eV Excitation Energy Research Articles

Charge carrier generation and recombination in cyclohexane solutions excited by vuv light

Abstract Fluorescence quenching by an electric field has been studied in cyclohexane solutions excited by photons in the energy range 7–10 eV. The threshold for internal photoionization of cyclohexane is found to be 8.45 eV. At 10.16 eV excitation energy, the ionization quantum yield is found to be φ i = 0.33, and the Gaussian distribution of the electron's range has a characteristic radius of 62 A.

A threshold-photoelectron fluorescence–photon coincidence study of radiationless transitions in the B2Π state of BrCN+

The fluorescence decay characteristics and intramolecular dynamics of selected vibronic energy levels of BrCN+ have been investigated by the T-PEFCO technique in which coincidences are counted between threshold photoelectrons and the fluorescence photons emitted by photoions produced by synchrotron radiation excitation in the 13.5–15 eV excitation energy range. The ν= 0 level of the Ã2Σ+ state fluoresces with a monoexponential decay constant τ= 2750 ± 100 ns. Selected vibrational levels of the B2Π state over the internal energy range Eν′= 0–5000 cm–1 exhibit a biexponential fluorescence decay. The results are interpreted in terms of the small-molecule resonance limit case of radiationless transitions. Values of the interelectronic state coupling matrix elements are derived and their variation with Eν′ is discussed. The threshold photoelectron spectrum (TPES) of BrCN was measured. Analysis of the 13.5–15 eV region of the TPES leads to revision of previous He I photoelectron spectroscopic assignments in this energy region.

Autoionization observed in the CO+ (A 2Π–X 2Σ+) and (B 2Σ+–X 2Σ+) fluorescence excitation spectra

The CO+ (A 2Π–X 2Σ+) and (B 2Σ+–X 2Σ+) fluorescence excitation spectra (FES) using VUV synchrotron radiation, have been measured from 16 to 38 eV excitation energy. Total photoionization cross section has also been measured from 17 to 20 eV for comparison. Sharp ionization onsets typical of direct ionization process are observed for both A 2Π and B 2Σ+ thresholds. New autoionization structures have been observed above 20 eV besides the broad shape resonance located at 32 eV in the (B–X) FES. Sharp structures observed above the A 2Π (v=0) threshold are interpreted in terms of electronically induced vibrational autoionization. Branching ratios for electronic autoionization of four Ryberg series converging to B into the CO+ A and X states have been measured. Those states decay preferentially into the CO+ A 2Π state as opposed to X 2Σ+.

Electron spectroscopy study of the 27.5 eV satellite line in acetylene excited by synchrotron radiation

Valence electron spectra from acetylene using synchrotron radiation with 50 and 70 eV excitation energy are presented. The 27.5 eV satellite line is resolved into two components. The intensities of these satellite lines are compared to the previously measured intensities at 1487 eV excitation energy. β values for the satellite lines and for all the valence levels are determined at 70 eV excitation enery. A previously unknown satellite line is found at 31.0 eV binding energy. This line corresponds to an onset of the double ionization continuum.

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−1 npπ)-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.

Mössbauer studies of after effects of Auger ionization following internal conversion in organo-129mtellurium compounds

The after effects of Auger ionization in diphenyl[129mTe]-telluride, dibenzyl[129mTe] telluride, and their dispersions in a solvent at 4.2 K, were compared with the same systems labeled with tellurium in the ground state, viz. 129Te. The Mössbauer emission, in the former case is preceded by Auger event and β decay, while in the latter case, it is preceded only by β decay. Three species were observed in the two compounds, I−, C6H5I or C6H5CH2I, and the third one has tentatively been identified as (C6H5)2I+ or (C6H5CH2)2I+. The formation of I− represents rupture of both bonds, i.e., complete fragmentation of the molecule. The probability of fragmentation was estimated with the help of matrix isolation experiments, as the electronically excited tellurium-129 ion formed by fragmentation of the molecule, following the Auger ionization, may interact with a neighboring organo-tellurium molecule in neat compounds, and enter combination. In about 85% of the events in case of diphenyl telluride and 55% in case of dibenzyl telluride, molecules escape complete fragmentation despite the fact that about 100 eV excitation energy may be deposited during charge neutralization. A novel mode for rapid disposal of energy is discussed.

Carbon K-shell excitation in small molecules by high-resolution electron impact

The excitation of 1s carbon electrons has been observed in CO, CH4, CF4, CO2, COS, C2H2and C2H4by means of the electron energy-loss technique with high resolution (70 meV in the 300 eV excitation energy range) and at an incident electron energy of 1.5 keV. The energies, widths and vibrational structures of excited states corresponding to the promotion of 1s carbon electrons to unoccupied valence and Rydberg orbitals have been obtained. The validity of the equivalent-core model, and the role of resonances caused by potential barriers, are discussed.

