Optimal Spectral Resolution for Infrared Studies of Solids and Liquids.

Due to a legacy originating in the limited capability of early computers, the spectroscopic resolution used in Fourier transform infrared spectroscopy and other systems has largely been implemented using only powers of two for more than 50 years. In this study, we investigate debunking the spectroscopic lore of, e.g., using only 2, 4, 8, or 16 cm-1 resolution and determine the optimal resolution in terms of both (i) a desired signal-to-noise ratio and (ii) efficient use of acquisition time. The study is facilitated by the availability of solids and liquids reference spectral data recorded at 2.0 cm-1 resolution and is based on an examination in the 4000-400 cm-1 range of 61 liquids and 70 solids spectra, with a total analysis of 4237 peaks, each of which was also examined for being singlet/multiplet in nature. Of the 1765 liquid bands examined, only 27 had widths <5 cm-1. Of the 2472 solid bands examined, only 39 peaks have widths <5 cm-1. For both the liquid and solid bands, a skewed distribution of peak widths was observed: For liquids, the mean peak width was 24.7 cm-1 but the median peak width was 13.7 cm-1, and, similarly, for solids, the mean peak width was 22.2 cm-1 but the median peak width was 11.2 cm-1. While recognizing other studies may differ in scope and limiting the analysis to only room temperature data, we have found that a resolution to resolve 95% of all bands is 5.7 cm-1 for liquids and 5.3 cm-1 for solids; such a resolution would capture the native linewidth (not accounting for instrumental broadening) for 95% of all the solids and liquid bands, respectively. After decades of measuring liquids and solids at 4, 8, or 16 cm-1 resolution, we suggest that, when accounting only for intrinsic linewidths, an optimized resolution of 6.0 cm-1 will capture 91% of all condensed-phase bands, i.e., broadening of only 9% of the narrowest of bands, but yielding a large gain in signal-to-noise with minimal loss of specificity.

An Interactive Spectral Analysis Tool for Chemical Identification and Quantification of Gas-Phase Species in Complex Spectra.

A spectral analysis tool has been developed to interactively identify and quantify individual gas-phase species from complex infrared absorbance spectra obtained from laboratory or field data. The SpecQuant program has an intuitive graphical interface that accommodates both reference and experimental data with varying resolution and instrumental lineshape, as well as algorithms to readily align the wavenumber axis of a sample spectrum with the raster of a reference spectrum. Using a classical least squares model in conjunction with reference spectra such as those from the Pacific Northwest National Laboratory (PNNL) gas-phase infrared database or simulated spectra derived from the HITRAN line-by-line database, the mixing ratio of each identified species is determined along with its associated estimation error. After correcting the wavelength and intensity of the field data, SpecQuant displays the calculated mixing ratio versus the experimental data for each analyte along with the residual spectrum with any or all analyte fits subtracted for visual inspection of the fit and residuals. The software performance for multianalyte quantification was demonstrated using moderate resolution (0.5 cm-1) infrared spectra that were collected during the time-resolved infrared photolysis of methyl iodide.

High-Field Magnetoelectric and Spin-Phonon Coupling in Multiferroic (NH4)2[FeCl5·(H2O)].

We combine high field polarization, magneto-infrared spectroscopy, and lattice dynamics calculations with prior magnetization to explore the properties of (NH4)2[FeCl5·(H2O)]─a type II molecular multiferroic in which the mixing between charge, structure, and magnetism is controlled by intermolecular hydrogen and halogen bonds. Electric polarization is sensitive to the series of field-induced spin reorientations, increasing linearly with the field and reaching a maximum before collapsing to zero across the quasi-collinear to collinear-sinusoidal reorientation due to the restoration of inversion symmetry. Magnetoelectric coupling is on the order of 1.2 ps/m for the P∥c, H∥c configuration between 5 and 25 T at 1.5 K. In this range, the coupling takes place via an orbital hybridization mechanism. Other forms of mixing are active in (NH4)2[FeCl5·(H2O)] as well. Magneto-infrared spectroscopy reveals that all of the vibrational modes below 600 cm-1 are sensitive to the field-induced transition to the fully saturated magnetic state at 30 T. We analyze these local lattice distortions and use frequency shifts to extract spin-phonon coupling constants for the Fe-O stretch, Fe-OH2 rock, and NH4+ libration. Inspection also reveals subtle symmetry breaking of the ammonium counterions across the ferroelectric transition. The coexistence of such varied mixing processes in a platform with intermolecular hydrogen- and halogen-bonding opens the door to greater understanding of multiferroics and magnetoelectrics governed by through-space interactions.

Spectroscopic Analysis of Vibronic Relaxation Pathways in Molecular Spin Qubit [Ho(W5O18)2]9-: Sparse Spectra Are Key.

