Fluorescence spectra of laser-excited YO molecules in a H2O2Ar flame

CW dye laser induced fluorescence emission and thermal emission spectra of YO-molecules in a 1 atm H₂O₂Ar flame of 2430 K were recorded simultaneously. Narrow band laser excitation was applied to four rotational lines in the (1, 1) Q-branch of the A²Π_{$\text{3}\text{2}$}→X²Σ⁺ transition and broadband excitation was applied to several separate Q-branches of the A²Π_{$\text{1}\text{2}$,$\text{3}\text{2}$}→X²Σ⁺ transitions. From the differences between the fluorescence emission spectra and thermal emission spectra, we conclude that collisional de-excitation of an excited vibronic level takes place by vibrational relaxation, decay to the electronic ground state and by intermultiplet transfer in order of increasing probability.     

Fluorescence of ammonia-d3 from its first excited singlet state

Structured emission spectra have been observed from ND₃ excited at 2139 Å and 2144 Å. The emission is short-lived (τ_{$\text{1}\text{2}$} < 10^?10 s) and has been assigned to the ND₃($\text{A}$) → ND₃($\text{X}$) fluorescence transition.     

Directional dispersion of excitons in anthracene,probed by electron spectroscopy

Anthracene single crystal foils have been investigated by electron energy loss in the range from 3 eV to 25 eV for various directions of the crystal excitation wave vector k. Experimental evidence for axial dispersion is obtained. Calculations in which a dielectric tensor is used in the oriented gas model explain the anisotropy in the energy loss data and allow the assignment of the losses in this whole energy range to ¹A_u and ¹B_u crystal excitations originating from molecular ¹A_1g → ¹B_1u and → ¹B_1u excitations.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum of molecular fluorine

Electron energy loss measurements and concommitant RPAE calculations are reported of the valence-shell dipole excitation spectrum of molecular fluorine. The measured spectrum is dominated by a series of strong features in the 12–16 eV interval which are in accord with X¹Σ_g⁺→¹Σ_u⁺ bands assigned in a previously reported high-resolution optical study. These features are attributed on basis of the present RPAE calculations to configuration mixing between 1π_g→npπ_u Rydberg and 3σ_g→3σ_u intravalence excitations. A depleted X→V_σ charge-transfer excitation is correspondingly observed at ≈17 eV, in good accord with the calculated values. The appearance of the σ→σ* transition in F₂ below the 3σ_g^?1 threshold is in marked contrast to the situation in other light diatomic molecules, in which cases σ→σ* transitions appear as intravalence shape resonances in photoionization continua. Assignments are also provided of weak, irregularly spaced X¹Σ_g⁺→¹Π_u excitations the origins of which are attributed to configuration mixing between 1π_g→npσ_u and 1π_u→nsσ_g Rydberg series.     

Observation of X1A1 vinylidene by photoelectron spectroscopy of the C2H2− ion

Photoelectron spectra of the vinylidene anion (C₂H₂^?) show vibrational structure in X¹A₁ vinylidene up 12 kcal/ mol above the vibrational ground state. Analysis yields an EA(C₂H₂$\text{X}$¹ A₁) of 0.47 ± 0.02 eV, and frequencies for the CC stretch and HCH bend. Absence of the ³B₂ state in the photoelectron spectra indicates the ¹A₁-³B₂ splitting in vinylidene is ? 1.7 eV.     

Energy transfer processes in reactions of He(2 3S) and Ne(3P0,2) with CS2

A comparative spectroscopic study in the visible and ultraviolet ranges was conducted on the flowing afterglows resulting from the reactions of He(2 ³S) and Ne(³P_0,2) metastables with CS₂. Penning ionization was found to be the predominant energy transfer process. However, electron—ion recombination within the afterglows constitutes a major secondary process and gives rise to the most intense emitting system, CS(A ¹ Π → X ¹Σ⁺). Both afterglows were found to produce the CS⁺₂($\text{B}$²Σ⁺_u-$\text{X}$²Π_g), CS⁺₂($\text{A}$²Π_u-$\text{X}$²Π_g) and CS(a ³Π-X ¹Σ⁺) emission systems as well as some atomic sulfur emission lines. Some intensity differences were observed and are interpreted in terms of energetics and the formation mechanisms of the emitting species. A moderately strong CS⁺(A ²Π_i-X ²Σ⁺) emission system was also observed in the ehlium afterglow. In addition, a weak, sharp group of bands in the 390–420 nm range in the helium afterglow has been determined to be due to the presence of a small amount of He⁺ ions. This group of bands consists of two overlapping emission systems and are identified as CS(B ¹Σ⁺ → A ¹Π) and CS⁺(B ²Σ⁺ → A ²Π_i).     

Collisional energy transfer to methane by octupole coupliing

A linear molecule or one electron atom interacting with a tetrahedral molecule is considered. Formulae are presented for various rotationally averaged E → R and V → R first order transition probabilities arising from dipole-octupole (R^?5) and quadrupole-octupole (R^?6) coupling. Relatively large amounts of rotational energy can be transferred in first order (ΔJ ? 3). With CH₄ as the octupolar partner, energy transfer upto 350 cm^?1 at room temperature is shown to proceed very efficiently. For the 5²P_{$\text{3}\text{2}$} → 5²P_{$\text{1}\text{2}$} transition in Rb, σ_qu is calculated to be ? 28 A² in quite good agreement with experiment and using an independent value of the octupole moment of CH₄. Energy transfer above 350 cm^?1 becomes rapidly less efficient. Among V → R transfer processes in the 500–700 cm^?1 range, the long range mechanism is almost certainly not the dominant one in the relaxation of CO₂⁺ (010) by CH₄ (σ_calc ≈ 10^?3 σ_obs) but will be important in the relaxation of SO₂.     

