This work was supported by the U.S. Army Research Office under Award Numbers W911NF-14-1-0241 and W911NF-17-1-0119. DMB is supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award Number DE-FC02- 04ER1553.

1. Constructing the Experimental Setup
Note: To run this experiment, assemble a system consisting of a plasma reactor where the reaction will take place, optical components for absorbance measurements, and the electronic lock-in amplification system to process the signal.
<ol>
	<li>Construct the plasma electrochemical cell.
	<ol>
		<li>Manufacture a reactor cell consisting of a transparent glass vessel, 50.8 mm (2 in) in diameter, with two optical windows at angles of approximately 20&#176; down from the normal plane.</li>
		<li>Construct a non-permeable lid containing four orifices, which will be used to introduce the plasma electrode, a platinum anode, and a small hose for flushing the head space with argon (Ar). 
		Note: The fourth orifice will allow Ar to vent from the head space.</li>
		<li>Form the anode by attaching a piece of platinum foil to a stainless-steel rod.</li>
		<li>Form the cathode by sharpening the end of a 1.58 mm (1/16 in) outer diameter, 0.178 mm (0.007 in) inner diameter stainless steel capillary. Connect an Ar hose to the blunt end of the capillary using appropriate fittings.</li>
	</ol>
	</li>
	<li>Construct the optical measurement apparatus.
	<ol>
		<li>Construct a laser source consisting of a 670 nm diode laser, an adjustable iris, and a 50 mm lens all mounted in a 30 mm optical cage system. Mount the cage to a goniometer, so that the entire system may be rotated at an angle of 19&#176; with the laser directed into one of the optical windows of the electrochemical cell. 
		Note: Other laser wavelengths may also be used, see Ref 16.</li>
		<li>Construct a photodetector consisting of a large area photodiode wired into a reverse-bias leakage circuit. Mount the photodetector to a goniometer so that it may receive reflected light through the optical window of the electrochemical cell opposite to the laser source. Additionally, mount to the detector a bandpass filter corresponding to the laser wavelength. 
		Note: A diagram of the photodetector circuit can be seen in Figure 1. For such a circuit, the output voltage is directly proportional to the laser intensity.</li>
	</ol>
	</li>
	<li>Construct the plasma switching circuit and lock-in amplifier circuit. 
	Note: The electronic components include a high voltage power supply, a high voltage switching circuit, a function generator, a voltmeter, and a lock-in amplifier.
	<ol>
		<li>Connect the high voltage power supply so that it may apply a direct current (DC) bias of approximately -2.5 kV between the plasma electrode and the anode to generate the plasma.</li>
		<li>Use a custom-built switching circuit to modulate the plasma current between high and low values at a carrier frequency of 20 kHz. 
		Note: A schematic of the circuit is shown in Figure 2. The high-voltage power supply drives the current through two ballast resistors connected in parallel. While a small current continuously flows through the 3 M&#937; resistor, the current through the 220 k&#937; is continuously switched by an insulated gate bipolar junction transistor (IGBT). The function generator produces the 20 kHz driving frequency, and is connected to the IGBT through an optical isolator, which isolates the high-voltage power supply from the function generator.</li>
		<li>Integrate the lock-in amplifier to allow the output of the photodetector to be connected to it. 
		Note: The lock-in amplifier will filter noise outside of the 20 kHz band. Its outputs, namely the amplitude and phase of the absorbance signal, should be recorded by a computer. An in-house program was used to record the output.</li>
