Infrared Absorption Signals Research Articles

Tunable acoustic graphene plasmon enhanced nano-infrared spectroscopy

Nano-infrared spectroscopy (nano-IR) technology can exceed the diffraction limit of light, achieving infrared spectroscopic detection with a spatial resolution of about 10 nm, which is an important technical means for studying the chemical composition and structure of molecules on a nanoscale. However, the weak infrared absorption signals of nanoscale materials pose a significant challenge due to the large mismatch between their dimensions and the wavelength of infrared light. The infrared absorption signals of molecular vibrational modes are proportional to the squares of the electromagnetic field intensities at their positions, implying that higher electromagnetic field intensity can significantly improve the sensitivity of molecular detection. Acoustic graphene plasmons (AGPs), excited by the interaction between free charges in graphene and image charges in metal, exhibit strong optical field localization and electromagnetic field enhancement. These properties make AGPs an effective platform for enhancing nano-IR detection sensitivity. However, the fabrication of graphene nanostructures often introduces numerous edge defects due to the limitations of nanofabrication techniques, significantly reducing the electromagnetic field enhancement observed in experiments. Here, we use finite element simulation to theoretically propose a tunable enhanced nano-IR detection platform based on nanocavity-acoustic graphene plasmons (n-AGPs), which utilizes a graphene/air gap/gold nanocavity structure. This platform avoids needing the nanofabrication of graphene, thereby preventing defects and contamination from being introduced in processes such as electron beam exposure and plasma etching. By plotting the dispersion of n-AGP, it is found that n-AGP has a high wavelength compression capability comparable to AGP (<i>λ</i><sub>0</sub>/<i>λ</i><sub>AGP</sub> = 48). Additionally, due to the introduction of the gold nanocavity structure, n-AGP possess an extremely small mode volume (<i>V</i><sub>n-AGP</sub> ≈ 10<sup>–7</sup><inline-formula><tex-math id="M5">\begin{document}$ {{ \lambda }}_{0}^{3} $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240489_M5.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240489_M5.png"/></alternatives></inline-formula>, <i>λ</i><sub>0</sub> = 6.25 μm). By calculating the electric field intensity distribution (|<i>E</i><sub>norm</sub>|) and the normalized electric field intensity spectrum (i.e. the relationship between frequency and |<i>E</i><sub><i>z</i></sub>|/|<i>E</i><sub><i>z</i>0</sub>|) of the n-AGP structure, it is evident that due to the high electron density on the gold surface, electromagnetic waves can be reflected from the boundaries of the gold nanocavity and resonantly enhanced within the nanocavity. At the resonant frequency of n-AGP (1800 cm<sup>–1</sup>), the electric field inside the cavity is enhanced by about 50 times. In contrast, at similar resonant frequencies, the electric field enhancement factor of Graphene plasmon (resonant frequency 1770 cm<sup>–1</sup>) and AGP (resonant frequency 1843 cm<sup>–1</sup>) are approximately 3 and 2 times, respectively, significantly lower than that of n-AGP. Furthermore, by placing a protein film (60 nm wide and 10 nm high) under the graphene, we calculate the spectral dip depths caused by Fano resonance between n-AGP and AGP with the vibrational modes of protein molecules, thereby validating the enhancement factors of different modes for protein vibrational mode infrared absorption. For the amide-I band of proteins, the detection sensitivity of n-AGP is about 60 times higher than that of AGP. Additionally, we find that by adjusting the structural parameters of the gold nanocavity, including cavity depth, width, and surface roughness, the response frequency band of n-AGP can be modulated (from 1290 to 2124 cm<sup>–1</sup>). Specifically, as the cavity depth increases, the electric field enhancement of n-AGP is improved, and the wavelength compression capability of n-AGP decreases, causing the resonant frequency to be blue-shifted (from 1793 to 2124 cm<sup>–1</sup>). As the cavity width increases, the resonant frequency of n-AGP is red-shifted (from 1793 to 1290 cm<sup>–1</sup>), and the effectiveness of the gold nanocavity boundary in reflecting the resonant electric field within the cavity diminishes, resulting in a decrease in the electric field enhancement factor. With the gradual increase in the roughness of the gold nanocavity bottom, the effective depth of the gold nanocavity increases, causing the n-AGP resonant frequency to be blue-shifted (from 1793 to 1861 cm<sup>–1</sup>) and the electric field enhancement factor to increase. Moreover, by adjusting the Fermi level of graphene (from 0.3 to 0.6 eV), we achieve dynamic tuning of n-AGP (from 1355 to 1973 cm<sup>–1</sup>). As the Fermi level of graphene increases, the wavelength compression capability of n-AGP decreases, resulting in a blue-shift in the resonant frequency. Finally, by optimizing the structural parameters and Fermi level of n-AGP, and placing protein particles of different sizes (20, 15, and 10 nm high, all 10 nm wide) into the graphene/gold nanocavity structure, we verify the protein detection capability of n-AGP-enhanced nano-IR. We find that n-AGP can detect the vibrational fingerprint features of the amide-I band and amide-II band. For protein films (60 nm wide and 10 nm high), the sensitivity increased by approximately 300 times, and for a single protein particle (10 nm wide and 10 nm high), the sensitivity increased by approximately 9 times. This enhanced structure based on n-AGP holds promise for providing an important detection platform for nanoscale material characterization and single-molecule detection, with broad application potential in biomedicine, materials science, and geology.

