Infrared Absorption Lines Research Articles

Atmospheric pressure as a natural climate regulator for a terrestrial planet with a biosphere

Lovelock and Whitfield suggested in 1982 that, as the luminosity of the Sun increases over its life cycle, biologically enhanced silicate weathering is able to reduce the concentration of atmospheric carbon dioxide (CO(2)) so that the Earth's surface temperature is maintained within an inhabitable range. As this process continues, however, between 100 and 900 million years (Ma) from now the CO(2) concentration will reach levels too low for C(3) and C(4) photosynthesis, signaling the end of the solar-powered biosphere. Here, we show that atmospheric pressure is another factor that adjusts the global temperature by broadening infrared absorption lines of greenhouse gases. A simple model including the reduction of atmospheric pressure suggests that the life span of the biosphere can be extended at least 2.3 Ga into the future, more than doubling previous estimates. This has important implications for seeking extraterrestrial life in the Universe. Space observations in the infrared region could test the hypothesis that atmospheric pressure regulates the surface temperature on extrasolar planets.

Infrared Absorption Spectra of t‐HNOH Radicals Generated on VUV Irradiation of NO in Solid Hydrogen

Photoproduct signature: Irradiation of solid hydrogen near 3 K containing NO with vacuum-UV light from synchrotron radiation yields new infrared absorption lines at 1241.7, 1063.6 and 726.2 cm(-1) (see figure). These new lines are assigned to vibrational modes of t-HNOH. This photoproduct is formed from electronically excited NO reacting with neighboring hydrogen in the solid sample.Irradiation of solid H(2) near 3 K containing NO with vacuum-ultraviolet light from a synchrotron yields new infrared absorption lines at 1241.7, 1063.6 and 726.2 cm(-1). The structures of four possible structural isomers: H(2)NO, t-HNOH, c-HNOH and NOH(2), their vibrational wavenumbers, IR intensities and D-isotopic shifts are calculated with density-functional theory according to B3LYP and PW91PW91/aug-cc-pVTZ methods. Based on the results of those calculations and of experiments with deuterium labeling, we assign the new lines to nu(4) (cis bending), nu(5) (N==O stretching) and nu(6) (out-of-plane deformation) modes, respectively, of t-HNOH. This photoproduct is formed through reaction of electronically excited NO with neighboring H(2) in the solid sample.

Rotor in a cage: Infrared spectroscopy of an endohedral hydrogen-fullerene complex

We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C(60). Narrow infrared absorption lines at the temperature of 6 K correspond to vibrational excitations in combination with translational and rotational excitations and show well-resolved splittings due to the coupling between translational and rotational modes of the endohedral H(2) molecule. A theoretical model shows that H(2) inside C(60) is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the relative concentration of ortho- and para-H(2).

Absorption Line Survey of $\mathrm{H}\,^{+}_{3}$ toward the Galactic Center Sources. II. Eight Infrared Sources within 30 pc of the Galactic Center

Infrared absorption lines of H þ , including the metastable R(3;3) l line, have been observed toward eight bright infrared sources associated with hot and massive stars located in and between the Galactic center cluster and the Quintuplet cluster 30 pc to the east. The absorption lines with high-velocity dispersion arise in the Galaxy’s central molecular zone (CMZ) as well as in foreground spiral arms. The temperature and density of the gas in the CMZ, as determined from the relative strengths of the H þ lines, are T ¼ 200Y300 K and n � 50Y200 cm � 3 . The detection of high column densities of H þ toward all eight stars implies that this warm and diffuse gaseous environment is widespread in the CMZ. The products of the ionization rate and path length for these sight lines are 1000 and 10 times higher than in dense and diffuse clouds in the Galactic disk, respectively, indicating that the ionization rate, � , is not less than 10 � 15 s � 1 and that L is at least on the order of 50 pc. The warm and diffuse gas is an important component of theCMZ,inadditiontothethreepreviouslyknowngaseousenvironments:(1)coldmolecularcloudsobservedbyradio emission of CO and other molecules; (2) hot (T ¼ 10 4 Y10 6 K) and highly ionized diffuse gas (ne ¼10Y100 cm � 3 ) seeninradiorecombinationlines,farinfraredatomiclines,andradio-wavescattering;and(3)ultrahot(T ¼10 7 Y10 8 K) X-ray emitting plasma. Its prevalence significantly changes the understanding of the environment of the CMZ. The sight line toward GC IRS 3 is unique in showing an additional H þ absorption component, which is interpreted as being due to either a cloud associated with circumnuclear disk or the ‘‘50 km s � 1 cloud’’ known from radio observations.Aninfraredpumpingschemeisexaminedasamechanismtopopulatethe(3,3)metastablelevelinthiscloud. Subject headingg Galaxy: center — ISM: clouds — ISM: lines and bands — ISM: molecules —

