Rotational Raman Spectroscopy Research Articles

Rationalizing In Situ Active Repair in Hydrogen Evolution Photocatalysis via Non‐Invasive Raman Spectroscopy

AbstractCurrently, most photosensitizers and catalysts used in the field of artificial photosynthesis are still based on rare earth metals and should thus be utilized as efficiently and economically as possible. While repair of an inactivated catalyst is a potential mitigation strategy, this remains a challenge. State‐of‐the‐art methods are crucial for characterizing reaction products during photocatalysis and repair, and are currently based on invasive analysis techniques limiting real‐time access to the involved mechanisms. Herein, we use an innovative in situ technique for detecting both initially evolved hydrogen and after active repair via advanced non‐invasive rotational Raman spectroscopy. This facilitates unprecedently accurate monitoring of gaseous reaction products and insight into the mechanism of active repair during light‐driven catalysis enabling the identification of relevant mechanistic details along with innovative repair strategies.

Rationalizing In Situ Active Repair in Hydrogen Evolution Photocatalysis via Non-Invasive Raman Spectroscopy.

Currently, most photosensitizers and catalysts used in the field of artificial photosynthesis are still based on rare earth metals and should thus be utilized as efficiently and economically as possible. While repair of an inactivated catalyst is a potential mitigation strategy, this remains a challenge. State-of-the-art methods are crucial for characterizing reaction products during photocatalysis and repair, and are currently based on invasive analysis techniques limiting real-time access to the involved mechanisms. Herein, we use an innovative in situ technique for detecting both initially evolved hydrogen and after active repair via advanced non-invasive rotational Raman spectroscopy. This facilitates unprecedently accurate monitoring of gaseous reaction products and insight into the mechanism of active repair during light-driven catalysis enabling the identification of relevant mechanistic details along with innovative repair strategies.

Time-Domain Rotational Raman Spectroscopy of the Ethylene Dimer and Trimer by Coulomb Explosion Imaging of Rotational Wave Packets.

We report rotational Raman spectroscopy of the ethylene dimer and trimer, based on time-resolved Coulomb explosion imaging of rotational wave packets. Rotational wave packets were created in the gas-phase ethylene clusters upon nonresonant ultrashort pulse irradiation. The subsequent rotational dynamics were traced as spatial distribution of monomer ions ejected from the clusters via the Coulomb explosion process induced by a strong probe pulse. The observed images of monomer ions show multiple kinetic energy components. The time-dependence of the angular distribution for each component was analyzed, and the Fourier transformation spectra, which correspond to rotational spectra, were obtained. A lower kinetic energy component was mainly attributed to a signal from the dimer and a higher energy component mainly from the trimer. We have successfully observed rotational wave packets up to a delay time of ∼20 ns and achieved a spectral resolution of 70 MHz after Fourier transformation. Owing to this higher resolution than the previous studies, improved rotational and centrifugal distortion constants were obtained from the spectra. In addition to improving the spectroscopic constants, this study opens the way for rotational spectroscopy of larger molecular clusters than dimers through Coulomb explosion imaging of rotational wave packets. Details of spectral acquisition and analyses of each kinetic energy component are also reported.

Simultaneous non-invasive gas analysis in artificial photosynthesis reactions using rotational Raman spectroscopy

A new instrument for gas analysis in artificial photosynthesis reactions is presented. The device uses rotational Raman spectroscopy to analyse multiple gases (H2, O2, N2, CO, CO2) simultaneously, non-invasively and with short analysis times (15 s).

Structure of benzene from mass-correlated rotational Raman spectroscopy.

We present high resolution rotational Raman spectra and derived geometry parameters for benzene. Rotational Raman spectra with sub-5 MHz resolution were obtained via high-resolution mass-correlated rotational alignment spectroscopy. Isotopologue spectra for C6H6, 13C–C5H6, C6D6, and 13C–C5D6 were distinguished through their correlated mass information. Spectra for 13C6H6 were obtained with lower resolution. Equilibrium and effective bond lengths were estimated from measured inertial moments, based on explicit assumptions and approximations. We discuss the origin of significant bias in previously published geometry parameters and the possibility to derive H,D isotope-specific bond lengths from purely experimental data.