	</ol>
	</li>
</ol>
2. Prepare NaClO4 Solution as a Conductive Background Electrolyte
<ol>
	<li>To prepare a 0.163 M NaClO4 solution, which is the concentration used in this experiment, dissolve 10 g of NaClO4 in 500 mL of deionized water. Note: NaClO4 was chosen as an electrolyte because it does not react with solvated electrons. The concentration of 0.163 M is only representative, and different concentrations can be used, typically on the order of 0.001-0.1 M.</li>
	<li>Pour 60 mL of NaClO4 into the plasma reactor.</li>
</ol>
3. Prepare Setup for Measurements
<ol>
	<li>Prepare the electrochemical cell set-up and purge the reactor.
	<ol>
		<li>Insert the anode through the appropriate orifice into the non-permeable lid. Partially submerge the anode beneath the surface of the solution.</li>
		<li>Connect the plasma electrode (cathode capillary) to a mass flow meter connected to an Ar tank and insert the capillary through the lid. Suspend the tip of the capillary approximately 1-2 mm above the surface of a solution. Use a camera to measure the distance between the capillary and the liquid surface.</li>
		<li>Connect the small hose to a mass flow meter connected to an Ar tank, and insert the hose through one of the orifices in the lid; this forms the purge line.</li>
		<li>With the cathode, anode, and Ar purge line in place, secure the non-permeable lid to the top of the reactor cell.</li>
		<li>Turn on the Ar flow through the purge line to approximately 250 cm3/min for at least 5 minutes to flush out air from the headspace of the plasma reactor. 
		Note: The flow rate and the duration of applied flow depend on the volume of the reactor and the volume of solution in the reactor; the values used here are representative. Failure to flush out air will lead to side reactions driven by dissolved oxygen in the solution (see Ref. 21).</li>
	</ol>
	</li>
	<li>Align the measurement.
	<ol>
		<li>Set the flow of Ar through the plasma electrode to roughly 10 cm3/min. Align the reactor so that the laser hits the plasma-liquid interface. Do this by observing light being scattered off the dimple by the flow of Ar gas.</li>
		<li>Close the flow of Ar to the plasma electrode and wait for the laser spot to return to its normal size. Once this happens, align the photodetector so that the laser hits the center of the detector.</li>
	</ol>
	</li>
	<li>Measure baseline signal intensity and prepare electrical system.
	<ol>
		<li>Connect the photodetector output to a voltmeter and measure the voltage given by the laser. Record this value, as it will be used later to normalize the measured absorbance signal. 
		Note: This value is directly proportional to the incident intensity I0.</li>
		<li>After measuring the incident intensity, disconnect the cable from the photodetector and connect it to the input of the lock-in amplifier to lock with the signal at the 20 kHz carrier frequency.</li>
		<li>Connect the transistor-transistor logic output of the function generator to the frequency input of the lock-in amplifier.</li>
		<li>Make sure the lock-in amplifier, function generator, and high-voltage power supply are turned on. 
		Note: The experiment is now ready to be initiated.</li>
	</ol>
	</li>
</ol>
4. Start Experiment and Data Collection
Note: An in-house program is used for data collection. Additionally, this system is automated to ensure precision and reduce human error. The underlying process of this automation is described in the following steps.
<ol>
	<li>Turn on the laser and begin measuring absorbance. Allow time for the noise to be integrated down to a sufficiently low level.</li>
