Conclusion
Photodissociative excitation processes of XeF2 were studied by absorption and photofragment fluorescence spectroscopies in the vacuum UV region 105 − 180 nm. The quantum yields were measured along with the dispersed fluorescence spectra for emissions from XeF*(B, C and D) excimers formed from XeF2 excited by linearly polarized, monochromatized synchrotron radiation. Fluorescence anisotropy, R, of XeF* fragment emission was also measured against excitation light wavelength in the region 105 − 165 nm.
The quantum yields for XeF(B, D) excimer formation are found to be very high, especially in the valence band region for λex < 130 nm where it reaches almost unity (~ 0.75), but strongly depends (0 − 0.75) on the excited states of XeF2, i.e., in some Rydberg bands it decreases and in 5πu 1/2→6sR state it varies with increasing bending quantum number. These results can be explained by an obvious reason that the Rydberg states are dominantly channeled into non-radiative dissociation and bending vibrations in the Rydberg states promote XeF* excimer formation.
The intense 5πu 3/2→6sR bands (30 − 40 Mb) which appear around λex = 143 nm exhibit no fluorescence excitation bands; even fluorescence dips are observed at λex = 141.04 and 142.11 nm. These findings indicate that the XeF2* molecules photoexcited via the 10σg→7σu valence transitions at the same excitation energies as the 5πu 3/2→6sR Rydberg bands tend to be relaxed and channeled into the same non-radiative dissociation channels as the Rydberg state. Hence, this suggests that this valence state can easily interact with the Rydberg states.
Few XeF(C) excimers are formed in the photodissociation process of XeF2, the ratio of XeF(D) to XeF(B) formation and the vibrational distribution of the XeF(B) excimers depend on the exciting light wavelength. That is, a large amount of excess energy may be needed to efficiently form XeF(D) excimers. The higher the excitation energy, the broader becomes vibrational distribution of XeF(B), i.e. the higher the vibrational content.
From the fluorescence measurements of the excimer fragments, new peaks which are assigned to 5πu→5dR transitions have been observed in the wavelength region of λex = 124.75 ~ 127.25 and 117.25 ~ 119.75 nm.
Based on the results of R values determined, the valence and Rydberg bands observed are assigned from the viewpoint of symmetry of the excited states. The R values for the very broad valence bands which are the 10σg→7σu band appearing around λex = 155 nm and underlie intense Rydberg bands at λex < 120 nm are found to be high, e.g., R = 0.085 at λex = 155 nm. The 5πu→nsR, ndσR series of Rydberg bands exhibit low R values (R ~ 0) but do not exhibit negatively large values.
