Absolute Generalized Oscillator Strengths Research Articles

Generalized oscillator strengths and integral cross sections of krypton studied by high-energy electron scattering

Absolute generalized oscillator strengths of the valence-shell excitations of krypton have been determined with an incident electron energy of 1500 eV and an energy resolution of about 80 meV. By employing the relative flow technique with a dilute target, the pressure effect is excluded and the accuracy of the measured generalized oscillator strengths is improved. By comparing the present generalized oscillator strengths with the previous experimental and theoretical results, it is found that the first Born approximation is not reached at the squared momentum transfer larger than 1 a.u.. The optical oscillator strengths have been obtained by extrapolating the measured generalized oscillator strengths to zero squared momentum transfer, which cross-checks the previous electron impact and photoabsorption results. With the aid of the BE-scaling method, the integral cross sections of krypton have been determined from the excitation threshold to 5000 eV. The present oscillator strengths and integral cross sections supplement the fundamental database of the electron scattering of krypton and provide a benchmark for the theoretical methods.

Investigation of the valence-shell excitations of molecular carbon disulfide by high-energy electron scattering

Absolute generalized oscillator strengths of the valence-shell excitations of carbon disulfide have been determined by a joint investigation of a fast electron impact experiment and equation of motion coupled cluster with singles and doubles (EOM-CCSD) calculations. Based on the present experimental and theoretical results, the transition nature of the excitations are elucidated, and some controversies about the transition assignments are resolved. By extrapolating the generalized oscillator strengths to zero momentum transfer, the corresponding optical oscillator strengths were determined, which provide a cross-check to the previous experimental and theoretical results. Moreover, the integral cross sections of the valence-shell excitations from their excitation thresholds to 5000 eV have been obtained by means of the BE-scaling method. The oscillator strengths and integral cross-sections complement the fundamental data of carbon disulfide and supply important applications in astrophysics and atmospheric physics.

Oscillator strengths and cross sections of the valence-shell excitations of CH[formula omitted]I studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CH3I are important for studying molecular photodissociation and monitoring the destruction of ozone in the Earth’s atmosphere. Based on the relative flow technique, absolute generalized oscillator strengths of the valence-shell excitations in CH3I have been determined by using an angle-resolved electron energy loss spectrometer. The measurement was carried out at an incident electron energy of 1.5 keV and an energy resolution of 80 meV. Optical oscillator strengths for the dipole-allowed transitions have been obtained at the zero limit of the squared momentum transfer, providing an independent cross check with the previous experimental results. The integral cross sections from the excitation threshold to 5000 eV have also been obtained by means of the BE-scaling method. The present experimental oscillator strengths and cross sections can supplement the fundamental spectroscopic data of CH3I and will play an important role in the studies of molecular photodissociation dynamics.

Vacuum ultraviolet electron impact excitation of the styrene molecule: cross sections and oscillator strengths

The vacuum ultraviolet electronic excitation of the styrene molecule has been studied in the 0–50 eV energy range, using angle-resolved electron-energy-loss spectroscopy at an incident energy of 1 keV. Intense new features have been observed at 10.3, 11.8, 13.7 and 17.1 eV. They were tentatively assigned to high-energy transitions originating from σ electrons or to double excitations involving π electrons. The absolute generalized oscillator strengths and absolute inelastic differential cross sections have been determined for the band centred at 5.3 eV, associated with the 2,3 1A′←1 1A′ transitions. The absolute elastic differential cross section has also been determined over an angular range of 2.5°–22.0°. The valence photoabsorption spectrum of styrene, derived from the electron-energy-loss spectrum, is compared to a previously measured benzene spectrum in the 3–45 eV energy range.

Electron-impact excitation of the6p7sP13state of Pb atom at small scattering angles

Electron-impact excitation of the $6p7s$ $^{3}P_{1}$ state of Pb atom has been investigated both experimentally and theoretically. Differential cross sections (DCSs) were measured at incident electron energies of ${E}_{0}=10$, 20, 40, 60, 80, and $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and small scattering angles up to $10\ifmmode^\circ\else\textdegree\fi{}$ using a crossed electron-atom beam technique. The forward scattering function method has been used for determination of the absolute generalized oscillator strengths and DCS values. Corresponding relativistic distorted wave calculations have been performed and compared with experimental results.