Graphical characterization of infrared absorption spectroscopic gas sensor using symmetrized dot pattern

A near-infrared gas sensor system was developed for methane (CH4) detection using symmetrized dot pattern (SDP) technique to transform the time waveforms from infrared absorption spectroscopy (IAS) into visible graphs. The basic principle of IAS was illustrated, and infrared absorption signals under different concentration levels were simulated based on the HITRAN (HIgh-resolution TRANsmission) database and LabVIEW software. For improving the detection sensitivity of SDP in the IAS system, the equations of SDP were updated by using a maximum detection value for normalization. The maximum detection value of the system was selected to normalize the radius and angles of SDP for increasing the resolution of the detection system. Graphs clearly show different trends under different concentration levels. Experimental tests on four CH4 samples at 40, 60, 80, and 100 ppm were carried out and SDP graphs were depicted. Through calibration experiment, the linearity between the maximum polar radius of IAS signals peak converted by SDP and the CH4 concentration reached 0.9888, and the detection limit of the system reached 67.41 part per billion (ppb).

Synthesis of bulk silicon oxynitride glass through nitridation of SiO2 aerogels and determination of Tg

AbstractThe development of transparent glass for use in high‐temperature applications is continuing. In this study, we synthesized bulk silicon oxynitride glasses (a‐Si(O,N)x) through the nitridation of SiO2 aerogels containing methyl (CH3‐) groups and evaluated their bulk properties, including their glass transition temperature (Tg). Tetramethyl orthosilicate and methyltrimethoxysilane were added into the precursor gels, and those gels were subjected to a supercritical CO2 drying process. The presence of CH3‐group in the gel avoided cracking during ammonolysis at 750°C–1400°C, and the transparency of the gel was remained even after ammonolysis at 1300°C. The ammonolysis successfully introduced nitrogen into the gels even at relatively low temperatures, for example, 750°C, and the highest nitrogen content (11.7 mass%) was achieved in the gel after ammonolysis at 1300°C. As the nitrogen‐related signals in electron spectroscopy indicated presence of nitride ions (N3−) after ammonolysis and the infrared absorption signals attributed to Si–N bonds were enhanced with the increase of nitrogen concentration, we successfully obtained oxynitride glasses. Those oxynitride glasses showed increase of Tg with their nitrogen concentration.