Vibrations play a prominent role in magnetic relaxation processes of molecular spin qubits as they couple to spin states, leading to the loss of quantum information. Direct experimental determination of vibronic coupling is crucial to understand and control the spin dynamics of these nano-objects, which represent the limit of miniaturization for quantum devices. Herein, we measure the magneto-infrared properties of the molecular spin qubit system Na9[Ho(W5O18)2]·35H2O. Our results place significant constraints on the pattern of crystal field levels and the vibrational excitations allowing us to unravel vibronic decoherence pathways in this system. We observe field-induced spectral changes near 63 and 370 cm-1 that are modeled in terms of odd-symmetry vibrations mixed with f-manifold crystal field excitations. The overall extent of vibronic coupling in Na9[Ho(W5O18)2]·35H2O is limited by a modest coupling constant (on the order of 0.25) and a transparency window in the phonon density of states that acts to keep the intramolecular vibrations and MJ levels apart. These findings advance the understanding of vibronic coupling in a molecular magnet with atomic clock transitions and suggest strategies for designing molecular spin qubits with improved coherence lifetimes.

Confirmation of PNNL Quantitative Infrared Cross-Sections for Isobutane.

The Pacific Northwest National Laboratory (PNNL) gas-phase database is a compilation of quantitative experimental (5, 25, and 50 °C) infrared spectra of ca. 500 molecules, designed for in situ, standoff or remote sensing of gases and vapors at or near atmospheric pressure. The data are characterized by calibration on both the wavenumber and intensity axes. Recent papers have called into question the PNNL intensity values for isobutane, [2-methylpropane, HC(CH3)3], suggesting discrepancies of 30-40%. In this study, we remeasure and re-examine the intensity values of isobutane using both similar and alternate methods to those used to generate the original PNNL database spectra. Indirect confirmation from literature data of homologous molecules and direct confirmation from new results confirm that for many band integrals across the isobutane spectrum, the original PNNL data are indeed accurate to within the reported 3% experimental uncertainty.

Absolute Band Intensity of the Iodine Monochloride Fundamental Mode for Infrared Sensing and Quantitative Analysis.

Iodine monochloride (ICl) is a potential off-gas product of molten salt reactors; monitoring this heteronuclear diatomic molecule is of great interest for both environmental and safety purposes. In this paper, we investigate the possibility of infrared monitoring of ICl by measuring the far-infrared absorption cross section of its fundamental band near 381 cm-1. We have performed quantitative studies of the neat gas in a 20 cm cell at 25, 35, 50, and 70 °C at multiple pressures up to ∼9 Torr and investigated the temperature and pressure dependencies of the band's infrared cross section. Quantitative measurements were problematic due to sample adhesion to the cell walls and windows as well as reactions/possible hydrolysis of ICl to form HCl gas. Effects were mitigated by measuring only the neat gas, using short measurement times, and subtracting out the partial pressure of the HCl(g). The integrated band strength is shown to be temperature independent and was found to be equal to 9.1 × 10-19 (cm2/molecule) cm-1. As expected, the temperature dependence of the band profile showed only a small effect over this limited temperature range. We have also investigated using the absorption data along with inverse least squares multivariate methods for the quantitative monitoring of ICl effluent concentrations under different scenarios using infrared (standoff) sensing and compare these results with traditional Beer's law (univariate) techniques.

Infrared Optical Constants from Pressed Pellets of Powders: I. Improved n and k Values of (NH4)2SO4 from Single-Angle Reflectance.

In combination with other parameters, the real, n(v∼), and imaginary, k(v∼), components of the complex refractive index, n^ = n + ik, can be used to simulate the optical properties of a material in different forms, e.g., its infrared spectra. Ultimately, such n/k values can be used to generate a database of synthetic reflectance spectra for the different morphologies to which experimental data can be compared. But obtaining reliable values of the optical constants n/k for solid materials is challenging due to the lack of optical quality specimens, usually crystals, large enough to measure. An alternative to crystals is to press the powder into a uniform disk. We have produced pellets from ammonium sulfate, (NH4)2SO4, powder and derived the pellets' n and k values via single-angle reflectance using a specular reflectance device in combination with a Fourier transform infrared spectrometer. The single-angle technique measures amplitude of light reflected from the material as a function of wavelength over a wide spectral domain; the optical constants are determined from the reflectance data using the Kramers-Kronig relationship. We investigate several parameters associated with the pellets and pellet formation and their effects upon delivering the most reliable n/k values. Parameters studied include pellet diameter, mass, and density (void space), drying, grinding, sieving, and particle size in the pellet formation, as well as pressing pressure and duration. Of these parameters, using size-selected mixtures of dried, small (<50 µm) particles and pressing at ≥10 tons for at least 30 min were found key to forming highly reflective samples. Comparison of two sets of previous literature n(v∼) and k(v∼) values obtained from crystalline (NH4)2SO4 both as crystal reflectance as well as extinction spectra of aerosols measured in a flow tube shows reasonable agreement, but suggests the present values, as confirmed from two independent techniques, represent a substantial improvement for n/k values for (NH4)2SO4, also demonstrating promise to measure the optical constants of other materials.

Spin-Lattice Coupling Across the Magnetic Quantum-Phase Transition in Copper-Containing Coordination Polymers.