Pure rotational Raman spectroscopy applied to N2/O2 analysis of air bubbles in polar firn

AbstractEarlier gas measurements of firn air (atmosphere in open pore channels) at polar sites have revealed the occurrence of gas fractionation phenomena during bubble close-off, in addition to well-known thermal and gravitational gas separation. Nevertheless, because of difficulties posed by measurement, little is known about the distribution of air constituents in already closed pores (bubbles) in firn. Herein, we describe the application of high-sensitivity pure rotational Raman spectroscopy, combined with sample immersion in the fluorocarbon-based inert fluid for removing the optical disturbance by diffused reflection. That application efficiently elicits information about nitrogen and oxygen composition ratios (N2/O2 or O2/N2) for each air bubble in firn. The developed methodology presents important implications for elucidating how gas records are formed and modified in the course of pore close-off in polar firn.

CRASY: correlated rotational alignment spectroscopy of pyridine. The rotational Raman spectrum of pyridine and asymmetric fragmentation of pyridine dimer cations.

We investigated the rotational Raman spectrum of pyridine monomers and pyridine dimers with mass-correlated rotational alignment spectroscopy (mass-CRASY) and ab initio calculations. The mass spectrum showed a strong signal for the protonated pyridine cation, which we assigned to asymmetric fragmentation of the dimer: ab initio calculations revealed facile proton transfer in the dimer cation and thermodynamically favorable asymmetric fragmentation. In the rotational spectrum correlated to the monomer mass channel, we assigned up to 40 lines for rotational states J ≤ 8. No spectrum could be assigned for the dimer, possibly due to the theoretically predicted presence of multiple dimer structures.

Shifted-excitation rotational Raman spectroscopy and Bayesian inference for in situ temperature and composition determination in laminar flames

Optical, non-invasive techniques for temperature and mole fraction measurements in flames and other reactive flows are often limited to applications under well-defined laboratory environments. These limitations mainly arise from various prerequisites of the techniques e.g. regarding the optical accessibility of the systems under investigation, its environmental conditions such as vibrations and temperature or the need of tracers inside the flow. In order to weaken these constrains, we present a robust, fiber-based sensor system, utilizing tracer-free spontaneous rotational Raman spectroscopy with tunable near-infrared (NIR) continuous-wave laser-excitation capable of a simultaneous point-wise determination of gas temperature and mole fractions in laminar spatially uniform non-sooting reactive flows. Benefits and limits of this method are evaluated investigating a laminar premixed methane/air flat-flame of a McKenna-type burner. In that case, the main rotational Raman-active and, therefore, detectable compounds are nitrogen, oxygen and carbon dioxide. For the subtraction of flame luminosity two different techniques were used: simple background subtraction (BGS) and shifted-excitation Raman difference spectroscopy (SERDS). For both methods, the corrected spectra are matched to simulated spectra via a least-square fit algorithm in order to extract the quantities of interest. Hereby, Bayesian inference enables the determination of the uncertainties of the results. The consideration of the covariance within the multi-variate analysis allows for the combination of the BGS and SERDS method to establish a more advanced analysis routine.

Rotational and Rotational-Vibrational Raman Spectroscopy of Air to Characterize Astronomical Spectrographs.

Raman scattering enables unforeseen uses for the laser guide-star system of the Very Large Telescope. Here, we present the observation of one up-link sodium laser beam acquired with the ESPRESSO spectrograph at a resolution λ/Δλ∼140 000. In 900s on source, we detect the pure rotational Raman lines of ^{16}O_{2}, ^{14}N_{2}, and ^{14}N^{15}N (tentatively) up to rotational quantum numbers J of 27, 24, and 9, respectively. We detect the ^{16}O_{2} fine-structure lines induced by the interaction of the electronic spin S and end-over-end rotational angular momentum N in the electronic ground state of this molecule up to N=9. The same spectrum also reveals the ν_{1←0} rotational-vibrational Q-branch for ^{16}O_{2} and ^{14}N_{2}. These observations demonstrate the potential of using laser guide-star systems as accurate calibration sources for characterizing new astronomical spectrographs.

