AB initio study of the low-lying excited states of cyclopropene by the equations-of-motion method

The lowest singlet and triplet excited states of cyclopropene have been investigated with the equations-of-motion method. The first spectral band is due to a singlet transition arising from π → 3s,3p excitations. The other two band systems are associated with clusters of transitions predominantly of Rydberg nature or strong admixtures of valence-valence and valence-Rydberg excitations. The first triplet transition is mainly of π → π character.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

Photoionization mass spectrometry of gaseous ethene and fluoroethenes in the energy range 280–300 eV

The photoionization mass spectra of the parent cations of ethene, fluoroethene, 1,1′-difluoroethene, cis-difluoroethene, trifluoroethene and tetrafluoroethene have been measured in the energy range 280–300 eV. Below, the C1s ionization limits strong resonances are observed, which are assigned to C(1s) → π^* and C(1s) → R transitions. The term values of the core excited valence states are strongly increased compared to those of π → π^* transitions. The 1s core hole causes negligible shifts in the Rydberg states. Ab initio MRD CI calculations are used to interpret the experimental results.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

AB initio and configuration interaction calculations for some σ → π* superexcited states of the trans-1,3-butadiene molecule

Some σ → π¹ superexcited states of the trans-1,3-butadiene molcule have been calculated in order to establish them as possible candidates for the 9.52 eV and 11.04 eV transitions observed in the electron impact spectra of this molecule. Four states have been solved self-consistently ( 7a_g→ 2a_u2a_gand 2b_g and 6b_u→ 2a_u, 2bg) and on the basis of extensive CI calculations of transition energies and oscillator strengths, we assign the 11.04 eV transition to the ¹B_g (6b_u→ 2a_u) state. The transition observed at 9.52 eV is more likely to be either a π (la_u) → π1 transition or the first member of a Rydberg series converging to the second ionization potential.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Non-empirical SCF and Cl Study of the ground and excited states of thioformaldehyde

Ab initio SCF and Cl calculations are reported for ground and various low-lying Rydberg and valence excited states of thioformaldehyde H₂CS. A double-zeta basis of near Hartree-Fock quality is employed in this work and the importance of polarization functions is also assessed. The calculations indicate uniformly larger CX bond lengths in this system than for H₂CO in the corresponding electronic states; they also lind potential minima for H₂CS non-planar nuclear conformations in the (n,π^*) and (π,π^*) excited states but in each case the calculated inversion barriers are seen to be smaller than those encountered in formaldehyde. The vertical transition energies to the various excited states studied are also found to be significantly smaller in H₂CS than in H₂CO but the order of electronic states is concluded to be virtually identical for the two systems. The lowest-lying excited states are the ^3,1(n,π^*) species calculated at 1.84 and 2.17 eV respectively; the first two allowed transitions are indicated to be the Rydberg species (n,s_R) and (n,px_R) at 5.83 and 6.62 eV. These are followed by the two allowed transitions σ → π^* and π → π^* at 7.51 and 7.92 eV respectively, both well below the first ionization limit in H₂CS. The much smaller splitting between the ^3,1(π,π^*) species in H₂CS than in H₂CO is attributed to the relatively diffuse charge distribution of the sulfur atom compared to that of oxygen.     

Multiconfigurational perturbation theory (CASPT2) applied to the study of the low-lying singlet and triplet excited states of cyclopropene

The electronic spectrum of cyclopropene has been studied using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the π and σ ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nature of the lowest energy singlet-singlet band and yield a conclusive assignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (σ → π*) state. The (π → π*) (V) state is interleaved among a number of Rydberg states in the most intense band of the system. The remaining spectral bands are due to Rydberg transitions of higher energy. The two lowest singlet-triplet transitions involve the same valence states. The results are in agreement with available experimental data and provide a number of new assignments of the experimental spectra.     

Low-energy electron-impact excitation spectra of formaldehyde,acetaldehyde and acetone

Threshold electron-impact excitation spectra, transmission spectra and n→n^* excitation functions are presented for formaldehyde, acetaldehyde and acetone. Formation of temporary negative ion states and their decay-channels are discussed. The electronic transition near 4 eV is shown to be due both to triplet and singlet n→n^* excitation. The energy positions of the ³n→n^* and first triplet Rydberg transition are accurately located. In formaldehyde a new transition is reported at 8.50 eV. Assignments are given for the observed electronic excitation processes.     