On the role of molecular clustering on infrared absorption line shapes of acetylene in a supersonic expansion

A supersonic expansion containing acetylene seeded into Ar and produced from a circular nozzle is investigated using CW/cavity ring down spectroscopy, in the 1.5 μm range. The results, also involving experiments with pure acetylene and acetylene-He expansions, as well as slit nozzles, demonstrate that the denser central section in the expansion is slightly heated by the formation of acetylene aggregates, resulting into a dip in the monomer absorption line profiles. Acetylene–Ar aggregates are also formed at the edge of the circular nozzle expansion cone.

Stress-Induced Splitting and Shift of Infrared Absorption Lines of Platinum–Hydrogen Complexes in Si

We have studied the stress-induced splitting and shift of infrared (IR) absorption lines of platinum–hydrogen complexes in Si by Fourier-transform IR (FT-IR) spectroscopy at 10 K combined with the application of uniaxial stress. We observed several peaks at 1873.0, 1880.7, 1891.8, 1922.3, 1929.9, and 1943.0 cm-1. These peaks may be due to the previously observed local vibrational modes (LVMs) of various platinum–hydrogen complexes, that is, the antisymmetric-stretching mode of (Pt–H2)0, the LVM of (Pt–H1)0, the symmetric-stretching mode of (Pt–H2)0, LVMs of (Pt–H3)- and (Pt–H3)0, and the LVM of an unidentified platinum–hydrogen complex, respectively. The peaks of (Pt–H1)0 and (Pt–H2)0 split into two components under <100 > compressive stress, while those of (Pt–H3)- and (Pt–H3)0 did not split. We measured the stress dependence of the two split components of the (Pt–H1)0 peak. The low-wavenumber component shifted by -3.2 cm-1/GPa, and the high-wavenumber component shifted by 4.2 cm-1/GPa. We show that these wavenumber shifts are equal to the piezospectroscopic tensor elements, A1 and A2, respectively. We propose a structural model of the Pt–H1 complex, where a hydrogen atom is bonded to one of four Si neighbors of the Pt atom in the {110} mirror plane perpendicular to the <110 > symmetric axis with C1h symmetry.

Infrared absorption line shapes in the classical limit: A comparison of the classical dipole and fluctuating frequency approximations

Infrared spectroscopy is a versatile technique for probing the structure and dynamics of condensed-phase systems. Simulating infrared absorption spectra with molecular dynamics (MD) offers a powerful means to establish a molecular-level interpretation of experimental results, as well as a basis for the parametrization of more accurate simulation force-fields. Two distinct methods for the calculation of infrared absorption line shapes of high-frequency (Planck's omega/k(B)T>>1) vibrational probes from MD simulations are examined: The classical dipole approximation (CDA) and the fluctuating frequency approximation (FFA). Although these two formalisms result in expressions for the infrared absorption line shape that appear very different, both approximations are shown to yield identical results for the infrared line shape of a harmonic system in the condensed-phase. The equivalence of the FFA and CDA is also demonstrated in the case where the transition dipole of the oscillator fluctuates in response to the environment (i.e., where the Condon approximation has been relaxed). Finally we examine the effects of solute anharmonicity and demonstrate that the CDA and FFA are not equivalent in general, and the magnitude of the deviations increases with anharmonicity. We conclude that the calculation of infrared absorption line shapes via the CDA is a promising alternative to the FFA approach in cases where it may be difficult or undesirable to employ the latter, particularly when the effects of anharmonicity are small.

