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.

Structure of 1-naphthol–water clusters in the S1 state studied by UV–IR fluorescence dip spectroscopy and ab initio molecular orbital calculations

IR spectra of trans-1-naphthol–(H2O)n (n=0–3) clusters in the S1 state were measured by UV–IR fluorescence dip spectroscopy. The observed dip spectra were compared with theoretical ones of various stable conformations predicted by ab initio MO calculations. From the comparison, linear (n=1) and cyclic hydrogen-bond structures (n=2 and 3) were concluded. The relation between the structures and photochemical reactivity was discussed.

Two-Photon Absorption in Rhodamine 6G Occurring in Concert with Fluorescence Depletion

Using two-color fluorescence dip spectroscopy, we observed two-photon absorption processes (TPA) in Rhodamine 6G occurring in concert with fluorescence depletion processes (FD). A nano-second pulse erase beam in the near-infrared region was applied to induce TPA together with FD. It was found that the fluorescence intensity strongly depends on the photon-flux of the erase beam and that the effective TPA cross-section for the nano-second laser pulse is smaller than that for a femto-second laser pulse. This phenomenon can be interpreted as resulting from FD induced by a multiple-excitation process.

Stochastic heating in asymmetric capacitively coupled RF discharges

Electron dynamics in a strongly asymmetric capacitively coupled radio-frequency (RF) discharge at low pressures is investigated by a combination of various diagnostics, analytical models and simulations. Electric fields in the sheath are measured phase and space resolved using fluorescence dip spectroscopy in krypton. The results are compared with a fluid sheath model. Experimentally obtained input parameters are used for the model. The excitation caused by beam-like highly energetic electrons is measured by phase resolved optical emission spectroscopy (PROES) and compared with the results of a hybrid Monte Carlo model based on the electric field resulting from the sheath model. The plasma itself is characterized by Langmuir probe measurements in terms of electron density, electron mean energy and electron energy distribution function (EEDF). The RF voltage and the current to the chamber wall are measured in parallel. At low pressures the plasma series resonance (PSR) effect is observed. It leads to high frequency oscillations of the current (non-sinusoidal RF current waveforms) and, consequently, to a faster sheath expansion. The measured current is compared with an analytical PSR model. Another analytical model using experimentally obtained input parameters determines the influence of beams of highly energetic electrons on the time averaged isotropic EEDF as measured by Langmuir probes. The main result is the observation of beams of highly energetic electrons during the sheath expansion phase, that are enhanced by the PSR effect. The paper shows that the nature of stochastic heating is closely related to electron beams and the PSR effect.

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.

Measurements of Electric-Field Strengths in Ionization Fronts during Breakdown

Using laser-induced fluorescence-dip Stark spectroscopy, we performed time-resolved, direct measurements of electric-field strengths during the breakdown phase of a low-pressure, pulsed discharge in xenon. With this experimental technique we could for the first time quantitatively measure the time evolution of the driving force of the plasma breakdown process: the electric field. Moving ionization fronts were measured with submicrosecond resolution. These ionization fronts were sustained by a spatially narrow, rapidly moving region of strong electric field.

Laser spectroscopic electric field measurement in krypton

A laser spectroscopic method for sensitive electric field measurements using krypton has been developed. The Stark effect of high Rydberg states of the krypton autoionizing series can be measured by a technique called fluorescence dip spectroscopy (FDS) with high spatial and temporal resolution. Calibration measurements have been performed in a reference cell with known electric field and they agree very well with numerical solutions of Schrödinger's equation for jl-coupled states. The application of this method has been demonstrated in the sheath region of a capacitively coupled radiofrequency (RF) discharge. The laser spectroscopic method allows us to add krypton as a small admixture to various low temperature plasmas.

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.

