Wave Cavity Research Articles

Following interfacial kinetics in real time using broadband evanescent wave cavity-enhanced absorption spectroscopy: a comparison of light-emitting diodes and supercontinuum sources

A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 micros signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

Real-Time Monitoring of Polyaniline Nanoparticle Formation on Surfaces

The formation of polyaniline (PAni) nanoparticles on silica surfaces has been monitored in situ using evanescent wave cavity ring-down spectroscopy (EW-CRDS). Aniline hydrochloride in aqueous solution at different concentrations was oxidatively polymerized using sodium persulfate. The process was found to involve the nucleation and growth of polymer nanoparticles, whose size and coverages were measured using tapping mode atomic force microscopy (TM-AFM). The formation of PAni was confirmed by replacing the silica surface with indium doped tin oxide (ITO) electrodes and subsequently running cyclic voltammetry experiments on the material deposited, which yielded the characteristic electrochemical response. The number of active groups (monomers) per polymer particle was estimated using the peak current of the cyclic voltammograms, with knowledge of the nanoparticle surface coverage from TM-AFM analysis. The quantity of material in each particle was consistent with TM-AFM height data, assuming hemispherical particle morphology. The polymerization process was found to be governed by surface-controlled kinetics, from a fixed number of particle sites of 35 (±5) μm−2. The reaction was first-order in aniline, with a rate constant for monomer addition of 0.7 (±0.4) × 10−7 cm s−1 in the limit of large particle size. For smaller nanoparticles, however, at the early stage of polymerization, the polymerization rate constant appeared to be size-dependent and to increase with decreasing nanoparticle radius (assuming a hemispherical geometry). This could also be due to a change in the morphology of the nanoparticles at the very shortest times but may indicate size-dependent polymerization kinetics.

Terahertz oscillations in mercury cuprate superconductors

It has been recently reported that the three-dimensional Bose-Einstein condensation of the quasi-particles is valid for the mercury cuprates at liquid helium temperature. In this study, the validity of the interlayer theory in three dimensions has been investigated for optimally oxygen-doped mercury cuprates at the temperature interval of 0–15 K. Furthermore, some thermodynamic and electrodynamics parameters of mercury cuprates have been calculated for both the under-doped and the over-doped samples at the vicinity of 4.2 K. Moreover, it has been determined that the superconducting system behaves as a terahertz wave cavity regardless of the oxygen doping concentration.

A study of OH radicals in an atmospheric AC discharge plasma using near infrared diode laser cavity ringdown spectroscopy combined with optical emission spectroscopy

Simultaneous measurements of absolute concentrations of H2O and OH radicals in an atmospheric AC discharge using continuous wave cavity ringdown spectroscopy (cw-CRDS) are reported. Formation of OH radicals and plasma temperatures are characterized by optical emission spectroscopy. The concentration of OH radical at the edge of the discharge plume at 380 K is measured by the cw-CRDS technique to be 1.1 ×1015 molecule cm-3. Ringdown measurements of the H2O (120-000) band and the OH first overtone around 1515 nm enable us to determine an OH generation yield, \(\Phi =\frac{N_{\rm OH} }{N_{\rm H_2 O}}\), to be 4.8 ×10-3, where NOH and \( N_{H_2 O} \) are the number densities of OH and H2O, respectively. The minimum detectable absorption coefficient of the cw-CRDS system is 8.9 ×10-9 cm-1, which corresponds to a 1σ detection limit of OH number density of 1.2 ×1013 molecule cm-3 in the discharge. This experimental approach is demonstrated for the first time ever in an AC discharge, and can be applied in general to a variety of atmospheric plasmas to help study OH formation mechanisms and OH-related plasma applications.

