Introduction
Since the discovery of the first noble-gas compounds in 1962, XeF2 has attracted great attention and its chemical activity, electronic and geometric properties have been investigated experimentally and theoretically because of its simple structure and unique electronic properties. The electronic configuration of XeF2 in the ground electronic state is
(8σg)2 (5σu)2 (9σg)2 (6σu)2 (4πu)4 (3πg)4 (10σg)2 (5πu)4 (7σu)0.From the photochemical point of view it is a special molecule since strong XeF* excimer emission has been detected in photolysis in the vacuum ultraviolet (VUV) region.
We study the photodissociative excitation processes of gaseous XeF2 in the excitation wavelength, ex, region from 105 (11.8 eV) to 180 nm (6.9 eV) applying various fluorescence spectroscopic techniques for the XeF* emissions along with absorption spectroscopy using monochromatized synchrotron radiation (SR). We have measured fluorescence excitation spectra for formation of XeF*(B, C and D) excimers. From the quantitative cross sections for photoabsorption, σabs, and photoemission, σem, the quantum yields for excimer formation have been determined for the first time in the wide wavelength region of λex = 105 − 180 nm. We have also observed the dispersed fluorescence spectra of XeF* formed at several excitation energies to obtain rough information on the branching fractions for forming XeF* excimers in the B, C and D states.
Another important technique applied in this report is the measurement of the XeF* fluorescence anisotropy defined as
where I|| and I⊥ are the intensities of polarized emissions parallel and perpendicular, respectively, to the electric vector of the incident light. The scalar information such as absorption and fluorescence excitation spectra alone does not provide us with enough dynamical information on photodissociation let alone the symmetry of the excited state. Measurement of fluorescence anisotropy however provides us vectorial information on photodissociative excitation processes since it reflects the nature of spatial distribution of the angular momenta of the excited fragments formed and thus the dynamics of dissociation, and the symmetry of the excited state as well as the lifetime of the excited molecule.
R = I|| − I⊥
I|| + 2 I⊥
In the present study, we have determined fragment XeF* fluorescence anisotropy as a function of the λex in the range of 105 − 180 nm. We attempt to resolve some of the problems associated with the symmetry and assignments of the excited states and the nature of the complicated potential energy surfaces leading to dissociation of photoexcited XeF2, based on the information on the anisotropy of polarized fluorescence and the electronic and vibrational distributions of the XeF* excimers obtained from the dispersed fluorescence spectra at some exciting wavelengths.
Schematic diagram of the standard enthalpies of formation for the XeF2 system and the photoexcitation energies for XeF2. On the left a schematic absorption spectrum of XeF2 is illustrated. Horizontal bars on the left correspond to the energy levels of the origins of the Rydberg bands.
