Rydberg and valence-shell character as functions of internuclear distance in some excited states of CH,NH, H2, and N2

SCF MO computations have been carried out on several excited states of CH and NH in which the excited MO 4σ is a Rydberg or near-Rydberg MO at internuclear distances R near that (R_e) of equilibrium in the ground state, but becomes an antibonding valence-shell MO as R increases toward dissociation. For the lowest ³Π_g state of H₂ and the lowest ³Π_g and ³Π_u states of N₂ the extent of 3dπ Rydberg character in the excited MO as a function of R for some R values ? R_e is evaluated by SCF MO computations.     

The vacuum ultraviolet spectrum of stannane

The vacuum UV spectrum of SnH₄ has been recorded up to 110nm (12.26 eV). This spectrum has been interpreted by ab initio calculations (SCF + CI), using a relativistic pseudopotential to describe the core electrons of the tin atom. The spectrum consists of a broad band composed of three maxima (at 8.73, 9.53 and 11.33 eV). The transition below 9.50 eV have been attributed to transitions to diffuse Rydberg states (6s and 6p). The other bands are due essentially to valence transitions.     

Orbital relaxation in the Rydberg series of the lithium atom. An excited state SCF calculation

Self-consistent-field (SCF ) calculations for a series of Rydberg states (1s²ns)²S of the Li atom are performed using the generalized Brillouin theorem (GBT) method. The calculated energy is a proper upper bound to the excited state energy. The SCF term values of the Rydberg states are almost the same as those of the frozen-core approximation ones. The orbital behavior shows that the core is slightly expanded by the penetration of the Rydberg orbitals, and the higher Rydberg orbitals can be very well represented by the modified hydrogen-like orbitals.     

Ab initio scf and CI study of the ground and excited states of the HN2 radical

Non-empirical SCF and CI calculations are reported for the HN₂, free radical in various low-lying electronic states. The nature of the angular and N-N and N-H stretching potential curves of each of these species is investigated, including a study of the dissociative behavior of such states. The ground state is found to be only very slightly bound with respect to NH stretch, in contrast to what is observed for isoelectronic HCO, The vertical electronic spectrum of HN₂, appears to be marked by a single long wavelength transition (1.95 eV) from the bent (124°) ‘A’ ground state to the linear ²Π excited species, but at least four other intra-valence and an additional n → 3s Rydberg species are indicated in the 5.5–8.0 eV absorbing region.     

Low lying optically active Rydberg transitions in propyleneimine

The circular dichroism spectrum of (?)-S-propyleneimine was recorded in the vapour phase and in hexane solution. SCF CI calculations of optical activity indicate that the absorption observed at ca. 50 000 cm^?1 corresponds to an n → 3s Rydberg type transition. The possible nature of the next absorption is also discussed.     

Rydberg states (n = 4–29) of azabicyclo [2.2.2] octane as studied by two-color fluorescence DIP and multiphoton ionization spectroscopies

Two-color fluorescence dip and multiphoton ionization (MPI) spectra have been observed for azabicyclo [2.2.2] octane in a supersonic jet. The spectra showed well-resolved structures consisting of five Rydberg series of n = 4–29. The five Rydberg series were assigned to s, p_z, p_xy, and two d orbitals. From the spectra obtained after exciting the molecule to various vibronic levels in the S₁ (3s) state, the Δν = 0 selection rule was obtained for the Rydberg-Rydberg transition. The same selection rule was found to be preserved also for the transition from the S₁ state to the ion. It was shown that the autoionization of the high Rydberg states to the ion is governed by Δν=?1. The existence of a very fast non-radiative channel was found for the Rydberg state from the ω₂ power dependence on the two-color MPI spectra.     

MRINDO/S-CI calculation on the electronic spectra of higher azines

Summary MRINDO/S calculation completed by singly excited configuration interaction was performed on the higher aziness-triazine,s-tetrazine and pentazine. The results enable the main characteristics of the observed electronic spectra to be interpreted. The importance of outer (Rydberg) atomic orbitals is stressed and it is found that a few singlet-singlet transitions of the higher azines lead to an excited state with considerable Rydberg character.     

A study of the electronic spectrum of halogen derivatives of ethylene

The spectra of tetrachloroethylene, monofluoroethylene, gem-, cis- and trans-difluoroethylene and trifluoroethylene were studied in the region 150–250 nm in the gas, solid and Kr matrix. The valence character of the π å π^* transition in the various compounds was verified and the Rydberg transitions were identified. The lowest energy band in the fluoroethylene was found to be a Rydberg transition. The vibronic structure of the π å π^* transition in tetrachloroethylene indicates that the excited state geometry is planar. The existence of a π å σ^* transition at lower energy than the π å π^* transition was verified for cis-difluoroethylene and trifluoroethylene.     

