Low-energy-electron production after $2p$ ionization of argon clusters

Irradiation of matter with x rays causes inner-shell ionization of atoms and molecules, followed by subsequent electronic relaxation leading to production of low-energy electrons. In this study we investigate the process of low-energy-electron production in Ar clusters by electron-ion multiple coincidence measurements. In addition to photoelectrons, low-energy electrons are observed after $2p$ ionization of argon clusters. We find that low-energy-electron production increases when the energy of photoelectrons exceeds the ionization potential of Ar. We experimentally identify the low-energy electrons produced by interatomic electronic decay processes and inelastic scattering of the photoelectrons and Auger electrons with the surrounding Ar atoms.

Figure 1

Processes in the present study. Step 1 (photoionization): By irradiation of soft x rays, an inner $2p$ electron is emitted. Step 2 (Auger decay): A $3s$ electron makes a discrete transition to the vacant $2p$ electron shell. A bound $3p$ electron receives the energy gained in this process and escapes from the atom. Step 3 (ICD): An outer $3p$ electron fills the vacancy of the inner $3s$ shell. The excess energy is transferred to a neighboring Ar atom, which then emits an electron. Step 4 (charge transfer and Coulomb explosion): The triply charged cluster goes through charge transfer, leading to production of three singly charged atoms. The cluster eventually dissociates due to Coulomb force and releases three singly charged Ar atoms. Step $3^{\prime }$ [ETMD(3)]: Instead of ICD, ETMD(3) may occur. In this process, an outer $3p$ electron of a neighboring Ar atom fills the vacancy of the inner $3s$ shell. The excess energy is transferred to another neighboring Ar atom, which then emits an electron. Step 1-2 (photoelectron inelastic scattering): When the photoelectron emitted in step 1 has enough energy to excite or ionize a neighboring Ar atom, inelastic scattering of the photoelectron by the neighboring atom occurs. Step 2-2 (Auger electron inelastic scattering): The Auger electron emitted in step 2 is scattered by the neighboring atom.

Figure 2

Total ion yield spectra measured with 0.3 MPa (blue dashed line) and 1.5 MPa (red solid line) stagnation pressures of Ar gases.

Figure 3

Electron spectra for Ar clusters (red solid lines) and Ar atoms (blue dashed lines) measured at (a) 258-eV and (b) 268-eV photon energies. The spectra for clusters and atoms are measured simultaneously and obtained in coincidence with three ${\mathrm{Ar}}^{+}$ and one ${\mathrm{Ar}}^{2+}$, respectively. The bin size of the electron kinetic energy is 0.1 eV.

Figure 4

Sum of kinetic energies of three ${\mathrm{Ar}}^{+}$ ions measured at 258-eV (red solid line) and 268-eV (blue dashed line) photon energies. Intensities are obtained by dividing counts at each kinetic energy by the total counts ($74309$ and $62952$ counts for 258 and 268 eV, respectively).

Figure 5

Partial electron spectra at (a) 258-eV and (c) 268-eV photon energies separated by the sum of ion kinetic energies. Also shown are the ratios of the partial electron spectra to the total electron spectra at (b) 258 eV and (d) 268 eV. The bin size of the electron kinetic energy is 0.2 eV.

Figure 6

Each component obtained from the fitting and subtraction for the electron spectra. Intensities are normalized, e.g., the integrated intensity of the photoelectrons around 10 eV is unity.