Laser-stimulated electron photodetachment
Advanced Diagnostics Navigation
- Atomic and molecular species, positive and negative ions
- Laser-Induced Fluorescence (LIF) and Two-Photon Absorption LIF (TALIF)
- Radar Resonant Enhanced Multi-Photon Ionization (Radar REMPI)
- Characterization of plasma, chemical composition and dynamic behavior
- Electron velocity distribution function, gas flow velocity and temperature
- Electric field and space potential
- Nanoparticle Diagnostics
- Surface Diagnostics
Laser-stimulated electron photodetachment (LSPD)
- Study of charging both nanoparticles and planar substrate surfaces
- Typical density for nanoparticles > 106 cm-3
- Surface charge density > 200 nC/m2
Laser-stimulated electron photodetachment (LSPD) is a diagnostic technique used to measure the charge on negatively charged planar surfaces and particles in gas discharge plasmas. This method involves using a laser to detach electrons from negatively charged particles (dust grains) and surfaces through the photodetachment process, enabling the measurement of particle charge or surface charge density. The technique is based on the detachment of surplus electrons responsible for surface charging, rather than bulk electrons from the material itself, by maintaining the incident photon energy below the material's work function. For studying surface charge decay and discharging dynamics, LSPD can be combined with a non-contact electrostatic voltmeter (Kelvin probe) or photodetached electron current measurements using transimpedance current amplifiers. LSPD enables the investigation of charging/discharging dynamics and surface charge trap states in common dielectric substrates such as SiO2, Al2O3, h-BN [1] as well as floating semiconductors and metals (Figure 1).
The utilization of different harmonics of Nd:YAG lasers, dye lasers, or optical parametric oscillators is also possible, allowing for the selection of various wavelengths depending on the surface properties of the materials under test. To determine the charge density and study the charging dynamics of nanoparticles, LSPD can be coupled with Langmuir probe techniques or microwave methods to detect the temporal evolution of detached electron density (Figure 2). Detached electrons generate a photoelectron current in the plasma, which is proportional to the number of electrons detached and, therefore, directly related to the charge of the nanoparticles [2]. With the particle number density determined by other methods (e.g., laser light extinction), real-time and in situ detection of the charge per particle can be achieved. Unlike many other techniques that infer charge from indirect measurements and require knowledge of plasma parameters and particle size, LSPD allows for a more straightforward measurement of the charge on negatively charged nanoparticles. Although the technique is relatively non-invasive and does not significantly disturb plasma dynamics, the possible detachment of electrons from background negative ions in reactive plasmas must be considered. The laser can also be focused on specific regions or particles in the plasma, enabling spatially resolved charge measurements.
[1] Y. Ussenov, M.N. Shneider, S. Yatom, and Y. Raitses, Laser-stimulated photodetachment of electrons from the negatively charged dielectric substrates, arXiv:2404.08773 [2] Y. Ussenov, M. Shneider, S. Yatom, Y. Raitses, Laser stimulated photodetachment of electrons from planar substrate surfaces and nanoparticles, 16th Dusty Plasma Workshop, University of Minnesota, MN, USA, May 19-22 (2024)