Theoretical study of the electronic spectrum of diimide by AB initio methods

A series of ab initio calculations is reported for the ground and low-lying valence and Rydberg states of diimide N₂H₂. Symmetric bending potential curves for both the cis and trans forms of this system have been obtained at the SCF level of treatment. In addition Cl calculations have been carried out for the trans-diimide ground state equilibrium nuclear conformation, using a configuration selection procedure described elsewhere; an associated energy extrapolation scheme is also employed which enables the effective solution of secular equations with orders of up to 40000. The ensuing Cl wavefunctions are interpreted in the discussion and the corresponding calculated energy differences between the various electronic states are compared with experimental transition energy results for both diimide and for related systems such as trans-azomethane. A more detailed analysis of the observed absorption bands in the ¹B_g-X¹A_g transition in N₂H₂ is also given, making use of calculated potential curve data as well as the pertinent Cl vertical energy difference. The dipole-forbiddenness of the excitation process is thereupon concluded to result in a distinct non-verticality for this electronic band system, causing its absorption maximum to occur at a position some 0.6 eV to the blue of the so-called vertical transition, i.e., that for which maximum vibrational overlap is obtained.     

Electronic spectrum of F<Subscript>2</Subscript>CO: theoretical calculations of vertical excitation energies and intensities

In this work, the linear response formalism with a triples-corrected CCSD reference wave function, LR-CCSDR(3), is applied to the calculation of vertical excitation energies of singlet states of the F₂CO molecule. A basis set of atomic natural orbitals augmented with a series of Rydberg functions has been used in the calculations. A large number of electronically excited states were calculated, and the valence, Rydberg, or mixed character of the states were investigated. In addition, the molecular quantum defect orbital (MQDO) method has been used to determine transition intensities involving Rydberg states. Excitation energies and transition intensities for Rydberg states with n > 3 are reported for the first time.     

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.     

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.     

The electronic absorption spectra of the alkyl-substituted derivatives of bis(benzene)chromium(0) in the vapour phase

The UV and visible absorption spectra of (arene)₂chromium(0) (arene = benzene (I), toluene (II), ethylbenzene (III), cumene (IV), tert-butylbenzene (V), mesitylene (VI) in the vapour phase have been investigated. Four band systems A,B,C and D are revealed in the spectra. The bands of the system with the shortest wavelengths, D, represent the Rydberg series. The first ionisation potentials IP_a1g, 5.18 and 5.01 eV respectively. The Rydberg bands correspond to the allowed electrodipole transitions from the highest occupied molecular orbital (MO) a_1g to the vacant MO of either the a_2u or e_1u type.System C corresponds to the intense band of the solution spectra. The electronic transition e_2g → e_2g obviously makes a great contribution to this system. System B is assigned to the transition from a_1g to vacant a_2u or e_1u MO, which can be Rydberg orbitals. System A can be assigned to the a_1g → e_2u transition or to the Rydberg transition, which is forbidden in the D_6h point group but becomes allowed upon reduction of symmetry.     

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.     

Theoretical study on the vertical electronic spectrum of carbonyl fluoride,F2CO

Ab initio self-consistent-field (SCF ) and configuration interaction (CI ) calculations on the ground and excited states of carbonyl fluoride (F₂CO) were carried out at its experimental ground-state equilibrium geometry. Vertical transition energies deduced from the CI results provide assignments for the electronic systems I–IV, experimentally observed by Workman and Duncan. The singlet excited state, ¹A₁ (π→π*), is found to be a mixed valence–Rydberg state and to he 1 to 1.2 eV above the suggested experimental value, irrespective of the choice of the basis used for the CI calculations.     

The magnetic circular dichroism spectrum of acetylene

The magnetic circular dichroism (MCD) spectrum of gas phase acetylene has been studied in the spectral region 320-140 nm. No MCD signal was detected in the region of the first transition. The observation of irregular MCD bands beginning at 185 nm confirm assignment of this latter band as the 0-0 band of the second transition. The discrete and relatively narrow bands in the region 155-140 nm exhibit pure A-term character, lending unambiguous confirmation to the assignment of ¹Π_u for the state. The observed absorption spectrum in this region is interpreted as consisting of two non-interacting systems, one involving the ¹Π_u Rydberg state and the other, a broad continuous background, cannot be assigned. It is however suggested that it may be due to the ¹Σ_u⁺ state of the π → π^* excited configuration.     

CH3I low-n Rydberg states in supercritical atomic fluids near the critical point

Vacuum ultraviolet photoabsorption spectra of CH₃I 6s and 6s′ Rydberg states doped into supercritical argon, krypton, and xenon perturbers were measured from low density to the density of the triple point liquid at noncritical temperatures and on an isotherm near the perturber critical temperature. A full line shape analysis of these spectra was performed using a single set of intermolecular potential parameters for each dopant/perturber system. The resulting perturber induced shift of the simulated adiabatic transition of the 6s and 6s′ Rydberg states is presented as a function of perturber number density, and this shift illustrates a perturber critical point effect on the excitation energies of the molecular low-n Rydberg states.     

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.     

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.