High sensitivity CW-CRDS of 18O enriched water near 1.6 μm

The absorption spectrum of 18O enriched water has been recorded by continuous wave cavity ring down spectroscopy between 5905.7 and 6725.7 cm −1 using a series of fibred DFB lasers. The investigated spectral region corresponds to the important 1.55 μm transparency window of the atmosphere where water absorption is very weak. The typical CRDS sensitivity (noise equivalent absorption of 5×10 −10 cm −1) allowed for the detection of lines with intensity as low as 10 −28 cm/molecule while the minimum intensity value provided by HITRAN in the considered spectral region is 1.7×10 −24 cm/molecule. The line parameters were retrieved with the help of an interactive least squares multi-lines fitting program assuming a Voigt function as line profile. Overall, 4510 absorption lines belonging to the H 2 18O, H 2 16O, HD 18O, HD 16O and H 2 17O water isotopologues were measured. Their intensities range between 3×10 −29 and 5×10 −23 cm/molecule at 296 K and the typical accuracy on the line positions is 1×10 −3 cm −1. 2074 of the observed lines attributed to H 2 18O, HD 18O and H 2 17O are reported for the first time. The transitions were assigned on the basis of variational calculations resulting in 288, 135 and 38 newly determined rovibrational energy levels for the H 2 18O, HD 18O and H 2 17O isotopologues, respectively. The new data set includes the band origin of the 4 ν 2 bending overtone of H 2 18O at 6110.4239 cm −1 and rovibrational levels corresponding to J and K a values up to 18 and 12, respectively, for the strongest bands of H 2 18O: 4 ν 2, ν 1+2 ν 2, 2 ν 2+ ν 3, 2 ν 1, ν 1+ ν 3, and ν 2+ ν 3. The obtained experimental results have been compared to the spectroscopic parameters provided by the HITRAN database and to the recent IUPAC critical review of the rovibrational spectrum of H 2 18O and H 2 17O as well as to variational calculations. Large discrepancies between the 4 ν 2 variationally predicted and experimental intensities have been evidenced for the H 2 18O and H 2 16O molecules.

Collective transverse cavity cooling of a dense molecular beam

Based on a classical point particle description, we study light induced transverse collimation and phase space compression of a fast molecular beam traversing the crossover region of a high-Q optical cavity and a standing-wave laser beam. The molecules scatter pump laser light, far detuned from any molecular transition, into the resonant cavity mode. We show that despite a very small probability for this process for a single particle, collective enhancement can lead to significant transverse cooling and collimation above the self-organization threshold. The analytical formulae for this threshold derived from homogeneous one-dimensional (1D) models give surprisingly good estimates for the pulsed 3D case. We compare ring and standing wave cavity geometries and show that phase space compression can be achieved even for extremely small particle field coupling if compensated by a large density and pump power. In the vicinity of the critical point the compression time can be reduced by tailored seeding of the cavity mode or using a second buildup cavity for the pump field.

Localization of rf breakdowns in a standing wave cavity

At SLAC, a 5-cell, normal-conducting, L-band (1.3 GHz), standing-wave (SW) cavity was built as a prototype positron capture accelerator for the ILC. The structure met the ILC gradient goal but required extensive rf processing. When rf breakdowns occurred, a large variation was observed in the decay rate of the stored energy in the cavity after the input power was shut off. It appeared that the breakdowns were isolating sections of the cavity, and that the trapped energy in those sections was then partitioned among its natural modes, producing a distinct beating pattern during the decay. To explore this phenomenon further, an equivalent circuit model of cavity was created that reproduces well its normal operating characteristics. The model was then used to compute the spectra of trapped energy for different numbers of isolated cells. The resulting modal patterns agree well with those of the breakdown data, and thus such a comparison appears to provide a means of identifying the irises on which the breakdowns occurred.

Evanescent Wave Cavity Ring-Down Spectroscopy as a Probe of Interfacial Adsorption: Interaction of Tris(2,2′-bipyridine)ruthenium(II) with Silica Surfaces and Polyelectrolyte Films