Nitrile Groups as Vibrational Probes: Calculations of the C≡N Infrared Absorption Line Shape of Acetonitrile in Water and Tetrahydrofuran

The C[TRIPLE BOND]N bond is a powerful probe of protein structure and dynamics because it absorbs in a region of the infrared spectrum apart from the other vibrations that occur naturally in proteins, and because its infrared absorption line shape is sensitive to specific characteristics of the local environment. Since the polarity experienced by the probe can differ dramatically within the protein, infrared spectroscopy of a C[TRIPLE BOND]N site-specifically labeled residue can be used to infer its local environment within the protein. It has been shown experimentally that the spectrum of acetonitrile in water is different in terms of peak position and width compared to acetonitrile in tetrahydrofuran (THF). An optimized quantum mechanics/molecular mechanics method for calculating accurate vibrational frequencies in condensed-phase was parametrized for acetonitrile in water. The transferability of the methodology to a different solvent was tested by computing the infrared line shapes of acetonitrile in both water and THF and comparing to experiment. The infrared absorption line shapes agree well with experiment in each case, and the trends observed experimentally are recovered. The accuracy of the methodology for two solvents of differing polarity indicates that this technique is suitable to study CN probes in proteins.

Line shapes of narrow optical bands: Infrared absorption byUcenters and heavier impurities in alkali halides

The shape of the bands for photon absorption and emission by the local constituents of a solid is governed mainly by processes involving many low-energy acoustic phonons. This applies not only to wide bands, such as those exhibited by F centers, but also to narrow ones, as those observed for infrared absorption by local vibration modes of U centers and heavier impurities. The line shapes are theoretically studied on a general basis to show they provide a nice example to illustrate the power of field theory and methods to reproduce experimental facts. To this aim, the phonon induced broadenings of infrared absorption lines by U centers in KCl and KBr, and by substitutional Ag+ in KI, were calculated to compare theoretical predictions with experiment. The agreement obtained between both is remarkable.

Theoretical study of the C iO i and I siC iO i defects in Si

The I SiC iO i (C4) defect consists of a Si self-interstitial trapped at the interstitial carbon–oxygen C iO i pair (C3). The C4 defect is characterized by two infrared absorption lines at 940 and 1024 cm −1. The former is C-related and the latter is O-related. The intensity of these lines begins to drop above 150 °C and they disappear around 200 °C. In this work, we present the results of first-principles calculations of the configurations, binding energies, vibrational spectra, and estimated gap levels of C3 and C4.

Performance of a Herriott cell, designed for variable temperatures between 296 and 20 K

We designed, fabricated and tested a multipath Herriott cell (or off-axis spherical mirror interferometer) to achieve low temperature absorption measurements. The cell is fabricated entirely from copper, and the 15 cm radius of curvature copper mirrors have gold coated reflective surfaces. The cell was tested at temperatures between 296 and 20 K with a folded absorption path length of 5.37 m utilizing a lead salt tunable diode laser. Short term temperature stability (1 h) of the Herriott cell is better than 0.005 K under normal operating conditions with a temperature uniformity better than 0.01 K (not measurable). The cell was tested by performing collisional cooling experiments on 13C 16O 2 in helium at temperatures between 70 and 20 K and by performing more traditional pressure broadening and shift measurements on molecular infrared absorption lines at temperatures between 300 and about 80 K on 13C 16O 2 and methane.

Infrared multiphoton ionization of superhot C60: Experiment and model calculations

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

Theoretical Study of the Electronic Structure and Stability of Titanium Dioxide Clusters (TiO2)n with n = 1−9

