Charging Mechanisms of Trapped Element-Selectively Excited Nanoparticles Exposed to Soft X Rays

Charging mechanisms of trapped, element-selectively excited free ${\mathrm{SiO}}_2$ nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O $1s$-excited ${\mathrm{SiO}}_2$ particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged ${\mathrm{SiO}}_2$ nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.

Figure 1

Charge state of a trapped ${\mathrm{SiO}}_2$ nanoparticle of 500 nm diameter upon x-ray illumination (500 eV) as determined by its secular motion frequency ${\omega }_r$ within an electrodynamic trap.

Figure 2

Step height distributions from charging curves at different photon energies. The height of the bars is obtained from the experimental results (cf. Fig. 1). The solid lines correspond to Poisson distributions, corresponding to the emission of secondary electrons. Contributions from direct photoionization and Auger processes are indicated in red. The average number of charging events per absorbed photon $\delta$ is indicated. See text for further details.

Figure 3

(a) Charging curves as a function of photon energy of a differently charged ${\mathrm{SiO}}_2$ nanoparticle. The curves A–C correspond to different initial and final charge states; (b) First derivative of the results shown in (a), corresponding to the charging current as a function of photon energy in the O $1s$ excitation regime. The left and right vertical dashed lines indicate the position of the maximum in curves (C) and (A), respectively.