We measured the infrared vibrational properties of two copper-containing coordination polymers, [Cu(pyz)2(2-HOpy)2](PF6)2 and [Cu(pyz)1.5(4-HOpy)2](ClO4)2, under different external stimuli in order to explore the microscopic aspects of spin-lattice coupling. While the temperature and pressure control hydrogen bonding, an applied field drives these materials from the antiferromagnetic → fully saturated state. Analysis of the pyrazine (pyz)-related vibrational modes across the magnetic quantum-phase transition provides a superb local probe of magnetoelastic coupling because the pyz ligand functions as the primary exchange pathway and is present in both systems. Strikingly, the PF6- compound employs several pyz-related distortions in support of the magnetically driven transition, whereas the ClO4- system requires only a single out-of-plane pyz bending mode. Bringing these findings together with magnetoinfrared spectra from other copper complexes reveals spin-lattice coupling across the magnetic quantum-phase transition as a function of the structural and magnetic dimensionality. Coupling is maximized in [Cu(pyz)1.5(4-HOpy)2](ClO4)2 because of its ladderlike character. Although spin-lattice interactions can also be explored under compression, differences in the local structure and dimensionality drive these materials to unique high-pressure phases. Symmetry analysis suggests that the high-pressure phase of the ClO4- compound may be ferroelectric.

Antiferroelectric Phase Transition in a Proton-Transfer Salt of Squaric Acid and 2,3-Dimethylpyrazine.

A proton-transfer reaction between squaric acid (H2sq) and 2,3-dimethylpyrazine (2,3-Me2pyz) results in crystallization of a new organic antiferroelectric (AFE), (2,3-Me2pyzH+)(Hsq-)·H2O (1), which possesses a layered structure. The structure of each layer can be described as partitioned into strips lined with methyl groups of the Me2pyzH+ cations and strips featuring extensive hydrogen bonding between the Hsq- anions and water molecules. Variable-temperature dielectric measurements and crystal structures determined through a combination of single-crystal X-ray and neutron diffraction reveal an AFE ordering at 104 K. The phase transition is driven by ordering of protons within the hydrogen-bonded strips. Considering the extent of proton transfer, the paraelectric (PE) state can be formulated as (2,3-Me2pyzH+)2(Hsq23-)(H5O2+), whereas the AFE phase can be described as (2,3-Me2pyzH+)(Hsq-)(H2O). The structural transition caused by the localization of protons results in the change in color from yellow in the PE state to colorless in the AFE state. The occurrence and mechanism of the AFE phase transition have been also confirmed by heat capacity measurements and variable-temperature infrared and Raman spectroscopy. This work demonstrates a potentially promising approach to the design of new electrically ordered materials by engineering molecule-based crystal structures in which hydrogen-bonding interactions are intentionally partitioned into quasi-one-dimensional regions.

Structure-Property Relations in Multiferroic [(CH3)2NH2] M(HCOO)3 ( M = Mn, Co, Ni).

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH3)2NH2] M(HCOO)3 ( M = Mn2+, Co2+, Ni2+) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH3)2NH2]Mn(HCOO)3 takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across TC reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH3)2NH2]Co(HCOO)3 reveals a surprising framework rigidity across the order-disorder transition due to modest distortions around the Co2+ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (and most likely, the Ni) compounds engage the formate bending mode to facilitate the transition to their fully saturated magnetic states, whereas the Co complex adopts a different mechanism involving formate stretching distortions to lower the overall magnetic energy. Similar structure-property relations involving substitution of transition-metal centers and control of the flexible molecular architecture are likely to exist in other molecule-based multiferroics.

Photoluminescent and magnetic properties of lanthanide containing apatites: NaxLn10-x(SiO4)6O2-yFy, CaxLn10-x(SiO4)6O2-yFy (Ln = Eu, Gd, and Sm), Gd9.34(SiO4)6O2, and K1.32Pr8.68(SiO4)6O1.36F0.64.

Single crystals of NaEu(9)(SiO(4))(6)O(2), Na(1.5)Eu(8.5)(SiO(4))(6)OF, Na(1.64)Gd(8.36)(SiO(4))(6)O(0.72)F(1.28), Gd(9.34)(SiO(4))(6)O(2), Ca(2.6)Eu(7.4)(SiO(4))(6)O(1.4)F(0.6), Ca(4.02)Sm(5.98)(SiO(4))(6)F(2), and K(1.32)Pr(8.68)(SiO(4))(6)O(1.36)F(0.64) and powders of NaEu(9)(SiO(4))(6)O(2), Na(1.5)Eu(8.5)(SiO(4))(6)OF, Eu(9.34)(SiO(4))(6)O(2), and Gd(9.34)(SiO(4))(6)O(2) were synthesized via flux growth in selected alkali-fluoride melts. All of the compounds adopt the apatite structure with space group P6(3)/m. Luminescence and magnetic data were collected on NaEu(9)(SiO(4))(6)O(2), Na(1.5)Eu(8.5)(SiO(4))(6)OF, Eu(9.34)(SiO(4))(6)O(2), and Gd(9.34)(SiO(4))(6)O(2). Luminescent data indicate that changing the cations and anions that surround the lanthanide site does not change the luminescent properties, making apatites versatile structures for optical materials.