THEORETICAL ASSIGNMENT OF THE VISIBLE SPECTRUM OF SINGLET METHYLENE

The potential energy surfaces of the lowest-lying singlet electronic states of methylene (CH2) are determined by internally contracted multireference configuration interaction calculations, using a full-valence reference space, with an extended Gaussian basis set. The rotation-vibration levels on these surfaces are calculated by diagonalizing the rovibrational Hamiltonian matrix in a contracted basis. The rovibronic mixing due to the strong Renner-Teller interaction in this system is treated through the Coriolis term in the kinetic energy operator, using geometry-dependent electronic angular momentum matrix elements calculated from ab initio wave functions. The agreement between experiment and this high-quality ab initio calculation is sufficiently good that the calculation can be used to assign the observed vibronic bands in this very complex spectrum, where 90% of the observed lines remain unassigned. Many of the previous vibronic band labels are found to be incorrect. Most of the K > O bands previously labeled b approximately 1B1 are actually predominantly a approximately 1A1 in character, and the vibrational numbering of their b approximately 1B1 components are also incorrect. This work demonstrates the importance of supplementing experimental data with good quality ab initio calculations.