Absolute oscillator strength densities below 100eV of N 2

The absolute optical oscillator strength density (OOSD) and generalized oscillator strength densities (GOSDs) below 100eV of N2 are determined with an incident electron energy of 2500eV and an energy resolution of 100 meV. The absolute generalized oscillator strengths (GOSs) for two transitions to the superexcited states at 23 and 31.4eV are determined and their momentum transfer dependence behaviours are discussed.

The 1S–1P electron excitations of Zn at small scattering angles

Electron impact excitation from the 41S ground state to the 41P and 51P states of zinc has been experimentally investigated at incident energies from 15 to 100eV and scattering angles up to 12°. The absolute generalized oscillator strengths and differential cross-sections are determined through normalization to the optical oscillator strengths. The present values are compared with limited number of available data.

Absolute Generalized-Oscillator-Strength Measurement of C1 s Preionization-Edge Electronic Excitations of C2F6

Absolute generalized oscillator strengths (GOSs) for the C 1 spreionization-edge transitions at 295.4 eV and 297.7 eV inperfluoroethane have been determined as functions of energy loss andmomentum transfer (K) at impact energy of 2.5 keV for the firsttime. The corresponding GOS profiles have been found to have thecharacteristic dipole-dominated shapes with a strong maximum at K = 0.The spectral features can be interpreted in terms of transitionsterminating at the σ* (C–F) molecular orbital.

Absolute Generalized Oscillator Strength Profiles of RydbergTransitions in C2F6

Absolute generalized oscillator strengths (GOSs) for the twoRydberg excitations at 12.1 eV and 13.5 eV in C2F6 have beendetermined as functions of energy loss andmomentum transfer (K) at impact energy of 2.5 keV. The GOS profiles forthese two Rydberg transitions to 3 p Rydberg orbital have thecharacteristic dipole-dominated shapes with a strong maximum at K = 0.

Generalized oscillator strengths for5s,5s′,and5pexcitations of krypton

The absolute generalized oscillator strengths (GOSs) for 5s, ${5s}^{\ensuremath{'}},$ $5p$ $[5/2{]}_{3,2},$ $5p$ $[3/2{]}_{1,2},$ and $5p$ $[1/2{]}_{0}$ transitions of krypton have been determined in a large ${K}^{2}$ region at a high electron-impact energy of 2500 eV. The positions of the minima and maxima of these GOSs have been determined. The present results show that the angular resolution and pressure effect have great influence on the position and the amplitude of the minimum for the GOS of ${5s+5s}^{\ensuremath{'}}$ transitions. When these effects are considered, the measured minimum position for the GOS of ${5s+5s}^{\ensuremath{'}}$ transitions is in excellent agreement with the calculation of Chen and Msezane [J. Phys. B 33, 5397 (2000)].

Non-dipole electron impact spectroscopy of a common local anesthetic: Generalized oscillator strengths of valence, Cl 2p and C 1s pre-edge transitions of chloroethane

Angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy has been used to determine absolute generalized oscillator strengths (GOSs) of discrete transitions in the pre-edge regions of the valence, Cl 2p and C 1s shells of chloroethane as functions of energy loss and momentum transfer. A new lowest-lying preionization-edge energy-loss feature at 7.2 eV has been clearly identified at a non-zero momentum transfer. The corresponding GOS profile is found to have a shape characteristic of non-dipole transitions that can be attributed to electronic excitations from nonbonding Cl 3p initial-state orbitals (4a″ and 13a′) to antibonding σ* C–Cl final-state orbital (14a′). The GOS profiles of the other low-lying valence-shell and Cl 2p and C 1s pre-edge features are found to have maximum intensity at zero momentum transfer, generally consistent with the spectral assignments to dipole-allowed transitions involving final-state Rydberg orbitals. Differences in and between the GOS profiles for the valence-shell and these inner-shell pre-edge features in C 2H 5Cl are identified. Although no discernible secondary extrema can be found in any of the inner-shell GOS profiles, subtle differences can be observed and are consistent with the proposed assignments. The present GOS results for C 2H 5Cl are compared with the available GOS data reported for other mono-chlorides, including CF 3Cl and CHF 2Cl.