A rotational Raman study under non-thermal conditions in pulsed CO2–N2 and CO2–O2 glow discharges

This work employs in situ rotational Raman spectroscopy to study the effect of N2 and O2 addition to CO2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO2 and 25%, 50% and 75% of N2 or O2 admixture, in a 5–10 ms on-off cycle, 50 mA plasma current and 6.7 mbar total pressure. The rotational temperature profile is not affected by adding N2, ranging from 400 to 850 K from start to end of the discharge pulse, while the addition of O2 decreases the temperature at corresponding time points. Molecular number densities of CO2, CO, O2 and N2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N2 containing mixtures show that CO2 conversion factor α increases from 0.15 to 0.33 when the content of N2 is increased from 0% to 75%, demonstrating the potential of N2 addition to enhance the vibrational pumping of CO2 and its beneficial effect on CO2 dissociation. Furthermore, the influence of admixtures on CO2 vibrations is examined by analysing the vibrationally averaged nuclear spin degeneracy. The difference between the fitted odd averaged degeneracy and the calculated odd degeneracy assuming thermal conditions increases with the addition of N2, demonstrating the growth of vibrational temperatures in CO2. On the other hand, the addition of O2 leads to a decrease of α, which might be attributed to quenched vibrations of CO2, and/or to the influence of the back reaction in the presence of O2.

Hybrid femtosecond/picosecond pure rotational anti-Stokes Raman spectroscopy of nitrogen at high pressures (1–70 atm) and temperatures (300–1000 K)

We demonstrate hybrid femtosecond/picosecond (fs/ps) pure-rotational coherent anti-Stokes Raman spectroscopy (RCARS) at high temperatures and pressures. Pulse-shaper-produced 40 fs pulses and bandwidth-narrowed, frequency-upconverted 5 ps pulses interact in a high-pressure cell containing N2 at 1–70 atm and 300–1000 K. Accurate experimental temperatures evaluated from fits to model rotational spectra confirm that the sensitivity and precision advantages of hybrid fs/ps RCARS can be exploited in characterizing combustion environments, even in the pressure regime where significant collisional energy transfer and line broadening cannot be neglected.

Excitation and relaxation of the asymmetric stretch mode of CO2 in a pulsed glow discharge

The excitation and relaxation of the vibrations of CO2 as well as the reduction of CO2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed: (1) time-resolved in situ Fourier transform infrared spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3–6.7 mbar and a current range of 10–50 mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T3) to the rotational temperature (Trot) is found to be only dependent on Trot, in the conditions under study. The decay rate follows the relation . Pressure and varying concentrations of CO and (presumably) atomic oxygen did not show to be of significant influence. In the active part of the discharge the excitation of T3 showed to be positively related to current and negatively to pressure. However, the contribution of current to vibrational excitation is ambiguous: the conversion of CO2 and therefore the fraction of CO in the discharge, is found to be strongly dependent on the current, with a conversion factor of 0.05–0.18 for 10–50 mA, while CO can contribute to the excitation through near-resonant collisions. A clear relation between the elevation of T3 and the dissociation of CO2 could not be confirmed, though conversion peaks are observed in the near afterglow, which motivate future experiments on vibrational ladder-climbing directly after termination of the discharge.

High-resolution rotational Raman spectroscopy of benzene.

We present a rotational Raman spectrum for benzene with single-MHz resolution, more than a 100-fold improvement on literature data and sufficient to partially resolve K-splitting in some bands. Spectra for a frequency range of 0 to 500 GHz were measured through the observation of a coherent rotational wave packet in the time domain over a time scale of 1 microsecond. Spectroscopic frequencies were referenced to a GPS-stabilized clock. Fitted molecular constants of B = 5689.2671(±52) MHz, DJ = 1178(±50) Hz, and DJK = -2300(±120) Hz agree with results from some high-resolution rovibrational and rovibronic spectra but contradict others.

In situ analysis of aerosols by Raman spectroscopy – Crystalline particle polymorphism and gas-phase temperature

We developed an in situ measurement system – based on linear Raman spectroscopy – to retrieve quantitative information on polymorphic modifications present in an aerosol. Besides, the Raman spectrum is utilized to determine the gas-phase temperature, which is a key parameter governing crystallinity. The particle polymorphism is obtained from the phonon modes of the particles, while pure rotational Raman spectroscopy is used for gas-phase thermometry. The approach is demonstrated for TiO2 aerosols in air and CO2 with different fractions of its two main polymorphs anatase and rutile. It forms the basis for future investigations in various gas-to-particle processes.