Chiral recognition between lactic acid derivatives and an aromatic alcohol in a supersonic expansion: electronic and vibrational spectroscopy

Jet-cooled diastereoisomeric complexes formed between a chiral probe, (+/-)-2-naphthyl-1-ethanol, and chiral lactic acid derivatives have been characterised by laser-induced fluorescence and IR fluorescence-dip spectroscopy. Complexes with non chiral alpha-hydroxyesters and chiral beta-hydroxyesters have also been studied for the sake of comparison. DFT calculations have been performed to assist in the analysis of the vibrational spectra and the determination of the structures. The observed 1 : 1 complexes correspond to the addition of the hydroxy group of the chromophore on the oxygen atom of the hydroxy in alpha-position relative to the ester function. Moreover, (+/-)-methyl lactate and (+/-)-ethyl lactate complexes with (+/-)-2-naphthyl-1-ethanol show an enantioselectivity in the size of the formed adducts: while fluorescent 1 : 1 complexes are the most abundant species observed when mixing (S)-2-naphthyl-1-ethanol with (R)-methyl or ethyl lactate, they are absent in the case of the SS mixture, which only shows 1 : 2 adducts. This property has been related to steric hindrance brought by the methyl group on the hydroxy-bearing carbon atom.

Plasma nonuniformities induced by dissimilar electrode metals

Nonuniformities in both sheath electric field and plasma excitation were observed around dissimilar metals placed on a rf electrode. Spatial maps of the rf sheath electric field obtained by laser-induced fluorescence-dip (LIF-dip) spectroscopy show that the sheath structure was a function of the electrode metal. In addition to the electric-field measurements, LIF, optical emission, and Langmuir probe measurements show nonuniform excitation around the dissimilar metals. The degree and spatial extent of the discharge nonuniformities were dependent on discharge conditions and the history of the metal surfaces.

Two-Point Separation in Far-Field Super-Resolution Fluorescence Microscopy Based on Two-Color Fluorescence Dip Spectroscopy, Part I: Experimental Evaluation

The two-point resolution of a novel two-color far-field super-resolution fluorescence microscopy was evaluated by measuring fluorescent beads 100 nm in diameter. This microscopy is based on a combination of two-color fluorescence dip spectroscopy and a phase-modulation technique for a laser beam. By simply introducing two-color laser light, the size of the fluorescent image of a bead was shrunk down to a diameter of 250 nm from the diffraction-limited image with a diameter of 360 nm. For two closely adjacent fluorescent beads with a separation distance of 350 nm, the two-color microscope clearly gave separated fluorescence images, while the conventional one-color fluorescence microscope could not resolve them. It has been proved that our technique breaks Rayleigh's diffraction limit.

Localization of electronic and vibrational energy in the jet-cooled m-cyanophenol/o-cyanophenol dimer: laser induced fluorescence and fluorescence-dip IR spectra

The mixed o-cyanophenol/m-cyanophenol dimer has been studied by laser-induced fluorescence and infrared fluorescence dip spectroscopy, in the region of the hydride stretch. DFT calculations have been carried out in conjunction with the experimental work. The most stable form of the dimer contains o-cyanophenol in its trans form and the cis isomer of m-cyanophenol. It exhibits a planar structure bound by a double hydrogen bond, similar to that observed in the homo (o-cyanophenol)2 or (m-cyanophenol)2 dimers. Its vibrational spectrum shows two strong bands assigned to the v(OH) stretch modes localized on each sub-unit. Electronic excitation localized on each moiety has been identified on the basis of dispersed fluorescence spectra. The lower-energy electronic state corresponds to the excitation of the cis m-cyanophenol sub-unit, while that localized on the trans o-cyanophenol moiety occurs 215 cm−1 higher in energy. No electronic energy transfer has been evidenced following excitation of the mixed dimer, up to 409 cm−1 excess energy.

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.

Theoretical investigation of the point-spread function given by super-resolving fluorescence microscopy using two-color fluorescence dip spectroscopy

The profile of the point spread function (PSF) in superresolution microscopy is studied theoretically. The fluorescence spot profile (i.e., the PSF) is determined by the focused beam patterns of the applied two-color lasers and the optical properties of the fluorescence-depletion process induced by the lasers (the pump and erase beams). In this study, the fluorescence-depletion process for the sample molecule is analyzed using a rate equation for a three-state model. Based on this result, we calculate the PSF for the case where the erase beam is modeled by a first-order Bessel function. In the case of an erase beam with a large photon flux, the obtained PSF has a Lorentzian-like shape, which seldom appears in traditional microscopy. In this work, we also investigated a possible relationship between the PSF and other parameters in the fluorescence-depletion process.

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.