Angle-resolved electron energy loss spectroscopy of valence-shell and Si 2p pre-edge excitation of SiF4: Bethe surface and absolute generalized oscillator strength measurement

Absolute generalized oscillator strengths (GOSs) of discrete transitions in the preionization-edge region of the valence and Si 2p inner shells of SiF4 have been determined as functions of energy loss and momentum transfer by using angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy. The GOS profiles of the pre-edge features are generally consistent with the spectral assignments based on the term values of the virtual and Rydberg states from earlier valence and inner-shell studies. In particular, the GOS profiles for these low-lying preionization-edge features in the valence shell are found to be dominated by a strong maximum at zero momentum transfer, consistent with the proposed assignment of predominantly dipole-allowed Rydberg and mixed valence-Rydberg transitions. In the case of the lowest-lying preionization-edge 1t1→6a1 feature, which is formally dipole-forbidden, the present work shows that such a shape for the GOS profile is, however, not exclusive to just dipole-allowed transitions. In the Si 2p shell, the GOS profiles for the well resolved, intense σ* resonance and three higher-lying Si 2p pre-edge features have been determined and are found to be largely dominated by dipole-allowed (Rydberg) excitations. Differences in and between the GOS profiles for the valence-shell and Si 2p pre-edge features in SiF4 are identified. No discernible secondary extrema can be found in any of these GOS profiles. The present GOS results for SiF4 are compared with those reported for other cage-like molecules, including CF4 and SF6.

Absolute generalized oscillator strength profiles of Si 2p shape resonances in SiF 4

Absolute generalized oscillator strengths (GOSs) of the Si 2p shape resonances in silicon tetrafluoride (SiF 4) have been determined as functions of momentum transfer by angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy. These GOS profiles reveal subtle details about the spectral nature of the underlying dipole-allowed Si 2p →2 e and Si 2p →7 t 2 continuum transitions, particularly with respect to the pre-edge Si 2p →6 a 1 σ ∗ resonance. The GOS profiles of the Si 2p shape resonances in SiF 4 are compared with those of the corresponding S 2p →2 t 2 g and S 2p →4 e g resonances in SF 6.

Absolute generalized-oscillator-strength measurement of preionization-edge electronic excitations in the valence and2pshells of argon

Absolute generalized oscillator strengths ~GOS’s! of the 3p6→3p5(4s ,4s8), 3p6→3p5(4p ,4p8), and 3p6→3p5(5s ,5s8,3d ,3d8) valence-shell electronic transitions as well as the 2p3/2→4s inner-shell transition in Ar have been determined as functions of energy loss and momentum transfer ~K! at an impact energy of 2.5 keV. Generally good agreement can be found between the present GOS measurement for the 3p →3p5(4s ,4s8) transitions ~at 11.8 eV! and the previous experimental profiles for K,1 a.u. Calculations based on the first Born approximation appear to be in reasonable agreement with the GOS data only in shape and also for K,1 a.u., while the Yukawa frozen-core calculations based on the Glauber approximation appear to converge to the present data even for K.1 a.u. The experimental and theoretical results all give consistent values for the K positions of the observed minimum and maximum in the GOS profile of the 3p →3p5(4s ,4s8) transitions. The GOS profile for the 3p6→3p5(4p ,4p8) transitions ~at 13.4 eV! in Ar has been determined and found to have a shape characteristic of nondipole excitation with a maximum at K ;0.7 a.u. Like the 3p6→3p5(4s ,4s8) transitions, the GOS profile for the 3p6→3p5(5s ,5s8,3d ,3d8) transitions ~at 14.2 eV! has the characteristic dipole-dominated shape with a strong maximum at K50 but a notably larger width and without any discernible extrema. Finally, the GOS profile for the 2p3/2→4s inner-shell transition ~at 244.4 eV! is dominated by a strong maximum at K50, characteristic of generally dipole-allowed excitations, but with a broad and rather complex shape. The present work illustrates the analytical value of GOS profiles for probing the intricate nature of the valenceand inner-shell electronic excitations.

Absolute generalized oscillator strengths for the vibronic bands ofA1Π,B1Σ+,C1Σ+, andE1Π transitions of carbon monoxide

Generalized oscillator strengths for the vibronic bands of ${\mathrm{A}}^{1}$\ensuremath{\Pi}, ${\mathrm{B}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$, ${\mathrm{C}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$, and ${\mathrm{E}}^{1}$\ensuremath{\Pi} have been determined by an angle-resolved electron energy loss spectroscopy at an incident electron energy of 1500 eV and in an angular range of 0.5\ifmmode^\circ\else\textdegree\fi{}--6.0\ifmmode^\circ\else\textdegree\fi{}. The corresponding absolute optical oscillator strengths obtained by extrapolating the generalized oscillator strengths to ${\mathrm{K}}^{2}$=0 are also reported. The present results have been compared with previous work, and some differences between them have been explained. The experimental generalized oscillator strengths for ${\mathrm{v}}^{|\mathrm{I}\mathrm{H}}$=0--8 of ${\mathrm{A}}^{1}$\ensuremath{\Pi}, ${\mathrm{v}}^{|\mathrm{I}\mathrm{H}}$=0--1 of ${\mathrm{B}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$, ${\mathrm{C}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$, and ${\mathrm{E}}^{1}$\ensuremath{\Pi} are reported.

