Laser-induced Fluorescence-dip Spectroscopy Research Articles

Laser-induced fluorescence-dip spectroscopy of Rydberg states of xenon for electric field measurement in plasma.

Laser-induced fluorescence-dip spectroscopy is considerably modified by using a special broad band dye laser. With a linewidth of the laser of 1nm, it is possible to detect a wide spectrum of the relevant Stark shifts in order to measure the weak electric fields in plasmas. Depending on the selection of the Rydberg level, the measuring range extends from about 100 V/cm to a few kV/cm. The main advantage of the newly developed method is the possibility to obtain the complete image of the field distribution in a wide spatial area over a few cm within one measurement step at a given time. This allows reliable electric field measurements for phenomena with poor pulse-to-pulse reproducibility on the microscale. The basics of the method and first experimental test results are presented in detail.

Measurements of fireball onset

Laser-based measurements of the characteristic features of fireball onset and stabilization in response to a stepped voltage applied to an anode immersed in a low pressure (100 mTorr) helium afterglow are reported. These include spatial and temporal evolution of metastable species, electron density, and electric field magnitude as measured by planar laser induced fluorescence, laser-collision induced fluorescence, and laser-induced fluorescence-dip spectroscopy, respectively. These measurements are found to be in qualitative agreement with recent particle-in-cell simulations and theoretical models [Scheiner et al., Phys. Plasmas 24, 113520 (2017)]. The measurements validate the simulations and models in which fireball onset was predicted to follow from the trapping of electrons born from electron impact ionization within a potential well created by a buildup of ions in the sheath. The experimental measurements also demonstrate transient features following the onset that were not present in previous simulations. New simulation results are presented which demonstrate that these features are associated with the abruptness of the voltage step used to initiate fireball onset. An abrupt step in the anode bias causes rapid displacement of ions and an associated plasma potential response following the sheath and fireball expansion.

Experimental Study of Stark Effects in Argon High Rydberg States

We examined the Stark effects of argon high Rydberg states in well-defined electric fields by laser-induced fluorescence-dip spectroscopy. The excitations to Rydberg states were carried out from the 4 p[3/2]2 state, which was populated by the excitation from the metastable 4s[3/2]2o state. We observed forbidden transitions from the 4 p[3/2]2 to np[3/2]1,2 states in electric fields. The forbidden transitions were caused by Stark mixing between the np[3/2]1,2 states and the neighboring (n-1)d[1/2]1o and (n-1)d[3/2]2o states. The energy levels of the np states shifted with an increase in the electric field strength. The magnitude of the Stark shift was proportional to the square of the electric field strength. In stronger electric fields, the dip spectra showed oscillating structures, indicating Stark splitting of energy levels. The intervals among the dip peaks induced by Stark splitting were proportional to the electric field strength.

MEASURING ELECTRIC FIELDS WITH LASER-INDUCED FLUORESCENCE-DIP STARK SPECTROSCOPY

The electric field is an important quantity in low-pressure gas discharges, driving many fundamental processes. Unfortunately, it is difficult to measure electric field distributions in plasmas directly. The goal of this research was to develop a diagnostic technique to measure electric fields in low-pressure xenon discharges. It was based on measuring Stark effects in Rydberg levels of xenon atoms by laser-induced fluorescence-dip spectroscopy. With our experimental arrangement we could measure electric fields ranging from 1000 to 4000 V/cm with an accuracy between 150 and 300 V/cm.

Electric field profiles around an electrical probe immersed in a plasma

Laser-induced fluorescence-dip spectroscopy was used to measure the spatial distribution of electric fields around a biased cylindrical probe in an argon rf discharge. Two-dimensional maps of the field profiles were obtained around the probe as functions of argon pressure, discharge power, probe bias, and phase of the rf cycle. Analysis of the radial dependence of the electric fields indicated that the ion density decreased in the vicinity of the probe for all cases, contrary to the orbit motion limited regime typically employed for Langmuir probe analysis. We also observed perturbation of the plasma by the presence of the probe that extended many times the measurable length scale of the sheath (∼5Δxsheath) into the plasma around the probe. Electric field distributions were also measured as functions of distance from the plasma sheath boundary. Coupling between the bounding sheath fields of the plasma and those formed around the biased probe was observed, even at distances greater than length scales of either sheath.

Distribution of electric field in the sheath of an electronegative plasma

The authors measured the distribution of electric field in the sheath formed between an electronegative Ar∕SF6 plasma and a biased electrode by laser-induced fluorescence-dip spectroscopy. It was found that the electric field in the sheath of an electronegative plasma had a stepwise structure, which was due to the reflection of negative ions at a localized distance from the electrode. The electric field observed in an electronegative plasma was compared with a theoretical calculation based on a fluid model.

