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Description
Electronic ground state hydrogen molecules at high vibrational states, $\mathrm{H}_{2}(\mathrm{X}^{1}\Sigma_{g}^{+},\nu \geq 5)$, have large cross sections for dissociative electron attachment, $\mathrm{H}_{2}(X^{1}\Sigma_{g}^{+},\nu)+\mathrm{e} \rightarrow \mathrm{H}_{2}^{-}(^{2}\Sigma_{g}^{+})\rightarrow \mathrm{H}^{-}+\mathrm{H}$, at low electron energies.
Thus, their production through electron impact excitation to electronic singlet states, i.e. $\mathrm{H}_{2}(\mathrm{X}^{1}\Sigma_{g}^{+},\nu)+\mathrm{e}\rightarrow \mathrm{H}_2(\mathrm{B}^{1}\Sigma_{u}^{+}, \mathrm{C}^{1}\Pi_{u}) \rightarrow \mathrm{H}_{2}(\mathrm{X}^{1}\Sigma_{g}^{+},\nu \geq 5)+hf$, is an integral step of $\mathrm{H}^-$ volume production in negative hydrogen ion sources.
The $\mathrm{H}^-$ ion beam intensity and vacuum ultraviolet (VUV) emission within the higher order Lyman-band corresponding to $\mathrm{H}_2(\mathrm{B}^{1}\Sigma_{u}^{+},\nu) \rightarrow \mathrm{H}_{2}(\mathrm{X}^{1}\Sigma_{g}^{+},\nu)$ transitions were measured from the LIISA $\mathrm{H}^-$ ion source at JYFL with varying source pressure and heating power.
It was observed that certain Lyman-band emission lines react differently to the variations of the given operational parameters.
The origin of the observed behavior was studied by comparing the experimental Lyman-band spectra to synthetic spectra produced by taking the ground state vibrational distribution from the literature and modeling the electronic excitation and subsequent decay processes with two deviant assumptions.
According to the first model, the $\mathrm{H}_{2}(\mathrm{B}^{1}\Sigma_{u}^{+}, \nu)$ vibronic state has a long lifetime, therefore allowing vibronic oscillations resulting to relaxation of the quantum mechanical wave function.
The second model postulates that the electronic excited state decays immediately i.e. the internuclear distance remains constant between the excitation and de-excitation events.
It was found that the relaxed model correlates better to the acquired experimental results when compared to the prompt decay model.
A relation between the extracted ion beam intensity and the specific Lyman-band emission lines reflecting the production rate of certain vibrational states will be discussed.
