Laser-induced Photodetachment Research Articles

Real-time in situ monitoring of dust particle growth in a low-pressure nanodusty plasma based on laser-induced photodetachment

Particulate matter air pollution in the form of ultrafine dust is a growing global concern. In this Letter, we will use a nanodusty Ar/HMDSO plasma as a model system for a heavily contaminated gas and we present the development of a technique for real-time in situ measurements of the dust particle size. The method is based on laser-induced photodetachment of bound electrons from the surface of dust particles. These photo-released electrons are measured as an increase in the free electron density of the plasma using microwave cavity resonance spectroscopy. We show that instead of reconstructing the entire resonance profile, the temporal response of a single microwave frequency was enough to perform the measurements. More specifically, the decay timescale of the cavity response can be interpreted as the re-charging timescale of the dust particles. Then, using a stochastic model, this timescale can be modeled, which eventually retrieves the dust particle size. We found good agreement between the predicted dust particle size and the average dust particle size obtained from ex situ scanning electron microscopy measurements. This method allows for the real-time monitoring of the dust particle size and a controlled production of nanometer-scale dust particles, which gives opportunities both for fundamental dusty plasma physics and models, as well as for applications in monitoring ultrafine dust air pollution.

A novel diagnostic for dust particle size in a low-pressure nanodusty plasma based on the decay of the electron density released by laser-induced photodetachment

One of the key parameters in low-pressure nanodusty plasmas is the dust particle size. In this work, we introduce a new method for the determination of the dust particle size in a nanodusty plasma, created in a mixture of argon and hexamethyldisiloxane. To this end, an ultraviolet (λ=266 nm) pulsed laser was used to release plasma-collected electrons from the nanoparticles. Subsequently, the response of the free electron density of the plasma was measured using microwave cavity resonance spectroscopy. Using a stochastic model for particle charging using orbital-motion limited (OML) theory, the predicted charging timescale can be directly compared to the experimentally measured decay timescale of the photo-released electron density. Good agreement was found between the experimentally predicted dust particle size and ex situ scanning electron microscopy (SEM) measurements. Furthermore, the sensitivity of the OML model to its input parameters was assessed. Finally, reversing the method can yield an estimate for the positive ion density based on the dust particle size from SEM.

Temporal afterglow between two pulses of repetitively pulsed argon-acetylene plasma: measuring electron and negatively charged species densities

The temporal afterglow between two pulses of a repetitively pulsed radio-frequency driven low-pressure argon-acetylene plasma is experimentally explored using laser-induced photodetachment combined with microwave cavity resonance spectroscopy. The densities of electrons and negatively charged species, i.e. anions and dust particles, are measured temporally resolved until 1.9 s in the temporal plasma afterglow. Two different plasma-on times are adjusted to investigate the dynamics of anions and dust particles in the afterglow phase. The measurements show that while electrons decay rapidly within the first few milliseconds of the afterglow phase, the negatively charged species reside much longer in the plasma after the plasma is switched off. The electron density decay is measured to be faster for a longer plasma-on time. This effect is attributed to an enhanced recombination rate due to a higher dust particle density and/or size. The density of negatively charged species decays within two different timescales. The first 20 milliseconds of the afterglow is marked with a rapid decay in the negatively charged species density, in contrast with their slow density decay in the second time scale. Moreover, a residual of the negatively charged species densities is detected as long as 1.9 s after extinguishing the plasma.

Charged-particles measurements in low-pressure iodine plasmas used for electric propulsion

This paper investigates iodine as an alternative propellant for space plasma propulsion. Measurements are taken in a low-pressure inductively-coupled plasma chamber used as the ionization stage of a gridded ion-engine. Langmuir probes are used to measure the electron density and the electron energy distribution functions spatial variations between the inductive coil and the extraction grids for several radio-frequency (RF) powers and mass flow rates. Measurements in iodine are compared to xenon, krypton and argon in order to evaluate performances of these various propellants for ionization (and therefore power) efficiency. At low mass flow rates, iodine is found to be the most efficient propellant, however, as the mass flow rate increases, the ionization cost in iodine increases rapidly due to both its molecular and electronegative nature. The ratio of negative ion to electron density is measured using laser-induced photodetachment in order to quantify the effect of iodine electronegativity. Finally, all measurements are compared to a previously published global (volume-averaged) model. The agreement between model and experiments is acceptable, however several modelling improvements are proposed.

Photoelectron Circular Dichroism of Electrosprayed Gramicidin Anions.