	<li>Set the high-voltage power supply to a voltage difference of approximately -2.5 kV to ignite the plasma. 
	Note: A photograph of the plasma discharge at a distance of 1 mm from the liquid surface is shown in Figure 3.</li>
	<li>Wait approximately half a minute for the amplitude measured by the lock-in amplifier to reach a steady-state and, thereafter, record the signal for approximately 2 min.</li>
	<li>Turn off the laser and wait for the laser signal to reach a steady-state. Subsequently, measure the absorbance for half a minute. 
	Note: This measurement will be subtracted from the previous absorbance measurements to account for the noise from the plasma.</li>
	<li>Turn off the plasma by switching off the high-voltage power supply.</li>
	<li>To repeat an experiment, make sure the plasma electrode is still aligned. To do this, retract the plasma electrode at least 1 cm and then open the flow of Ar through the capillary. Repeat steps 3.2 to 4.5.</li>
	<li>If no more experiments will be repeated, turn off all electronic instruments, take off the lid from the electrochemical cell, and dispose of the NaClO4 appropriately.</li>
</ol>
5. Data Analysis
Note: Output from the lock-in amplifier contains information about the amplitude R and phase &#981; of the 20 kHz absorbance signal. This can be represented by cosine and sine components, X and Y, respectively. Because the lock-in amplifier measures the modulated amplitude of the signal between the high and low currents set at step 1.3.2, the X and Y represent the differences between these two signals, and are used to measure the difference of absorbance between the low and high states, &#916;I.
<ol>
	<li>To properly analyze the data, normalize the X and Y time vectors by dividing them by the incident intensity voltage I0. 
	Note: This voltage, which was measured in Step 3.3.1, is directly proportional to the optical intensity, as are the signal components X and Y. Therefore, dividing X and Y by this value should yield dimensionless vectors representing the in-phase and out-of-phase components of the 20 kHz absorbance signal.</li>
	<li>Obtain the average of X/I0 and Y/I0 from the moment steady-state was reached after the plasma was turned on until the laser was turned off. 
	Note: Figure 4 shows the normalized absorbance RMS magnitude R measured throughout the experiment. The plasma was switched on after 30 s, the absorbance signal reached steady-state at a time of 50 s, and the plasma was switched off at a time of 150 s.</li>
	<li>Likewise, obtain the average of X/I0 and Y/I0 from the moment the laser was turned off until the plasma was turned off to obtain the cosine and sine components of the plasma noise. Note: In Figure 4, the plasma was turned off at a time of 150 s, and the noise measurement was the average absorbance detected from a time of 170 s to 200 s.</li>
	<li>To obtain the cosine and sine components of the absorbance signal, subtract the averages obtained in step 5.3 from the averages obtained in 5.2.</li>
	<li>To calculate the true absorbance, calculate the square root of the sum of the squares of the X and Y components of the signal obtained in step 5.4 as it is shown in Equation 1. 
	<img alt="Equation 1" src="/files/ftp_upload/56833/56833eq1v5.jpg" />&#160;(1)</li>
</ol>
6. Extraction of Parameters
<ol>
	<li>Calculate the concentration of solvated electrons in the solution by assuming a steady state between the rate at which solvated electrons are introduced into the solution by the plasma, and the rate at which they are consumed. 
	Note: The consumption of solvated electrons, in the absence of other reactions, occurs through the second order recombination of electrons as shown in Equation 2. 
