Knowledge Bank

The molecule acetylene has been recently detected from infrared spectra of several cool carbon $stars.1,2$ Since $C2H2$ is a useful probe of the chemical composition and temperature structure of the upper atmospheres and circumstellar shells of these objects, we have studied the bands observed in stellar spectra in the laboratory using the Ohio State University 10-m focal length Czerny-Turner vacuum grating spectrometer. Positions, accurate to $\pm 0.005cm-1$, have been measured for 3000 lines in the 3 $\mu m$ region using samples heated to 160 C. Since elevated temperatures and high resolution were used, the molecular constants have been improved for previously assigned bands as well as many new transitions have been observed. Absolute strengths of 100 lines in the 7 strongest bands in the 3 $\mu m$ region have been measured, including the $\nu 1+\nu 51$ band of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C13CH2$. Absolute strengths of 1.67 and $0.290cm-2$ $atm-1$ at 300 K have been determined for the integrated Q branch strength of the $\nu 1+\nu 51$ band at $4091cm-1$ and the $\nu 2+\nu 51$ band at $2701cm-1$, respectively. These values have been used to provide revised estimates of $3\pm 1\times 1018mol cm-2$, and $TR\cong 300KIRC+10\circ 216$. The total internal partition function of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C2H2$ has been evaluated in the temperature range 200 to 350 K from a tabulation of rotational and vibrational constants of all bands within $2200cm-1$ of the ground state. The present work has been combined with results from previous studies to deduce improved ground state constants for $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C13CH2$ of $B=1.148115\pm 18cm-1$ and $D=1.550\pm 11\times 10-6cm-1$.