Production of fluorine 2,4P→2P resonance radiation by electron impact on SF6, CF4, NF3, and CCl2F2

Abstract

We studied the emission of the fluorine (2p43s) 2,4P→(2p5) 2P resonance lines in the vacuum ultraviolet (VUV) at 95.5 and 97.5 nm, respectively, following dissociative excitation of SF6, CF4, NF3, and CCl2F2 by controlled electron impact under single collision conditions. Absolute photoemission cross sections and appearance potentials have been determined for the 95.5 nm 2P→2P multiplets for all four target gases. The apparent cross sections, which include cascade contribution from higher states, were measured to range from 2 to 5×10−18 cm2 at 200 eV impact energy. The cross sections were found to be heavily influenced by 3p→3s cascading with contributions ranging from 30% (SF6) to 75% (CF4). Subtraction of the cascade contributions yielded direct cross sections of 0.5× 10−18 cm2 (CF4), 1.4×10−18 cm2 (CCl2F2), 1.5×10−18 cm2 (SF6), and 2.9×10−18 cm2 (NF3) at 200 eV. The fluorine emissions are the result of the total fragmentation of the parent molecule for the targets SF6, CF4, and NF3. A much lower appearance potential and a different energy dependence of the cross section in the case of CCl2F2 indicates that partial fragmentation of this molecule plays a very important role. The findings for the 97.5 nm 4P→2P multiplet were essentially similar to those for the 2P→2P multiplet with the exception that accurate apparent emission cross sections could not be determined from a measurement of the 97.5 nm photon emission intensity. Due to their longer lifetime an appreciable fraction of the excited fluorine (2p43s) 4P atoms was found to drift out of the viewing area of our detection system before decaying radiatively. An attempt was made to extrapolate values for the 97.5 nm 4P→2P cross sections at 200 eV from a detailed analysis of the near-threshold region of the cross section for SF6, CF4, and NF3 This approach yielded FI 4P →2P cross sections comparable in magnitude to the FI 2P→2P cross sections.