	<img alt="Equation 2" src="/files/ftp_upload/56833/56833eq2v7.jpg" /> (2)
	<ol>
		<li>Use Beer&#39;s Law, as shown in Equation 3, to find the concentration of the solvated electrons as a function of an unknown penetration depth l, where &#949; is the molar extinction coefficient, and &#952; is the angle of incidence (defined as 19&#176; in step 1.2). 
		Note: The extinction coefficient of the solvated electron is ~ 19,000 L mol-1 cm-1 as described in Ref. 18. 
		<img alt="Equation 3" src="/files/ftp_upload/56833/56833eq3v5.jpg" />&#160;(3)</li>
		<li>To extrapolate the unknown penetration depth, combine Equation 3 with Equation 4, which assumes electrons are introduced at a rate proportional to the current density of the plasma, where k is the reaction constant of the second order recombination, q is the elementary charge, j is the current density, and NA is Avogadro&#39;s number. 
		Note: A factor of 2 is included in Equation 4 to denote that two electrons are consumed by the reaction in Equation 2. 
		<img alt="Equation 4" src="/files/ftp_upload/56833/56833eq4v5.jpg" /> (4) 
		The penetration length l and concentration [(e-)aq] can be determined from the measured signal using Equations 3 and 4.</li>
	</ol>
	</li>
</ol>
7. Reaction Rate Estimation
Note: When electrons solvate in a solution with unreactive electrolytes such as NaClO4, solvated electrons are only consumed by the reaction in Equation 2. However, solvated electrons have the capacity to reduce a wide variety of cations, anions, and neutral species. When any of these electron scavengers are dissolved in the aqueous solution, they react with the solvated electrons. This lowers their equilibrium concentration and leads to a reduction of the absorbance detected, which allows the TIRAS methodology to estimate the reaction rate constants of these reactions. When a new reaction is introduced, the rate balance becomes: 
<img alt="Equation 5" src="/files/ftp_upload/56833/56833eq5v5.jpg" />&#160;(5) 
where [(S)aq] is the concentration of the electron scavenger in the solution, and k2 is the reaction rate constant associated with its reaction. However, if the scavenger concentration is sufficiently large, Equation 5 can be simplified to: 
&#160;<img alt="Equation 6" src="/files/ftp_upload/56833/56833eq6v5.jpg" /> (6) 
Equation 3 can be then combined with Equation 6 to obtain a relationship between absorbance and the scavenger concentration. 
<img alt="Equation 7" src="/files/ftp_upload/56833/56833eq7v5.jpg" />&#160;(7)
<ol>
	<li>To measure the reaction rate constant of solvated electrons with an electron scavenger, start by dissolving the scavenger in the NaClO4 solution, which was prepared in step 1. 
	Note: The non-reactive NaClO4 in the solution ensures a solution conductivity high enough for plasma stability. The concentration of the scavenger should be high enough to compete with the second order recombination, otherwise, the reaction will not take place.</li>
	<li>Repeat steps 2.2 to 6.1.2 with different scavenger concentrations, and measure the difference in absorbance calculated in step 5.5, with respect to the pure NaClO4 solution.</li>
	<li>Make a plot of the absorbance as a function of [(S)aq]-1. 
	Note: If the scavenger concentration is sufficiently large for Equation 6 to be valid, plotting the absorbance as a function of [(S)aq]-1 will yield a straight line with a slope dependent on the rate constant k2.</li>
	<li>Extrapolate the reaction rate constant k2 from the slope of the line. 
	Note: For additional details, as well as examples of how this was applied to the reactions of NO2-(aq), NO3-(aq), and H2O2(aq), CO2(aq) with solvated electrons, see Ref. 16,19.</li>
</ol>