Fluorescence-infrared absorption dual-mode nanoprobes based on carbon dots@SiO2 nanorods for ultrasensitive and reliable detection of carcinoembryonic antigen

The level of carcinoembryonic antigen (CEA) in serum has the significant reference value for early diagnosis and treatment of various cancers. However, the CEA detection still suffers from the issue of limited sensitivity and reliability. Herein, a fluorescence (FL)-infrared absorption (IRA) dual-mode nanoprobe was fabricated based on carbon dots (CDs)@SiO2 nanorod for CEA detection. The FL and IRA signals display no mutual interference and can verify each other, ensuring the reliability of assay results. The highly sensitive FL signal originating from the CDs is enhanced by the surface passivation of SiO2 and improves the overall sensitivity of the detection. The detection range spans 9 orders of magnitude and the limit of detection reaches 794.6 ag mL−1, which are great superior to the commercial kits and most of the previous reports. Satisfactory recovery over the commercial kits was achieved in real serum samples. The ultrasensitive and reliable FL-IRA detection strategy sheds light on a new avenue toward promoting the practicability of the nanoprobes in clinical cancer diagnosis.

Tailoring grating strip widths for optimizing infrared absorption signals of an adsorbed molecular monolayer.

Metal structures with resonances in the mid-infrared spectral range enable an increased sensitivity for detecting molecular vibrational signals. 1D gold strip gratings have already proven potential in surface-enhanced infrared absorption (SEIRA) experiments, as grating resonances and local electric field enhancement can be spectrally tuned by changing the grating period. Here, we identify the grating strip width as another important design parameter, which is investigated for further optimization of molecular absorption signal enhancement in SEIRA experiments. Previous literature used gratings to increase light absorption in relatively thick polymer layers. Here, we demonstrate the capability of gold strip gratings fabricated on a CaF2 substrate to enhance the CH2 vibrational modes of a thiol-based monolayer of MHDA. An optimal choice of the strip width w = 1.33 μm enables a maximum vibrational signal enhancement factor of around 84, when normalized to microscopic GIR measurements of an MHDA monolayer on an extended gold surface. Numerical simulations demonstrate the broadband local field enhancement of gold strip gratings, which are suitable for enhancing multiple vibrational modes in a large hot-spot volume.

Fingerprints of polycyclic aromatic hydrocarbons (PAHs) in infrared absorption spectroscopy

We have analyzed a set of 51 PAHs whose structures have been hypothesized from mass spectrometry data collected on samples extracted from carbon particles of combustion origin. We have obtained relationships between infrared absorption signals in the fingerprint region (mid-IR) and the chemical structures of PAHs, thus proving the potential of IR spectroscopy for the characterization of the molecular structure of aromatic combustion products. The results obtained here for the spectroscopic characterization of PAHs can be also of interest in Materials Science and Astrophysics.

Monitoring bound HA1(H1N1) and HA1(H5N1) on freely suspended graphene over plasmonic platforms with infrared spectroscopy

Infrared (IR) spectroscopy provides fingerprinting of the energy and orientation of molecular bonds. The IR signals are generally weak and require amplification. Here we present a new plasmonic platform, made of freely suspended graphene, which was coating periodic metal structures. Only monolayer thick films were needed for a fast signal recording. We demonstrated unique IR absorption signals of bound proteins: these were the hemagglutinin area (HA1) of swine influenza (H1N1) and the avian influenza (H5N1) viruses bound to their respective tri-saccharides ligand receptors. The simplicity and sensitivity of such approach may find applications in fast monitoring of binding events.

Remote observations of eruptive clouds and surface thermal activity during the 2009 eruption of Redoubt volcano