The electronic structure and the stability of both neutral and singly charged (TiO2)n clusters with n = 1-9 have been investigated using the density functional B3LYP/LANL2DZ method. The lowest-lying singlet clusters tend to form some compact structures with one or two terminal Ti-O bonds, which are about 1.4-2.5 eV more stable than the corresponding triplet structures. For the lowest-lying structures, strong infrared absorption lines at 988-1020 cm(-1) due to terminal Ti-O bonds and below 930 cm(-1) due to Ti-O-Ti bridging bonds may be observed, with some characteristic lines at 530-760 cm(-1) due to 3-fold coordinated O-atoms that are comparable with the spectra of rutile and anatase bulk. The holes and excited electrons within triplet structures tend to be localized on the least coordinated O- and Ti-atoms, respectively, with some exceptions possibly due to the electron-hole interaction. The extra electrons within (TiO2)n- clusters and the holes within (TiO2)n+ clusters show a clearer preference of location on the least coordinated Ti- and O-atoms, respectively. For the lowest-lying (TiO2)n clusters, the cluster formation energy per TiO2 unit and the electron affinity tend to increase whereas the ionization potential tends to decrease with the cluster size n. On the other hand, the singlet-triplet and HOMO-LUMO gaps represent the lower and upper limits of the TiO2 bulk band gaps, respectively. The theoretical results agree well with the available experimental data and may be helpful for understanding the chemistry of small (TiO2)n clusters.

Determination of molecular line parameters for acrolein (C 3H 4O) using infrared tunable diode laser absorption spectroscopy

Acrolein (C 3H 4O) molecular line parameters, including infrared (IR) absorption positions, strengths, and nitrogen broadened half-widths, must be determined since they are not included in the high resolution transmission (HITRAN) molecular absorption database of spectral lines. These parameters are required for developing a quantitative analytical method for measuring acrolein in a single puff of cigarette smoke using tunable diode laser absorption spectroscopy (TDLAS). The task is complex since acrolein has many highly overlapping infrared absorption lines in the room temperature spectrum and the cigarette smoke matrix contains thousands of compounds. This work describes the procedure for estimating the molecular line parameters for these overlapping absorption lines in the wavenumber range (958.7–958.9 cm −1) using quantitative reference spectra taken with the infrared lead-salt TDLAS instrument at different pressures and concentrations. The nitrogen broadened half-width for acrolein is 0.0937 cm −1 atm −1 and to our knowledge, is the first time it has been reported in the literature.

Free electron laser studies of local mode linewidths in SrF 2 :Gd 3+ :H –

Infrared-absorption lines of the (X,Y) and Z local modes of hydrogenic C4v centres in rare-earth doped CaF2 and SrF2 have differing 10 K linewidths, attributed in part to one-phonon decay from the upper to the lower frequency local mode. However for SrF2 the lower frequency (Z) mode has the larger line width for Gd3+, Tb3+ and Dy3+ and one-phonon decay cannot be the primary cause for the differing line widths. This is borne out by free-electron laser 10K studies of the H– lines in SrF2:0.33%Gd3+, which yield (X,Y) and Z mode lifetimes of 43 and 30 ps suggesting similar homogeneous linewidths. The larger line-width for the Z mode is attributed to the greater sensitivity of this mode to rare-earth ion mismatch and to crystal disorder. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Determination of molecular parameters for 1,3-butadiene and propylene using infrared tunable diode laser absorption spectroscopy

A technique has been developed for the determination of molecular parameters, including infrared absorption line positions, strengths, and nitrogen-broadened half-widths for 1,3-butadiene (C 4H 6) and propylene (C 3H 6). The parameters for these two molecules are required for quantitation using Tunable Diode Laser Absorption Spectroscopy (TDLAS). These molecules have populations of highly overlapping infrared absorption lines in their room temperature spectra. The technique reported here provides a procedure for estimating the molecular parameters for these overlapping absorption lines from quantitative reference spectra taken with the TDLAS instrument at different pressures and concentrations. The system was developed for the quantitation of gaseous constituents in a single puff of cigarette smoke and this paper will describe the procedure and some of the factors that influence the accuracy of quantitation for 1,3-butadiene, including the approach taken to minimize the adverse effects of the absorption due to propylene in the same spectral region.

Hyperfine patterns of infrared absorption lines of Ho3+ C4v centres in CaF2

Hyperfine structure is observed on several infrared transitions to the 5I7 and 5I6 multiplets of the Ho3+ C4v centre in CaF2. Particularly complex and detailed hyperfine structures are observed for transitions to the Y3 level of the 5I7 multiplet, which include the appearance of ΔIz ≠ 0 transitions. The hyperfine lines of this level are satisfactorily accounted for by strong mixing of close-lying levels of this multiplet by the perpendicular hyperfine interaction, which leads to relaxation of the ΔIz = 0 selection rule and a redistribution of the hyperfine intensities over many transitions. An excellent correspondence is found between the measured and simulated spectra.