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The authors have nothing to disclose.

The results show that the measurement of absorbance of light at the plasma-liquid interface is an effective method to detect and measure the concentration of plasma-solvated electrons in an aqueous solution. The subsequent measurement at different wavelengths results in the measurement of the absorption spectrum. Though this experiment was done in an aqueous NaClO4 solution, the methodology should be valid for a great variety of other liquids, provided that electrons can solvate in the liquid.
<p class="jo...

Plasma-liquid interactions represent an area of growing interest in the plasma science and engineering community. The complex interface between plasmas and liquids, which contains a variety of highly reactive free radicals, has found applications in many areas including analytical chemistry, plasma medicine, water and wastewater treatment, and nanomaterial synthesis1,2,3,4,5,6. While there are various configurations that can be used to bring a plasma in contact with a liquid7, perhaps the simplest is the plasma analog of an electrolytic cell, where one of the standard metal electrodes is replaced with a plasma or gas discharge8. The plasma electrochemical cell consists of a reactor vessel, a submerged metal electrode, and a plasma discharge, which can function as either the cathode or anode (or both). When the plasma discharge is used as a cathode, gas-phase electrons generated in the plasma are injected into the solution. After the electrons enter the solution, their kinetic energy dissipates on the timescale of femtoseconds9,10,11 primarily through inelastic scattering off the solvent molecules. Once the electrons have reached a near-thermal kinetic energy, they trap and solvate in a cavity formed by surrounding solvent molecules. Depending on the solvent and temperature, these &#34;solvated&#34; electrons may be stable until they react with some reducible species in the solution or with another solvated electron. In aqueous solution, solvated electrons are also referred to as hydrated electrons12.
This process of solvation has long been known, and the detection of hydrated electrons generated by procedures such as pulse radiolysis or flash photolysis has been studied since the 1960s13,14,15. In traditional radiolysis and photolysis, the solvated electrons are produced via ionization of the solvent molecules; however, electrons solvated at the plasma-liquid interface are injected from the gaseous plasma16. Previous experiments have determined that hydrated electrons absorb red light near 700 nm13,14,17, which allows them to be experimentally studied via optical absorption spectroscopy. Other experiments have measured their diffusion constants, their reaction rates with hundreds of chemical species, their radius of gyration, and their charge mobility, among other properties of interest12,18.
Within the literature, several methods to detect solvated electrons have been reported, which can be separated mainly into two types: bulk dosimetry, where solvated electron presence is inferred from the bulk chemical analysis of their reaction products, and direct transient absorption spectroscopy, where the electrons&#39; absorbance is measured as the reaction takes place. The latter category, upon which the methodology presented here is based, has the advantage of direct and instant evidence, as well as the ability to monitor intermediate reactions.
The rationale behind the development of the total internal reflection absorption spectroscopy (TIRAS) methodology was to directly study the role of solvated electrons at the plasma-liquid interface. The reflection geometry was chosen, because the production of solvated electrons using a plasma discharge, as opposed to methods like radiolysis or photolysis, occurs at the interface between the plasma and the liquid. When a probe laser grazes the surface at a shallow angle of incidence, it is totally reflected back into the solution and out into a detector, less the small amount of light absorbed by the electrons. With no light escaping into the plasma, the experimental technique only measures free radicals in the liquid phase, just beneath the interface, and is thus a highly sensitive interfacial measurement technique. Additionally, the total internal reflection phenomenon has the advantage of eliminating noise from the changing of partial reflections due to surface fluctuations, which could otherwise dominate the signal.
The TIRAS protocol outlined in this article has three essential features. The first is a plasma electrochemical cell, which consists of a transparent glass beaker with two optical windows at angles of approximately 20&#176; facing downward and a controlled headspace of argon gas. The second feature is the optical measurement system, which includes a diode laser, an optical cage, and a photodiode detector. The laser provides the light that is absorbed by the solvated electrons, and is mounted in line with an adjustable iris and a 50 mm lens in an optical cage. This arrangement is mounted on a goniometer, which allows it to be rotated to a desired angle of incidence. The intensity of the transmitted light is then measured by the photodetector, which consists of a large area photodiode wired in a reverse-bias leakage circuit. Finally, because of their high reactivity, solvated electrons only penetrate ~10 nm into the solution, which yields an extremely small optical absorption signal of ~10-5 optical density. To ensure a sufficiently high signal-to-noise ratio, the third essential component is a lock-in amplification system, which consists of a plasma switching circuit and a lock-in amplifier. In the switching circuit, a solid-state relay circuit modulates the plasma current between a high and a low value at a carrier frequency of 20 kHz set by a function generator. This, in turn, also modulates the solvated electron concentration at the interface and their optical absorbance. The lock-in amplifier then takes the signal from the photodetector and filters all noise outside the carrier frequency.
The TIRAS method has great potential to reveal important chemical processes in plasma-liquid experiments, particularly in plasma electrochemistry. The reduction and oxidation pathways are primarily driven by a variety of short-lived radicals at the plasma-liquid interface, and the detection of the species is extremely important for understanding the interfacial chemistry. The in situ monitoring capabilities of TIRAS will help establish a greater understanding of the important electron-driven reactions involved at the plasma-liquid interface. TIRAS, for example, makes the measurement of reaction rates possible in the presence of electron scavengers. Previous studies have focused on the reduction of NO2-(aq), NO3-(aq), and H2O2(aq) scavengers dissolved in the aqueous solution16, as well as the reduction of dissolved CO2(aq)19. Other studies have focused on the effect of the plasma carrier gas on plasma-solvated electron chemistry20.

