Presscure-Induced a″ 1Σg+ ← X 1Σg+ Absorption in the Vacuum Ultraviolet Spectrum of Molecular Nitrogen

The ${\mathrm{a″}}^1{\Sigma }_g^{+}{\mathrm{–X}}^1{\Sigma }_g^{+}$ absorption band of N₂, which has been reported recently in a preliminary communication, has been investigated at nitrogen pressures from 10 to 80 torr. A study of the absolute absorption coefficient and of its pressure dependence shows definitely that the observed band arises as a pressure‐induced dipole transition at the quoted pressures. The integrated absorption represents an absolute oscillator strength of ${\text{f\hspace{0.17em}/\hspace{0.17em}p\hspace{0.17em}=\hspace{0.17em}3.2\hspace{0.17em}×\hspace{0.17em}10}}^{\text{−8}}{\mathrm{torr}}^{\text{−1}}$ . The large bandwidth (400 cm⁻¹) is attributed to the short duration of the dipole‐inducing collisions: 1.3 × 10⁻¹⁴ sec on the average. Thus the observed intensity can be interpreted in terms of an average oscillator strength of $\text{f\hspace{0.17em}=\hspace{0.17em}0.04}$ per molecule per collision. The apparent optical quadrupole absorption $f$ value of less than 10⁻⁷ corresponds to a very small quadrupole matric element compared to the one which determines the electron excitation cross section. A search for the $\mathrm{a″–X}$ quadrupole emission band in the electron‐beam‐excited spectrum at very low pressures has resulted in strong evidence for the presence of the 0–0 band of this transition, and tentative assignments of $O$ , $Q$ , and $S$ lines yield approximate values of ${\upsilon }_0\text{\hspace{0.17em}=\hspace{0.17em}99 005}{\mathrm{cm}}^{\text{−1}}$ and $B_0\text{′\hspace{0.17em}=\hspace{0.17em}1.85\hspace{0.17em}±\hspace{0.17em}0.15}{\mathrm{cm}}^{\text{−1}}$ .