Angle-resolved photoelectron spectroscopy of the DABCO molecule probed with VUV radiation

Abstract

We report a study of the diazabicyclo[2.2.2]octane (DABCO) molecule photoionized using VUV synchrotron radiation in combination with an ion--electron coincidence spectrometer. We determine accurately the adiabatic ionization energy to $7.199\pm0.006$~eV. Two vibrational progressions of DABCO cation ground state are resolved at $847~\text{cm}^{-1}\pm27~\text{cm}^{-1}$ and $1257~\text{cm}^{-1}\pm67~\text{cm}^{-1}$, which we assign to modes of $e'$ symmetry. Analysis of the photoelectron angular distribution shows that the anisotropy parameter depends on the vibrational excitation. This dependence of the $β$ parameter with the vibrational excitation is attributed to the scattering of the outgoing wavefunction mediated by high-lying Rydberg states.

Figures (4)

• Figure 1: Structure of the DABCO molecule
• Figure 2: 2D photoelectron matrix (upper panel) and SPES of DABCO (black trace, lower panel). The red solid line is the best fit to the data, composed of two Gaussian series (grey dashed and yellow dotted lines) assigned to two distinct vibrational progressions. Inset: photoelectron (black) and ion spectrum (blue) recorded near the ionization threshold. The grey and yellow dashed lines indicates the estimated position of the vibronic transitions. The x-axis is corrected by the Stark shift of respectively $6.21~\rm meV$ and $10.76~\rm meV$ for the main figure and the inset.
• Figure 3: a): Photoelectron anisotropy parameter. The grey stars are the data, the black thick line is the filtered data with a Savitsky-Golay filter with a span of 11 and a polynomial of order 3. The dashed lines indicate the position of the two Franck-Condon progressions listed in Table. \ref{['tab:nu']}. b): Deviation from the mean of the anisotropy parameter over the whole range between the threshold and $\approx$ 7.85 eV, calculated at $\bar{\beta}=~$0.83. c): Mean photoelectron anisotropy parameter, obtained by averaging the data over a $\pm~15~\text{meV}$ interval around the transitions. Error bars represent the standard deviation between measurements A and B (see Table. \ref{['tab:nu']}). The signal between the first two transitions of the first progression (shaded in the figure) is excluded from the analysis. The x-axis is corrected by the Stark shift of respectively $6.21~\rm meV$ and $7.55~\rm meV$ for the two measurements used in this figure.
• Figure 4: a) Photoelectron spectrum of the DABCO molecule between 7.1 eV and 11 eV. Inset: expanded view of the 88 Rydberg states line positions from. fujii_two-color_1984 The synchrotron energy resolution is schematically represented by the violet Gaussian-shaped profile (FWHM of 3 meV, corresponding to the highest energy resolution in our data). b) SPES of the DABCO obtained by integration over the first 300 meV (black line) shown with the integrated photoelectron yield over all the photoelectron energies (dark blue line) and corresponding ion yield (red dashed line). The x-axis is corrected by the Stark shift of $10.76~\rm meV$.