Many sophisticated approaches for analyzing properties of chiral matter have been developed in recent years. But in general, the available chiroptical methods are limited to either solvated or small gaseous molecules. Studying the chirality of large biopolymers in the gas phase, including aspects of the secondary structure, becomes accessible by combining the electrospray ionization technique with chiroptical detection protocols. Here, laser-induced photodetachment from gramicidin anions, a peptide consisting of 15 amino acids has been investigated. The angular distribution of photoelectrons is demonstrated to be sensitive to the substitution of protons by cesium ions, which is accompanied by a conformational change. The photoelectron circular dichroism (PECD) is -0.5% for bare gramicidin, whereas gramicidin with several Cs+ ions attached exhibits a PECD of +0.5%. The results are complemented and supported by ion mobility studies. The presented approach offers the prospect of studying chirality and the secondary structure of various biopolymers.

Laser-induced photodetachment of negative oxygen ions in the spatial afterglow of an atmospheric pressure plasma jet

Negative ions are an important constituent of the spatial afterglow of atmospheric pressure plasmas, where the fundamental plasma-substrate interactions take place that are vital for applications such as biomedicine, material synthesis, and ambient air treatment. In this work, we use laser-induced photodetachment to liberate electrons from negative ions in the afterglow region of an atmospheric pressure plasma jet interacting with an argon-oxygen mixture, and microwave cavity resonance spectroscopy to detect the photodetached electrons. This diagnostic technique allows for the determination of the electron density and the effective collision frequency before, during and after the laser pulse was shot through the measurement volume with nanosecond time resolution. From a laser saturation study, it is concluded that O− is the dominant negative ion in the afterglow. Moreover, the decay of the photodetached electron density is found to be dominantly driven by the (re)formation of O− by dissociative attachment of electrons with O2. As a consequence, we identified the species and process responsible for the formation of negative ions in the spatial afterglow in our experiment.

Generation of quantum vortices in photodetachment: The role of the ground-state wave function

Formation of quantum vortices in laser-induced photodetachment from negative ions is analyzed. The driving laser field consists of a single ultrashort pulse of circular polarization and the unperturbed ground-state wave function of the anion is found in either the $s$ or $p$ state. In particular, numerical illustrations for the photodetachment from ${\mathrm{H}}^{\ensuremath{-}},$ ${\mathrm{O}}^{\ensuremath{-}},$ ${\mathrm{K}}^{\ensuremath{-}},$ and a model ${\mathrm{A}}^{\ensuremath{-}}$ anion are presented. Special attention is paid to the symmetry of the ground-state wave function and ionization potential over the final vortex pattern. It is shown that the two-dimensional spectra of photoelectrons in momentum space comprise three well-defined regions: The low-energy (central) region, multiphotonlike zone, and supercontinuum. While the supercontinuum does not contribute to vorticity and the multiphoton zone depends only on the laser field characteristics, vortices in the low-energy region strongly depend on the bound-state wave function and its ionization potential.

Probing negative ions and electrons in the afterglow of a low-pressure oxygen radiofrequency plasma using laser-induced photodetachment

This paper reports on the further development and improvement of time-resolved laser-induced photodetachment in concert with microwave cavity resonance spectroscopy. The method is applied to measure—with microsecond time resolution—the decaying density of negative oxygen ions (O−, O, and O) and that of free electrons in the afterglow of a pulsed capacitively coupled radiofrequency-driven oxygen plasma. The afterglow behavior of the electrons shows a significant dependence on the gas pressure between 3 Pa and 6 Pa. For a pressure of 3 Pa, at which the plasma is in the so-called γ-mode, the decay of the negative ion density is slower than that of the electron density, eventually leading to the occurrence of a negative-ion-positive-ion plasma. At a slightly elevated pressure of 6 Pa (and higher), the plasma transits into the so-called α-mode, in which a short period of increased electron density is detected just after switching off the plasma. In the α-mode, the negative ion and electron densities decay within similar timescales, leading to the trapping of negative ions. In this pressure range, the decay of the additional electron density released by the photodetachment of negative ions occurs according to two distinct timescales. However, for increasingly elevated pressures above 10 Pa, the photodetachment signal is characterized by decay with an undershoot, which may indicate a temporary local disturbance of the plasma’s quasi-neutrality in the volume irradiated by the laser beam.