Volcanoes often erupt explosively and generate a variety of hazards including volcanic ash clouds and gaseous plumes. These clouds and plumes are a significant hazard to the aviation industry and the ground features can be a major hazard to local communities. Here, we provide a chronology of the 2009 Redoubt Volcano eruption using frequent, low spatial resolution thermal infrared (TIR), mid-infrared (MIR) and ultraviolet (UV) satellite remote sensing data. The first explosion of the 2009 eruption of Redoubt Volcano occurred on March 15, 2009 (UTC) and was followed by a series of magmatic explosive events starting on March 23 (UTC). From March 23–April 4 2009, satellites imaged at least 19 separate explosive events that sent ash clouds up to 18km above sea level (ASL) that dispersed ash across the Cook Inlet region. In this manuscript, we provide an overview of the ash clouds and plumes from the 19 explosive events, detailing their cloud-top heights and discussing the variations in infrared absorption signals. We show that the timing of the TIR data relative to the event end time was critical for inferring the TIR derived height and true cloud top height. The ash clouds were high in water content, likely in the form of ice, which masked the negative TIR brightness temperature difference (BTD) signal typically used for volcanic ash detection. The analysis shown here illustrates the utility of remote sensing data during volcanic crises to measure critical real-time parameters, such as cloud-top heights, changes in ground-based thermal activity, and plume/cloud location.

Direct Control of Electron Transfer to the Surface-CO Bond on a Pt/TiO2Catalytic Diode

We study CO adsorption on a multilayer catalytic diode in which electron transfer at the metal-semiconductor (Pt/TiO(2)) junction is controlled by an applied external voltage. The multilayer diode structure enhances infrared absorption signals from CO molecules adsorbed on the small area Pt surface. We find that the diode behaves like a Schottky junction and that changes in electron transfer at the junction are directly correlated with reversible shifts in the vibrational frequency of adsorbed CO. Infrared polarization and incidence angle dependent studies show that the magnitude of vibrational frequency shift varies with orientation of the molecules being probed and increases with proximity to the Pt/TiO(2) interface. The results demonstrate the ability to control the metal-adsorbate bond through external electronic modifications of a metal-support junction. The catalytic diode can potentially provide control of the surface chemical bond by an external voltage, providing a new approach for investigations in heterogeneous catalysis, sensors, and plasmonic devices.

Structural control of surface layer proteins at electrified interfaces investigated by in situ Fourier transform infrared spectroscopy

In situ Fourier Transform Infrared (FTIR) Spectroscopy complemented by Electrochemical Quartz Microbalance (EQMB) investigations allowed a detailed insight into the influence of the electrode potential on competing adsorption processes and bonding mechanisms of buffer ions and S-layer protein molecules of Lysinibacillus sphaericus CCM2177 at an electrified liquid/gold interface. The S-layer proteins adsorb on gold polarized positively of the point of zero charge by displacing perchlorate anions in the Helmholtz plane by their carboxylate groups. This is indicated by an increase of the peptide and carboxylate infrared absorption signals accompanied by a decrease of the perchlorate signal. S-layers interlinked laterally with Ca(2+) ions, positive of the point of zero charge, resulted in the formation of a crystalline layer participating in the Helmholtz layer. In contrast to the absence of the Ca(2+)-linkers, S-layers remain structurally intact also in the negative polarization domain where the Helmholtz layer is solely sustained by mainly solvated cations without participation of the negatively charged protein carboxylate functions.

Ozone Oxidation of Single Walled Carbon Nanotubes from Density Functional Theory

Ozone is known to react with single-walled nanotubes (SWNTs) to form oxide species on the nanotubes and, upon annealing, to etch the SWNTs. However, the mechanism of ozone attack is not known. We use gradient-corrected density functional theory to compute the potential energy surfaces for O3 dissociation on the sidewall of a pristine (8,8) SWNT. Two decomposition pathways were considered; the first involves the formation of a Criegee intermediate, with a barrier of 17 kcal/mol, followed by transformations leading to lactone, quinone, and carbonyl functional groups. The activation barriers for these transformations are below 23 kcal/mol. The cleavage of the lactone group, evolving CO and CO2, have barrier heights of 39.4 and 49.3 kcal/mol, respectively. This agrees well with experimental findings that the evolution of CO2 and CO occur at 600 K. The second decomposition pathway involves the direct cleavage of the ozonide, forming a singlet O2 and an ether or epoxide group on the SWNT. This pathway competes with the Criegee mechanism; the barrier for forming singlet O2 is 7.9 kcal/mol, which is 9.1 kcal/mol lower than the barrier to formation of the Criegee intermediate, indicating that formation of ether or epoxide groups is kinetically favored. However, formation of ester and carbonyl groups could proceed by subsequent addition of O3 on newly generated defect sites. Vibrational frequency calculations were carried out on cluster models in order to predict infrared absorption signals of local structures. The calculated results for C═O stretching frequencies agree well with experiments. Analysis of the calculated frequencies indicates that the unassigned experimental band at 1380 cm−1 is due to ester and ether groups, while the unassigned band at 925 cm−1 is due to epoxide groups. The vibrational frequency of the O+−O− stretch in the Criegee intermediate is in the range 1055−1096 cm−1.

