^{1,a)}and J. H. D. Eland

^{2,b)}

### Abstract

Single and double photoionizationspectra of formaldehyde have been measured at 40.81 and 48.37 eV photon energy and the spectrum of the doubly charged cation has been interpreted using high-level electronic structure calculations. The adiabatic double-ionization energy is determined as and the vertical ionization energy is 33 eV. The five lowest excited electronic states are identified and located. The potential-energy surfaces of the accessible states explain the lack of stable dications and the lack of vibrational structure. The experimental double-ionization spectrum can be decomposed into two distinct contributions, one from direct photoionization and the second from indirect double photoionization by an inner-valence shell Auger effect.

One of the authors (M.H.) would like to thank the NERSC (UC, Berkeley, USA) for computational time. We acknowledge the financial support of the EPSRC for the experimental part of the work.

I. INTRODUCTION

II. EXPERIMENTAL METHODS

III. METHODS FOR ELECTRONIC STRUCTURE CALCULATION

IV. RESULTS

A. Single ionization

B. Double photoionization

C. Theoretical results on

1. Excited electronic states of

2. On the bonding in this molecular dication

3. Six-dimensional potential-energy function of

V. DISCUSSION

VI. CONCLUSIONS

### Key Topics

- Carbon dioxide
- 44.0
- Ionization
- 25.0
- Photoionization
- 18.0
- Ground states
- 12.0
- Photons
- 10.0

## Figures

The photoelectron spectrum of formaldehyde at 48.37 eV photon energy (lower curve): the small peaks near 9 and 13 eV are artifacts. The upper curve shows the distribution of energies of electrons ejected as pairs in double ionization under the same conditions.

The photoelectron spectrum of formaldehyde at 48.37 eV photon energy (lower curve): the small peaks near 9 and 13 eV are artifacts. The upper curve shows the distribution of energies of electrons ejected as pairs in double ionization under the same conditions.

Complete double photoionization spectra of formaldehyde at 48.37 eV photon energy (25.6 nm) and 40.81 eV photon energy (30.4 nm). The dominance of band 1 is due to an inner-valence Auger effect, as explained in the text.

Complete double photoionization spectra of formaldehyde at 48.37 eV photon energy (25.6 nm) and 40.81 eV photon energy (30.4 nm). The dominance of band 1 is due to an inner-valence Auger effect, as explained in the text.

Two-dimensional electron pair energy distribution maps for double photoionization of formaldehyde at the indicated photon energies. The vertical scales show ionization energy (photon energy minus electron energy sum), while the horizontal axes show the lower of the two electron energies.

Two-dimensional electron pair energy distribution maps for double photoionization of formaldehyde at the indicated photon energies. The vertical scales show ionization energy (photon energy minus electron energy sum), while the horizontal axes show the lower of the two electron energies.

Spectra of from mainly direct double photoionization (lower curve) and mainly indirect double ionization (upper curve). The direct spectrum is extracted from the full electron pair distribution at 48.37 eV by choosing electron pairs of near equal energy. For the indirect spectrum, pairs where one electron has an energy below 0.25 eV are chosen, emphasizing both the inner-shell Auger process and the possibly distinct process giving very low-energy electrons, visible in Fig. 3.

Spectra of from mainly direct double photoionization (lower curve) and mainly indirect double ionization (upper curve). The direct spectrum is extracted from the full electron pair distribution at 48.37 eV by choosing electron pairs of near equal energy. For the indirect spectrum, pairs where one electron has an energy below 0.25 eV are chosen, emphasizing both the inner-shell Auger process and the possibly distinct process giving very low-energy electrons, visible in Fig. 3.

Definition of the internal coordinates of .

Definition of the internal coordinates of .

MRCI potential-energy curves of the singlet (upper trace) and the triplet (lower trace) states of along the CH coordinate. The other internal coordinates were kept fixed at their equilibrium values in . The thick vertical solid line corresponds to the center of the Franck-Condon region defined as the maximum of the squared ground-state wave function of .

MRCI potential-energy curves of the singlet (upper trace) and the triplet (lower trace) states of along the CH coordinate. The other internal coordinates were kept fixed at their equilibrium values in . The thick vertical solid line corresponds to the center of the Franck-Condon region defined as the maximum of the squared ground-state wave function of .

CASSCF electronic density difference for the equilibrium geometry of the dication (see Table I). The step between the contours is . The full lines correspond to and the dashed lines to .

CASSCF electronic density difference for the equilibrium geometry of the dication (see Table I). The step between the contours is . The full lines correspond to and the dashed lines to .

Two-dimensional cuts of the 6-D MRCI potential-energy function of for all eight permutations of internal coordinates. The remaining coordinates in each cut are fixed at their equilibrium values in . The contour intervals are except for the cut along the and coordinates, for which the step between the contours is . The lowest energy at ( for the cut along and ) corresponds to the first contour inside.

Two-dimensional cuts of the 6-D MRCI potential-energy function of for all eight permutations of internal coordinates. The remaining coordinates in each cut are fixed at their equilibrium values in . The contour intervals are except for the cut along the and coordinates, for which the step between the contours is . The lowest energy at ( for the cut along and ) corresponds to the first contour inside.

## Tables

Optimized equilibrium geometries, adiabatic excitation energies , vertical excitation energies , and dominant electronic configurations of the excited states of calculated at the cc-pV5Z/MRCI level of theory.

Optimized equilibrium geometries, adiabatic excitation energies , vertical excitation energies , and dominant electronic configurations of the excited states of calculated at the cc-pV5Z/MRCI level of theory.

Article metrics loading...

Full text loading...

Commenting has been disabled for this content