Higher Electronic States Research Articles

Nanoseconds UV laser study of radiationless deactivation from upper electronic excited states in solution: Tb 3+

Using 25 ns half width pulses of a frequency-quadrupled Nd laser at 265 nm Tb 3+ perchlorate in D 2O solution, or in borate glass, was excited to high electronic states and the fluorescence on transition from the upper 5D 3 and lower 5D 4 excited states to various sub-levels of the lowest 7F state were measured. The appropriate energy gap spacing of this rare earth ion results in lifetimes which enabled us to observe the decay of 3D 3 as well as the growing in and decay of 5D 4. Radiationless processes from 5D 3 direct to 7F or cascading to 5D 4 are discussed and correlated with previous theoretical and experimental results on related systems involving the interaction of the non-complexed solvated ion with the medium, and compared with related phenomena in glassy solids.

Antibody-hapten interactions: Circular and linear polarization of the fluorescence of dansyl bound to anti-dansyl antibodies

The fluorescence spectra of several dansyl derivatives (dansylamide, ϵ- N-dansyl- l-lysine, dansyl- l-alanine, and α- N-dansyl- l-alanine amide) bound to anti-dansyl antibodics (induced by an α- N-dansyl-poly d,l-alanine-poly l-lysine conjugate) are shifted by about 60 nm to the blue, and the quantum yields are markedly enhanced, compared to their respective fluorescence properties in water. The light emitted by the bound haptens is partly circularly polarized, reflecting the asymmetry induced in the bound chromophores by the antibody combining site. In contradistinction, the fluorescence spectrum of 1-dansyl-2-alanine diaminoethane bound to anti-alanine antibodies is similar to that of the free fluorophore in water and lacks circular polarization. These results imply that in this case the fluorophore of the hapten protrudes out of the site into the aqueous solvent. No circular dichroism is observed in the 300 to 400 nm region for the dansyl-anti-dansyl complex. Thus a change in the mode of interaction between the chromophore and its binding site takes place upon electronic excitation. The heterogeneity of the antibody binding sites is expressed by the dependence of the circular polarization of fluorescence on excitation wavelength. Differences in the circular polarization of luminescence were also observed when the residues attached to the dansyl group have been varied. This may reflect differences in the alignment of the fluorophore within the binding sites for the different dansyl derivatives. The linear polarization of dansylamide dissolved in glycerol is not constant across the emission band, indicating that the transition dipole moments related to the various vibronic states do not have the same spatial directions. Vibronic mixing of the emitting excited state with higher electronic states is thus indicated. Dansyl- l-alanine bound to anti-dansyl antibodies exhibitsan even more pronounced variation of the linear polarization across the emission band. In this case, the dependence of the linear polarization of the emitted light on excitation wavelength is anomalous, which is again a reflection of the heterogeneity of the population of the antibody molecules. The implications of these results to the studies of the fluorescence polarization of dansyl-protein complexes are discussed.

Heme-Spin Label Studies of Hemoglobin: I. PREPARATION AND PROPERTIES OF HEME-SPIN-LABELED FERRIHEMOGLOBIN