Structural and vibrational properties of {N,N} pairs and {N,H} complexes in Si

First-principles molecular-dynamics simulations are used to predict the structures and binding energies of interstitial nitrogen ${\mathrm{N}}_{\mathrm{i}},$ substitutional nitrogen ${\mathrm{N}}_{\mathrm{s}},$ the ${\mathrm{N}}_{\mathrm{i}}$-self-interstitial complex, the ${{\mathrm{N}}_{\mathrm{i}},{\mathrm{N}}_{\mathrm{i}}},$ ${{\mathrm{N}}_{\mathrm{i}},{\mathrm{N}}_{\mathrm{s}}}={{\mathrm{N}}_{2},\mathrm{V}},$ and ${{\mathrm{N}}_{\mathrm{s}},{\mathrm{N}}_{\mathrm{s}}}={{\mathrm{N}}_{2},{\mathrm{V}}_{2}}$ pairs (V is the vacancy). The interactions of N with H in Si are studied and the properties of several ${\mathrm{N},\mathrm{H}}$ complexes are predicted. The dynamical matrices yield many new local and pseudolocal vibrational modes associated with the impurities and their Si nearest neighbors. The unidentified ${\mathrm{N},\mathrm{H}}$-related infrared absorption line reported by Pajot et al. is assigned to the ${{\mathrm{N}}_{\mathrm{s}},\mathrm{H}}$ complex.

Combined electronic structure/molecular dynamics approach for ultrafast infrared spectroscopy of dilute HOD in liquid H2O and D2O.

We present a new approach that combines electronic structure methods and molecular dynamics simulations to investigate the infrared spectroscopy of condensed phase systems. This approach is applied to the OH stretch band of dilute HOD in liquid D2O and the OD stretch band of dilute HOD in liquid H2O for two commonly employed models of water, TIP4P and SPC/E. Ab initio OH and OD anharmonic transition frequencies are calculated for 100 HOD x (D2O)n and HOD x(H2O)n (n = 4-9) clusters randomly selected from liquid water simulations. A linear empirical relationship between the ab initio frequencies and the component of the electric field from the solvent along the bond of interest is developed. This relationship is used in a molecular dynamics simulation to compute frequency fluctuation time-correlation functions and infrared absorption line shapes. The normalized frequency fluctuation time-correlation functions are in good agreement with the results of previous theoretical approaches. Their long-time decay times are 0.5 ps for the TIP4P model and 0.9 ps for the SPC/E model, both of which appear to be somewhat too fast compared to recent experiments. The calculated line shapes are in good agreement with experiment, and improve upon the results of previous theoretical approaches. The methods presented are simple, and transferable to more complicated systems.

Calculated properties of nitrogen-vacancy complexes in beryllium- and magnesium-doped GaN

The properties of defect complexes consisting of a nitrogen vacancy with a substitutional beryllium or magnesium atom on neighboring lattice sites in hexagonal GaN are calculated using the AIMPRO local-densityfunctional theory method. Both types of defects VN-BeGa and VN-MgGa are bound with respect to their isolated constituents. They do not appear to have any electronic levels in the bandgap, and are expected to be neutral defects. Important structural differences are found. In its minimum energy configuration, the Be atom in the VN-BeGa complex lies nearly in the same plane as the three equivalent N atoms nearest to it. Thus, it has shorter Be-N bonds than the Ga-N distance in the bulk crystal, while the Mg atom in the VN-MgGa complex occupies a position closer the lattice site of the Ga atom it replaces. Hence, the VN-BeGa complex has a larger open volume than the VN-MgGa complex. This is consistent with positron annihilation experiments @Saarinen et al., J. Cryst. Growth 246, 281 ~2002!; Hautakangas et al., Phys. Rev. Lett. 90, 137402 ~2003!#. The frequency of the highest local vibrational mode of the VN-BeGa center is calculated to be within 3‐ 4 % of an infrared absorption line detected in Be-doped GaN @Clerjaud ~private communication!#.