%0 Journal Article
%T New ab Initio Potential Energy Surfaces for the Renner-Teller Coupled and States of CH2
%A Haitao Ma
%A Chunfang Zhang
%A Zhijun Zhang
%A Xiaojun Liu
%A Wensheng Bian
%J Advances in Physical Chemistry
%D 2012
%I Hindawi Publishing Corporation
%R 10.1155/2012/236750
%X New ab initio potential energy surfaces (PESs) for the two lowest-lying singlet ¡ä and ¡ä¡ä electronic states of CH2, coupled by the Renner-Teller (RT) effect and meant for the spectroscopic study, are presented. The surfaces are constructed using a dual-level strategy. The internally contracted multireference configuration interaction calculations with the Davidson correction, using the aug-cc-pVQZ basis set, are employed to obtain 3042 points at the lower level. The core and core-valence correlation effects are taken into account in the ab initio calculations with a modified optimized aug-cc-pCVQZ basis set for the higher-level points. The analytical representations of these PESs, with the inclusion of the nonadiabatic RT terms, are obtained by the nonlinear least-squares fit of the calculated points to three-body expansion. Quantum dynamical calculations are performed on these PESs, and the computed vibronic energy levels for the two singlet electronic states are in excellent agreement with experiment. 1. Introduction The CH2 biradical has been the subject of many theoretical and experimental studies, due to its distinct electronic characteristics and chemical and physical properties. It is the direct chemical precursor of the widely observed CH radical [1]. As a crucial link in the photodissociation sequence of cometary methane, CH2 plays a significant role in the chemistry of hydrocarbon combustion and the astrophysics of interstellar medium [2¨C5]. Since Herzberg and Johns [2] carried out a detailed analysis of the high-resolution absorption spectrum of the singlet CH2 in the near ultraviolet region half a century ago, several groups [6¨C11] have observed the direct absorption spectra or subsequent monitoring of fluorescence via laser-induced fluorescence, stimulated emission pumping, dispersed fluorescence, and so forth. Experimental investigations of the spectroscopy provided abundant and precise rovibronic energy levels [12¨C16]. Recently, bands in the CH2 - transition between 12500 and 13000£¿cm£¿1 were recorded at Doppler-limited resolution utilizing a transient frequency-modulation (FM) laser absorption spectrometer by Chang and coworkers [16]. An unexpected and particularly complicated rovibronic structure was detected for and vibronic levels. Comparison of the calculated spectra on the available potential energy surfaces (PESs) [17¨C19] with the experimental spectra of Herzberg and Johns [2] indicates that the labeling of some vibrational levels is quite inconsistent, which can be ascribed to the local perturbations from the vibrational resonances
%U http://www.hindawi.com/journals/apc/2012/236750/