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been used to study the interaction of the tris(2,2'-bipyridine)ruthenium(II) complex, [Ru(bpy)(3)](2+), at both native silica surfaces and surfaces modified with polyelectrolyte films. Both poly-l-lysine (PLL) and PLL/poly-l-glutamic acid (PGA) bilayer functionalized interfaces have been studied. Concentration isotherms exhibit Langmuir-type adsorption behavior on both silica and PGA-terminated surfaces from which equilibrium constants have been derived. The pH-dependence of the [Ru(bpy)(3)](2+) adsorption to silica and the PLL/PGA film has also been investigated. For the latter substrate, the effective surface pK(a) of the acid groups was found to be 5.5. The effect of supporting electrolyte was also investigated and was shown to have a significant effect on the extent of [Ru(bpy)(3)](2+) adsorption. A thin-layer electrochemical cell arrangement, in which a working electrode was positioned just above the substrate, was used to change the solution pH in a controlled way via the potential-pulsed chronoamperometric oxidation of water. By measuring the optical absorption using EW-CRDS during such experiments, the desorption of [Ru(bpy)(3)](2+) from the surface has been monitored in real time. Experiments were carried out at different cell thicknesses and at various pulse durations. By combining data from the EW-CRDS experiments with fluorescence confocal laser scanning microscopy (CLSM) to determine the pH at the substrate surface, the pK(a) of the PLL/PGA film could be ascertained and was found to agree with the static pH isotherm measurements. These studies provide a platform for the further use of electrochemistry combined with EW-CRDS to investigate dynamic processes at interfaces.

Pressure Broadening of H2O Absorption Lines in the 1.3 μm Region Measured by Continuous Wave-Cavity Ring-Down Spectroscopy: Application in the Trace Detection of Water Vapor in N2, SiH4, CF4, and PH3

A continuous wave cavity ring-down (cw-CRD) spectrometer has been developed for the measurement of trace levels of water vapor by absorption spectroscopy at wavelengths in the vicinity of 1358 nm and 1392 nm. The speed of data acquisition and selectivity make cavity ringdown spectroscopy potentially more useful than current techniques for measurement of trace water in process gases and vacuum environments used for semiconductor manufacture where water vapor contamination has a detrimental effect on the final product. The pressure broadening coefficients (gamma) for bath gases N(2), air, and Ar and semiconductor process gases SiH(4), PH(3), and CF(4) were determined for a range of absorption lines in the 2nu(1) and nu(1) + nu(3) bands of H(2)O. For the transitions investigated, the concentration of water vapor in the sample gas varied from 1.7 x 10(12) to 2.9 x 10(15) molecule cm(-3) in N(2) at a total pressure of <10 mbar and was mixed with the bath gas of increasing pressure up to approximately 200 mbar. The values of gamma quantify the reduction in peak absorption cross-sections with bath gas pressure and, thus, their effects on the detection limit of water vapor. For a CRD spectrometer with a ring-down time of tau = 12.0 mus measured with a precision of 0.6%, detection limits for the measurement of water vapor in 1 atm N(2) and of CF(4) were estimated to be 18 and 14 ppbv, respectively. Competing absorption by SiH(4) and PH(3) in the 1.3 mum wavelength region results in respective detection limits for water vapor of 98 and 319 ppbv (relative to 1 atm) in 0.2 atm SiH(4) and 0.37 atm PH(3).

NR activity in the accelerating cavities development and study for ILC positron source parameters

The critical part of the normal conducting (NC) positron preaccelerator (PPA) at the ILC positron source (PS) are capture sections that should operate with an accelerating gradient of up to 15 MV/m in combination with long RF pulse (∼1 ms). Developed at the Institute of Nuclear Research (INR) and now being constructed at DESY, the CDS booster cavity for the Photo Injector Test facility, DESY, Zeuthen, will operate under the same conditions and is a full-scale, high-RF-power prototype of the PS capture cavities. Cavity construction status and results of cavity tuning at a low RF level are presented. Other features of the standing wave cavities for the PPA, such as RF pulsed heating, advanced cooling, and beam loading, are discussed.

Noise caused by a finite extinction ratio of the light modulator in CW cavity ring-down spectroscopy

A model is presented for the effect of a finite extinction ratio of the light modulator used in continuous wave cavity ring-down spectroscopy (CW-CRDS) experiments. We present a simple analytical expression for the minimum isolation required to prevent a significant increase in the fluctuations of the cavity decay rate, which determine the sensitivity of the method. We also present systematic measurements of the signal to noise in CW-CRDS as a function of the effective isolation of the light modulator, and excellent agreement with the model is found.

