Search

Dynamic processes in catalysis are gaining increased attention and could very well be one of the next frontiers in surface science. One way to study such processes is to induce chemical changes on the surface for example by periodically adjusting the (electro)chemical potential in situ and identify the resulting spectral changes. Often this is referred to as Modulation Excitation Spectroscopy (MES). Using Ambient Pressure Photoelectron Spectroscopy data, we here discuss and compare the analysis of MES data using both Phase Sensitive Detection (PSD) and Fourier analysis. We discuss that PSD determines the component magnitude at a user-defined phase value while Fourier analysis provides the maximum oscillation amplitude and respective phase value of oscillating spectral features. We discuss advantages and disadvantages of the different analysis schemes and explore how the full time-evolution can be obtained. [Display omitted] • Phase Sensitive Detection and Fourier analysis are compared for the first time • Pros and cons and limitations of the PSD and Fourier analysis are discussed in detail • Fourier analysis enables on-the-fly detection of oscillating minority species in situ • For the special case of APXPS this enables structure-activity correlation • We hope our work will inspire many future studies of oscillating surface structures [ABSTRACT FROM AUTHOR]

A system was developed that allows different acoustic parameters to be measured inside the frequency audio range and that can be used in different propagation mediums. A simple configuration of fiber optic sensors based on multimodal interference (MMI) is used, due to the advantages they provide. To perform a spatial and frequency filtering according to the characteristics of the source of interest, a beamformer is applied to the data obtained and then phase sensitive detection is used to obtain the resulting signal. The simulations were made by replicating the existing conditions in the experiments subsequently made in the laboratory. The system showed that the combination of the proposed techniques increases the signal-to-noise ratio even in cases where the ambient noise exceeds the signal of interest several times. [ABSTRACT FROM AUTHOR]

Dual-phase lock-in amplifiers (LIAs) are designed to extract information (amplitude and phase) from signals buried in high noise levels. In spite of their popularity, their use has been traditionally limited to input sinusoidal waves with symmetric power supplies. This paper presents an algorithm that enables single-supply analog LIAs to properly process input square waves. Formed by linear equations, its computational implementation is much simpler than that of the traditional sinusoidal algorithm. Moreover, applied to battery-operated microcontrolled systems where square signals can be generated by the embedded microcontroller, it presents intrinsic advantages such as simplicity, versatility, and reduction in power and size. Experimental results validate the proposed algorithm, confirming its enormous possibilities in sensing applications. [ABSTRACT FROM PUBLISHER]

Abstract: A new type of differential surface plasmon (SPR) sensor integrated with a microfluidic system is presented. The working principle of the microfluidic device is based on hydrodynamic modulation of two laminar streams inside a microchannel to provide periodic changes of the environment on the SPR sensor. The modulated reflectance is then demodulated using a lock-in amplifier. The presented sensor provides sensitivities of index of refraction about 4×10−8 RIU together with a 4 orders of magnitude dynamic range. This method demonstrates a sensitive detection scheme which could be used for label-free detection. [Copyright &y& Elsevier]

Abstract: Using finite-difference time-domain method, we investigate phase and amplitude reflective properties of a metallic photonic crystal slab comprising a two-dimensional array of gold disks placed on top of a thin gold film resting on a dielectric substrate. Photonic crystal slab overcladding is a gaseous analyte. Throughout the paper we discuss prospectives of application of such photonic crystal slabs to sensing of changes in the gas refractive index. We start by studying the field distributions and spectral positions of two types of surface plasmons supported by such PhCs as a function of the photonic crystal lattice period, the gold film thickness, and the gold disk size. First, we find that the spectral positions of plasmon peaks depend almost linearly on the photonic crystal lattice constant, which is a manifestation of the delocalized nature of a plasmon extending over the PhC lattice. This is possible due to connectivity between metallic disks via an underlying thin metallic film. Second, we find that the width of plasmonic peaks is highly sensitive to the relative size of the metallic disks compared to the lattice period. This can be well explained via interaction between the localized plasmons situated at the disc edges in the adjacent unit cells. We therefore find that plasmons in the metallic PhC slabs featuring metallic discs connected by a metallic film exhibit both strong local and non-local properties. Third, we find that, generally, there are two types of plasmons supported by such metallic PhCs. One type is a plasmon with a large fraction of its field located in the gas overcladding, which is most suitable for sensing. In contrast, another plasmon type has its field concentrated at the interface between a thin metal film and a substrate; such plasmons are only weakly sensitive to the refractive index of the sensor overcladding, and, therefore, can serve as convenient references for the sensor measurements. Finally, we report sensor sensitivities to changes in the real part of the gas refractive index when using amplitude and phase-based detection strategies. We start by demonstrating that when measured in the sensor far field, the phase sensitive detection provides much lower detection limit ( RIU) compared to that of the amplitude-based detection ( RIU). We then find that sensor phase and amplitude sensitivities can be considerably enhanced when performing measurements using point detectors placed in the near field of a sensor. Particularly, we have established that both phase and amplitude sensitivities are maximal when measured in the sensor near field along the normal to the sensor surface going through the point of the highest symmetry of a photonic crystal unit cell ( point). Sensor resolution as high as RIU for the phase-based detection and RIU) for the amplitude-based detection are found, with the most dramatic enhancement observed for the phase detection approach. We believe that experimental verification of the sensor sensitivity enhancement using phase detection in the sensor near field can be accomplished by using scanning near-field optical microscopy. [Copyright &y& Elsevier]