Absolute generalized oscillator strengths of 4s, 4s', 4p+4p' excitations of argon determined by the angle-resolved electron-energy-loss spectrometer.

The absolute differential cross sections and generalized oscillator strengths of resolved 3p 61 S 0 ! 3 p 5 ( 2 P 1/2)4s ,3 p 5 ( 2 P 3/2)4s8 transitions in argon were determined by angle-resolved electronenergy-loss spectrometer at an incident electron energy of 1500 eV and scattering angles from 2° to 11.5°. Corrections of the experimental results for the angular factor and pressure effect have been made. These results are compared with other experimental and theoretical results. The absolute differential cross sections and generalized oscillator strengths of unresolved nondipole excitations 3 p 6 !3p 5 4p14p8 are reported. The profile of the generalized oscillator strength curve is similar to that of neon atoms. @S1050-2947~96!09809-5#

Electron-impact excitation of low-lying preionization-edgen→σ* and Rydberg transitions of CHF2Cl and CHFCl2: Absolute generalized oscillator strength measurement

Angle-resolved electron energy loss spectroscopy has been used to determine the absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of difluorochloromethane (CHF2Cl) and dichlorofluoromethane (CHFCl2) as functions of energy loss and momentum transfer at an impact energy of 2.5 keV. Absolute GOS profiles of the prominent low-lying preionization-edge energy loss features of CHF2Cl and CHFCl2 were determined and found to be consistent with the previous assignments of the underlying transitions made by VUV photoabsorption spectroscopy. In particular, the lowest-lying features at 8.0 eV in CHF2Cl and at 7.5 eV in CHFCl2 have been attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals to the C–Cl σ* antibonding orbital, in good accord with single-excitation configuration interaction (CI) excited-state calculations. The corresponding GOS profiles of these n(Cl 3p)→σ*(C–Cl) (HOMO→LUMO) transitions revealed an interesting trend of increased dipole character with increasing Cl content, i.e., from an essentially quadrupole-dominated profile, characterized by a maximum at K2∼0.9 a.u., in CHF2Cl to a mixed dipole-quadrupole profile in CHFCl2 and CHCl3. The CI calculations further showed that some of the underlying n(Cl 3p)→σ*(C–Cl) transitions in CHF2Cl, CHFCl2, and CHCl3, like the other chlorofluorocarbons: CF3Cl, CF2Cl2, CFCl3, and CCl4, could lead to dissociation of the C–Cl bond. In addition, the GOS profiles of the remaining low-lying preionization-edge features at 9.8 and 11.2 eV in CHF2Cl and at 9.4, 10.7, and 11.6 eV in CHFCl2 were also determined. These features have been previously assigned as Rydberg transitions originated from the nonbonding HOMOs. In particular, these experimental GOS profiles were found to be dominated by a strong maximum at K=0, which is indicative of strong dipole interactions. The weak secondary maxima observed at K2∼2.8–3.5 a.u. could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions. Together with our earlier work on CHF3 and CHCl3, the present work on the remaining members of the CHFmCl3−m (m=0–3) series, CHF2Cl and CHFCl2, provides further evidence for the empirical trends on the preionization-edge structures observed in the CFnCl4−n (n=0–4) series.

Electron-impact excitation of low-lying preionization-edge electronic and Rydberg transitions of fluoroform and chloroform: Bethe surfaces and absolute generalized oscillator strengths.

