Radiationless transitions between excited singlet states of biacetyl

Emission processes from lower excited states S₁ (fluorescence) and T₁ (phosphorescence) have been studied in the gas and liquid phases when biacetyl is excited into the second singlet state S₂. (In agreement with Kasha's rule no fluorescence is observed from the S₂ state.) In the liquid phase, when biacetyl is excited into the singlet states S₁ and S₂, no difference is observed between these emission processes. This phenomenon certainly results from an efficient nonradiative transition between the second excited singlet state S₂ and the first excited state S₁ with practically no excess vibrational energy. The quantum yield of this transition is almost unity and does not depend on the nature of the solvent. In the gas phase no emission processes are observed when biacetyl is excited into the S₂ state at low pressure (less than 10 mm Hg). High pressure of inert gas is necessary in order to observe these processes. As for excitation into the S₁ state with vibrational energy, loss of vibrational energy through collisions occurs from the S₂ state. The quantum yield of the S₂ → S₁ transition by excitation at 290 nm is estimated around 0.5–0.6 at 6 atm of inert gas (ethane, ethylene, or carbon dioxide).     

Anisotropy parameters for molecules excited by electron impact

Rotationally resolved excitation of the a¹Δ_g and b¹∑ _g ⁺ states of O₂ is considered. Results for the relevant state multipoles are given. The anisotropy produced by electron impact is visualised by figures of the angular distribution of the molecular axes in the excited states.     

Experimental investigation of excited state electron transfer reactions between some bicyclic molecules and tetracyanoquinodimethane (TCNQ)

Investigations on photoinduced electron transfer (ET) reactions between excited (ground) bicyclic electron donors 5,6,7,8-tetrahydro-2-naphthol (TH2N), 2-methoxy-5,6,7,8-tetrahydro naphthalene (2MTHN) and ground state (excited) acceptor tetracyanoquinodimethane (TCNQ) in fluid solutions of different polarity at the ambient temperature (300 K) by electronic absorption, steady state fluorescence and time-resolved spectroscopic methods in the time domain of nanosecond order have been carried out. It is suggested that in highly polar solvent acetonitrile (ACN), a loosely-structured transient geminate ion-pair complex (GIP) in the excited singlet state (S₁) is formed due to the ET encounter between the present donor TH2N or 2MTHN and TCNQ and this GIP complex rapidly dissociates into stable excited radical ions, as evidenced from steady state spectra. In polar DMF solvents, TCNQ exhibits an electronic absorption band of its anion without the presence of donor molecules. Both steady state and time-resolved data indicate that ET reactions between the present donors and acceptor TCNQ are largely impeded in the less polar solvent tetrahydrofuran (THF). In the highly polar solvent ACN, ET reactions between the donors and acceptor TCNQ have been suggested to be of adiabatic or intermediate between adiabatic and non-adiabatic types, from the observation of radical ion species in the electronic excited state. For some bicyclic donors and TCNQ acceptor systems, large negative ΔG, which is a measure of the gap between locally excited and radical ion-pair states, shows reaction occurs in highly exothermic regions. Further observations of −ΔG>λ, nuclear reorganization energy parameters and the decrement of ET rate (k_ET) with increasing exothermicity (more negative ΔG values) suggest the ET reaction for the bicyclic donor—TCNQ acceptor systems studied in the present investigation might occur in the Marcus inverted region. The possibility of building up efficient photoconducting materials with the present donor acceptor systems is suggested.     

An unusual electron impact induced fragmentation of methylbenzofurazans

The electron impact mass spectra of 4-and 5-methylbenzofurazans exhibit an ion corresponding to the loss of CHO from the molecular ion. Loss of NO from the molecular ion is quite unimportant, in contrast to the behaviour of benzofurazan and some other substituted benzofurazans, in which loss of NO is the dominant process.     

Cross sections for transitions between fine structure components of second group excited atoms induced by collisions with noble gas atoms

Cross sections of the transition ³ P ₁ ³ P ₀ for the atoms Zn, Cd, Hg upon collisions with inert gas atoms are calculated. It is shown that the transitions between fine structure components induced by nonadiabatic interaction are due to rotation of the molecule. A numerical estimation of the cross sections is performed.

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Zusammenfassung Es werden Wirkungsquerschnitte für den Übergang ³ P ₁ ³ P ₀ für die Atome Zn, Cd, Hg bei Stößen mit Edelgasatomen berechnet. Es wird gezeigt, daß die Übergänge zwischen Feinstrukturkomponenten, induziert durch nicht-adiabatische Wechselwirkung, durch Rotation des Moleküls bewirkt werden. Es wird eine numerische Abschätzung der Wirkungsquerschnitte durchgeführt.

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Résumé Calcul des sections efficaces pour la transition ³P₁ ³P₀ dans les atomes Zn, Cd, Hg par collision avec des atomes de gaz rare. On montre que les transitions entre les composantes de structure fine induites par interaction non adiabatique sont dues à la rotation de la molécule. Estimation numérique des sections efficaces.

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The authors are indebted to Dr. E. E. Nikitin for very valuable discussions.     

Two-beam linear magneto-optical spectroscopy of atomic transitions between short lived states

Two-beam, linear magneto-optical spectroscopy is a powerful tool for studying short-lived states. We present both measurements and a quantitative theoretical analysis of magneto-rotation observed in the forward scattering of a linearly polarised laser beam passing through an amplifying atomic medium placed in a longitudinal magnetic field. The probed transition connects two short-lived, excited atomic levels, the upper state (here the 7S _1/2 level of cesium) being prepared initially via another transition from ground state, excited by a linearly polarised pump beam. The probe polarisation undergoes three different magneto-optical processes: optical rotation, with separate contributions from the two transitions, and linear dichroism due to Hanle precession of the upper state alignment. Complete resolution of the hyperfine structures and ninety degree switching of the probe polarisation enable us to isolate all of these processes. To lowest order in optical thickness the relative intensities and lineshapes are well interpreted.     

