Forbidden Electronic Transitions Research Articles

Raman antiresonance: De-enhancement of Raman intensity by forbidden electronic transitions

The intensities of the Raman lines of the transition metal complexes Co(en)33+, PdCl4=, PdBr4=, and PdI4=, have been investigated as a function of excitation energy in the vicinity of vibronically allowed ligand field electronic transitions. Whereas resonance enhancement usually results in local maxima in Raman intensities at the energies of allowed electronic transitions, we observe minima at the energies of these electronically forbidden transitions. These minima are ascribable to interference between the weak scattering from the forbidden electronic states and strong preresonance scattering from higher energy allowed electronic states. The excitation profiles can be reproduced with a three-state scattering model, using reasonable parameters.

Nuclear Coordinate dependence of electronic transition moments in orbitally forbidden transitions: A new interpretation of deuterium isotope effects

The nuclear coordinate dependence of electronic transtion moments has been investigated for the purpose of finding new interpretations of deuterium isotope effects on spectral intensities and radiative decay rates in orbitally forbidden electronic transitions. By using “AO following nuclei” wavefunctions as the building block for the electronic wavefunction in the adiabatic BO vibronic wavefunction, the spin-free hamiltonian is diagonalized to generate eigenfunctions and eigen-energies. It is found that the electronic transtion moments based on these eigenfunctions show dependences upon the vibrational modes which are not directly involved in vibronic coupling. This leads to interpretations of the deuterium isotope effects in T 1 → S 0 radiative transitions of aromatic hydrocarbons and S 0 → S 1 absorption in pyrazine which are not based on the conventional Herzberg—Teller or non-BO coupling.

Electron-impact excitation of H2O and D2O at various scattering angles and impact energies in the energy-loss range 4.2–12 eV

The electron-impact excitation of H2O and D2O has been studied at electron energies close to threshold and at large scattering angles in order to enhance spin and/or symmetry forbidden electronic transitions; and at energies far from threshold and at small scattering angles to enhance optically allowed transitions. The energy-loss range covered is 4.2–12 eV. From a comparison of the present measurements and recent, accurate ab initio calculations, several new assignments of electronic transitions in both H2O and D2O have been made or suggested. Also suggested are future works which could be carried out in order to unravel the complex Rydberg spectra above 11 eV.

On the forbidden 2000 Å transition of trans-1,3-butadiene

The sharp, weak absorption bands that overlay the descending tail of the intense NV absorption system of trans-1,3-butadiene are shown to comprise a forbidden electronic transition that is made visible by vibrations of the a u symmetry species of the C 2h symmetry group to which this molecule belongs.

The 3400 Å band system of sulphur dioxide

The contour of the band of SO 2 at 29937 cm −1 has been shown to be of type-c, and an approximate excited state structure derived as r SO = 1.50 Å, valence angle = 112°. For a number of reasons it is proposed that the principal bands between 3150 and 3400 Å correspond to vibronically induced B 1-A 1 transitions of a 1A 2- 1A 1 forbidden electronic transition rather than to an allowed 1B 1- 1A 1 transition.

Luminescence from Thin Solid Films at 77°K under Low-Energy Electron Bombardment

A spectrometer system for the study of luminescence from thin solid films induced by low-energy electron bombardment is described. This technique shows promise in the study of higher energy and optically forbidden electronic transitions in parent molecules and in providing information concerning radiolytic products. Films of benzene and toluene exhibit monomer fluorescence onsetting at an excitation energy of 4.5 eV. In the case of toluene luminescence from the benzyl radical also appears. Benzene phosphorescence is observed in a cyclohexane matrix. For cyclohexane and n-hexane films, fluorescence is observed with maxima at approximately 200 and 210 nm, respectively. At impact energies above about 15 eV other emission bands appear in the alkanes which may involve excited radicals and ions. Water exhibits bands with λmax at 280 and 380 nm. The former is attributed to fluorescence of the hydroxyl radical, and the latter can plausibly be assigned to phosphorescence from either the quartet state of the OH radical or from triplet water.

Temperature Dependence of Phonon-Induced Electronic Transitions in Insulating Solids

The temperature dependence of the oscillator strength of a forbidden electronic transition at an impurity in an insulating solid is calculated, assuming that the transition is induced by a local linear electron-phonon interaction. The electronic states are allowed to couple, not merely to one configuration coordinate, but to all the modes of vibration of the imperfect crystal. The frequencies of these modes and their relative contributions to the oscillator strength are calculated using Green's-function techniques. In particular, the $4{d}^{10}$ to $4{d}^{9}5s$ transition of ${\mathrm{Ag}}^{+}$ in NaCl is considered. A model which represents the host ions as point charges is shown to be inconsistent with the experimentally determined temperature-dependent oscillator strength. Good agreement with experiment is obtained if one assumes that the short-range interaction between the ${\mathrm{Ag}}^{+}$ impurity and the host ions is important.