<table><tbody><tr><td>Function Generator</td><td>Protek</td><td>B8055</td><td></td></tr><tr><td>Lock-in Amplifier</td><td>Stanford Research Systems</td><td>SR830</td><td></td></tr><tr><td>High-Voltage Power Supply</td><td>Stanford Research Systems</td><td>PS325</td><td></td></tr><tr><td>Photodetector</td><td>Self-built</td><td></td><td></td></tr><tr><td>Flowmeter</td><td>Key Instruments</td><td>60310 R5</td><td></td></tr><tr><td>Flow controller</td><td>Omega Engineering</td><td>FMA 5400A/5500A</td><td></td></tr><tr><td>Camera</td><td>Dino-lite</td><td>Dinocapture 2.0</td><td></td></tr><tr><td>Voltmeter</td><td>Amprobe</td><td>AM-510</td><td></td></tr><tr><td>Optical Cage System</td><td>Thorlabs</td><td></td><td>30 mm cage system</td></tr><tr><td>Goniometers</td><td>Thorlabs</td><td>RP01 - &amp;Oslash;2</td><td>Manual rotation stage</td></tr><tr><td>Diode lasers</td><td>Thorlabs</td><td></td><td></td></tr><tr><td>Electrochemical cell</td><td>Adams &amp;amp; Chittenden Scientific Glass</td><td>Custom-made product</td><td></td></tr><tr><td>Stainless steel capillary</td><td>Restek</td><td></td><td>0.007 in. ID</td></tr><tr><td>SHV Coax Cable</td><td>SRS</td><td>Custom-made product</td><td></td></tr><tr><td>Sodium Perchlorate</td><td>Sigma-Aldrich</td><td></td><td>ACS reagent, &amp;ge;98.0%</td></tr><tr><td>Argon</td><td>Airgas</td><td>AR UHP300</td><td>Ultra-high purity</td></tr><tr><td>LabVIEW</td><td>National Instruments</td><td></td><td>Software used to generate in-house program used to collect data</td></tr></tbody></table>

total internal reflection absorption spectroscopy (tiras) for the detection of solvated electrons at a plasma-liquid interface

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 &#181;s), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons&#39; absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2-(aq)), nitrate (NO3-(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.

As mentioned in step 5 of the procedure, this experiment measures the cosine and sine components of the absorbance signal, the phase angle between them, and the magnitude of the signal. A plot of the magnitude of the signal and its two components is shown in Figure 4.
Occasionally, there will be measurements which may be not optimal or even unusable. This may be due to a misalignment of the laser wi...

Watch this Scientific Journal Video about Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface at JoVE.com

Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface

This article presents a total internal reflection absorption spectroscopy (TIRAS) method for measuring short-lived free radicals at a plasma-liquid interface. In particular, TIRAS is used to identify solvated electrons based on their optical absorbance of red light near 700 nm.

Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated ...

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Research

JoVE Journal

Chemistry

13.6K Views.  University of Notre Dame. Dit artikel presenteert een totale interne reflectie Absorptie spectroscopie (TIRAS) methode voor het meten van kortstondige vrije radicalen op het raakvlak van een plasma-vloeistof. In het bijzonder, TIRAS wordt gebruikt ter identificatie van de solvated elektronen op basis van hun optische absorptie van rood licht in de buurt van 700 nm.

Video: Total Internal Reflection Absorption Spectroscopy TIRAS for the Detection of Solvated Electrons at a Plasma-liquid Interface

Angle-resolved Photoemission Spectroscopy At Ultra-low Temperatures

The physical properties of a material are defined by its electronic structure. Electrons in solids are characterized by energy (&omega;) and momentum (k) and the probability to find them in a particular state with given &omega; and k is described by the spectral function A(k, &omega;). This function can be directly measured in an experiment based on the well-known photoelectric effect, for the explanation of which Albert Einstein received the Nobel Prize back in 1921. In the photoelectric effect the light shone on a surface ejects electrons from the material. According to Einstein, energy conservation allows one to determine the energy of an electron inside the sample, provided the energy of the light photon and kinetic energy of the outgoing photoelectron are known. Momentum conservation makes it also possible to estimate k relating it to the momentum of the photoelectron by measuring the angle at which the photoelectron left the surface. The modern version of this technique is called Angle-Resolved Photoemission Spectroscopy (ARPES) and exploits both conservation laws in order to determine the electronic structure, i.e. energy and momentum of electrons inside the solid. In order to resolve the details crucial for understanding the topical problems of condensed matter physics, three quantities need to be minimized: uncertainty* in photon energy, uncertainty in kinetic energy of photoelectrons and temperature of the sample.
In our approach we combine three recent achievements in the field of synchrotron radiation, surface science and cryogenics. We use synchrotron radiation with tunable photon energy contributing an uncertainty of the order of 1 meV, an electron energy analyzer which detects the kinetic energies with a precision of the order of 1 meV and a He3 cryostat which allows us to keep the temperature of the sample below 1 K. We discuss the exemplary results obtained on single crystals of Sr2RuO4 and some other materials. The electronic structure of this material can be determined with an unprecedented clarity.