Correlation between ESR and infrared absorption signals from platinum–hydrogen complexes in silicon

We present results of correlative measurements of platinum–hydrogen (Pt–H) complexes in silicon using infrared absorption (IR) and ESR measurements. Relative concentrations of the complexes in the samples were varied employing various gas pressures during hydrogen doping procedure. The ESR and IR results for the PtH2 and PtH3 complexes showed good correlation in various samples, supporting the existing identification of the IR peaks. The ESR and IR signals assigned to PtH pair, however, behaved differently. Our results suggest that the IR peak assigned previously to PtH pairs only, contains several unresolved components corresponding to PtmH (m⩾1) defects.

Synthesis and spectroscopic investigations of iron(III) complexes with chlorides and dianionic, symmetrically halogen substituted phthalocyanines as ligands

The synthesis of iron(III) complexes of general formula FeCl(R-pc), where R-pc are dianionic, symmetrically halogen substituted phthalocyanines at the positions 2,9,16,23 or 1,8,15,22, from the corresponding amino substituted derivatives is described (R=Cl, Br, I). The complexes are characterized by UV-visible and infrared spectra, powder X-ray diffraction and magnetic susceptibility measurements. The effect of substituents at the periphery and the basicity of the solvents used on the electronic spectra are discussed. The Q band of the electronic spectra for symmetrically halogen substituted derivatives are redshifted and the substituents at 2,9,16,23- positions are more effective in redshifting the Q bands than those at 1,8,15,22-positions. Depending upon the basicity of the solvents, the ligand-to-metal charge transfer (LMCT) transitions on Q band envelop shift to the higher energy region in the order of pyridine>DMF>DMSO. The infrared absorption signals for CH and metal-ligand vibrations appear to be sharper for 1,8,15,22 substituted derivatives than for 2,9,16,23 substituted ones.

Infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices

We report the study of infrared spectroscopy of intraband transitions in Ge/Si quantum dot superlattices. The superlattices, which were grown on (001) oriented Si substrates by a solid source molecular beam epitaxy system, are composed mainly of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of them and of the uncapped Ge dots grown on the surfaces of some of them were tested by cross-sectional transmission electron and atomic force microscopes, respectively. It is found that the Ge quantum dots have flat lens-like shapes. Infrared absorption signals peaking in the mid-infrared range were observed using Fourier transform infrared and Raman scattering spectroscopy techniques. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions within the valence band of the Ge quantum dots in the superlattices. The fact that the intraband absorption is strongly polarized along the growth axis of the superlattices signifies that the Ge quantum dots with flat lens-like shapes perform as Ge/Si-based quantum wells. This study demonstrates the application potential of these kinds of Ge/Si quantum dot superlattices for developing mid-infrared photodetectors.