Axial diagnosis of radio-frequency capacitively coupled Ar/O<sub>2</sub> plasma

The capacitively coupled Ar plasma containing oxygen, driven by a radio frequency of 27.12 MHz, is investigated by laser-induced photo-detachment technique assisted with a Langmuir probe. The plasmas with different amounts of oxygen are obtained by changing the flow of Ar and oxygen, each of which is controlled by a mass flow controller. The axial distribution of plasma characteristic can be measured by changing the relative axial position of the Langmuir probe between the parallel electrodes. The electron density and electron temperature are calculated from the current-voltage curve measured by the scanning Langmuir probe, and the electronegativity is obtained from the current curves of the probe with the laser-induced photo-detachment technique. The negative ion density can be calculated from the electron density and the electronegativity. It is shown that with oxygen flow rate increasing, the dissociative attachment of oxygen molecules with electrons will consume the electrons with the middle energy in the electron energy probability function (EEPF) measured by Langmuir probe. The EEPF evolves from Druyvesteyn to Maxwellian distribution due to the thermalization by the e-e interaction with applied power increasing. It is worth mentioning that a depression in the EEPF curve will appear when discharging high-pressure Ar gas containing oxygen. This depression can also be caused by the dissociative attachment of oxygen molecules with electrons where the threshold energy is around 4.5 eV. The axial profile of the electron density is calculated from the EEPF changing from a linear rise in pure Ar plasma to a flater phase of the distribution due to the negative ions such as oxygen introduced into the plasma. The electron temperature changes a little at different axial positions. The rise of negative ion density nearby the sheath of powered electrode is due to the dissociative attachment caused by the collision of oxygen molecules with energetic electrons. In addition, the axial distribution of electronegativity takes on a shape of spoon, which results from the consequence of generation and loss of negative ions in the process of the ambipolar-electric-field-driven diffusion to the plasma center.

Axial diagnosis of electron and negative ion behaviors in capacitively coupled O2-containing Ar plasma driven by 27.12 MHz

Capacitively coupled O2-containing Ar plasma driven by a radio frequency (RF) of 27.12 MHz has been investigated. The electron energy probability function (EEPF) was measured with a Langmuir probe. The electronegativity was measured with a laser-induced photodetachment (LIPD) technique in combination with a Langmuir probe. The probe measurement results show a transition of the EEPF from bi-Maxwellian to single-Maxwellian and finally to a Druyvesteyn distribution as RF input power or discharge pressure was increased. This transition indicates the evolution of the heating mode in the Ar plasma by changing the discharge conditions. Adding electronegative O2 gas into Ar plasma leads to the deviation of the EEPF from the pure Ar plasma case. This deviation becomes more serious at high pressure due to the inelastic collisions of electrons with oxygen molecules. Additionally, the addition of O2 not only lowers the electron density in the axial direction but also smoothens the electron density distribution close to the powered electrode in comparison to the linear electron density with the axis in the Ar plasma case. LIPD measurement results show that electronegativity in 5% O2-containing Ar plasma tends to be high as close to the powered electrode and to be a V-shaped distribution along the axis direction with the increase in the pressure. This behavior of the negative ion distribution may be caused by the combined effects of recombination of negative and positive ions and the pseudo-γ mode of negative ions with oxygen neutrals.

Characterization by laser-induced photodetachment of anions formed during dust particle growth in a magnetically confined very low-pressure argon–acetylene plasma

Anions in low-pressure magnetically confined Ar–C2H2 plasmas were investigated using the laser-induced photodetachment technique. In particular, the influence of the magnetic field (B) was studied. We observed that the density of negatively-charged species n– increases with B and slightly decreases when the C2H2 percentage grows. These phenomena are the result of changes in the plasma kinetics. We also observed that n– decreases as the pressure increases from 2 to 10 mTorr due to the corresponding rise of the collision frequency, which leads to enhanced losses of the negatively-charged species. In parallel, the photodetachment cross-section was deduced from the energy dependence of the photodetachment signal and was found to be close to that of C2H− anions, which indicates that these anions are likely to dominate the negative ion population.

Interference effect in the photodetachment from H- ion in a few-cycle laser pulse

