Knowledge Bank

Laser induced fluorescence probing of vibrationally excited acetylene molecules created by excitation of the second overtone of the C-H stretching vibration provides a measure of the state mixing in the ground electronic state and directly determines the frequencies of two of the ungerade vibrations in the electronically excited state. Excitation of the second C-H stretching overtone with light at about $1.0\mu$ prepares a single angular momentum state of $C2H2(3\nu CH$), and ultraviolet photons at about 2900 A promote it to its first electronically excited single state ($\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Au$). Detecting the emission as a function of the vibrational overtone excitation laser wavelength provides the vibrational spectrum, which clearly shows mixing between zero-order states having largely C-H stretching character and ones having mostly C-C stretching and trans-bending character. We can determine the rotational constants of the perturbing states and identify them as combinations of zero-order stretching and bending vibrations. The two-photon excitation to the electronically excited state through the Bu vibrational state directly accesses the ungerade vibrations in the electronically excited molecule. Using the angular momentum information from the rotational state selection in the first step of the double resonance excitation, we are able to analyze spectra obtained by varying the wavelength of the ultraviolet laser to obtain the wavenumbers of the out-of-plane torsion $\nu 4(746cm-1)$ and the symmetric bend $\nu 8(768cm-1)$.