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 (X) over tilde (1)A(1) electronic state, spectroscopic features are observed that belong to five additional states: the low-lying (a) over tilde B-3(1) and (b) over tilde (3)A(2) states, as well as two excited singlets, (A) over tilde (1)A(2) and (B) over tilde B-1(1), and a higher-lying triplet, (c) over tilde (3)A(1). Term energies (T-0, in cm(-1)) for the excited states obtained from the data are: 10 354 +/- 11 ((a) over tilde B-3(1)); 11 950 +/- 30 ((b) over tilde (3)A(2)); 20 943 +/- 11 ((c) over tilde (3)A(1)); and 13 677 +/- 11 ((A) over tilde (1)A(2)). Strong vibronic coupling affects the (A) over tilde (1)A(2) and (B) over tilde B-1(1) states as well as (a) over tilde B-3(1) and (b) over tilde (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 (B) over tilde B-1(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 angstrom by cavity ringdown spectroscopy (and coincident with a diffuse interstellar band) is carried by the (B) over tilde B-1(1) state of H2CCC. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3696896]