We theoretically study the electron detachment of negative hydrogen ions in a three-cycle linearly polarized laser field with a wavelength of 2150 nm in the context of the strong field approximation (SFA). The numerical integration and the saddle-point (SP) methods are both used in our calculations. The results show that both the energy spectra and the momentum spectra of the photoelectrons detached from negative hydrogen ions, obtained from these two methods, accord very well with each other for the laser intensities of 1.31011 W/cm2 and 6.51011 W/cm2, respectively. It is found that there is an obvious stripe-like structure along the vertical direction of the momentum spectra when the laser intensity is 6.51011 W/cm2. To explore the main origin which leads to the specific structures of the momentum spectra, we divide the interferences of the electronic wave packets emitted at different times during the laser pulse into the intra-cycle interference and the inter-cycle interference based on the SP method. Inter-cycle interference arises from the coherent superposition of electron wave packets released at complex times during different optical cycles, whereas intra-cycle interference comes from the coherent superposition of electron packets released in the same optical cycle. It is found that when only considering the inter-cycle interference, the main structures of the momentum spectra accord well with the above-threshold detachment (ATD) rings, which indicates that the inter-cycle interference corresponds to ATD rings of the photoelectron spectrum. But when only considering the intra-cycle interference, there are stripe-like structures with left-right asymmetry along the vertical direction of the momentum spectra. So the main structures of the momentum spectra of the photoelectrons are attributed to the interplay of the intra-and inter-cycle interferences. In addition, to intuitively explain the reason why the momentum spectra depend on the intensity of the laser field, we analyze the influence of the intensity of the laser field on the inter-cycle interference of quantum wave packets. It is found that the phase difference of the inter-cycle interference depends on the intensity of the laser field, which may lead to the difference among the momentum spectra of the photoelectrons at different laser intensities. Moreover, the influences of the intra-and inter-cycle interferences on the energy spectrum of the photoelectrons are also analyzed. It is found that the main oscillatory patterns and the peak positions of the energy spectra are mainly determined by the inter-cycle interference. Finally, the effects of the duration of laser pulse on the intra-and inter-cycle interferences are discussed. It seems that the main structures of the momentum spectra accord well with the ATD rings in multi-cycle laser pulses. So it is concluded that in multi-cycle laser pulses, the inter-cycle interference dominates while the intra-cycle interference is suppressed. The work in this paper is meaningful for further understanding the quantum interference effect and the optical control of the laser-induced photodetachment of negative ions.

Laser-Assisted Hα Spectroscopy for Measurement of Negative Ion Density in a Hydrogen Plasma

A novel laser-assisted Hα spectroscopy is proposed to measure negative ion density in a hydrogen plasma. The laser-induced photodetachment of negative ions leads to a decrease in Hα intensity due to blocking of the mutual neutralization channel associated with generation of H (n=3) atoms. The relationship between the reduced Hα intensity and the negative ion density is investigated experimentally and analytically. It is observed that the reduced Hα intensity follows the trend in the negative ion density as a function of pressure, indicating that this spectroscopy holds promise for determining the negative ion density. In addition, a departure from linearity between the reduced Hα intensity and the negative ion density is also analyzed because it can affect the quantitative determination of the negative ion density in the laser-assisted Hα spectroscopy. The departure is found to be attributed to the change in the mutual neutralization reaction rates depending on plasma conditions.

Anion dynamics in the first 10 milliseconds of an argon–acetylene radio-frequency plasma

The time evolution of the smallest anions (C2H− and H2CC−), just after plasma ignition, is studied by means of microwave cavity resonance spectroscopy (MCRS) in concert with laser-induced photodetachment under varying gas pressure and temperature in an argon–acetylene radio-frequency (13.56 MHz) plasma. These anions act as an initiator for spontaneous dust particle formation in these plasmas. With an intense 355 nm Nd : YAG laser pulse directed through the discharge, electrons are detached only from these anions present in the laser path. This results in a sudden increase in the electron density in the plasma, which can accurately and with sub-microsecond time resolution be measured with MCRS. By adjusting the time after plasma ignition at which the laser is fired through the discharge, the time evolution of the anion density can be studied. We have operated in the linear regime: the photodetachment signal is proportional to the laser intensity. This allowed us to study the trends of the photodetachment signal as a function of the operational parameters of the plasma. The density of the smallest anions steadily increases in the first few milliseconds after plasma ignition, after which it reaches a steady state. While keeping the gas density constant, increasing the gas temperature in the range 30–120 °C limits the number of smallest anions and saturates at a temperature of about 90 °C. A reaction pathway is proposed to explain the observed trends.

Negative Ions in Helicon Oxygen Discharges

AbstractThe negative oxygen ion density is studied experimentally in the high‐density helicon device VINETA by laserinduced photodetachment. The measurements indicate a negative ion density being higher than the electron density in contrast to previous experiments in helicon oxygen discharges. The dependency of the relative negative ion density on the oxygen gas pressure and rf‐power is in good qualitative agreement with observations in low temperature RF inductive and capacitive discharges, but differ quantitatively by one order of magnitude. To understand the basic physics in the helicon discharges, the measurements are compared to a zero dimensional reaction rate model of the electronegative oxygen discharge. The particle densities are calculated via rate equations estimated fromcollisional cross sections assumingMaxwellian electron energy distribution functions. The model shows that the electron temperature is the most sensitive parameter governing the formation of negative oxygen ions. Additionally, the differences to previous experimental measurements in rf discharges can be explained by different electron temperature and plasma losses to the walls (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental investigation of double layers in expanding plasmas