A rotational Raman study under non-thermal conditions in a pulsed CO2 glow discharge

The implementation of in situ rotational Raman spectroscopy is realized for a pulsed glow discharge in CO2 in the mbar range and is used to study the rotational temperature and molecular number densities of CO2, CO, and O2. The polarizability anisotropy of these molecules is required for extracting number densities from the recorded spectra and is determined for incident photons of 532 nm. The spatiotemporally-resolved measurements are performed in the same reactor and at equal discharge conditions (5–10 ms on–off cycle, 50 mA plasma current, 6.7 mbar pressure) as in recently published work employing in situ Fourier transform infrared (FTIR) spectroscopy. The rotational temperature ranges from 394 to 809 K from start to end of the discharge pulse and is constant over the length of the reactor. The discharge is demonstrated to be spatially uniform in gas composition, with a CO2 conversion factor of 0.15 ± 0.02. Rotational temperatures and molecular composition agree well with the FTIR results, while the spatial uniformity confirms the assumption made for the FTIR analysis of a homogeneous medium over the line-of-sight of absorption. Furthermore, the rotational Raman spectra of CO2 are related to vibrational temperatures through the vibrationally averaged nuclear spin degeneracy, which is expressed in the intensity ratio between even and odd numbered Raman peaks. The elevation of the odd averaged degeneracy above thermal conditions agrees well with the elevation of vibrational temperatures of CO2, acquired in the FTIR study.

Optimization design of the tuning method for FBG spectroscopy based on the numerical analysis of all-fiber Raman temperature lidar

Abstract All fiber Raman temperature lidar for space borne platform has been proposed for profiling of the temperature with high accuracy. Fiber Bragg grating (FBG) is proposed as the spectroscopic system of Raman lidar because of good wavelength selectivity, high spectral resolution and high out-of-band rejection rate. Two sets of FBGs at visible wavelength 532 nm as Raman spectroscopy system are designed for extracting the rotational Raman spectra of atmospheric molecules, which intensities depend on the atmospheric temperature. The optimization design of the tuning method of an all-fiber rotational Raman spectroscopy system is analyzed and tested for estimating the potential temperature inversion error caused by the instability of FBG. The cantilever structure with temperature control device is designed to realize the tuning and stabilization of the central wavelengths of FBGs. According to numerical calculation of FBG and finite element analysis of the cantilever structure, the center wavelength offset of FBG is 11.03 nm/°C with the temperature change in the spectroscopy system. By experimental observation, the center wavelength offset of surface-bonded FBG is 9.80 nm/°C with temperature changing when subjected to certain strain for the high quantum number channel, while 10.01 nm/°C for the low quantum number channel. The tunable wavelength range of FBG is from 528.707 nm to 529.014 nm for the high quantum number channel and from 530.226 nm to 530.547 nm for the low quantum number channel. The temperature control accuracy of the FBG spectroscopy system is up to 0.03 °C, the corresponding potential atmospheric temperature inversion error is 0.04 K based on the numerical analysis of all-fiber Raman temperature lidar. The fine tuning and stabilization of the FBG wavelength realize the elaborate spectroscope of Raman lidar system. The conclusion is of great significance for the application of FBG spectroscopy system for space-borne platform Raman lidar.

Rotational excitation of molecules with long sequences of intense femtosecond pulses

We investigate the prospects of creating broad rotational wave packets by means of molecular interaction with long sequences of intense femtosecond pulses. Using state-resolved rotational Raman spectroscopy of oxygen, subject to a sequence of more than 20 laser pulses with peak intensities exceeding $10^{13}$ W/cm$^{2}$ per pulse, we show that the centrifugal distortion is the main obstacle on the way to reaching high rotational states. We demonstrate that the timing of the pulses can be optimized to partially mitigate the centrifugal limit. The cumulative effect of a long pulse sequence results in high degree of rotational coherence, which is shown to cause an efficient spectral broadening of probe light via cascaded Raman transitions.