Calculation of the intensities of the vibrational components of the ammonia ultra-violet absorption bands

The intensities of the vibrational components of the A ← X and B ← X band systems of NH3 have been calculated from vibrational functions appropriate to the best one-dimensional potential curves for the inversion mode. For the A ← X system the experimental data is fitted by an expression: The Franck-Condon approximation in which the Q 2 matrix element is neglected gives a poor fit to the data. We deduce that the electronic transition moment increases as the planar molecule is bent. For the B ← X system, which is a forbidden electronic transition, the experimental data are fitted well by the matrix element 2 which arises from the leading term in the Taylor expansion of the electronic transition moment. Using the same potential functions for ND3, and a reduced mass based on a pure bending mode, gives good agreement for the intensities of the corresponding ND3 bands. New experimental values for the extinction coefficients of the A ← X system have been obtained.

Electron impact excitation by the SF 6 scavenger technique : I. Nitrogen

The nitrogen excitation spectrum was investigated by electron impact using sulphur hexafluoride scavenging as the detection method. Improvement of the resolution of the experimental data was obtained by the application of a smoothing and deconvolution method. The formation of a transient negative molecular ion N 2 − is observed, in its ground electronic state at 2.4 eV, and, probably, in an excited state around 11.8 eV, autoionizing towards the E 2Σ g + state of the molecule. As a threshold technique, the method allows the identification of forbidden electronic transitions to the a′Σ u −, B r3 Σ u − and w 1Δ u states, around 9.5 eV, which are more important than the well-known Lyman-Birge-Hopfield system.

Dynamically Induced Forbidden Electric-Dipole Transitions of Np4+ in ThO2

Forbidden electronic electric-dipole transitions in the optical spectrum of Np4+ in ThO2 are induced by a dynamic distortion to the Oh symmetry. The resulting dynamic line broadening can be described to a first approximation by an averaged Hamiltonian B2αO20, which predicts linewidths in reasonably good agreement with experiment for B2 = 150 cm−1.

Systematische Untersuchungen der dynamischen Protonenpolarisation in organischen Kristallen mit eingebauten Radikalzentren

Abstract In order to produce high proton polarizations, several organic hydrocarbons have been doped with stable free radical molecules. Through the "solid state effect" a part of the electronic polarization in a magnetic field has been transferred to the protons of these materials. For this purpose, the "forbidden electronic transitions" of the combined electron-proton system have been saturated at about 37 GHz. The enhancement factor of the dynamic proton polarization was measured by pulsed NMR at 56 MHz. In phenanthrene with about 4 w.% bis-diphenylen-phenylallyl (BPA) a maximum polarization of 10.5% has been obtained at 1.5 °K and 13 200 gauss. This value may be increased by extending the temperature and magnetic field range. Systematic investigations in numerous organic systems have been carried out and have particularly yielded data on the frequency dependence and on the saturation behaviour of the enhancement curves. - Spin temperature theories of dynamic polarization have been considered to explain the results and to find possibilities for improving the polarization. In particular, the exchange interaction of the radicals and the distinction between relaxing and polarizing paramagnetic centres had to be taken into account.

Molecular-Beam Measurements of the Emission Spectrum and Radiative Lifetime of N2 in the Metastable E 3Σg+ State

Electron-impact excitation of a molecular beam of nitrogen was used to produce the metastable E 3Σg+ state. This state, which lies 11.87 eV above the ground state, has recently been observed in a number of electron-impact experiments. This paper reports the emission spectrum from the molecular beam, and identifies three forbidden electronic transitions, E 3Σg+ → A 3Σu+, E 3Σg+ → B 3Πg, and E 3Σg+ → C 3Πu. Approximate absolute transition probabilities are determined. The radiative lifetime of the E state is 270 ± 100 μsec, and the equilibrium internuclear distance is 1.16 ± 0.02 Å.

The effect of intermolecular interaction on the intensity of weak or forbidden electronic transitions

The effect of a solvent on the intensity of a forbidden electronic transition of a solute is studied by applying first order perturbation theory to the system consisting of a solute molecule A and its perturber B. The interaction energy between A and B is expressed in terms of the Longuet-Higgins and Salem formulation of intermolecular forces, with the result that there are two mechanisms by which the forbidden transition gains intensity—(a) borrowing from allowed transitions of A by virtue of the static electric field of the solvent B, and (b) borrowing from intense transitions of the solvent B by virtue of terms related to the dispersion interaction between A and B. Second order perturbation terms are shown to add nothing new, and a cross interaction is important if the forbidden transition of A is already vibronically perturbed. The solvent enhancement of the 0-0 band of the 2600 Å transition of benzene is discussed as an example. The static electric fields of polar solvents such as water and alcohols induce intensity borrowing from allowed benzene transitions, mainly 1 E 1 u - 1 A 1 g . In carbon tetrachloride and other chlorinated solvents the theory supports the Robinson mechanism by which intensity is borrowed from a transition of the solvent.

