ã2T2 fluorescence at ca. 350–700 nm from SiCl4/GeCl4 for photon energies above the adiabatic ionisation energy of the C̃2T2 state of SiCl+4/GeCl+4 (15.0 and 14.6 eV, respectively), (d) CCl A2Δ–X2Π fluorescence at ca. 276–280 nm from CCl4 excited in the range 14–18eV, and (e) Si* and Ge* atomic emission lines at wavelengths below 310 nm from SiCl4/GeCl4 excited in the range 19–25 eV. These assignments are confirmed by action spectra in which the energy of the vacuum–UV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. By using the single-bunch mode of synchrotron radiation, lifetimes of all the emitting states that fall in the range ca. 4–100 ns have been measured. The MCl∗2 products are formed by photodissociation of low-lying Rydberg states of MCl4; the thresholds for their production therefore relate to energies of Rydberg states of the parent molecule. The CCl*, Si* and Ge* products are formed by sequential, multi-step photodissociation of MCl∗2; the thresholds for these emissions correspond to the thermodynamic thresholds to form the emitting product with either three (in the case of CCl*) or four (in the case of Si*/Ge*) chlorine atoms.