The simulated photoelectron spectrum of 1-propynide

The negative ion photoelectron spectrum of 1-propynide is computed by employing the multimode vibronic coupling approach. A three-state quasidiabatic Hamiltonian, H(d), is reported, which accurately represents the ab initio determined equilibrium geometries and harmonic frequencies of the ground X (2)A(1) state as well as the low-lying Jahn-Teller distorted components of the A (2)E excited state. It also reproduces both the minimum energy crossing point (MECP) on the symmetry-required (2)E(x)-(2)E(y) conical intersection seam and the MECP on the same symmetry (2)A(1)-(2)E(x) conical intersection seam. H(d) includes all terms through second order in internal coordinates for both the diagonal and off-diagonal blocks. It is centered at the (2)E(x)-(2)E(y) MECP and is determined using ab initio gradients and derivative couplings near both the (2)E(x)-(2)E(y) MECP and the X (2)A(1) equilibrium geometry. This construction is enabled by a recently reported normal equation based algorithm. The C(3v) symmetry of the system is used to significantly reduce the computational cost of the ab initio treatment. This H(d) is then expressed in a vibronic basis that is chosen for its ability to reduce the dimension of the vibronic expansion. The vibronic Hamiltonian matrix is diagonalized to obtain a negative ion photoelectron spectrum for 1-propynide-h(3). The determined spectrum compares favorably with previous spectroscopic results. In particular, the lines attributable to the (2)E state are found to be much weaker than those corresponding to the (2)A(1) state of 1-propynyl. This diminution of the (2)E state is attributable principally to the (2)E(x)-(2)A(1) conical intersection rather than an intrinsically small electronic transition moment for the production of the (2)E state.