Surface Assembly and Redox Dissolution of Silver Nanoparticles Monitored by Evanescent Wave Cavity Ring-Down Spectroscopy

The adsorption kinetics of Ag nanoparticles on a silica surface modified with poly-l-lysine (PLL) have been measured in situ by following the interfacial optical absorbance at 405 nm by evanescent wave cavity ring-down spectroscopy (EW-CRDS). Sensitivity toward nanoparticle detection is enhanced due to the plasmon resonance of the Ag nanoparticles. The redox-dissolution kinetics of the immobilized nanoparticles have been investigated by two distinct approaches. First, IrCl62− was generated electrochemically from IrCl63− by a chronoamperometric potential step in a thin-layer cell configuration formed between the silica surface and a Pt macroelectrode. The oxidative dissolution kinetics were obtained by monitoring the EW-CRDS signal as the nanoparticles dissolved. The reaction kinetics were extracted by complementary finite element modeling of diffusional and reaction processes. The second method of dissolution investigated involved the injection of IrCl62−(aq) directly at the surface by means of a microcap...

Automated System for Monitoring Trace C2H2 in Ambient Air by Cavity Ring-Down Spectroscopy Combined with Sample Preconcentration

A fully automated instrument combining a continuous wave cavity ring-down spectrometer and dual-trap sample preconcentration has been implemented for monitoring C2H2 mixing ratios in ambient air. A distributed feedback diode laser operating in the near-infrared region (lambda approximately 1534.973 nm in air) detects C2H2 in absorption via the P(17) rotational line of the (v1 + v3) vibrational combination band. The instrument is shown to be capable of fast, quantitative, and precise monitoring of C2H2 mixing ratios, with a detection limit of approximately 8 pptv (parts per trillion by volume). It thus has potential to be deployed for analysis of air samples in many rural and urban environments. In situ measurements were carried out at 30 min intervals over periods of up to 15 h on several days for indoor and outdoor air samples. For indoor air monitored on a Sunday, the C2H2 mixing ratio was stable at 1.45 +/- 0.04 ppbv (parts per billion by volume). On weekdays, both indoor and outside air analyses showed peaks in the range 2-4 ppbv in the early morning and late afternoon that coincided with periods of busy road traffic.

On the direct formation of HO2 radicals after 248 nm irradiation of benzene C6H6 in the presence of O2

We present in this work the direct observation of HO2 radicals after irradiation of benzene C6H6 at 248 nm in the presence of O2. HO2 radicals have been unambiguously identified using the very selective and sensitive detection of continuous wave cavity ring-down spectroscopy (cw-CRDS) coupled to a laser photolysis reactor. HO2 radicals were detected in the first vibrational overtone of the OH stretch at 6638.20 cm-1, using a DFB diode laser. This reaction might be important because 248 nm photolysis of H2O2 has often been used in the past for studying the OH•-initiated degradation of C6H6, often using a large excess of C6H6 over H2O2. The possible importance of the title reaction with respect to these former laboratory studies has been quantified through comparison with HO2• signals obtained from 248 nm photolysis of H2O2: one obtains under our conditions (excess O2 and total pressure of 6.6 kPa helium) from the 248 nm irradiation of identical initial concentrations [C6H6]=[H2O2] the following relative initial radical concentrations: [HO2•]=(0.28±0.05)×[OH•]. Experiments with various O2 concentrations have revealed that the origin of the HO2 radicals is not the reaction of H-atoms with O2, but must originate from the reaction of O2 with excited C6H6*. The quantum yield of C6H6* formation has been deduced to ϕ=0.2±0.1.

Determination of the OH radical in atmospheric pressure dielectric barrier discharge plasmas using near infrared cavity ring-down spectroscopy

The hydroxyl radical (OH) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. The present work reports the determination of the hydroxyl radicals in atmospheric dielectric barrier discharge plasmas via near infrared continuous wave cavity ring-down spectroscopy. The P-branches of OH X2Πi (ν' = 2 ←ν′′ = 0) bands were used for its number density measurements. The minimum measurable absorption coefficient is about 3 × 10-8 cm-1 in DBD plasmas. At certain experimental conditions (a.c. frequency of 70 kHz, 6700 ppm H2O in He, 1 atm), when the peak-to-peak discharge voltage varied from 6 kV to 10.4 kV, the determined OH radical concentration increased from (2.1 ± 0.1) × 1013 molecules cm-3 to (3.7 ± 0.1) × 1013 molecules cm-3. The plasma gas temperature, derived from the Boltzmann plots of OH rotational population distributions, ranged from 312 ± 10 K to 363 ± 10 K when the discharge voltage was raised in the above range. The influences of O2 and N2 addition on the production of OH radicals have been also investigated.

