Photoionization of Vibrationally Excited Nitrogen

A quadrupole mass spectrometer with a photoionization source has been used in conjunction with a flowing nitrogen afterglow to obtain the photoionization spectrum of vibrationally excited nitrogen (${\mathrm{N}}_2^{*}$ ) at a spectral resolution of 0.03 nm in the interval 79.58–84.20 nm. This wavelength interval lies below the photoionization threshold of ${\mathrm{N}}_2\text{\hspace{0.17em}(v″=0)}$ . More than 50 new preionized bands and an underlying continuum have been observed. Many of the new bands are attributed to preionized transitions between ${\mathrm{N}}_2{\mathrm{\hspace{0.17em}(X\hspace{0.17em}}}^1{\Sigma }_g^{+}\text{,\hspace{0.17em}v″=1\hspace{0.17em}}\mathrm{to}\text{\hspace{0.17em}4)}$ and Rydberg states belonging to series that converge either to ${\mathrm{N}}_2^{+}{\mathrm{\hspace{0.17em}(A\hspace{0.17em}}}^2{\Pi }_u)$ or to vibrationally excited levels of ${\mathrm{N}}_2^{+}{\mathrm{\hspace{0.17em}(X\hspace{0.17em}}}^2{\Sigma }_g^{+})$ . From estimates of the concentrations of various vibrationally excited states of N₂ and measurements of the ionizing photon flux as a function of wavelength, we have estimated the photoionization coefficients of the new preionized bands and continuum. The relative probabilities of various transitions from ${\mathrm{N}}_2^{*}$ to the preionized Rydberg states are different from transitions to the same upper states from ${\mathrm{N}}_2\text{\hspace{0.17em}(v″=0)}$ owing to different Franck—Condon factors. The ${\mathrm{N}}_2^{*}$ continuum is a sum of continua from all of the vibrational states, each of which rises only slowly from threshold because of the unfavorable transition probability for direct ionization of N₂ when $\text{Δ v ≠ 0}$ . Many of the preionized bands can be attributed in large part to a specific vibrationally excited level, and the application of photoionization as a sensitive tool for studying kinetic processes involving ${\mathrm{N}}_2^{*}$ is proposed. A detection limit of 10¹¹ molecules/cm³ is obtainable with the present apparatus. Preliminary experiments on the quenching of ${\mathrm{N}}_2^{*}$ by N₂O and CO₂ are reported.