Extensive multireference configuration interaction calculations were carried out in order to obtain complete two-dimensional (2D) potential energy surfaces for the amidogen (NH2) radical as functions of both N–H bond lengths keeping the bond angle fixed at its experimental ground state equilibrium value. The eight lowest-lying states (four of each symmetry, A′ and A″) were treated mainly for the purpose of using these surfaces in subsequent studies of the photodissociation dynamics. In analogy with the neighboring dihydrides CH2 and H2O the photodissociation of NH2 into NH+H (hydrogen abstraction) takes place preferentially after excitation of the first two Rydberg s states (3 2A′/2 2A1 and 2 2A″/2 2B1) found closely together at about 7.6 eV. The transition dipole moments connecting the ground state with these two states are large (0.44 a.u. and 0.66 a.u.) in the Franck–Condon region, but the behavior of the potentials in the dissociation channel is quite different. The 3 2A′/2 2A1 state is weakly repulsive whereas the 2 2A″/2 2B1 state is strongly repulsive. This will result in differences in the dissociation dynamics for the two states. The next higher state which should play a role in the NH2 photodissociation is the 4 2A″/3 2B1 Rydberg s state at 9.4 eV, because of its large transition dipole moment with the ground state (0.36 a.u.). Close to this state, many Rydberg p states were found. Due to the high density of states in the region above 9.0 eV, interactions of these states are expected and should lead to complicated dissociation dynamics. Contrary to CH2, the two low-lying valence states for NH2 are found at lower energies [2.2 eV (1 2A1) and 6.5 eV (1 2B2)], well separated from the first members of the Rydberg series. These states are less important for the photodissociation of NH2, compared with CH2, because the first state is bound and the transition to the other is dipole-forbidden in C2v symmetry. For H2O, the valence states are missing.

Motivated by the possible importance of OBrO in atmospheric photochemistry, multireference configuration interaction calculations of the low-lying excited states were carried out to obtain information about the electronic vertical spectrum up to excitation energies of about 6 eV from the ground state, including the transition dipole moments, and about possible photodissociation pathways, based on one-dimensional cuts through the potential energy surfaces for dissociation into BrO + O and Br + O2, respectively. In addition, for probing the angle dependence the bending potentials were also calculated. From all computed eight doublet states (two/four of each symmetry in C 2 v   C s ) only the 12A2 state at 2.7 eV possesses a large transition dipole moment with the 12B1 ground state, whereas for all other states this quantity is very small or zero. Therefore the 12A2 state should play a decisive role in OBrO photochemistry. Close to the 12A2 state two other states were found at 2.4 eV (12B2) and 2.5 eV (12A1) so that interactions of these three states should certainly influence possible dissociation processes. For this reason, besides direct adiabatic photodissociation of the 12A2 state into BrO + O also predissociation via these close-lying states can be expected, leading to a very complex photodissociation mechanism for excitation energies around 2.5 eV. Moreover, in this energy range photodissociation into Br + O2 is only possible through the 12B2 state (after initial excitation of the 12A2 state) because only for this state a small barrier of 0.7 eV relative to its minimum is estimated from the calculation of a simplified C 2 v minimum energy path. For the 12A1 and 12A2 states, rather large barriers are predicted. The next higher-lying states, with excitation energies of 3.9 eV (22A1) and 4.5 eV (22B2) are well separated from lower- and higher-lying states and from each other, but due to their small transition dipole moments, they should be probably of minor importance for the OBrO photochemistry. The last two states considered in our study are predicted to lie close together at 6.0 eV (22A2) and 6.1 eV (22B1) and are strongly repulsive upon dissociation into BrO + O. Finally, it should be noted that our calculations demonstrate the expected qualitative similarity to the results previously obtained for the corresponding OClO system.

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