Abstract: We determine the linewidth of the hyperfine structure a 10 component of R(56) 32-0 transition of 127I2 at 532nm using the dependence of the peak amplitude of the third-derivative signal on the modulation width. We also use the same method to investigate pressure broadening and power broadening of the a 10 component. [Copyright &y& Elsevier]

Modulation spectroscopy can be an adequate tool to separate a weak system response from a huge background absorption, provided the process under consideration enables a periodic external stimulation. Phase sensitive detection (PSD) is then used to demodulate the periodic system response. We introduce a new method of PSD that can be used in modulated excitation (ME)-FTIR spectroscopy without the need of a lock-in amplifier or spectrometer build-in hardware. An advantage of this method is that only standard procedures included in all FTIR instruments, such as the measurement of time-resolved spectra, are required. The principal mathematical formalism of the used PSD is described.As an example we show phase-resolved spectra of the amid I′ region obtained from temperature modulated FTIR spectroscopic experiments of RNase A. The advantage of the ME compared to a relaxation process is shown by the power of separation of overlapping absorption bands. [Copyright &y& Elsevier]

Infrared spectroscopy is typically not used to establish the oxidation state of metal-based catalysts. In this work, we show that the baseline of spectra collected in diffuse reflectance mode of a series of Pd/Al2O3 samples of increasing Pd content varies significantly and reversibly under alternate pulses of CO or H2 and O2. Moreover, these baseline changes are proportional to the Pd content in Pd/Al2O3 samples exhibiting comparable Pd particle size. Similar measurements by X-ray absorption spectroscopy on a different 2 wt.% Pd/Al2O3 confirm that the baseline changes reflect the reversible reduction-oxidation of Pd. Hence, we demonstrate that changes in oxidation state of metal-based catalysts can be determined using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and that this behavior is part of the spectral changes that are returned by experiments under operando conditions. [ABSTRACT FROM AUTHOR]

Methane is a well-established fuel molecule whose production from CO 2 through methanation garners increasing interest as an energy storage solution. While often produced with Ni based catalysts, other metals are of interest thanks to higher robustness and activity-selectivity numbers. The Rh/CeO 2 catalyst has shown appreciable properties for CO 2 methanation and its structural dynamics has been studied in situ. However, the reaction pathway is unknown. Here, we present infrared modulation excitation spectroscopy measurements with phase sensitive detection of a Rh/CeO 2 catalyst adsorbate composition during H 2 pulsing (0–2 vol.%) to a constant CO 2 (0.5 vol.%) feed. Various carbonyl (CO) and carbonate (b-CO 3 /p-CO 3 ) ad-species clearly respond to the hydrogen stimulus, making them potential reaction intermediates. The different CO ad-species are likely intermediates for product CO and CH 4 but their individual contributions to the respective formations are not unambiguously ascertained. As for the carbonate dynamics, it might be linked to the reduction/oxidation of the CeO 2 surface upon H 2 pulsing. Formate (HCOO) ad-species are clearly visible but appear to be, if not spectators, linked to slow side reactions possibly also affected by CeO 2 redox processes. [ABSTRACT FROM AUTHOR]

This paper presents the quadrature sinusoidal oscillators for a phase sensitive detection (PSD) system. The proposed oscillators are design by using the commercially available ICs (LT1228). The core oscillator consists of three LT1228s: two grounded capacitors and one resistor. By adding four resistors without the requirement of additional active devices, the amplitudes of two quadrature waveforms become adjustable. The quadrature output nodes are of low impedance, which can be connected to the impedance sensor or other circuits in a phase sensitive detection system without the need of buffer devices. The amplitudes of the quadrature waveform are equal during the frequency of oscillation (FO) tuning. The frequency of oscillation is electronically and linearly controlled by bias current or voltage without affecting the condition of oscillation (CO). Furthermore, the condition of oscillation is electronically controlled without affecting the frequency of oscillation. The performances of the proposed oscillators are experimentally tested with ±5 voltage power supplies. The frequency of the proposed sinusoidal oscillator can be tuned from 8.21 kHz to 1117.51 kHz. The relative frequency error is lower than 3.12% and the relative phase error is lower than 2.96%. The total harmonic distortion is lower than −38 dB (1.259%). The voltage gain of the quadrature waveforms can be tuned from 1.97 to 15.92. The measurement results demonstrate that the proposed oscillators work in a wide frequency range and it is a suitable choice for an instrument-off-the-shelf device [ABSTRACT FROM AUTHOR]

This paper presents a low-power fully integrated quadrature signal generator for system-on-chip (SoC) impedance spectroscopy applications. It has been designed in a 0.18 μm-1.8 V CMOS technology as a self-contained oscillator, without the need for an external reference clock. The frequency can be digitally tuned from 10 to 345 kHz with 12-bit accuracy and a relative mean error below 1.7%, thus supporting a wide range of impedance sensing applications. The proposal is experimentally validated in two impedance spectrometry examples, achieving good magnitude and phase recovery results compared to the results obtained using a commercial LCR-meter. Besides the wide frequency tuning range, the proposed programmable oscillator features a total power consumption lower than 0.77 mW and an active area of 0.129 mm2, thus constituting a highly suitable choice as stimulation module for instrument-on-a-chip devices. [ABSTRACT FROM AUTHOR]