The overall goal of this method is to determine the low-energy electronic structure of solids at ultra-low temperatures using Angle-Resolved Photoemission Spectroscopy with synchrotron radiation.

The physical properties of a material are defined by its  electronic structure. Electrons in solids are characterized by energy (&omega;)  and momentum ...

Engineering

In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of materials that are inaccessible using conventional synthesis techniques. Coupling soft landing with in situ characterization using secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS) enables analysis of well-defined surfaces under clean vacuum conditions. The capabilities of three soft-landing instruments constructed in our laboratory are illustrated for the representative system of surface-bound organometallics prepared by soft landing of mass-selected ruthenium tris(bipyridine) dications, [Ru(bpy)3]2+ (bpy = bipyridine), onto carboxylic acid terminated self-assembled monolayer surfaces on gold (COOH-SAMs). In situ time-of-flight (TOF)-SIMS provides insight into the reactivity of the soft-landed ions. In addition, the kinetics of charge reduction, neutralization and desorption occurring on the COOH-SAM both during and after ion soft landing are studied using in situ Fourier transform ion cyclotron resonance (FT-ICR)-SIMS measurements. In situ IRRAS experiments provide insight into how the structure of organic ligands surrounding metal centers is perturbed through immobilization of organometallic ions on COOH-SAM surfaces by soft landing. Collectively, the three instruments provide complementary information about the chemical composition, reactivity and structure of well-defined species supported on surfaces.

In Situ SIMS and IR Spectroscopy of Well-defined Surfaces Prepared by Soft Landing of Mass-selected Ions

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of novel materials. Coupled with analysis by in situ secondary ion mass spectrometry (SIMS) and infrared reflection absorption spectroscopy (IRRAS), soft landing provides unprecedented insights into the interactions of well-defined species with surfaces.

Soft landing of mass-selected ions onto surfaces is a powerful approach for the highly-controlled preparation of materials that are inaccessible using ...

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Applying X-ray Imaging Crystal Spectroscopy for Use as a High Temperature Plasma Diagnostic

X-ray spectra provide a wealth of information on high temperature plasmas; for example electron temperature and density can be inferred from line intensity ratios. By using a Johann spectrometer viewing the plasma, it is possible to construct profiles of plasma parameters such as density, temperature, and velocity with good spatial and time resolution. However, benchmarking atomic code modeling of X-ray spectra obtained from well-diagnosed laboratory plasmas is important to justify use of such spectra to determine plasma parameters when other independent diagnostics are not available. This manuscript presents the operation of the High Resolution X-ray Crystal Imaging Spectrometer with Spatial Resolution (HIREXSR), a high wavelength resolution spatially imaging X-ray spectrometer used to view hydrogen- and helium-like ions of medium atomic number elements in a tokamak plasma. In addition, this manuscript covers a laser blow-off system that can introduce such ions to the plasma with precise timing to allow for perturbative studies of transport in the plasma.

X-ray spectra provide a wealth of information on high temperature plasmas. This manuscript presents the operation of a high wavelength resolution spatially imaging X-ray spectrometer used to view hydrogen- and helium-like ions of medium atomic number elements in a tokamak plasma.

X-ray spectra provide a wealth of information on high temperature plasmas; for example electron temperature and density can be inferred from line ...

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose of efficiently driving non-equilibrium chemistry is discussed. The advantage of a flowing plasma reactor is that continuous chemical processes can be driven with the flexibility of startup times in the seconds timescale. The plasma approach is generically suitable for conversion/activation of stable molecules such as CO2, N2 and CH4. Here the reduction of CO2 to CO is used as a model system: the complementary diagnostics illustrate how a baseline thermodynamic equilibrium conversion can be exceeded by the intrinsic non-equilibrium from high vibrational excitation. Laser (Rayleigh) scattering is used to measure the reactor temperature and Fourier Transform Infrared Spectroscopy (FTIR) to characterize in situ internal (vibrational) excitation as well as the effluent composition to monitor conversion and selectivity.

