Tracing intermolecular Coulombic decay of carbon-dioxide dimers and oxygen dimers after valence photoionization

We have conducted an experimental study on the photo double ionization (PDI) of carbon-dioxide dimers at photon energies of 37 and 55 eV and oxygen dimers at photon energies of 38, 41.5, and 46 eV, while focusing on the dissociation dynamics upon single-photon absorption. The investigation was performed by applying the cold-target recoil-ion momentum spectroscopy method in order to collect and record the three-dimensional momenta of the ionic fragments and emitted electrons from the dissociating dimer in coincidence. The kinetic-energy release upon fragmentation and the electron angular distributions in the laboratory and body-fixed frames, as well as the relative electron-electron emission angle, show unambiguous experimental evidence of intermolecular Coulombic decay (ICD) in carbon-dioxide dimers upon photoionization below and above the double-ionization threshold of ${\mathrm{CO}}_2$ monomers. The PDI of oxygen dimers is less conclusive and shows contributions from ICD and knock-off ionization mechanisms. As for atomic dimers, the present results reveal that ICD in ${\mathrm{CO}}_2$ dimers after valence PDI can also serve as a source for low-energy electrons, known to be very relevant in biological systems, cells, and tissues.

Figure 1

(a) Yield of the ${\mathrm{CO}}_2{}^{+}+{\mathrm{CO}}_2{}^{+}$ dimer breakup at 55 eV photon energy as a function of the kinetic-energy release (KER) and the kinetic energy of either emitted electrons. (b) Differential cross section (DCS) in KER distributions for the ${\mathrm{CO}}_2{}^{+}+{\mathrm{CO}}_2{}^{+}$ fragmentation channel at photon energies of 37 eV [green (gray) line] and 55 eV (black line), showing the same shape and a maximum located at 3.8 eV. (c) Differential cross section (DCS) in electron energy distribution of the PDI of ${\mathrm{CO}}_2$ dimers for a photon energy of 55 eV (black line), showing two distinct contributions from the low-energy and high-energy electrons featuring the ICD process and for a photon energy of 37 eV [green (gray) line]. (d) Electron energy correlation map showing the yield of the ${\mathrm{CO}}_2{}^{+}+{\mathrm{CO}}_2{}^{+}$ dimer breakup as a function of the energy of the first and the second detected electron for a photon energy of 55 eV. The estimated energy maps of the emitted electrons for direct PDI (diagonal black lines) and ICD (black dots) processes, taken from Tables 2 and 3 for a photon energy of 55 eV, are overlaid in order to identify the states involved and their yields. The yields in (a) and (d) are given on a logarithmic color scale.

Figure 2

(a) Differential cross section (DCS) in kinetic-energy release distributions for the ${\mathrm{O}}_2{}^{+}+{\mathrm{O}}_2{}^{+}$ fragmentation channel for the PDI of ${\mathrm{O}}_2$ dimers with photon energies of 38 eV (black line), 41.5 eV [red (dark gray) line], and 46 eV [green (gray) line], showing the same shape and a maximum located at 4.55 eV. The singlet, triplet, and quintet of $({\mathrm{O}}_2{)}_2$ ground states are marked as dashed lines. (b)–(d) Electron energy correlation map showing the yield of the ${\mathrm{O}}_2{}^{+}+{\mathrm{O}}_2{}^{+}$ dimer breakup as a function of the energy of the first and the second detected electron for a photon energy of (b) 38 eV, (c) 41.5 eV, and (d) 46 eV. The estimated energy of the emitted electrons for direct PDI (diagonal black lines) and ICD (black dots) processes, taken from Tables 5 and 6, is overlaid in order to identify the implicated states and their yields. The yields in (c) and (d) are given on a logarithmic color scale.

Figure 3

Angular distribution of the (a) high-energy (13–25 eV) electron and (b) low-energy (0–7 eV) electron with respect to the polarization axis (horizontal) of the PDI of ${\mathrm{CO}}_2$ dimers for a photon energy of 55 eV. The (red) line shows a fit $\{1+0.5\beta [3{\cos }^2\left(\theta \right)-1]\}$ to the data points. The values of the asymmetry parameter $\beta$ for (a) and (b) are equal to $0.515\pm 0.048$ and $0.064\pm 0.066$, respectively. All error bars represent one standard deviation in the statistical uncertainty.

Figure 4

Angular distribution of the (a) high-energy (5–15 eV) electron and (b) low-energy (0–4 eV) electron with respect to the polarization axis (horizontal) of the PDI of ${\mathrm{O}}_2$ dimers for a photon energy of 46 eV. The (red) line shows a fit of $\{1+0.5\beta [3{\cos }^2\left(\theta \right)-1]\}$ to the data points. The value of the asymmetry parameter $\beta$ for (a) and (b) are equal to $0.332\pm 0.028$ and $0.002\pm 0.017$, respectively. All error bars represent one standard deviation in the statistical uncertainty.

Figure 5

Emission direction of the ICD electron, i.e., low-energy electron, with respect to the photoelectron, i.e., high-energy electron (fixed to the right as indicated by the black arrow), from the PDI of ${\mathrm{CO}}_2$ dimers with a photon energy of 55 eV. The (red) line is a circle fitted to the data points to guide the eye. All error bars represent one standard deviation in the statistical uncertainty.

Figure 6

Relative angle between the two emitted electrons from the PDI of ${\mathrm{O}}_2$ dimers with a photon energy of 46 eV for (a) electrons with equal energy sharing and (b) electrons with unequal energy sharing. The higher-energy electron emission direction is fixed to the right, as indicated by the black arrows. The (red) line is a circle fitted to the data points to guide the eye. All error bars represent one standard deviation in the statistical uncertainty.

Figure 7

Angular distribution of the (a) high-energy (13–25 eV) electron and (b) low-energy (0–7 eV) electron with respect to the dimer axis (horizontal) of the PDI of ${\mathrm{CO}}_2$ dimers for a photon energy of 55 eV. The (red) line is a Legendre polynomial series up to fourth-order fit to the data points. All error bars represent one standard deviation in the statistical uncertainty.

Figure 8

Angular distribution of the (a) high-energy (5–15 eV) electron and (b) low-energy (0–4 eV) electron with respect to the dimer axis (horizontal) of the PDI of ${\mathrm{O}}_2$ dimers for a photon energy of 46 eV. The (red) line is a Legendre polynomial series up to fourth-order fit to the data points. All error bars represent one standard deviation in the statistical uncertainty.