Photodetachment Signal Research Articles

Photoelectron Imaging Signature for Selective Formation of Icosahedral Anionic Silver Cages Encapsulating Group 5 Elements: M@Ag12- (M = V, Nb, and Ta).

An assembly of 13 atoms can form highly symmetric architectures like those belonging to D3h, Oh, D5h, and Ih point groups. Here, using photoelectron imaging spectroscopy in combination with density functional theory (DFT) calculations, we present a simple yet convincing experimental signature for the selective formation of icosahedral cages of anionic silver clusters encapsulating a dopant atom of group 5 elements: M@Ag12- (M = V, Nb, and Ta). Their photoelectron images obtained at 4 eV closely resemble one another: only a single ring is observed, which is assignable to photodetachment signals from a 5-fold degenerate superatomic 1D electronic shell in the 1S21P61D10 configuration of valence electrons. The perfect degeneracy represents an unambiguous fingerprint of an icosahedral symmetry, which would otherwise be lifted in all of the other structural isomers. DFT calculations confirm that Ih forms are the most stable and that D5h, Oh, and D3h structures are not found even in metastable states.

Advancement of Langmuir probe-based laser photo-detachment technique for negative ion density measurement in a high-power helicon plasma source.

In the pursuit of precise diagnostics for measuring negative ion density in a helicon plasma source (HPS), a new approach utilizing a radio frequency (RF) broadband transformer-based Langmuir probe is developed specifically for laser photo-detachment (LPD) analysis. This inductively coupled LPD technique is useful for high power RF systems in which capacitive RF noise is in the same scale as the pulsed photo-detachment signal. The signal acquired by this transformer-based probe is compared against the conventional Langmuir probe-based LPD technique, revealing a remarkable enhancement in signal fidelity through an improved signal-to-noise ratio (SNR) achieved by the RF broadband transformer methodology. In addition, the localized hydrogen negative ion density measurements obtained through this probe are harmoniously aligned with the line-averaged negative ion density derived from the cavity ringdown spectroscopy (CRDS) technique. These concurrence measurements highlight the RF broadband transformer-based approach's accuracy in capturing localized negative ion density during helicon mode operation in an HPS setup. Furthermore, the correlation of negative ion density values with RF input exhibits a consistent trend in tandem with background plasma density. Notably, both CRDS and LPD measurements ascertain negative ion densities ranging from ∼5 to 6×1016m-3 under an RF power of 500-700W and a pressure of 8 × 10-3mbar, all under the influence of a 55G axial magnetic field. These specific parameters represent the optimal operational configuration for effective negative ion production with the present experimental HPS setup. Due to its better SNR, the RF broadband transformer-based Langmuir probe emerges as a useful tool for LPD diagnostics, particularly in the presence of pervasive RF noise.

In-situ measurement of dust charge density in nanodusty plasma

A dust grain immersed in a low-pressure gas discharge obtains a permanent negative surface charge due to the high mobility of electrons compared to that of ions. This charge essentially governs all fundamental processes in dusty and complex plasmas involving dust grains, neutrals, (an)ions and electrons and—consequently—virtually all industrial applications of these types of plasmas are affected and steered by it. In this work, we have measured the surface charge by application of laser-induced electron detachment from nanosized dust grains in concert with microwave cavity resonance spectroscopy and laser light extinction. The main result is that the electron release is governed by photodetachment rather than by thermionic emission, and that recharging of the dust grains occurs on timescales that are well in agreement with the orbital-motion-limited (OML) theory. The total surface charge density residing on the dust grains inside the laser volume follows from the saturation of the photodetachment signal, which was used in combination with dust density values derived from extinction measurements to estimate the mean dust charge. The negative dust charge on the 140 nm (average) diameter dust grains in this work is obtained to be in the range of 273 – 2519 elementary charges, of which the lower bound matches well with analytical predictions using the OML theory.

Laser photo-detachment combined with Langmuir probe in magnetized electronegative plasma: how the probe size affects the plasma dynamic?

