Ground and low-lying excited states of propadienylidene H2C=C=C: obtained by negative ion photoelectron spectroscopy

A joint experimental-theoretical study has been carried out on electronic states of propadienylidene (H2CCC), using results from negative-ion photoelectron spectroscopy. In addition to the previously characterized X̃1A1 electronic state, spectroscopic features are observed that belong to five additional states: the low-lying ã3B1 and b̃3A2 states, as well as two excited singlets, Ã1A2 and B̃1B1, and a higher-lying triplet, c̃3A1. Term energies (T0, in cm−1) for the excited states obtained from the data are: 10 354±11 (ã3B1); 11 950±30 (b̃3A2); 20 943±11 (c̃3A1); and 13 677±11 (Ã1A2). Strong vibronic coupling affects the Ã1A2 and B̃1B1 states as well as ã3B1 and b̃3A2 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 B̃1B1 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 B̃1B1 state of H2CCC.