Photoabsorption in formaldehyde: Intensities and assignments in the discrete and continuous spectral intervals

Theoretical investigations of total and partial-channel photoabsorption cross sections in molecular formaldehyde are reported employing the Stieltjes–Tchebycheff (S–T) technique and separated-channel static-exchange (IVO) calculations. Vertical one-electron dipole spectra for the 2b2(n), 1b1(π), 5a1(σ), 1b2, and 4a1 canonical molecular orbitals are obtained using Hartree–Fock frozen-core functions and large basis sets of compact and diffuse normalizable Gaussians to describe the photoexcited and ejected electrons. The calculated discrete excitation spectra provide reliable zeroth-order approximations to both valence and Rydberg transitions, and, in particular, the 2b2(n) →nsa1, npa1, npb2, and nda2 IVO spectra are in excellent accord with recent experimental assignments and available intensity measurements. Convergent (S–T) photoionization cross sections in the static-exchange (IVO) approximation are obtained for the 15 individual partial channels associated with ionization of the five occupied molecular orbitals considered. Resonance features in many of the individual-channel photoionization cross sections are attributed to contributions from valencelike a1σ* (CO), a1σ* (CH), and b2σ* (CH)/πy* (CO) molecular orbitals that appear in the photoionization continua, rather than in the corresponding one-electron discrete spectral intervals. The vertical electronic cross sections for 1A1→1B1, 1B2, and 1A1 excitations are in generally good accord with previously reported CI (S–T) predictions of continuum orbital assignments and intensities, although some discrepancies due to basis-set differences are present in the 1B1 and 1B2 components, and larger discrepancies apparently due to channel coupling are present in the 1A1→1A1 cross section. Partial-channel vertical electronic cross sections for the production of the five lowest parent-ion electronic states are found to be in general agreement with the results of very recent synchrotron-radiation photoelectron branching-ratio measurements in the 20 to 30 eV excitation energy interval. Most important in this connection is the tentative verification of the predicted orderings in intensities of the partial- channel cross sections, providing support for the presence of a strong ka1σ* (CO) resonance in the (5a1−1)2A1 channel. Finally, the total vertical electronic cross sections for absorption and ionization are in general accord with photoabsorption measurements, photoionization–mass–spectrometric studies, and the previously reported CI (S–T) calculations. Although further refined calculations including vibrational degrees of freedom and autoionization line shapes are required for a more precise quantitative comparison between theory and experiment, the present study should provide a reliable zeroth-order account of discrete and continuum electronic dipole excitations in molecular formaldehyde.

Photoabsorption in molecular nitrogen: A moment analysis of discrete-basis-set calculations in the static-exchange approximation

Theoretical investigations of photoexcitation and ionization cross sections in molecular nitrogen are reported employing the recently devised Stieltjes–Tchebycheff moment-theory technique in the static-exchange approximation. The coupled-channel equations for photoabsorption are separated approximately by identifying the important physically distinct excitation processes associated with formation of the three lowest electronic states of the parent molecular ion. Approximate Rydberg series and pseudospectra of transition frequencies and oscillator strengths are constructed for the seven individual channel components identified using Hartree–Fock ionic core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Detailed comparisons of the theoretically determined discrete excitation series with available spectral data indicate general accord between the calculated and observed excitation frequencies and oscillator strengths, although there are some discrepancies and certain Rydberg series have apparently not yet been identified in the measured spectra. The total Stieltjes–Tchebycheff vertical photoionization cross section obtained from the discrete pseudospectra is in excellent agreement with recent electron–ion coincidence measurement of the cross section for parent–ion production from threshold to 50 eV excitation energy. Similarly, the calculated vertical partial cross sections for the production of the three lowest electronic states in the parent molecular ion are in excellent accord with the results of recent electron–electron coincidence and synchrotron–radiation branching ratio measurements. The origins of particularly intense resonancelike features in the discrete and continuum portions of the photoabsorption cross sections are discussed in terms of excitations into valencelike molecular orbitals. Small discrepancies between theory and experiment are attributed to specific autoionization processes and channel couplings not included in the calculations. In contrast to previously reported model or local potential studies, the present results employ the full nonlocal and nonspherical molecular Fock potential in ab initio photoabsorption calculations. The excellent agreement obtained between theory and experiment in molecular nitrogen suggests that highly reliable photoabsorption cross sections for diatomic molecules can be obtained from Hilbert space calculations and the Stieltjes–Tchebycheff method in the static-exchange approximation under appropriate conditions.

