Analysis of the Near Ultraviolet Absorption Spectrum of Acetylene

Abstract

The absorption spectrum of acetylene in the region 1970—2470A has been photographed in the fourth and fifth orders of a twenty‐one‐foot grating spectrograph with absorbing paths of 0.007 to 70 m‐atmos. It has been possible to extend the arguments which led King and Ingold to conclude that in the upper electronic state the molecule is bent and has C_2h symmetry. The upper electronic state is A_u or, possibly A_g. The effective moment of inertia I_a of the excited state is small, causing sub‐band separations of 50 to 150 cm^—1. These sub‐bands all have P, Q, and R branches, are well resolved, and have the appearance of bands of diatomic molecules. The following rotational constants of the upper state are derived: $$A_0{\text{=12.94,\hspace{0.17em}\hspace{0.17em}B}}_0{\text{=1.1247,\hspace{0.17em}\hspace{0.17em}C}}_0\text{=1.0297\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}.$$ If the C–H distance is assumed to be between 1.070 and 1.090A, one finds $$r_{\mathrm{cc}}{\text{=1.38}}_8\mathrm{A}\mathrm{,\hspace{0.17em}\hspace{0.17em}[open\; phi]}\mathrm{CCH}{\text{=120}}^{\circ }.$$ Ground‐state combination differences are in agreement with precise values derived from infrared data. Some new lower state B values are obtained. The O–O vibrational transition is shown to be at 42 197.69 cm^—1. The sub‐bands form ν₄″ progressions and ν₃′ progressions. Analysis gives two upper state vibrational frequencies $${\nu }_3(\mathrm{CCH}\text{)=1047.70\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\mathrm{\hspace{0.17em}\hspace{0.17em}}\mathrm{and}{\mathrm{\hspace{0.17em}\hspace{0.17em}\nu }}_2(\mathrm{CC}\text{)=1389\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}.$$ The following lower state vibrational constants are obtained: $${\omega }_4^0{\text{=608.2}}_6{\mathrm{,\hspace{0.17em}\hspace{0.17em}x}}_{44}{\text{=+3.29,\hspace{0.17em}g}}_{44}{\text{=+0.1}}_7\hspace{0.17em}{\mathrm{cm}}^{\text{−1}}.$$ The fundamental frequency ν₄″ is the sum of these constants, that is, 611.7₂ cm^—1.