Selection rules for electric multipole transition of diatomic molecule in scattering experiments

et al. 2022

J. Phys. Chem. A

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A joint experimental and theoretical investigation of the valence shell excitations of carbon tetrachloride has been performed by fast electron scattering and time dependent density functional theory calculations. At a collision energy of 1.5 keV and an energy resolution of about 70 meV, the dipole-forbidden transition of a1σ* ← 2t1 has been clearly observed at large momentum transfers, and its excitation energy of 6.15 eV and line width of 0.72 eV have been determined. Two new features are also recognized at 9.97 and 10.26 eV. The generalized oscillator strengths of the excited states at 5–11.3 eV have been determined from the measured spectra. The calculated generalized oscillator strength of the a1σ* ← 2t1 transition with the vibronic effect shows better agreement with the experiment, and the vibronic effect also accounts for its nonzero intensity at zero squared momentum transfer. The optical oscillator strengths of the valence shell excitations have also been obtained by extrapolating the generalized oscillator strengths to the limit of zero squared momentum transfer. The integral cross sections have been systematically determined from the threshold to 5000 eV by means of the BE-scaling method. The present oscillator strengths and cross sections provide the fundamental data of carbon tetrachloride and have important applications in photochemical modeling for atmospheric physics.

Section: Experimental and Calculation Methodsmentioning

confidence: 72%

Generalized Oscillator Strengths for the Valence Shell Excitations in Carbon Tetrachloride Studied by Fast Electron Impact

et al. 2022

J. Phys. Chem. A

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“…For clarification, we use (ν′ = 0) following a state to represent its vibronic ground state. Being different from the photoabsorption spectra where the dipole-allowed transitions dominate, the present electron energy loss spectra include the singlet dipole-forbidden transitions to 3b 2 1 A 2 , 4pb 2 1 A 2 , and 3db 2 1 A 2 states due to the relaxation of the selection rules at nonzero scattering angles in the fast electron collision experiment. , For example, the broad feature in 5.0–7.5 eV shown in Figure consists of both the excitations to the dipole-forbidden 3b 2 1 A 2 state and the dipole-allowed 4sa 1 1 B 1 state. In the range of 7.8–8.1 eV, the previous assignments to the transitions are somewhat controversial.…”

Section: Methodsmentioning

confidence: 73%

“…Being different from the photoabsorption spectra where the dipole-allowed transitions dominate, the present electron energy loss spectra include the singlet dipole-forbidden transitions to 3b 2 1 A 2 , 4pb 2 1 A 2 , and 3db 2 1 A 2 states due to the relaxation of the selection rules at nonzero scattering angles in the fast electron collision experiment. 43,44 For example, the broad feature in 5.0−7.5 eV shown in Figure 1 18,19 However, due to the lack of the transition at 7.85 eV in their works, they assigned the transition at 7.90 eV as the one previously observed at 7.85 eV. By analyzing the present spectra at different scattering angles, it is found that all of the four excitations at 7.85, 7.90, 7.98, and 8.04 eV exist in our spectra.…”

Section: ■ Methodsmentioning

confidence: 99%

“…ment, 45 the existence of the triplet excitations excludes the possibility to extract quantitative information of the 3b 2 1 A 2 state such as its excitation energy and line width. A high incident electron energy of 1.5 keV is used in this work, so only the singlet states can be approached due to the restriction of the selection rules, 43,44 which means that only 3b 2 1 A 2 and 4sa 1 1 B 1 states exist between 5.0 and 7.5 eV. At small angles with small squared momentum transfer, the dipole-allowed 4sa 1 1 B 1 dominates, so it is understood that the fitted excitation energy and line width are in good agreement with the photoabsorption measurement.…”

Section: ■ Methodsmentioning

confidence: 99%

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The Study of the Low-Lying Valence-Shell Excitations of Hydrogen Sulfide by Fast Electron Impact

et al. 2020

J. Phys. Chem. A

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The valence-shell excitations of hydrogen sulfide have been studied by fast electron impact at a collision energy of 1.5 keV and an energy resolution of about 70 meV. By analyzing the variations of intensity and shape of the feature in the range of 5.0−7.5 eV at different scattering angles, the excitation energy of 5.85 ± 0.01 eV and the line width of 0.80 ± 0.01 eV of the 3b 2 1 A 2 state have been determined. The generalized oscillator strengths of the valence-shell excitations in the energy range of 5.0−9.2 eV of hydrogen sulfide have been determined from the measured spectra. The corresponding optical oscillator strengths have been obtained by extrapolating the generalized oscillator strengths to the limit of zero squared momentum transfer. The integral cross sections have also been systematically determined from the threshold to 5000 eV by means of the BE-scaling method. The presently obtained oscillator strengths and integral cross sections have significant applications in the studies of planetary atmospheres and interstellar gases.

“…The following measurement of the C 1s near-edge photoabsorption spectroscopy of C 2 H 4 indicated the importance of vibronic coupling which provides a mechanism for localized core-hole [22]. However, electron impact method can detect not only the dipole-allowed but also the dipole-forbidden transitions, which can be used to probe the (1sσ g ) −1 (1π g ) 1 state if the core-hole is delocalized [25]. Note that measuring only the excitation processes leads to an incoherent sum of the gerade and ungerade core-hole states, i.e., the inner-shell excitations should include the dipole-allowed (1sσ u ) −1 (1π g ) 1 and dipole-forbidden (1sσ g ) −1 (1π g ) 1 components.…”

Section: Introductionmentioning

confidence: 99%

Probing the delocalized core-hole via inner-shell excitation in N₂

et al. 2022

New J. Phys.

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The dispute about whether the 1s core-hole is localized on one atom or delocalized over both in a homonuclear diatomic molecule has continued for decades, which has been extensively studied by the photoelectron and electron-ion coincidence spectroscopies. For N2, if the 1s core-hole is delocalized, the K-shell excitation into the 1πg orbital should split into two components, i.e., the dipole-allowed transition from the ungerade 1σu state and the dipole-forbidden transition from the gerade 1σg state. However, only the dipole-allowed transition has been verified up to now. Herein, we report the inner-shell electron energy loss spectra of N2 at different scattering angles with an incident electron energy of 1500 eV and an energy resolution of 65 meV. The vibrational structures of both the dipole-allowed (1sσu)-1(1πg)1 and dipole-forbidden (1sσg)-1(1πg)1 states of N2 have been observed at different momentum transfers. The splitting between the (1sσu)-1(1πg)1 and (1sσg)-1(1πg)1 states with the reverse symmetry is determined to be 67±7 meV. Moreover, the momentum transfer dependence behavior of the transition intensity ratio agrees with the theoretical predictions, as increasing to a maximum and the decreasing. The experimental results clearly shows that the inner most electrons can be described by 1σg and 1σu, which indicates that the inner-shell 1s core-hole of N2 is delocalized over both two N atoms based on the excitation processes.