Forbidden Vibrational Transitions in the Electron-Impact Spectra of Molecular Oxygen and Nitrogen

The excitation of forbidden vibrational transitions by electron impact has been studied at 45-eV electron kinetic energy. The relative intensities of these transitions in N2 and O2 referred to the elastic peak, are independent of the scattering angle between 3° and 14°. For comparison we have also included data on vibrational transitions in CO and NO at one angle (4°). These are allowed by electric-dipole selection rules. In molecular oxygen we have also observed the forbidden electronic transitions a 1Δg ← X 3Σg− and b 1Σg+ ← X 3Σg− although the latter can be barely detected above background.

Intensity Enhancement of Forbidden Electronic Transitions by Weak Intermolecular Interactions

This paper discusses the phenomenon where an originally ``forbidden'' electronic transition in a molecule (M) becomes strikingly enhanced through weak interactions with a neighboring perturber (P). The purpose of the paper is to present a general mechanism by which both symmetry-forbidden and multiplicity-forbidden transitions of a molecule in a perturbing environment can gain intensity. The mechanism for intensity enhancement is based on perturbation theory, diagrams being introduced in order to simplify the discussion. The paper emphasizes the fact that the enhanced transition moment may be gained not only from strong transitions in the molecule itself, but also from transitions in the perturbing environment. A number of examples of intensity enhancement are presented and analyzed. The so-called Ham bands of benzene in carbon tetrachloride, for example, are shown to arise from intensity borrowed from the solvent. In spin-forbidden transitions a requirement for enhancement is that either P is paramagnetic (O2 effect) or that strong spin—orbit coupling in the excited states of P exists (external heavy-atom effect). Besides spin-forbidden and symmetry-forbidden transitions, double and multiple transitions made allowed through weak intermolecular interactions are discussed. Double transitions in molecular oxygen are analyzed, and it is concluded that intensity is borrowed from the Schuman—Runge band system. In this case an intermolecular interaction energy of only 5 cm−1 is required to give the observed intensity, so the transition can hardly be thought of as being due to the ``O4 molecule.'' A study of these weak interactions allows knowledge to be gained about molecular eigenfunctions in the important but often neglected regions of intermolecular space.

The electronic spectrum of cyanoacetylene: Part II. Analysis of the 2300-Å system

The diffuse banded absorption spectrum of cyanoacetylene in the gas phase between 2300 Å and 2000 Å has been measured and analyzed. The spectrum results from a forbidden electronic transition from the linear ground state with 1Σ + symmetry to a linear excited state with either 1Σ − or 1Δ symmetry, made allowed by π-type vibrations. The values of these for the excited state of HCCCN DCCCN are: ν 5′, 290 235 cm −1 ; ν 6′, 414 407 cm −1 ; ν 7′, 226 (215) cm −1 . Other excited state vibrations that have been identified are: ν 2′ ( σ +), 2120 2090 cm −1 ; ν 4′ ( σ +), 953 930 cm −1 . The electronic origin of the transition is at 44 221 44 248 cm −1 . The diffuseness of the bands precluded rotational analysis.

Forbidden Electronic Transitions in XeF2 and XeF4

Transition strengths have been measured for the weak 2330 Å band in XeF2 (f=0.002) and for the two weak bands in XeF4 at 2280 Å (f=0.009) and 2580 Å (f=0.003). To investigate the origins of these weak transitions, the possibilities of vibronic and singlet—triplet transitions in XeF2 and XeF4 were examined. Using the Herzberg—Teller theory of vibronic transitions and a molecular orbital treatment of excited electronic states, estimated strengths of the relevant vibronic transitions have been calculated to be f=0.001 for both XeF2 and XeF4. The vibronic band in XeF2 borrows intensity from the symmetry allowed 1A1g→1A2u transition at 1580 Å (f=0.45), while in XeF4 the major contribution to the vibronic band is from the symmetry allowed 1A1g→1Eu transition at 1325 Å (f=0.8). A temperature dependence of the intensity of the 2330 Å band in XeF2 has been observed and found to be less than that predicted by the Herzberg—Teller theory. The estimated strength of the singlet—triplet transition in XeF2 corresponding to the singlet—singlet transition at 1580 Å is shown to be small (f≦10−4) in spite of a heavy atom effect; the small transition strength persists because of the lack of nearby excited states of the required symmetry. In XeF4 the triplet excited state 3Eu corresponding to the singlet—singlet transition 1A1g→1Eu at 1840 Å (f=0.22) is permitted by group theoretical selection rules to mix with its own singlet state. Using an intermediate coupling scheme the estimated intensity of this singlet—singlet transition is calculated to be f=0.007. The theoretical estimates of the symmetry and spin forbidden transition strengths are used for the assignment of the weak electronic transitions in the xenon fluorides.

