Angular distributions of photoelectrons and interatomic-Coulombic-decay electrons from helium dimers: Strong dependence on the internuclear distance

In the present paper, we show that the absorption of a single photon can singly ionize both atoms of a helium dimer (He${}_2$): ionization with simultaneous excitation of one atom followed by de-excitation via interatomic Coulombic decay leads to the ejection of an electron from each of the the two atoms of the dimer. Using the Cold Target Recoil Ion Momentum Spectroscopy technique (COLTRIMS), we obtained angular distributions of these electrons in the laboratory frame and the molecular frame. We observe a pronounced variation of these distributions for different regions of kinetic-energy releases of the ions.

Figure 1

TOF in the spectrometer of two detected ions. The curved diagonals result from the Coulomb breakup of helium dimers and other molecules in the residual gas. A simulation of the spectrometer (green curves) marks the expected positions for the different charge-to-mass ratios. The vertical and horizontal features results from random coincidences. The double diagonal is an electronic artifact of the detector.

Figure 2

The kinetic energy of measured electrons as a function of the KER at a photon energy of 68.86 eV. This spectra allows a clear separation of ICD electrons and photoelectrons.

Figure 3

Angular distribution of photoelectron with respect to the polarization vector $\epsilon$ at a photon energy of 68.86 eV. The data are integrated over all KERs, and data points between 0${}^{°}$ and 90${}^{°}$ are mirrored into the other three quadrants.

Figure 4

Photoelectron angular distribution with respect to the polarization axis at a photon energy of 68.86 eV for two different regions of KER. (A subset of the data shown in Fig. 3. Data points between 0${}^{°}$ and 90${}^{°}$ are mirrored into the other three quadrants.) The solid line shows a fit of Eq. (3) to the data, yielding $\beta$ of $+$0.11 (a) and $\beta$ of $-$0.52 (b).

Figure 5

Top: calculated kinetic energy distribution for states of $\Sigma$ and $\Pi$ symmetry (from [11]). Bottom: Asymmetry parameter $\beta$ of the photoelectron as function of the KER. The dotted line indicates a measurement for helium atoms by Lindle et al. [23]. The dashed line shows a theory of $\beta$ for 2$p$ states only [25].

Figure 6

Distribution of the angle $\vartheta$ between the dimer axis and the emission direction of the ICD electrons. (Distribution is integrated over all values of KER. Data points between 0${}^{°}$ and 90${}^{°}$ are mirrored into the other three quadrants.)

Figure 7

Angular distributions of the ICD electron with respect to the dimer axis (horizontal) for different regions of KER. (Datapoints between 0${}^{°}$ and 90${}^{°}$ are mirrored into the other three quadrants.) The continuous red lines represent a simple model. The upper panel shows the relative contributions of different transitions to the KER [11]. Selected areas are marked in this plot as the gray bars (a–i).