UV Continuum and Hydrogen Line Emission in Hard Ionized High Pressure Neon

Abstract

Hard ionized (e.g. electron-beam excitation, pre-ionized discharge) high pressure (>0.5 bar) neon emits a distinct ultraviolet continuum in the 200–450 nm spectral range, which has not yet been unequivocally attributed to any particular neon transition. The intensity of this continuum scales with squared neon pressure, and thus can directly be linked to a molecular neon precursor. Concurrently, hydrogen line emission, particularly a very strong Lyman-α (121.57 nm), as well as Hα (656.3 nm) and Hβ (486.1 nm) lines can be observed. This work shows that no hydrogen line emission and, particularly, no UV-continuum emission can be observed in highly purified hydrogen-free neon. Only after deliberately adding small amounts of hydrogen into the buffer gas neon, the hydrogen lines, and particularly the UV continuum, are being emitted. Furthermore, the intensity dependence of all structures on hydrogen partial pressure essentially coincide and show a distinct maximum at about 0.7 Pa, largely independent of the absolute neon base-pressure. The UV continuum thus clearly originates from excited hydrogen species, and is attributed here to the \({{a}^{3}}\Sigma _{g}^{ + } \to {{b}^{3}}\Sigma _{u}^{ + }\) hydrogen excimer transition. Excitation of the upper \({{a}^{3}}\Sigma _{g}^{ + }\) level is proposed as energy transfer from neutral neon excimer molecules: while part of the \({\text{Ne}}_{2}^{*}\) energy is sufficient to dissociate and excite hydrogen molecules into an H + H (n = 2) configuration, leading to successive Lyman-α radiation, Ne₂ excimer molecules close to the so called “classical left turning point” lack the necessary energy, but can readily excite hydrogen molecules directly from the \({{X}^{1}}\Sigma _{g}^{ + }\) ground state into the \({{a}^{3}}\Sigma _{g}^{ + }\) bound molecular state.