In order to investigate the protein conformation in the vicinity of the heme and the spin states of the heme-iron in dissolved hemoglobins, a nitroxide spin label was covalently attached to one of the propionic acid groups of the porphyrin ring. The optical spectra of the heme-spin labeled ferrihemoglobins were identical to those of native hemoglobins indicating that the spin labeling did not affect significantly the electronic structure of the prosthetic group. The EPR spectra of the nitroxide moiety in the fluoride, cyanide, and azide derivatives of the heme-spin-labeled hemoglobin and of the corresponding acid methemoglobin in solution were not identical, suggesting that the protein conformation in the vicinity of the label is different in each of these hemoglobin derivatives. The resonance amplitude of the nitroxide in heme-spinlabeled hemoglobin was sensitively influenced by the high spin heme-iron located in the center of the porphyrin ring due to the magnetic dipolar interactions between them. The degree of the dipolar interaction depended on the magnetic moment and electron-spin relaxation time of the heme-iron, as well as the distance between the nitroxide and heme-iron. From the strength of this interaction, the distance between the heme-iron and the nitroxide radical was estimated as 11.8 A. Since the spin label attached to heme is sensitive to changes in the magnetic moment of the iron, the heme-spinlabeled ferrihemoglobin can also be used for studying the thermal equilibrium between high and low spin electronic states of the heme-iron. By comparing the nitroxide resonance amplitudes of the hydroxide form with those of the high spin acid met form and the low spin cyanide form of hemoglobin, the ratio of high to low spin components of the hydroxide was calculated as 53:47 at 0°. This ratio was increased at higher temperatures due to the shift of the equilibrium composition in favor of the high spin form. Optical and EPR spectra of free spin-labeled protohemin were also investigated. EPR spectra of protohemin were sensitive to dimer formation of hematin in aqueous alkaline solution. Two empirical indices are presented for convenient numerical expression of the mobility of a nitroxide label from its EPR line shape.

ChemInform Abstract: ELECTRIC FIELD SPECTRA AND DIPOLE MOMENTS OF 1,3‐DIAZAAZULENE

Nitrogen heterocycles of azulene have currently been under study to examine the effects of nitrogen substitution in the azulene system. We have determined the dipole moments of three different electronic states of 1,3‐diazaazulene: the ground state, the first excited singlet state (1ππ*, B1), and the phosphorescent state (3ππ*). Measurements have been performed on the origin band and several prominent vibronic bands of the 1B1 ← 1A1 transition. The dipole moment of the ground state was determined from dielectric constant measurements, and the change in dipole moments upon excitation to higher electronic states was measured from the pseudo‐Stark effect of molecular crystals on diazaazulene in a naphthalene host. The measured values are μ(GS) = 2.98 ± 0.01 D, |Δμ(GS−S1)| = 1.80 ± 0.05 D, and |Δμ(GS—T1)| = 1.10 ± 0.07 D. The measurements enable us to examine the role of nitrogen atoms participating the charge distribution of the σ core and the π‐electron cloud in the 1B1 ← 1A1 transition in comparison with t...

Herzberg‐Teller Interaction and Vibronic Coupling in Molecular Crystals I. The Simple Model

AbstractThe vibronic absorption spectrum of a molecular crystal, corresponding to the formation of an exciton in a state forbidden by the intramolecular symmetry, is investigated. It is assumed that intensity is borrowed from a higher electronic state with help of the Herzberg‐Teller mechanism. Non‐totally and totally symmetric intramolecular vibrations are included into this consideration. Formulae for relative intensities of different vibronic lines in the spectrum are obtained. The one‐ and two‐phonon regions of the spectrum are discussed.

Electric field spectra and dipole moments of 1,3-diazaazulene

Nitrogen heterocycles of azulene have currently been under study to examine the effects of nitrogen substitution in the azulene system. We have determined the dipole moments of three different electronic states of 1,3-diazaazulene: the ground state, the first excited singlet state (1ππ*, B1), and the phosphorescent state (3ππ*). Measurements have been performed on the origin band and several prominent vibronic bands of the 1B1 ← 1A1 transition. The dipole moment of the ground state was determined from dielectric constant measurements, and the change in dipole moments upon excitation to higher electronic states was measured from the pseudo-Stark effect of molecular crystals on diazaazulene in a naphthalene host. The measured values are μ(GS) = 2.98 ± 0.01 D, |Δμ(GS−S1)| = 1.80 ± 0.05 D, and |Δμ(GS—T1)| = 1.10 ± 0.07 D. The measurements enable us to examine the role of nitrogen atoms participating the charge distribution of the σ core and the π-electron cloud in the 1B1 ← 1A1 transition in comparison with the known dipole moment of the corresponding first excited singlet state of azulene. Discussion on the direction of the change in dipole moment and the relative magnitude of the dipole moments of the origin bands of S1 and T1 states as well as the vibronic bands 264, 539, and 575 cm−1 are presented.