In-Situ Measurement of Colloidal Gold Adsorption on Functionalized Silica Surfaces

Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been applied to study, in situ, the deposition kinetics of commercially available gold colloids on functionalized silica surfaces from quiescent solution. Neither 5 nor 20 nm citrate-stabilized nanoparticles were observed to adsorb on clean silica surfaces. Adsorption on a poly-l-lysine-functionalized surface, however, occurs readily and irreversibly with the kinetics of adsorption differing markedly for the two particle sizes studied. 5 nm particles adsorb to form a highly disperse submonolayer of individual particles with atomic force microscope images showing no evidence of aggregation. The controlled growth of multilayer nanoparticle/polyelectrolyte films is demonstrated by alternately depositing colloidal particles and poly-l-lysine films. The deposition of multilayer nanoparticle films increases the sensitivity of the functionalized surface to changes in the solvent refractive index. The adsorption kinetics of the 20 nm colloid is more complex than that of the smaller colloid with adsorbed particles acting as nucleation sites for subsequent aggregation with the result that the interfacial absorbance continues to increase indefinitely with time.

Detection of HO2 Radicals in the Photocatalytic Oxidation of Methyl Ethyl Ketone

The possibility of HO2 formation and its diffusion into the gas phase during the photocatalytic degradation of methy ethyl ketone (MEK) on TiO2 has been studied using the very sensitive and selective detection method of continuous wave cavity ring down spectroscopy (cw-CRDS), coupled to a continuous photolysis reactor. No HO2 radicals could be detected, given a detection limit of 3.2 × 109 cm-3.

Determination of the HO2 radical in dielectric barrier discharge plasmas using near-infrared cavity ring-down spectroscopy

The hydroperoxyl radical (HO2) plays an important role in combustion systems, atmospheric chemistry and the removal of air pollutants by non-thermal plasmas. This work reports the determination of the hydroperoxyl radical in dielectric barrier discharge plasmas via near-infrared continuous wave cavity ring-down spectroscopy. HO2 radicals were observed in discharges of HCHO/O2/H2O/N2 mixtures around 6625.7 cm−1 in the first H–OO stretching overtone, (2, 0, 0)–(0, 0, 0), of its ground electronic state . At certain discharge conditions (ac frequency of 5 kHz, peak-to-peak voltage of 6.5 kV, 1900 ppm HCHO, 20% O2, 3.5% H2O in N2, Ptotal = 30 Torr), HO2 radical concentration was determined to be 1.0 × 1013 molecules cm−3. The temporary evolution of HO2 concentration was obtained using the ‘time window’ method. The effects of oxygen concentration, water concentration, the discharge voltage and discharge gas pressure on the concentration of HO2 radicals have been investigated. The detection limit of our setup for the HO2 radical is ∼1 × 1011 molecules cm−3.

On fiber optic probe hydrophone measurements in a cavitating liquid

The measurement of high-pressure signals is often hampered by cavitation activity. The usage of a fiber optic probe hydrophone possesses advantages over other hydrophones, yet when measuring in a cavitating liquid large variations in the signal amplitude are found; in particular when the pressure signal recovers back to positive values. With shadowgraphy the wave propagation and cavity dynamics are imaged and the important contributions of secondary shock waves emitted from collapsing cavitation bubbles are revealed. Interestingly, just adding a small amount of acidic acid reduces the cavitation activity to a large extent. With this treatment an altered primary pressure profile which does not force the cavitation bubbles close to fiber tip into collapse has been found. Thereby, the shot-to-shot variations are greatly reduced.

Photothermal measurement of thermal diffusivity in carbonyl iron powder suspensions

We have measured the thermal diffusivity of micron size Carbonyl Iron Powder suspension in a silicone oil base fluid, at various Carbonyl Iron Powder concentrations by means of a photothermal technique called Thermal Wave Cavity. Using literature data (density and specific heat capacity) we can determine the effective thermal conductivity. We compare our experimental results with various theoretical models for the effective thermal conductivity in heterogeneous materials, previously deployed in the literature.