Absolute generalized oscillator strengths (GOSs) of valence-shell electronic transitions of ${\mathrm{CHF}}_{3}$ and ${\mathrm{CHCl}}_{3}$ as functions of energy loss (0--150 eV) and momentum transfer (i.e., the Bethe surfaces) have been determined using angle-resolved electron energy-loss spectroscopy at an impact energy of 2.5 keV. The assignments for the prominent low-lying preionization-edge energy-loss features of ${\mathrm{CHF}}_{3}$ and ${\mathrm{CHCl}}_{3}$ were inferred from the term values reported previously and from their characteristic experimental GOS profiles. In particular, the GOS profiles of the low-lying Rydberg transitions (which originated from the nonbonding highest occupied molecular orbitals) below the ionization edge at 11.1, 11.9, 12.7, and 13.7 eV in ${\mathrm{CHF}}_{3}$ and at 8.5, 9.6, and 10.6 eV in ${\mathrm{CHCl}}_{3}$ were determined. These GOS profiles were found to be dominated by a strong maximum at zero-momentum transfer, which is characteristic of dipole interaction. Weak secondary maxima (and minima) were also observed and could be interpreted qualitatively in terms of the spatial overlaps between the initial-state and final-state orbital wave functions.In addition, the low-lying feature at 7.2 eV in ${\mathrm{CHCl}}_{3}$ could be attributed predominantly to electronic excitations from the Cl 3p nonbonding (n) orbitals (2${\mathit{a}}_{2}$, 9e, 9${\mathit{a}}_{1}$, and 8e) to a C-Cl \ensuremath{\sigma}* antibonding orbital (10${\mathit{a}}_{1}$), according to a single-excitation configuration-interaction (CI) excited-state calculation. The experimental GOS profile of this low-lying feature was found to have a shape that is characteristic of a mixture of dipole-allowed and nondipole interactions, with maxima at momentum transfers of 0 and \ensuremath{\sim}0.9 a.u., respectively. Furthermore, the CI calculation indicated that some of these n(Cl 3p)\ensuremath{\rightarrow}\ensuremath{\sigma}*(C-Cl) excitations in ${\mathrm{CHCl}}_{3}$, like other Cl-containing freons [${\mathrm{CF}}_{\mathit{n}}$${\mathrm{Cl}}_{4\mathrm{\ensuremath{-}}\mathit{n}}$ (n=0--3) and ${\mathrm{CHF}}_{\mathit{m}}$${\mathrm{Cl}}_{3\mathrm{\ensuremath{-}}\mathit{m}}$ (m=1,2)], could also lead to dissociation of the C-Cl bond. \textcopyright{} 1996 The American Physical Society.

Atomic-site-specific nonoptical excitation spectroscopy of chlorofluoromethanes CF4−nCln (n=0–4): Absolute generalized oscillator strength measurement of below-edge electronic excitations of C 1s and Cl 2p shells

The absolute transition probabilities, more commonly known as absolute generalized oscillator strengths (GOSs), have been determined for selected electronic excitations from the C 1s or Cl 2p shell to the lowest-lying σ* orbitals for chlorofluoromethanes CF4−nCln (n=1–4) using angle-resolved electron energy loss spectroscopy at 2.5 keV impact energy. The GOS profiles of these inner-shell transitions were found to have shapes characteristic of predominant dipole-allowed transitions. Furthermore, the proportions of dipole-allowed components of these GOS profiles appeared to increase with successive chlorination for these molecules. In particular, the GOS profile of the C 1s→σC–Cl* [lowest unoccupied molecular orbital (LUMO)] transition of CF3Cl was found to contain relatively more intense higher momentum-transfer (K) components than those of the corresponding features in CF2Cl2 and CFCl3 (and CCl4). The GOS profiles for the Cl 2p3/2,1/2→σC–Cl* transitions of CF3Cl were found to contain a well-defined secondary maximum at K2∼1.2 a.u., characteristic of predominant nondipole interaction, in addition to the primary dipole-dominated peak at zero momentum transfer. Within the present statistics, our GOS measurements have not, however, revealed any discernible difference between the Cl 2p3/2→11a1 and Cl 2p1/2→11a1 features that could be attributed to spin effects. Furthermore, the dipole-dominated GOS profile was found to become more concentrated in the lower momentum transfer region for the Cl 2p3/2→σC–Cl* transition in CCl4 than that in CFCl3. The present work illustrates the possibilities of atomic-site-specific investigation of the excited states by means of GOS measurements of electronic excitations from localized inner-shell initial states.

Momentum-transfer dependence of S 2p excitations of sulfur hexafluoride by angle-resolved electron-energy-loss spectroscopy.

Absolute generalized oscillator strengths (GOS's) of S 2p excitations of ${\mathrm{SF}}_{6}$ have been determined as a function of momentum transfer K, using angle-resolved electron-energy-loss spectroscopy at 2.5-keV impact energy. The GOS for the dipole-forbidden S 2p\ensuremath{\rightarrow}6${\mathit{t}}_{1\mathit{u}}$ transition was found to be significantly enhanced at large K, in contrast to the generally decreasing trend observed for the dipole-allowed discrete and shape-resonance states of the S 2p shell. A simple empirical analysis based upon the Lassettre series expansion showed that this state has a dominant quadrupole contribution.