Rotationally inelastic scattering of jet cooled H2O with Ar: State-to-state cross sections and rotational alignment effects

State-to-state rotationally inelastic scattering cross sections of H2O with Ar are measured under single-collision conditions in crossed supersonic jets at a collision energy of 480(90) cm−1. The H2O is initially prepared in its lowest ortho (101) and para (000) rotational levels by supersonic cooling in a Ne expansion, and then excited in the intersection region by single collisions with a second pulsed jet of Ar atoms. Column-integrated densities of H2O in both initial and final scattering states are monitored via direct absorption of narrow bandwidth (Δν≈0.0001 cm−1) infrared light from a continuous wave (cw) F-center laser. Absolute inelastic cross sections for state-to-state collisional energy transfer out of para and ortho initial states are determined from the dependence of infrared absorption signals on collider gas densities. Overall, the results can be approximately characterized by an exponential decrease in cross section with the magnitude of rotational energy transferred, i.e., as suggested by exponential energy gap models. However, at the state-to-state level, a highly structured, nonmonotonic dependence on energy is observed, which indicates a propensity for rotational excitation around the A (in-plane, perpendicular to C2) and C (out-of-plane) principal axes. This preferential state-to-state scattering dynamics reflects an intramolecular alignment of J in the body-fixed frame and is in good qualitative agreement with theoretical classical trajectory predictions. A rigorous comparison is made via full quantum close-coupling scattering calculations on empirical and ab initio Ar-H2O potential energy surfaces, which successfully reproduce all the state-to-state trends observed, but at the more quantitative level appear to overestimate the intramolecular alignment effects experimentally observed.

Scattering dynamics in HF+He, Ne, and Ar: State-to-state cross sections, Dopplerimetry, and alignment measurement via direct infrared laser absorption in crossed supersonic jets

Absolute state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of HF with He, Ne, and Ar at mean center-of-mass collision energies of 480, 390, and 350 cm−1, respectively. HF seeded in Ar diluent gas is cooled into the J=0 ground rotational state in a pulsed supersonic expansion, followed by single collision rotational excitation with rare gas atoms from a second pulsed supersonic jet. The column-integrated densities of HF in both the initial and final scattering states are probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), continuously tunable, color center laser. Total inelastic cross sections for collisional loss out of J=0 and collisional excitation into J>0 states are determined in absolute units from the dependence of infrared absorption signals on collider gas concentration. Full close coupling scattering calculations are performed on several ab initio and empirical potential energy surfaces for each of the three HF+rare gas systems. Agreement for He+HF and Ar+HF integral cross sections is remarkably good, but significant discrepancies are noted for the less accurately determined Ne+HF surface. Photoelastic polarization modulation of the IR laser is used to probe for rotational alignment in the scattered HF flux; the measurements set an upper polarizance limit for collisionally populated J=1 HF molecules [probed on P(1)] of |P|<2%. High resolution IR laser Dopplerimetry reveals velocity structure in the collisionally excited J=1 Doppler profiles, which is in excellent qualitative agreement with theoretical predictions of rainbow features in the J=1←0 state-to-state differential cross section.

Rotationally inelastic scattering in CH4+He, Ne, and Ar: State-to-state cross sections via direct infrared laser absorption in crossed supersonic jets

Absolute integral state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of CH4 with the rare gases He, Ne, Ar, at center of mass collision energies of 460±90, 350±70, and 300±60 cm−1, respectively. CH4 seeded in Ar buffer gas is cooled in a pulsed supersonic expansion into the three lowest rotational levels allowed by nuclear spin statistics corresponding to A(J=0), F(J=1), and E(J=2) symmetry. Rotational excitation occurs in single collisions with rare gas atoms from a second pulsed supersonic jet. The column integrated densities of CH4 in both initial and final scattering states are subsequently probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), single mode color center laser. Total inelastic cross sections for collisional loss out of the J=0, 1, and 2 methane states are determined in absolute units from the linear decrease of infrared absorption signals as a function of collider gas concentration. Tuning of the ir laser source also permits probing of the collisionally excited rotational states with quantum state and velocity resolution; column integrated scattering densities are measured for all energetically accessible final states and used to infer absolute inelastic cross sections for state-to-state energy transfer. The observed trends are in good qualitative agreement with quantum state resolved pressure broadening studies; however, the dependences of the rotationally inelastic cross sections on nuclear spin modification (i.e., J) and rotational inelasticity (i.e., ΔJ) is not well predicted by conventional angular momentum or energy gap models. More rigorous comparison with the quantum state-resolved scattering data is obtained from full close coupled scattering calculations on trial potential energy surfaces by Buck and co-workers [Chem. Phys. Lett. 98, 199 (1983); Mol. Phys. 55, 1233, 1255 (1985)] for each of the three CH4+rare gas systems. Agreement between theory and experiment for He+CH4 is nearly quantitative, but some discrepancies are noted for the heavier rare gases. Finally, the possible influence of sequential collision channels on the measurement of the smallest observable cross sections is investigated via a master equation analysis.

