Intracluster polymerization in acrylonitrile cluster anions studied by photoelectron spectroscopy

Photoelectron spectra of acrylonitrile cluster anions, (AN)n− (2⩽n⩽8), were measured at photon energies of 2.33 and 3.49 eV. A broad peak in each spectrum was approximated by a Gaussian function which reflects the amplitude of the vibrational wavefunction for the ground state of the anion. The measured vertical detachment energy decreases abruptly from 2.1 eV at n = 2 to 0.9 eV at n = 3 and increases gradually. This n-dependence is explained by the anion structure that the anion with n ⩽ 5 is composed of a polymerized cyclic trimer anion, (AN)3−, and AN, while that with n ⩾ 6 contains a polymerized cyclic trimer unit, (AN)3, in addition to (AN)3− and AN. The dimer anion, (AN)2−, should have an acyclic geometry.