All-valence-electron CI treatment of the electronic spectrum of trans-butadiene

An all-valence-electron CI treatment is reported for the low-lying valence and Rydberg states of butadiene. All singly- and doubly-excited configurations relative to a series of the leading terms in a given CI expansion are taken into account, with resulting secular equation orders of as high as 150 000. The agreement between calculated and experimental transition energies is invariably better than 0.2 eV where comparison is possible, with all low-lying valence triplet and Rydberg singlet excited states being unambiguously assigned. The valence-shell excitation to the 2 ¹A_g species is concluded to correspond to the 7.06 eV band system, while the forbidden singlet—singlet transition reported by McDiarmid is assigned as x₂ → 3s. The possibility of an avoided crossing between Rydberg valence ¹B_u excited states having a determining influence on the appearance of the broad intense V₁—N absorption is also discussed.     

Excited electronic states of 1,3,5-cycloheptatriene

The electron-impact energy-loss spectrum of 1,3,5-cycloheptatriene has been measured at impact energies of 30,50, and 75 eV, and scattering angles varying from 5° to 80 °. Singlet → triplet transitions were observed at 3.05 eV and 3.95 eV. No evidence for the weak transition at 2.1 eV previously reported on the basis of threshold electron-impact studies was found. Single → singlet transitions were observed at 4.85 eV and 6.40 eV in good agreement with the optical spectrum and semi-empirical calculations.     

The valence electronic excited states of trans-1,3-butadiene and trans,trans-1,3,5-hexatriene from generalized valence bond and configuration interact

Self-consistent ab initio generalized valence bond (GVB) and configuration interaction (Cl) calculations are presented for the ground and valence electronic excited states of trans-1,3-butadine and all trans-1,3,5-hexatrine. Previous workers have suggested that (all trans) polyenes exhibit a parity-forbidden valence excited state (2¹ A_g at an energy just below that of the first dipole-allowed (1¹ B_u) state. We find such valence excited electronic states for butadiene (ΔE = 7.06 eV) and hexatriene (ΔE = 5.87 eV), but in both cases the excitation energy is considerably higher than the dipole-allowed transitions (zero-zero transitions at 5.95 eV and 4.95 eV, respectively). The lower two triplet states are found at 3.35 eV and 5.08 eV for butadie and at 2.71 eV and 4.32 eV in hexatrine, in good agreement with experimental values (3.2–3.3 eV and 4.92 eV for butadiene and 2.66 eV and 4.1–4.2 eV for hexatrine). Considering the states formed by removing one electron from the π space we found ion states at 8.95 eV and 11.40 eV for butadiene and at 8.33 eV, 10.53 eV, and 11.60 eV for hexatriene, in godo agreement with experimental results (9.0 eV and 11.5 eV for butadiene and 8.45 eV, 10.43 eV and 11.6 eV for hexatriene).     

Application of the MC SCF method to the π → π* excitation energies of ethylene

The MC SCF method is employed to calculate the N → T and N → V π → π^* vertical excitation energies of ethylene. To obtain accurate excitation energies it is found to be necessary to utilize an expanded valence space containing two π and two π^* orbitals. Relatively small MC SCF calculations, allowing at most one-electron excitations from the sigma space, are found to yield excitation energies and spatial extents of the excited states in excellent agreement with the predictions of large multi-reference or iterative-natural-orbital CI calculations. These results show that within an MC SCF framework σ-σ correlation is unimportant for describing the π → π^* processes. We also conclude that the neglect of the effects of unlinked cluster terms in some of the CI calculations may have introduced small, but important, errors in the excitation energies and predictions of the spatial extent of the V state.     

Triplet states of the amide group. Trapped electron spectra of formamide and related molecules

Trapped electron (TE) spectra are obtained using ion cyclotron resonance detection of scavenged electrons. The lowest singlet-triplet transitions, ³(n→π^*), in formamide (HCONH₂) and N,N-dimethyl formamide (HCONMe₂) are found at vertical energies of 5.30 and 5.00 eV, respectively. An unresolved band containing the ³(π→π^*) and ³(n→3s) states appears at higher energies, centered at 6.60 and 6.00 eV, respectively. The TE spectra of formaldehyde (HCHO), acetaldehyde (MeCHO) and acetone (Me₂CO) are obtained for comparison and are used along with results from ab initio theoretical calculations in establishing assignments. Singlet-triplet transitions dominate the spectra of all of these carbonyl containing molecules, to the exclusion of low lying singlet-singlet transitions. This is in agreement with other TE spectra and the expectation that (dσ/dE) will be higher near threshold for singlet-triplet as compared to singlet-singlet transitions.     

