VACUUM-UV FLUORESCENCE SPECTROSCOPY OF SIF4 IN THE RANGE 10-30 EV

The vacuum-UV and visible spectroscopy of SiF4 using fluorescence exci tation and dispersed emission techniques is reported, The fluorescence excitation spectrum has been recorded following excitation with synch rotron radiation from the BESSY I, Berlin source in the energy range 1 0-30 eV with an average resolution of similar to 0.05 eV. By compariso n with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing sta te of a fragment have been assigned, Dispersed emission spectra have b een recorded at the energies of all the peaks in the excitation spectr a. Four different decay channels are observed: (a) SiF3 fluorescence i n the range 350-650 nm for photon energies around 13.0 eV, (b) SiF2 (a ) over tilde(3)B(1)-(X) over tilde(1)A(1) phosphorescence in the range 360-440 nm for photon energies in the range 15.2-18.0 eV, (c) SiF2 (A ) over tilde(1)B(1)-(X) over tilde(1)A(1) fluorescence in the range 21 0-270 nm for photon energies in the range 17.0-20.0 eV, and (d) emissi on from the (D) over tilde(2)A(1) State of SiF4+ predominantly in the range 280-350 nm for photon energies greater than 21.5 eV. These assig nments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection at a specific (di spersed) wavelength. Using the single-bunch mode of the synchrotron, l ifetimes of all the emitting states have been measured, The lifetimes of the unassigned emitting state in SiF3, the (A) over tilde(1)B(1) st ale of SiF2, and the (D) over tilde(2)A(1) State of SiF4+ are 3.9 +/- 0.7, 11.2 +/- 1.5, and 9,16 +/- 0.02 ns, respectively. This is the fir st measurement of the lifetimes of these excited stares in SiF, and Si F2. The decay from the (a) over tilde(3)B(1) State of SiF2 has a fast component of 2.6 +/- 0.4 ns. We conclude that the lifetime of the (a) over tilde(3)B(1) State of SiF2, is either as low as 2.6 ns or too hig h (tau>similar to 200 ns) to measure with the timing profile of the si ngle-bunch made of BESSY 1. If the latter interpretation is correct, a s seems likely for st spin-forbidden phosphorescence to the (1)A(1) gr ound stale, the 2.6 ns component could be the lifetime of intersystem crossing from higher vibrational levels of the (a) over tilde(3)B(1) s tate of SiF2 into its ground state. (C) 1997 American Institute of Phy sics.