Laser photo-detachment combined with a Langmuir probe (LP) is used to diagnose negative ion properties in electronegative plasmas. The technique relies on the combined use of a laser pulse and an LP. The laser pulse converts negative ions into electron–atom pairs, while the LP tracks the temporal evolution of electron current (laser photo-detachment signal) that is analyzed to retrieve the negative ion density. Although an external magnetic field is frequently used to enhance the negative ion production and extraction, the data analysis often neglects the effects of the magnetic field on the probe current. This work investigates the response of an electronegative plasma to a laser pulse in the presence of an external magnetic field through a two-dimensional particle-in-cell/Monte Carlo collision model. The results show that a low electron density region surrounding the probe, called a flux-tube, can form for a probe size comparable with or larger than the electron Larmor radius. The formation of the flux-tube strongly affects the components of the laser photo-detachment signal, leading to an important oscillation of probe current during the plateau phase, i.e. the amplitude of the AC component of the probe current is in the same magnitude order of the DC component of this current, and an important overshoot in comparison to the current rise. Numerical results are qualitatively compared to measurements obtained from the RAID negative ion source.

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.

High-Resolution Photoelectron Spectroscopy of Vibrationally Excited OH.

The effect of vibrational pre-excitation of anions on their photoelectron spectra is explored, combining slow photoelectron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) with tunable IR radiation to pre-excite the anions. This new IR cryo-SEVI method is applied to OH- as a test system, where the R(0) transition of the hydroxyl anion (3591.53 cm-1) is pumped. Vibrational excitation induces a 30% depletion in photodetachment signal from the v = 0, J = 0 ground state of the anion and the appearance of all five allowed, rotationally resolved photodetachment transitions from the OH- (v = 1, J = 1) level, each with peak widths between 1 and 2 cm-1. By scanning the IR laser, IR cryo-SEVI can also serve as a novel action technique to obtain the vibrational spectrum of OH-, giving an experimental value for the R(0) transition of 3591(1.2) cm-1.

Ultraslow radiative cooling of Cn - (n = 3-5).

Ultraslow radiative cooling lifetimes and adiabatic detachment energies for three astrochemically relevant anions, Cn - (n = 3-5), are measured using the Double ElectroStatic Ion Ring ExpEriment (DESIREE) infrastructure at Stockholm University. DESIREE maintains a background pressure of ≈10-14 mbar and temperature of ≈13 K, allowing storage of mass-selected ions for hours and providing conditions coined a "molecular cloud in a box." Here, we construct two-dimensional (2D) photodetachment spectra for the target anions by recording photodetachment signal as a function of irradiation wavelength and ion storage time (seconds to minute time scale). Ion cooling lifetimes, which are associated with infrared radiative emission, are extracted from the 2D photodetachment spectrum for each ion by tracking the disappearance of vibrational hot-band signal with ion storage time, giving 1e cooling lifetimes of 3.1 ± 0.1 s (C3 -), 6.8 ± 0.5 s (C4 -), and 24 ± 5 s (C5 -). Fits of the photodetachment spectra for cold ions, i.e., those stored for at least 30 s, provide adiabatic detachment energies in good agreement with values from laser photoelectron spectroscopy on jet-cooled anions, confirming that radiative cooling has occurred in DESIREE. Ion cooling lifetimes are simulated using a simple harmonic cascade model, finding good agreement with experiment and providing a mode-by-mode understanding of the radiative cooling properties. The 2D photodetachment strategy and radiative cooling modeling developed in this study could be applied to investigate the ultraslow cooling dynamics of a wide range of molecular anions.

Eclipse laser photo-detachment: does the blocking wire distort the collected laser photo-detachment signal?

The eclipse laser photo-detachment (LPD) method has been introduced to avoid Langmuir probe (LP) surface ablation. This method uses, besides a laser pulse and a LP, a blocking wire (BW) to create a shadow around the LP. The BW masks the probe surface from laser photons avoiding, therefore, probe ablation. In return, the BW prevents photo-detachment within the shadowed area breaking the axial symmetry of the plasma surrounding the probe. This work studies the response of an electronegative plasma to laser pulse, in eclipse LPD experiment, via a two dimensional Particle-In-Cell/Monte Carlo Collision model. The plasma dynamics and LPD signals are simulated in the presence and the absence of BW. Our results show that potential patterns form around the LP after negative ions photo-detachment. In the absence of BW, the formed potential patterns accelerate electrons towards the LP; while in the presence of the BW, the potential pattern formed within the shadowed area obstructs the electron transport in the latter direction. Thus, the LPD signal is significantly affected by the presence of the BW.

