The Infrared Absorption Spectra of Acetylene, Ethylene and Ethane

Abstract

The spectra of acetylene, ethylene and ethane have been investigated with gratings in the region between 2 and 15µ. Each gas shows a more or less characteristic type of structure for its vibrational-rotational bands. The three main absorption regions of acetylene have been resolved into individual lines which are alternately intense and faint. These lines have the same average spacing in the three bands. The band at 13.7µ shows a strong unresolved Q branch, while the bands at 7.5 and 3.0µ lack evident Q branches. The two minor absorption regions of acetylene have also been investigated. The molecular moment of inertia has been computed from the spacing of the fine structure of the principal acetylene bands. The moment of inertia has also been computed from the classical theory on the assumption of a linear molecular model. The values obtained by the two methods are in agreement.For ethylene, seven regions of absorption have been investigated. Two of these have been resolved into individual lines, with a strong Q branch present in one case and absent in the other. Four of the remaining regions reveal envelopes of an identical type, which is characterized by a sharp Q branch together with P and R branches. The bands found in the region of 2.3µ apparently lack Q branches. This latter group has been partially resolved.The main absorption regions of ethane reveal bands, each consisting of a single succession of absorption maxima. The spacingof these maxima is different for each of the bands, but the structure obtained is for the most part of a regular nature.