Methods to enhance absorption signals in infrared reflectance spectroscopy: A comparison using optical simulations

We use optical simulations to determine the substrate and experimental geometry that should be used to enhance infrared absorption signals from thin films in a reflectance geometry. Our goal is to highlight those configurations which are suitable for the real-time study of surface and subsurface chemical bonding during growth or processing. The enhancement techniques reviewed are the use of metal substrates, multiple internal reflection (MIR), and an optical cavity substrate. For each technique, we simulate the absorption strength for Si–H vibrational modes in and on hydrogenated amorphous silicon films as a typical example. The techniques are discussed in terms of (i) the choice of the substrate and flexibility in measurement geometry; (ii) the sensitivity in both s and p polarizations; (iii) the effect of film refractive index (n) and extinction coefficient (k) on the line shape and absorption intensity; and (iv) the sensitivity as a function of film thickness. For the real-time study of high-refractive-index films, the optical cavity substrate and MIR are best suited. MIR is most applicable to study surface reactions during epitaxial chemical vapor deposition (CVD) on high-refractive-index crystals having low bulk absorption in the infrared region. The optical cavity substrate is best suited to study film growth on low-refractive-index material. Metal substrates work well for low-refractive-index materials, e.g., studies of adsorbate layers and metal CVD. These techniques are complementary to each other and provide a means of studying a wide variety of material–substrate combinations.

CO(v,J) product state distributions from the reaction O(3P)+OCS→CO+SO

The title reaction was studied by probing CO[v,J] product state distributions with a tunable infrared diode laser. Only the fastest O(3P) atoms formed by 355 nm photolysis of NO2, i.e., those corresponding to NO[2Π1/2, v=0], have sufficient energy to surmount the activation barrier. Thus these experiments were performed under monoenergetic, near-threshold conditions. The CO[v,J] distributions were extracted from the relative slopes of the transient infrared absorption signals. Little vibrational excitation is observed. The vibrational branching ratio is [v=1]/[v=0]≊0.05; CO[v≥2] was not detected. In contrast, rotational degrees of freedom are much hotter. The nascent CO[v=0] rotational distribution is well characterized by a temperature of TR=4400±390 K. Total CO internal excitation, on the average 3150 cm−1, accounts for 16% of the energy available to products. Surprisal analyses based on several models of energy disposal show that the nascent distributions are nonstatistically cold. The results are consistent with direct attack at the S atom to form an OSCO intermediate, followed by rapid C–S bond scission which exerts a torque on the CO product.

Hard mode Spectroscopy: Experimental studies of structural phase transitions

Abstract The frequencies of phonons with energies well above those of corresponding soft modes depend on the evolution of the structural order parameter as a function of temperature, pressure or chemical composition. The relative changes of the phonon frequencies are usually below 2% of their total energies (hence “hard modes” in contrast to “soft modes”) which allows the treatment of the coupling between the phonon frequencies ω and the order parameter Q in perturbation theory The exponent m = 1, or 2 is determined by symmetry considerations. For modes which are optically active at T > Tc one finds B ≪ A in most cases. Similar correlations are given for the Raman scattering cross sections and the intensities of infrared absorption signals. The correlation length of hard modes is, in general, comparable with that of bond distances so that the order parameter is measured on an atomistic length scale. This property of Hard Mode Infrared Spectroscopy allows the quantitative investigation of short range order...