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.

Photo-detachment signal analysis to accurately determine electronegativity, electron temperature, and charged species density

Laser pulse induced photo-detachment combined with Langmuir probing has been introduced to diagnose plasma electronegativity. This technique uses a laser pulse to convert negative ions into electron-atom pairs and tracks the change of electron saturation current by a Langmuir probe. The existing model determines plasma electronegativity as the ratio of electron saturation current before and after detachment. However, this model depends on various assumptions and neglects the formation of a potential barrier between the laser channel and surrounding electronegative plasma. In this letter, we present a new analytical model to analyze photo-detachment signals in order to improve the accuracy of electronegativity measurements and extend this technique for measuring electron temperature and charged species density. This analytical model is supported by Particle-In-Cell simulation of electronegative plasma dynamics following laser photo-detachment. The analysis of the signal, detected on a simulated probe, shows that the present analytical model determines electronegativity, electron temperature, and plasma density with a relative error of ∼20%, ∼20%, and ∼50%, respectively, whereas the electronegativity obtained from a previous model is underestimated by an order of magnitude.

Negative ion production and beam extraction processes in a large ion source (invited).

Recent research results on negative-ion-rich plasmas in a large negative ion source have been reviewed. Spatial density and flow distributions of negative hydrogen ions (H(-)) and positive hydrogen ions together with those of electrons are investigated with a 4-pin probe and a photodetachment (PD) signal of a Langmuir probe. The PD signal is converted to local H(-) density from signal calibration to a scanning cavity ring down PD measurement. Introduction of Cs changes the slope of plasma potential local distribution depending upon the plasma grid bias. A higher electron density H2 plasma locally shields the bias potential and behaves like a metallic free electron gas. On the other hand, the bias and extraction electric fields penetrate in a Cs-seeded electronegative plasma even when the electron density is similar. Electrons are transported by the penetrated electric fields from the driver region along and across the filter and electron deflection magnetic fields. Plasma ions exhibited a completely different response against the penetration of electric fields.

Charged particle flows in the beam extraction region of a negative ion source for NBI.

Experiments by a four-pin probe and photodetachment technique were carried out to investigate the charged particle flows in the beam extraction region of a negative hydrogen ion source for neutral beam injector. Electron and positive ion flows were obtained from the polar distribution of the probe saturation current. Negative hydrogen ion flow velocity and temperature were obtained by comparing the recovery times of the photodetachment signals at opposite probe tips. Electron and positive ions flows are dominated by crossed field drift and ambipolar diffusion. Negative hydrogen ion temperature is evaluated to be 0.12 eV.

Particle-In-Cell simulation of laser photodetachment in capacitively coupled radio frequency oxygen discharges

Particle-In-Cell simulations with Monte Carlo collision of capacitively coupled radio frequency oxygen discharges are used to study the appearance and characteristics of two experimentally observed electronegative modes, the high electronegative mode for low peak-to-peak voltage, and the low electronegative mode for high peak-to-peak voltage. For the high electronegative mode, the simulated laser photodetachment signal agrees very well with the experiment. The simulation identifies the dominant transport processes for high electronegativities: electrons flow fast out of the perturbed region, where the laser pulse generates laser detachment of negative ions. Negative ions are not streaming inward, but are produced within this region by dissociative attachment after the laser pulse.

Optimum circuit design for the detection of laser photodetachment signals

Laser photodetachment is a widely used diagnostic technique for the detection of negative ions in low-temperature plasma sources. In one common method electrons, photodetached from the negative ions by an incident laser beam, are collected by a Langmuir probe biased above the local plasma potential. The photodetachment current signal, transmitted through the Langmuir probe feedthrough and interconnecting cables (transmission circuit), is converted into a voltage signal by a detection circuit. The negative ion density and temperature (using the two-pulse technique) are obtained from signal processing. Circuit analysis of the signal acquisition circuit, both transmission and detection parts, is used to obtain the design criteria for the undistorted transmission of the photodetachment signal. The effects of changing the circuit parameters from their optimum values are investigated experimentally in a conventional low-pressure magnetron system. The results show that an inappropriate choice of the circuit parameters can lead to significant distortion of the temporal photodetachment signal, leading to large measurement errors of the negative ion density and temperature.

