Quantitative analysis of gas phase molecular constituents using frequency-modulated rotational spectroscopy

A new instrumentation for simultaneous terahertz and mid-infrared spectroscopy in corrosive gaseous mixtures

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200-350 K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200-350 K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42 m and by a THz radiation beam (82.5 GHz to 1.1 THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.

High band-width mid-infrared frequency-modulated Faraday rotation spectrometer for time resolved measurement of the OH radical

We present a novel mid-infrared frequency-modulated Faraday rotation spectrometer (FM-FRS) for highly sensitive and high bandwidth detection of OH radicals in a photolysis reactor. High frequency modulation (up to 150 MHz) of the probe laser using an electro-optical modulator (EOM) was used to produce a modulation sideband on the laser output. An axial magnetic field was applied to the multi-pass Herriott cell, causing the linearly polarized light to undergo Faraday rotation. OH radicals were generated in the cell by photolyzing a mixture of ozone (O₃) and water (H₂O) with a UV laser pulse. The detection limit of OH reaches 6.8 × 10⁸ molecule/cm³ (1σ, 0.2 ms) after 3 and falling to 8.0 × 10⁷ molecule/cm³ after 100 event integrations. Relying on HITRAN absorption cross section and line shape data, this corresponds to minimum detectable fractional absorption (A_min) of 1.9 × 10^-5 and 2.2 × 10^-6, respectively. A higher signal-to-noise ratio and better long-term stability was achieved than with conventional FMS because the approach was immune to interference from diamagnetic species and residual amplitude modulation noise. To our knowledge, this work reports the first detection of OH in a photolysis reactor by FM-FRS in the mid-infrared region, a technique that will provide a new and alternative spectroscopic approach for the kinetic study of OH and other intermediate radicals.

Accurately Measuring Molecular Rotational Spectra in Excited Vibrational Modes

Although gas phase rotational spectroscopy is a mature field for which millions of rotational spectral lines have been measured in hundreds of molecules with sub-MHz accuracy, it remains a challenge to measure these rotational spectra in excited vibrational modes with the same accuracy. Recently, it was demonstrated that virtually any rotational transition in excited vibrational modes of most molecules may be made to lase when pumped by a continuously tunable quantum cascade laser (QCL). Here, we demonstrate how an infrared QCL may be used to enhance absorption strength or induce lasing of terahertz rotational transitions in highly excited vibrational modes in order to measure their frequencies more accurately. To illustrate the concepts, we used a tunable QCL to excite v₃ R-branch transitions in N₂O and either enhanced absorption or induced lasing on 20 v₃ rotational transitions, whose frequencies between 299 and 772 GHz were then measured using either heterodyne or modulation spectroscopy. The spectra were fitted to obtain the rotational constants B₃ and D₃, which reproduce the measured spectra to within the experimental uncertainty of ± 5 kHz. We then show how this technique may be generalized by estimating the threshold power to make any rotational transition lase in any N₂O vibrational mode.

A quantitative analysis method based on collision broadening for trace gas using terahertz heterodyne spectrometer

Quantitative analysis of trace gases is an important research field in analytical chemistry. The terahertz electronic spectrometer is one of the most powerful tools for detecting trace gas. Here, a terahertz spectrometer based on frequency multiplier chain and heterodyne detection was presented. The rotational spectra of acetonitrile (CH₃CN) gas were measured in the 290-370 GHz frequency band with 100 kHz spectral resolution. The spectrometer demonstrated excellent spectral specificity and the extrapolated limit of detection for CH₃CN gas of 1.4 ppm. Furthermore, a novel quantification method of trace gas was proposed based on broadening mechanisms. The CH₃CN self- and nitrogen (N₂)- collisional broadening coefficients were obtained experimentally for verifying the method. The CH₃CN concentration of the validation group was calculated, and the relative error was 0.1%. The error analysis of the different number of measurements of the method was carried out. The method could provide a new perspective for trace gas quantitative analysis.

Advancing Plasmon-Induced Selectivity in Chemical Transformations with Optically Coupled Transmission Electron Microscopy

Nanoparticle photocatalysts are essential to processes ranging from chemical production and water purification to air filtration and surgical instrument sterilization. Photochemical reactions are generally mediated by the illumination of metallic and/or semiconducting nanomaterials, which provide the necessary optical absorption, electronic band structure, and surface faceting to drive molecular reactions. However, with reaction efficiency and selectivity dictated by atomic and molecular interactions, imaging and controlling photochemistry at the atomic scale are necessary to both understand reaction mechanisms and to improve nanomaterials for next-generation catalysts. Here, we describe how advances in plasmonics, combined with advances in electron microscopy, particularly optically coupled transmission electron microscopy (OTEM), can be used to image and control light-induced chemical transformations at the nanoscale. We focus on our group's research investigating the interaction between hydrogen gas and Pd nanoparticles, which presents an important model system for understanding both hydrogenation catalysis and hydrogen storage. The studies described in this Account primarily rely on an environmental transmission electron microscope, a tool capable of circumventing traditional TEM's high-vacuum requirements, outfitted with optical sources and detectors to couple light into and out of the microscope. First, we describe the H₂ loading kinetics of individual Pd nanoparticles. When confined to sizes of less than ∼100 nm, single-crystalline Pd nanoparticles exhibit coherent phase transformations between the hydrogen-poor α-phase and hydrogen-rich β-phase, as revealed through monitoring the bulk plasmon resonance with electron energy loss spectroscopy. Next, we describe how contrast imaging techniques, such as phase contrast STEM and displaced-aperture dark field, can be employed as real-time techniques to image phase transformations with 100 ms temporal resolution. Studies of multiply twinned Pd nanoparticles and high aspect ratio Pd nanorods demonstrate that internal strain and grain boundaries can lead to partial hydrogenation within individual nanoparticles. Finally, we describe how OTEM can be used to locally probe nanoparticle dynamics under optical excitation and in reactive chemical environments. Under illumination, multicomponent plasmonic photocatalysts consisting of a gold nanoparticle "antenna" and a Pd "reactor" show clear α-phase nucleation in regions close to electromagnetic "hot spots" when near plasmonic antennas. Importantly, these hot spots need not correspond to the traditionally active, energetically preferred sites of catalytic nanoparticles. Nonthermal effects imparted by plasmonic nanoparticles, including electromagnetic field enhancement and plasmon-derived hot carriers, are crucial to explaining the site selectivity observed in PdH_x phase transformations under illumination. This Account demonstrates how light can contribute to selective chemical phenomena in plasmonic heterostructures, en route to sustainable, solar-driven chemical production.