This article describes a flowing microwave reactor that is used to drive efficient non-equilibrium chemistry for the application of conversion/activation of stable molecules such as CO2, N2 and CH4. The goal of the procedure described here is to measure the in situ gas temperature and gas conversion.

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...

Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active metal-oxide surfaces in contact with light-absorbing dyes is an ongoing thorny and elusive challenge in physical chemistry. With this in mind, a molecular-dynamics (MD) simulation were performed by using optimized empirical potentials for a well-representative and prototypical dye-sensitized solar cell (DSC) solvated by a widely-studied room temperature ionic liquid (RTIL), in the guise of a [bmim]+[NTf2]- RTIL solvating an N719-sensitizing dye adsorbed onto 101 anatase-titania. In doing so, important insights were gleaned into how using a RTIL as the electrolytic hole acceptor modulates the dynamical and vibrational properties of a N719 dye, estimating the spectra for the DSC photo-active interface via Fourier transformation of mass-weighted velocity autocorrelation functions from MD. The acquired vibrational spectra were compared with the experiment spectra and those sampled from ab initio molecular dynamics (AIMD); in particular, various empirical-potential spectra generated from MD provide insight into how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction. In any event, careful fitting of empirical force-field models has been shown to be an effective tool in handling DSC vibrational properties, when validated by AIMD and an experiment.

A dye-sensitized solar cell was solvated by RTILs; using optimized empirical potentials, a molecular dynamics simulation was applied to compute vibrational properties. The obtained vibrational spectra were compared with experiment and ab initio molecular dynamics; various empirical potential spectra show how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction.

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active ...

Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas

Langmuir probes have long been used in experimental plasma physics research as the primary diagnostic for particle fluxes (i.e., electron and ion fluxes) and their local spatial concentrations, for electron temperatures, and for electrostatic plasma potential measurements, since its invention by Langmuir in the early 1920s. Emissive probes are used for measuring plasma potentials. The protocols exhibited in this work serve to demonstrate how these probes may be built for use in a vacuum chamber in which a plasma discharge may be confined and sustained. This involves vacuum techniques for building what is essentially an electrical feedthrough, one that is rotatable and translatable. Certainly, complete Langmuir probe systems may be purchased, but they can also be built by the user at considerable cost savings, and at the same time be more directly adapted to their use in a particular experiment. We describe the use of Langmuir probes and emissive probes in mapping the electrostatic plasma potential from the body of the plasma up to the sheath region of a plasma boundary, which in these experiments&#160;is created by a negatively biased electrode immersed within the plasma, in order to compare the two diagnostic techniques and assess their relative advantages and weaknesses. Although Langmuir probes have the advantage of measuring the plasma density and electron temperature most accurately, emissive probes can measure electrostatic plasma potentials more accurately throughout the plasma, up to and including the sheath region.

The main goal of this work is to make it easier for research groups unfamiliar with Langmuir probes and emissive probes to use them as plasma diagnostics, especially near plasma boundaries. We do this by demonstrating how to build the probes from readily available materials and supplies.

Langmuir probes have long been used in experimental plasma physics research as the primary diagnostic for particle fluxes (i.e., electron and ion fluxes) ...

Localized Surface Plasmon Resonance Imaging to Detect Protein Secretions from a Single Cell

Source: Raghu, D. et al., <a href="https://app.jove.com/v/53120">A Label-free Technique for the Spatio-temporal Imaging of Single Cell Secretions.</a> J. Vis. Exp.&#160;(2015)
This video demonstrates localized surface plasmon resonance imaging to detect protein secretions from a single cell. Using a microfluidic setup and a peptide-bound plasmonic nanostructure chip, changes in reflected light intensity at the resonance angle reveal interactions with secreted proteins from a single cell.

Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface

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