Investigations of Stark effects in xenon Rydberg states by laser-induced fluorescence-dip spectroscopy

Stark effects in Rydberg states of xenon atoms were investigated both experimentally and theoretically. The experimental part consisted of laser-induced fluorescence-dip spectroscopy. Using a (2+1)-photon excitation scheme, xenon atoms were excited from the ground state to high-lying Rydberg ns and nd levels. Measurements were made in a controllable electric field environment, produced by applying a pulsed voltage to two parallel metal electrodes. For energy levels with principal quantum numbers ranging from 12 to 18, Stark shifts of up to 4.8 cm{sup -1} were observed for electric fields ranging from 0 to 4000 V/cm. Additionally, mixing of energy levels in high electric fields was measured for nd levels. The experimental results were compared to a theoretical calculation based on solving the Schroedinger equation for a perturbed Hamiltonian. The calculation method proved to be very accurate for predicting Stark effects in Rydberg nd levels, while for ns levels the agreement was only moderate, probably due to deviations from the assumption of a hydrogenlike atom that is used in the calculation. Finally, the feasibility of using measurements of Stark shifts of Rydberg levels as a diagnostic for electric fields in low-pressure discharges was discussed.

Two-dimensional profiles of electric fields in a radio frequency argon plasma above nonuniformities present on a surface

Laser-induced fluorescence-dip spectroscopy was used to measure two-dimensional (2-D) maps of the electric field present in an argon discharge above a ratio frequency-powered, nonuniform surface. Electric fields were obtained from experimentally measured Stark shifts of the energy of argon Rydberg states. The 2-D maps of the electric fields demonstrated that nonuniformities present on an electrode have long-range effects on the structure of the sheath.

Radio frequency sheath formation and excitation around a stepped electrode

Plasma and sheath structure around a rf excited stepped electrode is investigated. Laser-induced fluorescence dip spectroscopy is used to spatially resolve sheath fields in an argon discharge while optical emission and laser-induced fluorescence are used to measure the spatial structure of the surrounding discharge for various discharge conditions and step-junction configurations. The presence of the step perturbs the spatial structure of the fields around the step as well as the excitation in the region above the step.

Radiofrequency sheath fields above a metal-dielectric interface

Two-dimensional maps of the sheath electric fields formed around a metal-dielectric interface were measured in a radio frequency (rf) argon plasma using laser-induced fluorescence-dip spectroscopy. Experimentally determined Stark shifts of the argon Rydberg 13d[3∕2]1 state were used to quantify the electric fields in the sheath as functions of the rf cycle, voltage, and pressure. Both the structure of the sheath fields and the discharge characteristics in the region above the electrode depend on the discharge conditions and the configuration of the surface. Dissimilar materials placed adjacent to each other result in electric fields with a component parallel to the electrode surface.

Electric fields in a sheath near a metal–dielectric interface

Spatially resolved electric fields in the sheath region near a metal–dielectric junction were measured in a radio-frequency-driven argon plasma. The fields were determined by observing the Stark shifted transitions to the 13d[3∕2]1 Rydberg state by laser-induced fluorescence-dip spectroscopy. Calibration of the Stark shifts for the 13d[3∕2]1 Rydberg state were experimentally obtained in a separate apparatus. Maps of the electric fields illustrate that the structure of the sheath formed around the junction depended on both the surface material and on the configuration of the surface.

Sensitive measurements of electric field distributions in low-pressure Ar plasmas by laser-induced fluorescence-dip spectroscopy

Laser-induced fluorescence-dip (LIF-dip) spectroscopy of Ar was used for measuring the distributions of sheath electric fields in low-pressure, inductively-coupled Ar plasmas. A sensitive detection limit of 3 V/cm obtained by LIF-dip allowed the measurement in the presheath region. The distributions of electric fields observed experimentally were compared with those of theoretical calculations based on a simple fluid model. As a result, reasonable agreement between the experiment and the theory was obtained in the electric fields in the sheath region, while the electric fields in the presheath region observed experimentally were higher than the theoretical results.

Observation of the Electric Field Structure in the Sheath of Electronegative Plasmas by Laser-Induced Fluorescence-Dip Spectroscopy

We have measured the distribution of the electric field in the sheath formed between electronegative Ar⁄SF6 plasmas and a biased electrode by laser-induced fluorescence-dip spectroscopy. It has been found for the first time that the electric field in the sheath of an electronegative plasma has a multistage structure, which may be due to the reflections of negative ions and electrons at different distances from the electrode surface.