Excitation of the 4.3-μm bands ofCO2by low-energy electrons

Abstract

Rate coefficients for the excitation of the 4.3-μm bands of ${\mathrm{CO}}_2$ by low-energy electrons in ${\mathrm{CO}}_2$ have been measured using a drift-tube technique. The ${\mathrm{CO}}_2$ density [(1.5 to 7) × ${10}^{17}$ molecules/${\mathrm{cm}}^3$] was chosen to maximize the radiation reaching the detector. Line-by-line transmission calculations were used to take into account the absorption of 4.3-μm radiation. A small fraction of the approximately ${10}^{-8\mathrm{}}$ W of the 4.3-μm radiation produced by the approximately ${10}^{-7\mathrm{}}$-A electron current was incident on an InSb photovoltaic detector. The detector calibration and absorption calculations were checked by measuring the readily calculated excitation coefficients for vibrational excitation of ${\mathrm{N}}_2$ containing a small concentration of ${\mathrm{CO}}_2$. For pure ${\mathrm{CO}}_2$ the number of molecules capable of emitting 4.3-μm radiation produced per cm of electron drift and per ${\mathrm{CO}}_2$ molecule varied from ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ at $\frac{E}{N}=6\mathrm{}\times {10}^{-17\mathrm{}}\mathrm{V} {\mathrm{cm}}^2 \mathrm{to} 5.4\times {10}^{-16\mathrm{}}{\mathrm{cm}}^{-2\mathrm{}}$ at $\frac{E}{N}=4\mathrm{}\times {10}^{-16\mathrm{}}$ V ${\mathrm{cm}}^2$. Here $E$ is the electric field and $N$ is total gas density. The excitation coefficients at lower $\frac{E}{N}$ are much larger than estimated previously. A set of vibrational excitation cross sections is obtained for ${\mathrm{CO}}_2$ which is consistent with the excitation coefficient data and with most of the published electron-beam data.