Double layers (DLs) have been observed in a plasma reactor composed of a source chamber attached to a larger expanding chamber. Positive ion beams generated across the DL were characterized in the low plasma potential region using retarding field energy analyzers. In electropositive gases, DLs were formed at very low pressures (between 0.1 and 1mTorr) with the plasma expansion forced by a strongly diverging magnetic field. The DL remains static, robust to changes in boundary conditions, and its position is related to the magnetic field lines. The voltage drop across the DL increases with decreasing pressure; i.e., with increasing electron temperature (around 20V at 0.17mTorr). DLs were also observed in electronegative gases without a magnetic field over a greater range of pressure (0.5 to 10mTorr). The actual profile of the electronegative DL is very sensitive to external parameters and intrusive elements, and they propagate at high negative ion fraction. Electrostatic probes measurements and laser-induced photodetachment show discontinuities in all plasma parameters (electron density, electron temperature, negative ion fraction) at the DL position. The voltage drop across the electronegative DL is about 8V, is independent of the gas pressure and therefore of the electron temperature.

Population dynamics of molecular hydrogen and formation of negative hydrogen ions in a magnetically confined low temperature plasma

An attempt is made to unravel the influence of vibrational population distributions of molecular hydrogen in the electronic ground state on the formation of negative hydrogen ions in the volume of a magnetically confined low temperature plasma. The densities of rovibronically excited molecules are determined by laser-induced fluorescence spectroscopy and optical emission spectroscopy. Energy distributions of electrons are measured with a Langmuir probe and densities of negative ions are found by laser-induced photodetachment. A global model is presented that describes the stationary state of the molecular plasma. An overall view of the discharge is obtained on the basis of this model. This allows us to cross-check the obtained results for coherence. It also allows us to assess the relevance of specific processes and their dependences on the rovibronic molecular states and the discharge current. The influence of associative recombination of a hydrogen atom of the plasma bulk with a hydrogen atom adsorbed at the wall is discussed with respect to the influence of this process on the densities of other plasma particles, in particular the density of the negative ions.

Evolution of charged particle densities after laser-induced photodetachment in a strongly electronegative RF discharge

The temporal and spatial evolutions of charged particle densities after laser induced photodetachment in a SiH/sub 4//H/sub 2/ RF discharge have been modeled by a particle-in-cell/Monte Carlo (PIC/MC) method. Two dips in the evolution of the electron density and the positive ion density, which are observed by experiments in the literature, are explained. The images indicate that the two dips occur at different moments in time.

Local and fast relaxation phenomena after laser-induced photodetachment in a strongly electronegative rf discharge.

A one-dimensional self-consistent particle in cell/Monte Carlo method is used to study the local (at the laser impact region) and fast relaxation phenomena after laser-induced photodetachment in a strongly electronegative SiH(4)/H(2) rf discharge. The relaxation process of the local densities of the charged plasma species has been studied in association with the time evolution of the local electric field. The phenomena predicted theoretically about the relaxation processes, such as the potential well, the electrostatic oscillation, the long lasting potential structure, the distortion of the early potential perturbation on the measurement of negative ion temperature, and the depression in the positive ion density profile at the edges of the laser impact region, have been confirmed by our simulation results. Compared to the relaxation in weakly electronegative discharges, the local and even the global electric field in strongly electronegative discharges, has been weakened strongly after photodetachment. The relaxation of the local electric field lasts 100 rf cycles with the recovery of the local electron density and the local electron energy. The electrostatic oscillation exhibited as the deviation in quasineutrality, is very strong and continues over several rf cycles in our case. The large dip in the center of the positive ion density profile, observed in the experiment, is also reproduced by our model.

Laser-induced photodetachment in high-density low-pressure SF6 magnetoplasmas

Using laser-induced photodetachment (LIPD), we investigate in some detail how different discharge parameters affect the negative ion fraction in high-density low-pressure SF6 magnetoplasmas sustained by the propagation of electromagnetic surface waves. A plane electrostatic probe is used for collection of the photodetached electrons. Careful testing of the LIPD technique itself is carried out prior to systematic measurements and adequate laser fluence conditions are determined. Negative ions are found to outnumber electrons several times, even at mTorr and submTorr pressures, indicating the important electronegative character of the discharge. The dependence of the negative ion fraction on gas pressure, argon admixture, microwave power, and axial and radial position in the reactor is interpreted on the basis of different negative ion formation and loss mechanisms. The negative ion fraction is found to be maximum in conditions and regions of minimal electron temperature and positive ion density.