Ab initio MRD-CI study of the rydberg states of methylene

Potential curves have been calculated for the low-lying Rydberg states of CH₂ as well as for a number of its valence-shell species by employing the ab initio MRD-CI method. The first Rydberg transition is found to occur with a vertical energy of 6.38 eV (1b₁ → 3s), but the corresponding upper state is believed to be strongly predissociated since it correlates directly with the CH(²II) + H(²S_g) ground state fragments at lower energy. The assignment of the first observed Rydberg transition at 8.757 eV by Herzberg as 1b₁ → 3dπ is confirmed almost quantitatively in the calculations, while the corresponding minimum 1P value is computed to be 10.21 eV compared to the experimental result of 10.3 ± 0.1 eV. The dissociation energy of methylene in its ground state is calculated to be 4.47 eV, and this result also fits in well with experimental evidence, which determines a lower limit for this quantity of D₀ > 4.23 eV. Finally, it is found that none of the Rydberg states nor any of the higher-lying valence-shell species of methylene are of sufficiently low energy to play a significant role in the experimental determination of the ¹A₁-³B₁ splitting of this system.     

Electronically excited and ionized states of the chlorine molecule

Potentials curves for the ground and excited states of the chlorine molecules and its positive and negative ions have been calculated by means of the MRD-CI method. The standard AO basis employed consists of 74 functions including two atomic d and one set of s and p bond species, and the results at the corresponding full CI level are estimated for each state via a perturbation correction. Special emphasis is placed upon the treatment of Rydberg-valence mixing in this system, which phenomenon is found to be essential to the understanding of Cl₂ electronic absorption spectrum. All singlet states which correlate with the lowest dissociation limit plus many others which go to ionic Cl⁺+Cl^? or Rydberg Cl+Cl asymptotes are given explicit consideration. Among the triplet species of Cl₂ which dissociate into the ground state atoms only the ³Π_u state is not repulsive. The calculated D₀ value for the ground state is 2.455 eV compared to the experimental value of 2.475 eV, while the vertical ionization energy and electron affinity are found to be 11.48 and 2.38 eV respectively, also in very good agreement with the corresponding measured data of 11.50 and 2.51 ± 0.1 eV. In addition to Cl₂ laser line is confirmed to result from a ³Π_g → ³Π_u emission, whereby the calculated downward vertical transition energy of 4.86 eV fits in quite well with the known location of this line at 4.805 eV. The first two dipole-allowed transitions from the ground state of chlorine involve ¹Σ_u⁺ and ¹Π_u states which are calculated to be nearly isoenergetic, and these results also match very well with the location of the first absorption band in this spectrum. Finally quite similarly as in O₂ it is found that an avoided crossing between Rydberg and valences states produces a relatively steep potential well for an upper state (2 ¹Σ_u⁺), whose location concides with that of a second absorption band recently observed in synchrotron radiation studies.     

Multireference configuration interaction studies on higher valence and Rydberg states of OClO, ionization potentials, and electron detachment energies

MRCI results are reported for the vertical excitation energies (VEE) and oscillator strengths f of doublet states of OClO up to 11 eV, including 3b(1) → 4s, 4p, 3d, 5s, 5p, 4d, and most 1a(2), 8a(1), 5b(2) → 4s and 4p Rydberg states. The lowest Rydberg states 3b(1) → 4s and 3b(1) → 4p(x) have mixed valence-Rydberg character. The observed spectral bands were reassigned to include valence states which have generally higher oscillator strengths. The well-known valence state 1(2)A(2) has a VEE of 3.63 eV, and a relatively high f of 0.042. Overall, the calculated oscillator strengths are in good agreement with measured values. The lowest quartet state, 1(4)B(2), lies at 6.95 eV. Quartet Rydberg states start with 1a(2) → 4s at 9.28 eV. According to calculated vertical ionization potentials (VIP) of OClO, the second VIP at 12.59 eV is reassigned from 1(3)B(1) to 1(3)B(2) (ionization from 1a(2), rather than 8a(1)), and the third VIP at 12.63 eV from 1(1)B(1) to 1(3)B(1) (ionization from 8a(1)). Vertical electron detachment energies of OClO(-) have been calculated up to 8.9 eV. There is good agreement with experimental values.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Dielectronic recombination of lithium-like Ar15+

Dielectronic recombination (DR) of Ar¹⁵⁺(1s ²2s) ions was studied in a single-pass merged-beams experiment at the UNILAC (universal linear accelerator) of GSI. Absolute recombination rates and cross sections were measured for electron-ion center-of-mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with 2s → 2p (Δn=0) and 2s → 3? (Δn=1) core excitations and even individual terms in the 1s ²3?3?′ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions, effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2 _{p 1/2}? and 2 _{p 3/2}? Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s ²3?n?′) Rydberg series.