Fast and highly sensitive fiber-enhanced Raman spectroscopic monitoring of molecular H2 and CH4 for point-of-care diagnosis of malabsorption disorders in exhaled human breath.

Breath gas analysis is a novel powerful technique for noninvasive, early-stage diagnosis of metabolic disorders or diseases. Molecular hydrogen and methane are biomarkers for colonic fermentation, because of malabsorption of oligosaccharides (e.g., lactose or fructose) and for small intestinal bacterial overgrowth. Recently, the presence of these gases in exhaled breath was also correlated with obesity. Here, we report on the highly selective and sensitive detection of molecular hydrogen and methane within a complex gas mixture (consisting of H2, CH4, N2, O2, and CO2) by means of fiber-enhanced Raman spectroscopy (FERS). An elaborate FERS setup with a microstructured hollow core photonic crystal fiber (HCPCF) provided a highly improved analytical sensitivity. The simultaneous monitoring of H2 with all other gases was achieved by a combination of rotational (H2) and vibrational (other gases) Raman spectroscopy within the limited spectral transmission range of the HCPCF. The HCPCF was combined with an adjustable image-plane aperture pinhole, in order to separate the H2 rotational Raman bands from the silica background signal and improve the sensitivity down to a limit of detection (LOD) of 4.7 ppm (for only 26 fmol H2). The ability to monitor the levels of H2 and CH4 in a positive hydrogen breath test (HBT) was demonstrated. The FERS sensor possesses a high dynamic range (∼5 orders of magnitude) with a fast response time of few seconds and provides great potential for miniaturization. We foresee that this technique will pave the way for fast, noninvasive, and painless point-of-care diagnosis of metabolic diseases in exhaled human breath.

Comparative study of in situ N2 rotational Raman spectroscopy methods for probing energy thermalisation processes during spin-exchange optical pumping

Spin-exchange optical pumping (SEOP) has been widely used to produce enhancements in nuclear spin polarisation for hyperpolarised noble gases. However, some key fundamental physical processes underlying SEOP remain poorly understood, particularly in regards to how pump laser energy absorbed during SEOP is thermalised, distributed and dissipated. This study uses in situ ultra-low frequency Raman spectroscopy to probe rotational temperatures of nitrogen buffer gas during optical pumping under conditions of high resonant laser flux and binary Xe/N2 gas mixtures. We compare two methods of collecting the Raman scattering signal from the SEOP cell: a conventional orthogonal arrangement combining intrinsic spatial filtering with the utilisation of the internal baffles of the Raman spectrometer, eliminating probe laser light and Rayleigh scattering, versus a new in-line modular design that uses ultra-narrowband notch filters to remove such unwanted contributions. We report a ~23-fold improvement in detection sensitivity using the in-line module, which leads to faster data acquisition and more accurate real-time monitoring of energy transport processes during optical pumping. The utility of this approach is demonstrated via measurements of the local internal gas temperature (which can greatly exceed the externally measured temperature) as a function of incident laser power and position within the cell.

The conformational stability of gaseous 1-butene studied by femtosecond nonlinear spectroscopy and ab initio calculations

1-Butene was investigated by rotational femtosecond degenerate four-wave mixing spectroscopy (fs DFWM) under supersonic expansion conditions as well as by quantum chemical calculations. Fs DFWM is a time-resolved rotational Raman spectroscopy, which allows for precise determination of rotational constants. The experimental fs DFWM spectrum was successfully reproduced by a fitted simulation using a single structure ascribed to the gauche conformer of 1-butene. The obtained rotational constants A = 22.6 ± 1.7 GHz, B = 4.1554 ± 0.0004 GHz and C = 4.0550 ± 0.0004 GHz are in excellent agreement with the ones from microwave spectroscopy. The cis conformer was not observed in the fs DFWM spectrum recorded under the low-temperature conditions of a supersonic expansion. The absence of the cis form along with simulations of a fs DFWM spectrum indicate that the cis conformer is at most equally but most likely less stable than the gauche rotamer. The experimental findings were supplemented by high level quantum chemical calculations, which predict the gauche form to have lower total energy than the cis structure.