Generalized oscillator strengths for two transitions in CO 2 at incident electron energies of 300, 400 and 500 eV

Generalized oscillator strengths have been obtained for two transitions in CO 2 from inelastic electron scattering determinations as a function of angle. Excitation energies for the two transitions are 11.04 and 11.38 eV, respectively. This spectral region was studied previously by Lassettre and Shiloff 1 but at a resolution too low to resolve the two transitions. In the present experiments the two are clearly resolved. Results are compared with the previous experiments of Foo et al. 2. Since substantial discrepancies are found both with the previous results of Lassettre and Shiloff and that of Foo et al., the accuracy of angular trends has been checked by comparing elastic scattering with the accurate determinations of Bromberg. 6 Agreement is good. Multiple scattering is shown to be absent by varying the pressure. Generalized oscillator strengths are compared with the theoretical calculations of McKoy and collaborators. The change in the shape of the CO 2 spectrum in the region of 7–10 eV excitation energy is also studied.

Angular resolved photoemission from PbS (100) for 16.85 eV and 21.22 eV excitation energy

Angular resolved photoelectron spectra are presented for the narrow-gap semiconductor PbS. Two different photon energies and two different azimuthal orientations of the (100) surface are employed. The experimental results provide a further test for the validity of the one-dimensional density of states approach in the case of the lead chalcogenides. The experimental binding energies of 10 three-dimensional critical points are compared with the prediction of two independent band structure calculations.

Triplet states of ketene by trapped electron spectroscopy

The threshold electron impact excitation spectrum of ketene is reported. The spectrum is interpreted in comparison with results of an ab initio frozen core calculation. Triplet states ( 3A 2, 3A 1, 3B 1) are observed at 3.8, 5.0 and 5.8 eV excitation energy.

Plasmon excitation energies in ZnO, CdO, and MgO

A study has been made to experimentally determine the plasmon excitation energies in the oxides of Zn, Cd, and Mg using the reflection electron energy loss technique. The measurements were made on clean single-crystal surfaces using electrons of 250–1000-eV energy. Distinction was made between surface- and bulk-plasmon excitation by recording the changes which occurred in the energy loss spectrum as the surface was selectively contaminated. Surface-plasmon excitation energies of 17.0, 15.9, and 14.7 eV and bulk-plasmon excitation energies of 18.8, 22.8, and 22.0 eV were measured for ZnO, CdO, and MgO, respectively. Comparison is made with similar data obtained from optical reflectivity and transmission electron energy loss measurements.

Internal energy effects on the reaction of D3+with CH4

A tandem mass spectrometer has been used to measure the distribution of ionic products resulting from the reaction of D3+ with CH4 as a function of the average number of D3+-D2 collisions occurring in the source. These results provide information on the amount of internal excitation energy in D3+. It is shown that at least 69% of the D3+ ions are formed with 0.6 eV or more excitation energy. These results are compared with an approximate D3+ excitation energy distribution based upon a statistical model which assumes a random distribution of energy among the vibrational and translational states of the products of the reaction of D2+ with D2. Reasonable agreement was obtained.

Generalized oscillator strengths through the water vapor spectrum to 75 eV excitation energy; electron kinetic energy 500 eV

Generalized oscillator strengths (GOS) as functions of momentum change are reported for four peaks in the electron impact spectrum of water vapor. Differential generalized oscillator strengths are also reported at seven points in the continuum. Extrapolated (to ΔP=0) oscillator strengths are given at even finer intervals. Most of the experiments were done at about 500 eV kinetic energy, but two transitions were studied at 400 eV kinetic energy to test the Born approximation.

Thermochemical and Theoretical Investigations of the Sodium-Oxygen System. II. Properties of NaO and Its Ions from Hartree-Fock Molecular Orbital Studies

Accurate Hartree-Fock wavefunctions, computed by the method of Roothaan, are reported for the 2II and 2Σ states of NaO, for the 3Σ state of NaO+, and for the 3II, 3Σ, and 1Σ states of NaO−. Spectroscopic constants and dipole and quadrupole moments have also been calculated for each of the molecules and ions. Several properties of the Na–O species have been deduced. These properties include the vertical and adiabatic ionization potentials (7.7 and 7.4 eV), electron affinity (1.1 eV), dissociation enthalpy (2.7 eV), several thermodynamic functions for NaO, and the Π 3→Σ1 (1.7 eV) and Π 3→Σ3 (0.2 eV) excitation energies for the NaO− system. Thermodynamic data for selected reactions involving NaO and NaO+ are given, together with a brief discussion of the bond enthalpies in NaO2 and NaO2+.

Detection and Identification of Triplet States of H2O by Electron Impact

It has been previously (1-4) that the angular behavior of the differential cross section for exciting molecular electronic states by low-energy electron impact is a useful tool in distinguishing transitions which are optically spin forbidden from those which are spin allowed. We have used this method to investigate water vapor and have detected and identified at least two singlet→triplet transitions. One is a broad feature having a maximum intensity at about 4.5 eV excitation energy and the other has a sharp minimum at 9.81 eV. The first one had been detected previously (5-10) and the triplet state character of the upper state involved has been implied (5). The second one had not previously been detected. We show that the differential cross section for both of these excitation processes have an angular dependence characteristic of singlet→triplet transitions.