Calculation of carbon-13 chemical shifts in simple hydrocarbons

The experimental carbon-13 chemical shifts of acetylene, dimethyl acetylene, and allene are reported and compared with existing values for methane, ethane, ethylene, methylacetylene, and benzene. The relative importance of first-, second-, and third-order perturbation terms in the shift calculation is considered. It is concluded that second-order terms dominate the shielding but significant third- and possibly fourth-order terms indicate that overly refined second-order calculations are not warranted when higher-order terms are neglected. A comparison of the average energy method for calculating chemical shifts with the sum-over-states or coupled Hartree-Fock perturbation method is made, and the two methods are found to be similar in many details. The effects of symmetry and of electron radial character as specified by the Slater orbital exponent were examined and found to affect significantly the shielding expression. The methods presented are applicable to larger molecules where both INDO and MINDO, widely used MO programs may be employed. While the dominant structural features are revealed in chemical shielding calculations, this work indicates that excessive refinement is not warranted so long as one neglects higher-order effects, variations in orbital radial character, and details of admixtures of higher electronic states by the magnetic field into the electronic description of the molecule.

Multiphoton Ionization and Dissociation of Molecular Hydrogen at 1.06 μm

This paper reports the experimental investigation of the multiphoton dissociation and ionization of the hydrogen molecule under the influence of Nd-glass laser radiation at 1.06 \ensuremath{\mu}m. The higher electronic states of the potential-energy curves of ${\mathrm{H}}_{2}$ are found to play the dominant role. An eleven-photon process is the initial step in the formation of ${\mathrm{H}}^{+}$ and a twelve-photon process is the initial step in the formation of $\mathrm{H}_{2}^{}{}_{}{}^{+}$.

Higher electronic states of ReF 6

The ultraviolet absorption of ReF 6 comprises a broad ( δ − = 3760 cm −1) structureless band centered at 47 687 cm −1 and two structured systems originating at 49 322 and 56 404 cm −1. The first is interpreted as a charge transfer transition. The other two are interpreted as members of a Rydberg series converging on an ionization potential of 7.99 eV. The molecule, in the Rydberg states, is a slightly expanded octahedron

Optical radiation produced by 1100 eV He +, He ∗, and He O in various gases

Optical spectra are observed associated with collisional excitation by He +, He ∗, He O, Penning ionization by He ∗, and charge transfer by He +. Metastable He ∗ is collisionally excited to higher electron states, producing HeI emission, much more readily than is ground state He O.

Comments of the vibronic linewidth of an isolated resonance of a higher electronic state

Abstract The linewidth of a higher electronic transition interacting with a lower continuum of vibronic states is calculated using a simple model. The calculated linewidth is two orders of magnitude smaller than observed.

Vibronic Coupling in the Exciton States of the Rigid-Lattice Model of Molecular Crystals

The vibrational structure of the Frenkel exciton states of a molecular crystal is investigated by a variational technique using a rigid-lattice-model Hamiltonian. The spirit of the weak coupling formalism is strictly adhered to by constructing crystal wavefunctions from products of isolated molecule wavefunctions so that the only parameters of the theory are Franck—Condon factors and vibrational frequency shifts. The basis set consists of all vibronic, i.e., single particle, states belonging to the same electronic state transforming according to a given wave vector, plus a set of two-particle states in which vibronic and ground vibrational excitation occupy different sites. Interactions between single-particle states are treated in detail, as are interactions between one- and two-particle states. However, part of the interaction between certain two-particle states is neglected and this makes the treatment progressively less accurate for higher vibrational states and stronger coupling. Electron exchange effects, coupling of excitons to photons, higher electronic states, and ion-pair states are not considered. It is shown how each element of the conventional weak coupling energy matrix is supplemented by terms which account for the interaction between one- and two-particle states as well as interactions among the two-particle states. For a given wave vector each single-particle state is accompanied by a family of two-particle bands which rapidly increases in size for higher vibrational states. In the weak coupling limit the families are widely separated, while intermediate coupling corresponds to two-particle bandwidths comparable with the vibrational spacing. The onset of strong coupling corresponds to overlapping of adjacent families.