Electronic States of p-Benzoquinone. V. Vibrational Analysis of the Vapor Absorption Spectrum around 4500-A Region

The absorption spectrum of p-benzoquinone vapor in the region between 4080 and 5100 A was photographed and measured. Although the theoretical calculation as well as the experimental crystal spectrum indicate that two singlet-singlet electronic transitions exist in this region, prominent vibrational structure of the vapor spectrum can be interpreted as caused by a single forbidden electronic transition. If we assume that the hydrogen vibrations are not effective as perturbing vibrations which make the forbidden electronic transitions allowed through the vibrational-electronic interaction, this fact suggests the following two sets of assignments for the symmetries of the forbidden transition and the perturbing allowed transition: (1) the spectrum is a superposition of 1Au, 1B2u←1Ag transitions perturbed by a 1B2u←1Ag transition, (2) the spectrum is caused by a single 1B1g←1Ag transition being allowed through vibrational-electronic interaction with the allowed 1B1u←1Ag transition. Vibrational structure alone cannot exclude the second possibility, but a discussion supporting the first assignment is given. This assignment (1) is in agreement with the theoretical calculation of the energy levels. Most prominent bands are assigned to particular vibronic transitions. Fundamental frequencies in the excited electronic state 436, 796, 1109, and 1220 cm—1 are obtained corresponding probably to the ground-state values of 444, 770, 1144, and 1667 cm—1.

Infra-red spectra of compressed gases

TN the infra-red spectra of compressed gases bands due to forbidden vibrational, rotational and electronic transitions are observed with an absorbance proportional to the square of the density. Also simultaneous transitions do occur in which two molecules are simultaneously excited. Both phenomena are caused by mutual deformation of molecules at short distances (collision pairs), mainly by electric polarization. The influence of pressure on the infra-red absorption spectrum of a gas or a vapour up to pressures of a few atmospheres is restricted to line broadening which causes the rotational fine structure of the vibration bands to disappear. At higher pressures in some cases some change in the integrated absorption has been observed too, for example, in CH, by addition of N, (20 per cent at 600 atm) and A (1). At pressures of 10-100 atm and above new phenomena are observed. First vibrational, rotational and electronic transitions which are strictly forbidden for the isolated molecule do nevertheless occur and give rise to pressure-induced infra-red absorption bands. Secondly in mixtures of gases new bands due to simultaneous transitions are observed at frequencies which are the sum and difference of frequencies of both component. molecules. These simultaneous transitions are also observed in liquid mixtures. The pressure-induced and simultaneous transitions are both caused by the mutual deformation of the charge distribution of the molecules at very short distances, that is to say these bands in compressed gases are due to collision pairs. The absorbance of the pressure induced bands in pure gases is proportional in first approximation to the square of the pressure (density). For the pressure-induced bands in gas mixtures and for the simultaneous transitions the absorbance is proportional to the product of the pressures (densities) of both components. This quadratic dependence permits to distinguish pressure-induced transitions from weak bands arising from overtones and combination frequencies, but also from weak magnetic dipole and electric quadrupole transitions.

FORBIDDEN TRANSITIONS IN DIATOMIC MOLECULES: IV. THE a′3Σ+ ← XlΣ+ AND e3Σ− ← X1Σ+ ABSORPTION BANDS OF CARBON MONOXIDE

Two examples of forbidden electronic transitions in heteronuclear molecules, 3Σ+ – 1Σ+ and 3Σ− – 1Σ+, are studied in the spectrum of the CO molecule. The bands have been obtained in absorption in the region 1750 to 1230 Å with absorbing paths up to 400 cm-atm., using the fourth and fifth orders of a 3 meter vacuum spectrograph. In most of the 3Σ+ – 1Σ+ bands all four predicted branches are observed. For the 3Σ− – 1Σ+ bands, a somewhat unusual structure is predicted: two branches of O and S form and three branches of Q form. In the best-resolved bands, four branches are observed which closely fit the predicted branches, two of the Q branches being very nearly coincident and unresolved in the present spectra. The rotational and vibrational constants of the upper states, a′3Σ+ and e3Σ− of the two band systems have been determined. Some of these data had previously been obtained from perturbations in other band systems of CO. On the whole, the perturbation data agree satisfactorily with the more precise data obtained directly in the present study.