Knowledge Bank

The $\Pi u-\Sigma g+$ bands $(2\nu 4+\nu 3)1$ at 1940.002 and $1959.697cm-1$ and $3\nu 51$ at $2169.166cm-1$ together with fourteen accompanying “hot” bands of $\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}C2H2$ were measured using a high-resolution grating spectrometer equipped with an InSb photo-voltaie detector. The “hot” bands are the $\Sigma +-\Pi ,\Sigma --\Pi$ and $\Delta -\Pi$ transitions of $3\nu 4+\nu 41,2\nu 4+2\nu 5-\nu 51\nu 4+3\nu 5-\nu 41$ and $4\nu 5-\nu 51$; in addition, the R-branches of the $\nu 2+\nu 51-\nu 41$ and $\nu 2+\nu 51-\nu 41\Pi -\Pi$ bands were also observed. The information obtained on the strongly interacting upper states involved in the transitions analyzed has made it possible to arrive at a reliable set of spectroscopic constants (anharmonic and rotation-vibration coupling constants) for the bending modes of the acetylene molecule.