Ground and low-lying excited states of propadienylidene (H 2 C=C=C:) obtained by negative ion photoelectron spectroscopy

A joint experimental-theoretical study has been carried out on electronic states of propadienylidene (H 2 CCC), using results from negative-ion photoelectron spectroscopy. In addition to the previously characterized \documentclass[12pt]{minimal}\begin{document}${\tilde{X}}^1A_1$\end{document} X ̃ 1 A 1 electronic state, spectroscopic features are observed that belong to five additional states: the low-lying \documentclass[12pt]{minimal}\begin{document}${\tilde{a}}^3B_1$\end{document} a ̃ 3 B 1 and \documentclass[12pt]{minimal}\begin{document}${\tilde{b}}^3A_2$\end{document} b ̃ 3 A 2 states, as well as two excited singlets, \documentclass[12pt]{minimal}\begin{document}${\tilde{A}}^1A_2$\end{document} A ̃ 1 A 2 and \documentclass[12pt]{minimal}\begin{document}${\tilde{B}}^1B_1$\end{document} B ̃ 1 B 1 , and a higher-lying triplet, \documentclass[12pt]{minimal}\begin{document}${\tilde{c}}^3A_1$\end{document} c ̃ 3 A 1 . Term energies ( T 0 , in cm −1 ) for the excited states obtained from the data are: 10354±11 ( \documentclass[12pt]{minimal}\begin{document}${\tilde{a}}^3B_1$\end{document} a ̃ 3 B 1 ); 11950±30 ( \documentclass[12pt]{minimal}\begin{document}${\tilde{b}}^3A_2$\end{document} b ̃ 3 A 2 ); 20943±11 ( \documentclass[12pt]{minimal}\begin{document}${\tilde{c}}^3A_1$\end{document} c ̃ 3 A 1 ); and 13677±11 ( \documentclass[12pt]{minimal}\begin{document}${\tilde{A}}^1A_2$\end{document} A ̃ 1 A 2 ). Strong vibronic coupling affects the \documentclass[12pt]{minimal}\begin{document}${\tilde{A}}^1A_2$\end{document} A ̃ 1 A 2 and \documentclass[12pt]{minimal}\begin{document}${\tilde{B}}^1B_1$\end{document} B ̃ 1 B 1 states as well as \documentclass[12pt]{minimal}\begin{document}${\tilde{a}}^3B_1$\end{document} a ̃ 3 B 1 and \documentclass[12pt]{minimal}\begin{document}${\tilde{b}}^3A_2$\end{document} b ̃ 3 A 2 and has profound effects on the spectrum. As a result, only a weak, broadened band is observed in the energy region where the origin of the \documentclass[12pt]{minimal}\begin{document}${\tilde{B}}^1B_1$\end{document} B ̃ 1 B 1 state is expected. The assignments here are supported by high-level coupled-cluster calculations and spectral simulations based on a vibronic coupling Hamiltonian. A result of astrophysical interest is that the present study supports the idea that a broad absorption band found at 5450 Å by cavity ringdown spectroscopy (and coincident with a diffuse interstellar band) is carried by the \documentclass[12pt]{minimal}\begin{docum