Equations of motion method: Excitation energies and intensities in formaldehyde

Abstract

We have used the equations of motion method to study the excitation energies and intensities of electronic transitions in formaldehyde. The calculated excitation energies and oscillator strengths agree well with experiment and suggest explanations for some unusual features recently observed in the optical absorption and electron scattering spectrum of formaldehyde in the vacuum ultraviolet.

Simultaneous calculation of Rydberg and valence excited states of formaldehyde

“…This observation is shared by previously reported theoretical data [5,8,11,12]. Whereas the EOM-based data [11,12] for these three states are in acceptable agreement with our MR-CISD and MR-AQCC/LRT oscillater strengths, the MR-DCI [8] and the generalized valence bond CI [5] method predict weaker transition intensities, especially for the n±3s excitation. On the other hand, the reported oscillator strengths for the n±3d excitations from EOM calculations [12] are by up to 4 times larger than those derived from CI methods.…”

“…Such comparisons have to be regarded sometimes with caution since computed vertical excitation energies do not have a directly observable experimental counterpart. We recon®rm the ®nding that the vertically excited n±3p z state is lower in energy than the n±3p y state, a fact ®rst observed by Yeager and McKoy [11] and Harding and Goddard [5] and reemphasized by Hachey et al [6]. At the MR-CISD and MR-CISD+Q levels using the MINVAL reference space and the [431/ 21] basis set, the energetic splitting of these two states is rather small ( Table 3).…”

“…A large number of quantum chemical calculations have been performed on excited states of formaldehyde, of which we mention the pioneering investigations by Whitten and Hackmeyer [3] and Buenker and Peyerimho [4] as well as more recent multireference con®guration interaction (MR-CI) [5±9], complete-active-space perturbation theory to second order (CASPT2) [10] and equation-of-motion (EOM) calculations [11,12]. The aforementioned puzzling experimental ®ndings concerning irregularities in certain Rydberg series and the missing p±p à state have been explained by Hachey et al [6] in their MR double excitation CI (MRD-CI) calculations by interactions between Rydberg and valence states, associated in particular with the stretching coordinate of the CO bond.…”

Electron-impact rotationally elastic total cross sections for H2CO and HCOOH over a wide range of incident energy (0.01–2000 eV)

“…The present calculated first electronic excitation energy is 4.44 eV which finds excellent agreement with the theoretical data of Kaur and Baluja [22] and in Refs. [20,21,[25][26][27]. The rotational constant obtained in the present calculation is 9.406 cm −1 and is also in excellent agreement with experimental and theoretical data from the National Institute of Standards and Technology (NIST) [24] .…”

Photo-Absorption Studies on Formaldehyde Using Synchrotron Radiation at Indus I

“…Since 1970, several theoretical calculations have been performed to predict the accurate location and assignment of various valence and Rydberg states. [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Most of these calculations, which focus on different aspects such as geometrical structure, vertical and adiabatic excitation energies, and frequencies of excited states, have been reviewed by Jose et al [39] It may be seen from literature that the only strong valence transition observed is from the ground state to the first excited singlet state in 4.08 eV region assigned to the à 1 A 2 ÀX 1 A 1 band system and that it consists of an extended progression of vibronic bands. [8,21] Some of the Rydberg states are well located, but there is ambiguity in respect of the vertical ordering of some low-lying Rydberg transitions, viz 1 B 2 (n-3p z ), 1 A 1 (n-3p y ), and the absence or weak structures of other excited valence states, viz 1 A 1 (p À p à ), 1 B 1 (r À p à ), that are expected to lie in the same energy region.…”