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Full three-dimensional adiabatic potential energy surfaces are presented for the lowest five 3A‘ and five 3A’ states of CH2. Both the 1 3A’ and 2 3A‘ states are dissociative with respect to the C–H coordinates, consistent with our earlier two-dimensional results. All higher lying states are found to be bound for this coordinate, although the barrier toward dissociation is small for some states. In terms of angle dependence, the 1 3A’ state shows a flat behavior, but tends towards larger angles as dissociation proceeds. Most excited 3A’ states are somewhat bent with only a small barrier to linearity. Transition dipole moments connecting the ground state with the excited triplet states are presented as well. The 1 3A’ state is the only state of that symmetry with a large transition dipole moment in the Franck–Condon region. Other 3A’ states exhibit large values only if one bond is stretched compared with the ground state equilibrium geometry. The 1 3A‘, 3 3A‘, and 4 3A‘ states are also slightly bent with a small barrier to linearity. However, the 2 3A‘ state has an absolute minimum at very small angles (less than 60°), and shows a considerable local minimum (∼1.5 eV) for the linear configuration. The 5 3A‘ state prefers the linear shape. The 3 3A‘ state has the largest transition dipole moment function in the Franck–Condon region, but the transition moments to other 3A‘ states can exhibit large values outside this region. The 2 3A‘ and 3 3A‘ states undergo an avoided crossing in the Franck–Condon region, so that a coupled states treatment is necessary for a correct description of the photodissociation dynamics. In order to provide the corresponding transition dipole moments in an appropriate form, a transformation to the principal axes of inertia was performed. The adopted transformations are discussed in... [ABSTRACT FROM AUTHOR]

We present quantitative results on photodissociation of CH2 (X 3B1) and its isotopomers CHD and CD2 through the first excited triplet state (1 3A1). A two-dimensional wave packet method employing the light–heavy–light approximation was used to perform the dynamics. The potential energy surfaces and the transition dipole moment function used were all taken from ab initio calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks in experimental CH2 and CD2 3+1 resonance enhanced multiphoton ionization spectra, provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape of the total absorption spectra of vibrationless and vibrationally excited CH2 , trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD. [ABSTRACT FROM AUTHOR]

The photodissociation processes of CH2 into CH and H have been studied using ab initio multireference configuration-interaction methods. Two-dimensional potential energy surfaces of the ten lowest triplet states correlating with the seven lowest states of CH have been calculated as functions of bond angle and one C–H bond distance, keeping the other C–H distance fixed at the equilibrium CH2 value. Transition dipole moments connecting the excited states with the ground state have been obtained as well. It is shown that efficient photodissociation of CH2 into CH (X 2Π)+H can occur by absorption from the ground X 3B1 (1 3A‘) state into the 1 3A1 (1 3A’) state at about 6.3 eV. Photodissociation into excited CH (a 4Σ-)+H can take place through the 1 3A2 (2 3A‘) and 2 3B1 (3 3A‘) states, although in a more complex manner since several avoided crossings occur along the reaction path. The 1 3A2 state is a so-called low-angle state, which has an equilibrium bond angle of less than 60° and correlates directly with C(3P)+H2. At 180°, when the molecule has D∞h or C∞v symmetry, interesting crossings between the ground and low-lying surfaces are found. Altogether, these crossings and correlations are predicted to lead to complicated dissociation dynamics for most of the states. The higher-lying states of CH2 can photodissociate either directly into excited states of CH, or they can be predissociated by the repulsive 1 5A2 (1 5A‘) state, which correlates with CH (a 4Σ-)+H. [ABSTRACT FROM AUTHOR]

Quantitative results on photodissociation of CH2(X 3B1) through the first excited (1 3A1) triplet state, producing CH (X 2Π)+H(2S), are presented. A three-dimensional time dependent quantum mechanical method was adopted to perform the dynamics using ab initio potential energy surfaces and an ab initio transition dipole moment function. The calculations were performed for J=0, where J is the angular momentum associated with the overall rotation of the nuclei. Comparison with calculations in which the bending angle was kept fixed at its ground state equilibrium value shows that a two-dimensional treatment suffices for obtaining the absorption spectrum. On the other hand, a three-dimensional calculation is necessary for correctly predicting the final rotational state distribution of the CH fragment. [ABSTRACT FROM AUTHOR]