Isotope shifts and hyperfine structures investigation of doubly excited levels in SrI

We measured isotope shifts and hyperfine structure of visible transitions of stable strontium isotopes by means of Doppler-free saturated absorption spectroscopy. In particular, we investigated transitions between excited states where the upper level involves two excited electrons. We report hyperfine coupling constants for the levels 5p4d¹D₂, 5p4d³F₂, 5s4d³D₁, 5s6s³S₁, 5s5p³P₁ and, for some of the studied transitions, we separate the specific mass and volume contribution to the isotope shifts.     

Forbidden electric quadrupole transitions between even OIII levels

The relativistic CI method is used to determine transition probabilities for electric quadrupole transitions between the 1s ² 2s ² 2p ² (³ P ₀,³ P ₂,¹ D ₂) and 1s ² 2p ^{4 3} P ₂ even levels of OIII. The variation of the results with the number of configurations used is discussed, both for the Coulomb and the Babushkin (length) gauge.     

An improved Born approximation for the electron impact ionization of atomic hydrogen

A two-electron continuum wave function satisfying exact asymptotic boundary conditions (see [1]) is simplified in the high energy limit for asymmetric kinematics. The long range correlation between the fast escaping electron and the target atom in a low continuum state is properly taken into account, and corrects therefore the major shortcoming of the ordinary Born approximation. This improved Born approximation has been employed to calculate triply differential ionization cross sections at intermediate energies.     

One photon two electron excitations between doubly excited states of helium

Variational calculations using Hylleraas coordinates have been performed for the first time for estimating the energies of 3dnf((1,3)D(o)) state of helium for n=4,5,6. We predict absorption peaks at 12.219, 12.647, and 12.857 eV for the (3)D(o) series converging to N=3 ionization threshold of He(+) which can be expected to be observed in the experiment of single photon double excitation of lowest (3)P(e) state of helium placed in synchrotron radiation.     

Secondary hydrogen-deuterium isotope effects on electron impact induced fragmentations

Secondary hydrogen-deuterium isotope effects have been observed in the mass spectra of cis-4-t-butylcyclohexyl iodide, 5-iodononane and 2-iodopropane. Under conditions which suppress competing and second generation fragmentations, β-deuterium substitution decreases the intensity ratio \documentclass{article}\pagestyle{empty}\begin{document}$ ([{\rm M} - {\rm I]}^{\rm + } /[{\rm M]}^{\mathop + \limits_ \cdot } ) $\end{document}, a result analogous to a normal isotope effect. The decrease is larger in the spectrum of cis-4-t-butylcyclohexyl iodide-trans-2-d than in the spectrum of the cis-2-d derivative. Since these effects parallel those in the better understood solvolysis reaction, both effects may have a common origin. In contrast, deuteration of more remote positions in cis-4-t-butylcyclohexyl iodide and 5-iodononane increases the indicated intensity ratio, an apparent inverse isotope effect. Although similar effects have been observed in solvolysis reactions, the mass spectral effect may be attributable to an increase in the nonfixed energy available for fragmentation. These results suggest that secondary isotope effects can be readily measured in certain cases, and that they may eventually become useful probes into the mechanisms of mass spectral fragmentations.     

On the electron impact induced hydroxyl loss from o-nitrostyrene

Unimolecular hydroxyl (OD^˙) loss from regio- and stereo-specifically labelled o-nitrostyrenes 1a, 1c and 1d results in the formation of an ion which upon collisional activation gives identical mass spectra. Suggestions are made which aim at explaining: (i) the loss of stereochemical integrity of the diastereotopic methylene hydrogens in the course of hydroxyl elimination; and (ii) to account for the collision induced losses of CO and HNC from the [M—hydroxyl]⁺ ion.     

Rearrangements in the electron impact induced fragmentations of sulfonyl chlorides

The major fragmentation pattern obsrved in the mass spectra of simple alkane- and arylsulfonyl chlorides may be rationalized by loss of a chlorine atom from the molecular ion, followed by loss of SO₂ with concomitant carbocation formation. The mass spectra of α-mesyl sulfonyl chlorides and napthalenesulfonyl chlorides exhibit ions resulting from chlorine atom migration to the α-carbon atom with concomitant loss of SO₂. The mass spectra of α-mesyl sulfonyl chlorides also show ions which involve chlorine atom migration to the β-sulfonyl group.     

Novel electronic transitions within excited doublets of dimers

We consider the electronic transition between the split components of the excited electronic doublet of a dimer species when the monomer species have permanent dipole moments. We show that the transition moment is given in terms of the permanent dipole moments of the ground and excited states of the monomers. Extension to ionic dimers and transitions between exciton bands in molecular crystals are suggested.     

The electron impact induced fragmentation of 1,3-diphenyl-2-pyrazoline

The electron impact induced fragmentation of 1,3-diphenyl-2-pyrazoline was studied by nitrogen-15, carbon-13 and deuterium labelling, substitution in the para position of one of the phenyl rings by fluoro and methoxy groups, and ion kinetic energy spectroscopy, as well as accurate mass measurements. In the fragmentation pathway observed all the ions in the mass spectrum of 1,3-diphenyl-2-pyrazoline (except m/e 103) originate from the molecular ion, which in all compounds is the base peak. The formation of prominent fragment ions is discussed.     

Translational energy distribution of the excited hydrogen atom produced by controlled electron impact on HCl

The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the ²Π state of HCl⁺ produce the fast component and Rydberg states converging to the repulsive HCl⁺ states which cross the ²Σ⁺ state produce the slow component.