The ionic states of difluoromethane: A reappraisal of the low energy photoelectron spectrum including <i>ab initio</i> configuration interaction computations

Palmer, Michael H.; Biczysko, Małgorzata; Baiardi, Alberto; Coreno, Marcello; Simone, Monica de; Grazioli, Cesare; Hoffmann, Søren Vrønning; Jones, Nykola C.; Peterson, Kirk A.

doi:10.1063/1.4998150

Cited by 3 publications

(14 citation statements)

References 46 publications

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“…PES footprint. 4 The multiplet of 14 maxima in the X 2 B1 state, 4 mean separation 1069 cm -1 , would be readily observed if present in band I of the VUV spectrum. The PES will fit under band I, after a lowering of its energy by 3.948 eV; this PES shift to superimpose the VUV spectrum, yields a potential Rydberg state having n = 3 with δ = 1.14, in agreement with earlier spectral work.…”

Section: B Theoretical Interpretation Of the Overall Vuv Spectrum Fomentioning

confidence: 99%

“…Term value = IE -Em (1) where IE is each adiabatic ionization energy, E m is the m-th energy level (both in eV). R is the contains three main bands in the 12 to 21 eV region, 4 and these have been assigned to seven IE. 4 The present VUV range makes Rydberg states from several of these IE likely to occur.…”

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

“…R is the contains three main bands in the 12 to 21 eV region, 4 and these have been assigned to seven IE. 4 The present VUV range makes Rydberg states from several of these IE likely to occur.…”

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

“…Earlier results have been reassigned to the current orientation. 4 The X 1 A1 ground state contains 13 doubly occupied molecular orbitals (DOMOs, 26e); however, we limit the numbering to the valence shell, and the resulting MOs are 1a1 2 to 4a1 2 , 1a2 2 , 1b1 2 to 2b1 2 , 1b2 2 to 3b2 2 . The highest occupied (HOMO), 2b1 2 , is crucial in much of the Rydberg state analysis.…”

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

“…1(B), enhances these weak details. 4 Depending on the individual VUV spectral peak shape, either Gaussian functions were fitted to the region being subtracted, 4 or when a portion of the peak contained structure, such as the onset of some bands, then a Boltzmann sigmoid peak was subtracted from that region. The choice of function is completely pragmatic, in achieving the best fit to the broad structure.…”

Section: Experimental and Computational Proceduresmentioning

confidence: 99%

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The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations

Palmer

Hoffmann

Jones

et al. 2018

The Journal of Chemical Physics

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The vacuum ultraviolet (VUV) spectrum for CHF from a new synchrotron study has been combined with earlier data and subjected to detailed scrutiny. The onset of absorption, band I and also band IV, is resolved into broad vibrational peaks, which contrast with the continuous absorption previously claimed. A new theoretical analysis, using a combination of time dependent density functional theory (TDDFT) calculations and complete active space self-consistent field, leads to a major new interpretation. Adiabatic excitation energies (AEEs) and vertical excitation energies, evaluated by these methods, are used to interpret the spectra in unprecedented detail using theoretical vibronic analysis. This includes both Franck-Condon (FC) and Herzberg-Teller (HT) effects on cold and hot bands. These results lead to the re-assignment of several known excited states and the identification of new ones. The lowest calculated AEE sequence for singlet states is 1B ∼ 1A < 2B < 1A < 2A < 1B < 3A < 3B. These, together with calculated higher energy states, give a satisfactory account of the principal maxima observed in the VUV spectrum. Basis sets up to quadruple zeta valence with extensive polarization are used. The diffuse functions within this type of basis generate both valence and low-lying Rydberg excited states. The optimum position for the site of further diffuse functions in the calculations of Rydberg states is shown to lie on the H-atoms. The routine choice on the F-atoms is shown to be inadequate for both CHF and CHF. The lowest excitation energy region has mixed valence and Rydberg character. TDDFT calculations show that the unusual structure of the onset arises from the near degeneracy of 1B and 1A valence states, which mix in symmetric and antisymmetric combinations. The absence of fluorescence in the 10.8-11 eV region contrasts with strong absorption. This is interpreted by the 2B and 1A states where no fluorescence is calculated for these two states, which are only active in absorption. The nature of the two states, 1B and 2B, is fundamentally different, but both are complex owing to the presence of FC and HT effects occurring in different ways. The two most intense bands, close to 12.5 and 15.5 eV, contain valence states as expected; the onset of the 15.5 eV band shows a set of vibrational peaks, but the vibration frequency does not correspond to any of the photoelectron spectral (PES) structure and is clearly valence in nature. The routine use of PES footprints to detect Rydberg states in VUV spectra is shown to be inadequate. The combined effects of FC and HT in the VUV spectral bands lead to additional vibrations when compared with the PES.

