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

Abstract

Rate coefficients for the excitation of the 4.3-\ensuremath{\mu}m 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) \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ 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-\ensuremath{\mu}m radiation. A small fraction of the approximately ${10}^{\ensuremath{-}8}$ W of the 4.3-\ensuremath{\mu}m radiation produced by the approximately ${10}^{\ensuremath{-}7}$-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-\ensuremath{\mu}m radiation produced per cm of electron drift and per ${\mathrm{CO}}_{2}$ molecule varied from ${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ at $\frac{E}{N}=6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}\mathrm{V} {\mathrm{cm}}^{2} \mathrm{to} 5.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{\ensuremath{-}2}$ at $\frac{E}{N}=4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ 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.