Vibrational Relaxation and Electronic Quenching of the C 3Πu (υ′ = 1) State of Nitrogen

Abstract

A molecular lifetime apparatus was used to study energy transfer processes of the $C^3{\Pi }_u$ state of nitrogen. Kinetic and luminosity measurements as a function of pressure indicate a cross section ${\text{σ(3)\hspace{0.17em}=\hspace{0.17em}2.5\hspace{0.17em}±\hspace{0.17em}0.7 Å}}^2$ for vibrational relaxation of ${\mathrm{N}}_2{\mathrm{(C}}^3{\Pi }_u{)}_{\text{υ′=1}}$ by ground‐state nitrogen molecules. A useful result of the kinetic analysis is that although the observed lifetime (including quenching) decreases with $\mathrm{\upsilon \prime }$ , vibrational relaxation reduces the gross $C$ state luminescence decay to a single exponential, characteristic of the $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ level. Natural radiative lifetimes and electronic quenching cross sections, $\text{σ(2)}$ , were determined for the $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ and $\text{υ′\hspace{0.17em}=\hspace{0.17em}1}$ levels of the $C$ state: $$\text{υ′\hspace{0.17em}=\hspace{0.17em}0: τ\hspace{0.17em}=\hspace{0.17em}40.5\hspace{0.17em}±\hspace{0.17em}1.3}\mathrm{nsec\; and}{\text{σ(2)\hspace{0.17em}=\hspace{0.17em}1.98\hspace{0.17em}±\hspace{0.17em}0.02 Å}}^2;$$ $$\text{υ′\hspace{0.17em}=\hspace{0.17em}1: τ\hspace{0.17em}=\hspace{0.17em}44.4\hspace{0.17em}±\hspace{0.17em}1.4}\mathrm{nsec\; and}{\text{σ(2)\hspace{0.17em}=\hspace{0.17em}1.42\hspace{0.17em}±\hspace{0.17em}0.71 Å}}^2.$$ Estimates of electronic and vibrational deactivation cross sections for the $\text{υ′\hspace{0.17em}=\hspace{0.17em}2}$ level are $${\text{υ′\hspace{0.17em}=\hspace{0.17em}2: σ(2)\hspace{0.17em}=\hspace{0.17em}3.9\hspace{0.17em}±\hspace{0.17em}0.8 Å}}^2\mathrm{and}{\text{σ(3)\hspace{0.17em}=\hspace{0.17em}1.6\hspace{0.17em}±\hspace{0.17em}0.4 Å}}^2.$$ The efficiencies for excitation of the $\text{υ′\hspace{0.17em}=\hspace{0.17em}0}$ and $\text{υ′\hspace{0.17em}=\hspace{0.17em}1}$ levels by the secondary electronsproduced by fission fragments are in the ratio 1.0:0.44. It is argued that the $C^3{\Pi }_u\text{(υ′\hspace{0.17em}=\hspace{0.17em}1)}$ state deactivates in two ways through a common $N_4$ intermediate, viz., $$C^3{\Pi }_u{\text{(υ′\hspace{0.17em}=\hspace{0.17em}1)\hspace{0.17em}+\hspace{0.17em}X}}^1{\Sigma }_g^{+}\text{(υ′\hspace{0.17em}=\hspace{0.17em}0)\hspace{0.17em}±\hspace{0.17em}}{\mathrm{N}}_4{\mathrm{(C}}_{\text{2υ}}\mathrm{)\to }{\displaystyle {lim}^{\mathrm{I}}}{B}^3{\Pi }_g{\mathrm{(\upsilon ″\hspace{0.17em}=\hspace{0.17em}\xi )\hspace{0.17em}+\hspace{0.17em}X}}^1{\Sigma }_g^{+}\text{(υ″\hspace{0.17em}≥\hspace{0.17em}0)}$$ $$\to {\displaystyle {lim}^{\mathrm{II}}}{C}^3{\Pi }_u{\text{(υ″\hspace{0.17em}=\hspace{0.17em}0)\hspace{0.17em}+\hspace{0.17em}X}}^1{\Sigma }_g^{+}\text{(υ″\hspace{0.17em}=\hspace{0.17em}0)}$$ and that the strong vibrational relaxation is accounted for by a nonadiabatic mechanism originally proposed by Nikitin for vibrational relaxation of ground‐state NO.