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Section: B Theoretical Interpretation Of the Overall Vuv Spectrum Fomentioning

confidence: 99%

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

Section: Rydberg States Have Term Values Given By Equationmentioning

confidence: 99%

Section: Experimental and Computational Proceduresmentioning

confidence: 99%

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The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations

Palmer

Hoffmann

Jones

et al. 2018

The Journal of Chemical Physics

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A comparison of methods for theoretical photochemistry: Applications, successes and challenges

Hill

Coote

2019

Annual Reports in Computational Chemistry

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The photoelectron spectra of the isomeric 1- and 2-methyltetrazoles; their equilibrium structures and vibrational analysis by ab initio calculations

Palmer

Coreno

Simone

et al. 2018

The Journal of Chemical Physics

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New synchrotron based studies of the photoelectron ionization spectra (PES) for the isomeric 1- and 2-methyltetrazoles (1- and 2-MeTet) show markedly higher resolution than previous reports. The unusual spectral profiles suggest that a considerable overlay of the ionic states occurs for both molecules. Under these circumstances of near degeneracy of two or more ionic states, mutual annihilation of vibrational fine structure occurs for all except the strongest vibrational states; the PES just reflects the resultants rather than full spectra. Theoretical determination of the adiabatic ionization energies (AIEs) proved a challenge; the most successful method was second order Møller-Plesset perturbation theory (MP2). These calculations suggest that the lowest PES bands for both isomers contain ionization both from lone pair σ-orbitals (A') on the N-atoms (LP) and π-orbitals (A″). The lowest experimental AIEs are as follows: 1-MeTet is 10.315 eV assigned to 1A', while 2-MeTet is 10.543 eV assigned to 1A″. Franck-Condon analysis shows that the lowest ionization energy regions of both spectra are dominated by IE from the LPA' manifold, even though the A″ states have a higher absolute intensity. In this example, we have utilized a VUV Rydberg state to assist simplification of the PES; more frequently, the PES assignment is simpler and assists the location of Rydberg states in the VUV. The very slow spectral onset for 2-MeTet demonstrates the importance of vertical ionization energy calculations since maxima are more readily measured than slow onsets. These were performed at the equilibrium structure of the XA' state, using both multi-reference multi-root configuration interaction and the ionization potential variant of the equations-of-motion coupled cluster method, with single and double excitations (EOMIP-CCSD). This enabled the principal ionization bands to be identified over a wider range of energy. Attempts to study the higher ionic states by EOMIP-CCSD showed that several states of each symmetry are close to degenerate for 1-MeTet, in particular. A multi-configuration self-consistent field study confirmed the small separation of ionic states, but state switching during the optimization process largely disabled this method.

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The ionic states of difluoromethane: A reappraisal of the low energy photoelectron spectrum including ab initio configuration interaction computations

Cited by 3 publications

References 46 publications

The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations

The valence and Rydberg states of difluoromethane: A combined experimental vacuum ultraviolet spectrum absorption and theoretical study by ab initio configuration interaction and density functional computations

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

The photoelectron spectra of the isomeric 1- and 2-methyltetrazoles; their equilibrium structures and vibrational analysis by ab initio calculations

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