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.

Wave frequency dependence of H− ion production and extraction in a transformer coupled plasma H− ion source at SNU

The effect of rf wave frequencies on the production of H(-) ion is investigated in a transformer coupled plasma H(-) ion source at Seoul National University. A Langmuir probe is installed to measure the plasma density and temperature, and these plasma parameters are correlated to the extracted H(-) beam currents at various frequencies. The Langmuir probe is also used to measure the density of H(-) ions at the ion source by generating photodetachment with an Nd:YAG laser. The extracted H(-) currents decrease to a minimum value until 13 MHz and then, increase as the driving frequency increases from 13 MHz while the relative H(-) population measured by photodetachment monotonically decreases as the driving rf frequency increases from 11 MHz to 15 MHz. A potential well formed at the extraction region at high frequencies of more than 13 MHz is considered responsible for the increased H(-) beam extraction even with a lower photodetachment signal. The variation in the driving rf frequency not only affects the density and temperature of the plasma but also modifies the plasma potential with the existence of a filtering magnetic field and consequently, influences the extracted H(-) current through the extraction as well as formation of H(-) ions.

Using the resonance hairpin probe and pulsed photodetachment technique as a diagnostic for negative ions in oxygen plasma

In this work the resonance hairpin probe technique has been used for detection of photoelectrons generated during photodetachment experiments performed to determine negative ion density in an inductively coupled oxygen plasma. An investigation of the temporal development of the photoelectron population was recorded with the hairpin probe located inside the laser beam region and at various points outside the beam. Varying the external microwave frequency used to drive the probe resonator allowed the local increase in electron density resulting from photoelectrons to be determined. At a fixed probe frequency, we observed two resonance peaks in the photodetachment signal as the photoelectron density evolved as a function of time. Inside the laser beam the resonance peaks were asymmetric, the first peak rising sharply as compared with the second peak. Outside the laser beam region the peaks were symmetric. As the external frequency was tuned the resonance peaks merge at the maximum electron density. The resonance peak corresponding to maximum density outside the beam occurs at a delay of typically 1–2 µs as compared with the centre of the beam allowing an estimate of the negative ion velocity. Using this method, negative ion densities were measured under a range of operating conditions inside and outside the beam.

The electron affinity of tungsten

The electron affinity of tungsten has been measured using laser photodetachment threshold spectroscopy in a collinear geometry. The electron affinity was determined to 6583.6(6) cm-1 by observing the onset of the process when W- ions in the $5d^56s^2$ 6S5/2 ground state are photodetached producing neutral W atoms in the $5d^46s^2$ 5D0 ground state. The measured value is in agreement with previous measurements and improves the accuracy by almost two orders of magnitude. Further, a photodetachment signal below the ground state photodetachment threshold was found, which indicates the existence of a bound excited state in W-.

Study on transport of negative ion plasma using dc laser photodetachment method

Transport of negative ion containing plasma was studied in a hydrogen plasma by injecting a semiconductor laser to make an electron density perturbation by photodetachment of negative ions. Change due to laser irradiation on electron saturation current to a Langmuir probe and that on electron current extracted through an orifice biased at the anode potential were measured phase sensitively by a lock-in-amplifier. The measured transport velocity of the negative ion containing plasma was about 1.4x10(5) cm/s for both cases. The photodetachment signal measured through the orifice had given a larger ratio of negative ion density to electron density. Change in transport of negative ion containing plasma around the Langmuir probe has been also investigated by detecting photodetachment signal onto electron current extracted through an orifice.

Photodetachment diagnostics of a volume production type negative ion source with a diode-laser

Time evolution of photodetachment perturbation signal induced by a diode laser was observed in an O(2) plasma. Photodetachment current collected by a Langmuir probe was directly measured and recorded by a digital oscilloscope. After integrating the recorded signal data, the waveform of the photodetachment current showed a time dependence resembling an error function. The waveform had changed its shape in accordance as the position between the probe and the laser beam axis was changed. These characteristics of the photodetachment signal are well explained by a diffusion model. The method has the possibility to yield information on various negative ion containing plasmas, but requires quiescence in the electron saturation current with the fluctuation level less than 10(-4).