Light emission from shock-heated sulphur dioxide. Part 2.—Ultra-violet emission

The thermal emission from shock-heated SO2 stretches from 2500–5000 A. It can be resolved into contributions from the 3B1, 1B2 and a third (1B1 ?) higher electronic state. The latter has its origin near 2800 A and predissociates from levels with about 55 kcal/mole vibrational energy. Possible earlier observations of absorption and emission from this state are discussed. Kinetics of electronic excitation are described for each state. The radiative lifetimes are 2 × 10–4, 4 × 10–6 and 2 × 10–7 sec respectively.

Photoionization and Absorption Cross Sections and Fluorescence of CF4

Absorption and photoionization cross sections for CF4 were measured in the 600-to-1000-Å region using a 1-m Seya-type scanning ultraviolet monochromator with a resolution of 0.7 Å. The Hopfield continuum of He2 was used as the light source. The cross sections obtained show that from 600 to 1000 Å the absorption is continuous with broad maxima at 910 Å, 780 Å, and 660 Å, and broad minima at 830 Å and 760 Å. The appearance potential of ions was located at 15.56±0.01 eV (796.7 Å). Photoionization efficiencies were computed and the results indicate that processes other than ionization, such as dissociation or excitation to higher electronic states, are appreciable. Maximum fluorescent emission from the irradiated gas was observed to occur when the wavelength of the incident radiation was 905 Å.

On the energy losses of electrons in molecular nitrogen

Very numerous critical potential determinations of atoms have been made, but there is a distinct lack of such data regarding molecules. The work to be described is a contribution to remedying this deficiency for nitrogen. It will be useful here to state the kind of molecular processes which our apparatus will recognize. There is first of all molecular excitation and ionization—as in the case of the ato —complicated, however, in excitation by the fact that there is not only the possibility of a change of electronic energy, but also of vibrational and rotational energies. In these experiments the resolving power of the apparatus is only adequate in the recognition of electronic and vibrational energy changes. So that only excitations from the lowest vibrational level of the normal electronic state to vibrational levels of higher electronic states will, in this paper, be considered.

The absorption spectra of some polyatomic molecules containing methyl and ethyl radicals

Although measurements on the ultra-violet absorption spectra of polyatomic molecules have rapidly multiplied in recent years, probably in no case has the structure of the entire spectrum been satisfactorily and completely interpreted. From the chemical point of view, investi­gations have been mainly directed to the study of “predissociation” processes and their correlation with the primary processes of photo­ chemical change, whilst in addition some knowledge has been gained in regard to the products of photodissociation and energies of linkage. A more careful examination of the matter has now shown that the inferences to be drawn from predissociation phenomena must be made with care, and in many cases additional measurements—for example of fluorescence or of quantum efficiencies—have to be made before the interpretations become unambiguous. From the physical standpoint, only a few band systems have been analysed in detail ( e . g ., ClO 2 , Urey and Johnston*; SO 2 , Watson and Parker,) and even in these the interpretations given may not be accurate. One aspect of the matter which has not yet received much attention, is the nature and type of the vibrations excited in polyatomic molecules. This may prove to be of considerable importance in connection with chemical kinetics. The chief difficulty in the analysis of the spectra of polyatomic mole­cules usually arises from their complexity, whilst the frequent occurrence of purely continuous spectra which may or may not overlap band systems often makes it impossible to derive much knowledge of the molecular excited states. In such cases as the latter, it may be that further in the ultra-violet, i . e ., in the Schumann region, discrete band systems may lead to knowledge of higher electronic states, but this region has so far been little explored. Herzberg and Teller* have recently attempted to con­struct selection rules for electronic and vibrational transitions in poly­atomic molecules, but even when such rules as these are applied and when the infra-red and Raman frequencies are well known, the analysis of most band systems still remains very difficult.