Knowledge Bank

An MCSCF/CI computation has been made of the vertical and adiabatic electronic spectra of nitrous oxide. Twenty-configuration MCSCF and one hundred-configuration ancillary CI wavefunctions have been optimized for each state within about 11 eV of the ground state by a rigorous search of the single and double excitations in the valence space. In the case of the diffuse singlet and triplet pi states a singly occupied diffuse orbital and diffuse basis functions are included in the calculation. The 4s3p Dunning/Huzinaga basis set used in the configuration selection process has been augmented variously with diffuse and polarization functions for the vertical spectrum. Limited surface scans at $180\circ$ with the 4s3p basis set correlated wavefunctions yield corrections necessary to obtain the adiabatic excitation energies to linear excited states or state components. Bending potential energy curves were computed to enable an estimate of the adiabatic corrections for non-linear excited states. The correlation energy recovered in the present study is about 0.03 a.u. more than in a preliminary MCSCF study reported at this conference in $1974,1$ but the MCSCF excitation energies are about the same. The present 4s3p1d MCSCF/CI vertical excitation energies identify and place the $a3\Sigma +,b3\Delta ,c3\Sigma -,A1\Sigma -$, and $B1\Delta$ states at 6.5, 7.1, 7.5, and 7.7 eV, respectively. Bond relaxation at $180\circ$ for, e.g., the a and B states lowers their excitation energies to 5.3 eV and 6.3 eV, respectively. Zero-point energies have not been removed. Bending to a most stable angle of about $130\circ$ lowers these states or state components by about 1.3-1.9 eV more yielding adiabatic excitation energies of approximately 3.8, 4.4, 4.8, and 4.4 eV for the $a3A\prime (3\Sigma +),b3A\prime \prime (3\Delta ),11A\prime \prime (1\Sigma -)$,and $21A\prime (1\Delta )$ states, respectively. The 4s3p1d MCSCF/CI vertical excitation energy of the $21\Sigma +$ state is 9.6 eV after zero-point correction. The latter result is in excellent agreement with experiment, as is the result for the $C1\Pi$ state. Vertical and off-vertical transitions to the lowest five excited states apparently account for the broad peak beginning at about 5 eV and peaking at about 6.7 eV (cf. calculated vertical position of the a state) in the electron energy-loss $measurements.2$ Vibrational corrections place the A and B states at about 7.1 and 7.3 eV, respectively, compared to a weak, spin-allowed transition observed spectroscopically at 6.8 $eV.3$ The calculated excitation energy for the latter two singlet states will have to be averaged over the zero-point energy vibrational motion of $N2O$ in the X state to obtain definitive comparison with experiment.