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.     

Integral cross sections for electronic excitation in KHg collisions

The energy dependence of the integral cross section for the electronic excitation in collisions of K and Hg is investigated for energies between 50 eV and 1500 eV. By the measurement of the spectra of the emitted light the 4²P_{$\text{3}\text{2}$} and the 4²P_{$\text{1}\text{2}$} states of potassium are found to be dominant. For these the energy dependence of the cross sections is studied in detail. By the measurement of the polarization the contributions to the 4²P_{$\text{3}\text{2}$} state are differentiated with respect to |m_j|.     

Surface-hopping model for near-resonant electronic energy transfer

A surface-hopping model is applied to near-resonant electronic energy transfer in the NFBi and O₂I systems. Multiple surface crossings occur in NFBi at ca. 8 A, corresponding well with measured transfer cross section of 200 A². A Landau-Zener model yields the temperature dependence of the thermally averaged cross section for the laser pumping reaction, O^*₂(a¹Δ) + I(²P_{$\text{3}\text{2}$}) → O₂(X³Σ^?_g) + 1^*(²P_{$\text{1}\text{2}$}).     

The excited states of cycloporane. MCD spectrum,and CD spectrum of an optically active derivative

The MCD spectrum of gas phase cyclopropane and the CD spectrum of trans 1,2-dimethyl cyclopropane were measured in the spectral region 210–140 nm. The absorption spectra are also reported. Comparison of CD and absorption spectra of dimethyl cyclopropane and consideration of the anisotropy factors g = Δε/ε as a guide to the assignment of magnetic dipole allowed transitions led to the ordering of the states of the first excited configuration as A′₁, A′₂, E′ in order of increasing energy. The magnetic moments of the two observed allowed excited states of cyclopropane were determined from the MCD spectrum. LCAO MO level calculations of the MCD parameters $\text{A}$₁/$\text{D}$₀ for the lowest three excited E′ states were carried out and the results were used to discuss the assignment of the allowed transitions. It is concluded that configuration interaction is of considerable importance in the low energy excited states of cyclopropane.     

Dynamic metal-ligand coupling in the infrared MCD spectra of the cobalt(II) tetrahalides

The axial single-crystal magnetic circular dichroism spectra of Cs₃ZnCl₅/Co²⁺ and Cs₃ZnBr₅/Co²⁺ have been measured over the 4000–7000 cm^?1 region of the ⁴A₂ → ⁴T₁ (F) transition at ambient and liquid-helium temperature. The B/D and C/D terms obtained give transition moment ratios, 〈t₂∥m∥t₂〉/〈e∥m∥t₂〉, in accord with the value required (?3^{$\text{1}\text{2}$}/2^{$\text{1}\text{2}$}) by a dynamic ligand-polarisation model for d-electron transition probabilities in tetrahedral metal complexes.     

Chemically reacting bismuth and nitrous oxide in a heat pipe oven

The chemical reactions between bismuth and oxidants in a heat pipe oven are described. Strong chemiluminescence (A²Π_{$\text{1}\text{2}$} → X²Π_{$\text{1}\text{2}$}) was observed in the reaction with N₂O and the vibrational temperature of the A²Π_{$\text{1}\text{2}$} state was measured to be 3150 ± 300 K.     

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.     

Determination of the dipole moment of thioformaldehyde in the a3A2 excited state by laser phosphorescence excitation spectroscopy

Doppler-limited phosphorescence excitation spectra have been recorded at various electric fields for two rotational transitions in the $\text{a}$³A₂-$\text{X}$¹A₁ 0-0 band of H₂CS. Stark splittings were resolved, and were used to determine the dipole moment in the excited electronic state. The value found, 0.57(3) D, is of the order expected by comparison with dipole moments determined for other states of H₂CS, but rather lower than that predicted by ab initio calculations.     

Magnetic and optical investigation of Mg6MnO8

Magnetic susceptibility measurements and optical reflectance spectra of the pure compound Mg₆MnO₈ are reported and discussed. The magnetic moment of 3.94 ± 0.08 μ_B and the Weiss temperature of ?20 ± 5 K are consistent with the structure of the compound which has paramagnetic Mn⁴⁺ ions (d³ configuration) in an octahedral environment with only weak antiferromagnetic interaction. The first spin-allowed ⁴A_2g → ⁴T_2g transition has been assigned to the band observed at 20.4 kK; the remaining bands in the reflectance spectrum also have been assigned.     

Fine structure transition cross sections for several alkali + rare gas systems

The energy dependence, E_c.m. </ 0.2 eV, of the inelastic total cross sections for the ²P_{$\text{1}\text{2}$} → ²P_{$\text{3}\text{2}$} fine structure transition of the lowest excited states of the alkali atoms are calculated for the following systems: Na, K, Rb + He, Ne, Ar and Cs + He. Encouraging agreement between theory and experiment is obtained.     

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.