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

Stanton, John F.; Garand, Etienne; Kim, Jongjin; Yacovitch, Tara I.; Hock, Christian; Case, Amanda S.; Miller, Elisa M.; Lu, Yu J.; Vogelhuber, Kristen M.; Wren, Scott W.; Ichino, Takatoshi; Maier, John P.; McMahon, Robert J.; Osborn, David L.; Neumark, Daniel M.; Lineberger, W.C.

A joint experimental-theoretical study has been carried out on electronic states of propadienylidene (H 2CCC), using results from negative-ion photoelectron spectroscopy. In addition to the previously characterized X 1A 1 electronic state, spectroscopic features are observed that belong to five additional states: the low-lying a 3B 1 and b 3A 2 states, as well as two excited singlets, A 1A 2 and B 1B 1, and a higher-lying triplet, c 3A 1. Term energies (T 0, in cm -1) for the excited states obtained from the data are: 10 354±11 (a 3B 1); 11 950±30 (b 3A 2); 20 943±11 (c 3A 1); and 13 677±11 (A 1A 2). Strong vibronic coupling affects the A 1A 2 and B 1B 1 states as well as a 3B 1 and b 3A 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 B 1B 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 Bv 1B 1 state of H 2CCC. © 2012 American Institute of Physics.