Ν1 Symmetric Stretch Research Articles

Self-assembly of silver nanoparticles as high active surface-enhanced Raman scattering substrate for rapid and trace analysis of uranyl(VI) ions

A facile surface-enhanced Raman scattering (SERS) substrate based on the self-assembly of silver nanoparticles on the modified silicon wafer was obtained, and for the first time, an advanced SERS analysis method basing on this as-prepared substrate was established for high sensitive and rapid detection of uranyl ions. Due to the weakened bond strength of OUO resulting from two kinds of adsorption of uranyl species (“strong” and “weak” adsorption) on the substrate, the ν1 symmetric stretch vibration frequency of OUO shifted from 871cm−1 (normal Raman) to 720cm−1 and 826cm−1 (SERS) along with significant Raman enhancement. Effects of the hydrolysis of uranyl ions on SERS were also investigated, and the SERS band at ~826cm−1 was first used to approximately define the constitution of uranyl species at trace quantity level. Besides, the SERS intensity was proportional to the variable concentrations of uranyl nitrate ranging from 10−7 to 10−3molL−1 with an excellent linear relation (R2=0.998), and the detection limit was ~10−7molL−1. Furthermore, the related SERS approach involves low-cost substrate fabrication, rapid and trace analysis simultaneously, and shows great potential applications for the field assays of uranyl ions in the nuclear fuel cycle and environmental monitoring.

Infrared diode laser spectroscopy of O2—N2O van der Waals complex in the ν1 symmetric stretch region of N2O

The rovibrational spectrum of O2—N2O van der Waals complex is measured in the ν1 symmetric stretch region of N2O monomer using a tunable diode laser spectrometer. The complex is generated by a slit-pulsed supersonic expansion with gas mixtures of O2, N2O, and He. Both a- and b-type transitions are observed. The effective Hamiltonian for an open-shell complex consisting of a diatomic molecule in a 3Σ electronic state and a closed-shell partner is used to analyze the observed spectrum. Molecular constants in the vibrationally excited state are determined accurately. The band-origin of the spectrum is determined to be 1284.7504(25) cm−1, red-shifted from that of the N2O monomer by ∼ 0.1529 cm−1.

A high pressure infrared spectroscopic study of PbCO3-cerussite: constraints on the structure of the post-aragonite phase

We have measured the infrared spectrum of aragonite-structured PbCO3-cerussite to 41 GPa at 300 K in the diamond anvil cell. We observed a phase transition from an orthorhombic to a trigonal structure beginning at ~15 GPa, manifested by a splitting of the ν2-out-of-plane bending vibration and a broadening and dramatic decrease in amplitude of the ν1-symmetric stretching vibration of the carbonate group. While the locations of the ν1-symmetric stretching and ν4-in-plane bending bands are similar between the low- and high-pressure phases, their mode shifts and peak shapes change markedly near the transition. In particular, the ν1 symmetric stretch has an essentially zero pressure shift in the high pressure phase, and its dramatically enhanced peak width indicates that it may be symmetry forbidden. The decreased mode shifts of the carbonate vibrations after the phase transition suggest that the carbonate group is less compressible in the new structure. The spectral changes observed are consistent with a small, trigonal unit cell, with space group $${P\bar{3}{1c}}$$ and two formula units, instead of a previously proposed orthorhombic cell with sixteen formula units. This structure is identical to that of the high-pressure phase of BaCO3, and likely CaCO3 as well. Our results thus indicate that the post-aragonite, high-pressure phase of divalent-cation carbonates may be a comparatively high-symmetry trigonal structure.

Collision effects in nitrogen and methane coherent anti-Stokes Raman isotropic Q-branch spectra at high densities

Using coherent anti-Stokes Raman spectroscopy (CARS) the spectral shift and width of the collisionally narrowed Q-branch structures of nitrogen and the ν1 symmetric stretch vibration in methane were investigated at high densities. The gas samples either contained the pure substance or, for the case of nitrogen and methane, were diluted with argon, methane and carbon monoxide or argon and nitrogen, respectively, in the pressure range 50–2000 bar and at temperatures between 300 and 700 K. The simultaneous recording of spectra at ambient conditions ensured a frequency measurement accuracy of 0.07 cm−1. Contributions to the line shapes and frequency shifts are determined that originate from narrowing of the rotational structure and from vibrational dephasing in nitrogen, methane, and its mixtures. The results are compared with quasiclassical calculations of the band shape and shift to determine thermally averaged collision cross sections for energy relaxation and vibrational dephasing as a function of temperature. In the investigated density regime, for nitrogen the band shape is dominated by collisional narrowing. The peak position of the band does not strongly depend on composition of the sample and the maximum red shift of the Raman frequency diminishes with increasing temperature. For methane at densities above 50 amagat effects from rotational relaxation are no longer detectable and dephasing collisions are dominant. In addition to vibration–translation relaxation, vibrational energy transfer is an important process for line broadening at high densities. The frequency shift of the Q-band strongly depends on mixture composition and temperature.

CH3 state distributions form the reactions of O(1D) with saturated and chlorinated hydrocarbons

CH3 product state distributions arising from the reaction of O(1D) with CH4, C2H6, C3H8, and i-C4H10 were characterized using resonantly enhanced multiphoton ionization (REMPI). The vibrational distributions in the ν1 symmetric stretch and in the ν2 ‘‘umbrella’’ mode are noninverted in all cases. The results are compared to statistical adiabatic channel-phase space theory (SACM-PST) calculations. The ν2 excitation is much less excited than statistically expected for the reactions with CH4 and C2H6 but nearly statistical in the case of the heavier hydrocarbons. For all reactions but the one with C2H6, the ν1 excitation is similar to the distribution statistically expected. The CH3 rotational distribution from the reaction of O(1D) with CH4 is much hotter than room temperature as comparison with simulated rotational band contours showed. From the integrated CH3 band intensities, we obtained approximate reaction cross sections for the CH3 channel with a decrease in the order CH3≳C2H6≳C3H8≳i-C4H10. In the reactions of O(1D) with CH3Cl, CH3CH2Cl, and CH3CH2CH2Cl, we observed CH3 as a primary product. CH3–ν1 excitation is small in all cases but ν2 excitation is considerable with even an inverted v=1/v=0 distribution in the case of the reaction with C2H5Cl. The relative reaction cross sections for the CH3 channel decrease in the order CH4≫CH3Cl≳CH3CH2Cl≳CH3CH2CH2Cl.

High resolution study of the ν1 vibration of CH3 by coherent Raman photofragment spectroscopy

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational–rotational Q-branch structure of the ν1 symmetric stretch of methyl radicals produced by 266 nm laser photolysis of methyl iodide. Spectra were recorded in both flow cells and free jet expansions at instrumental resolutions ranging from 0.25 to 0.005 cm−1. Due to the high recoil velocity of the CH3 fragment, Doppler and collisional broadening of the transitions is appreciable. Even at the highest resolution such broadening of the transitions leads to interference effects among the closely spaced Raman transitions that influence both the line positions and intensities in the observed CARS spectra. The molecular parameters (cm−1) obtained from the analysis are ν1=3004.42(4), αB1=0.0851(8), αC1=0.0475(7), DN1−DN0=−0.000 046(8), DNK1−DNK0=0.000 083(20), and, with assumptions, DK1−DK0=−0.000 039. These results and infrared data in the literature yield a CH bond length of 1.08378(5) Å for the (1000) state and, with some assumptions, an equilibrium bond length Re of 1.076 Å for this prototypic case of sp2 bonding.

Examination of the 2A′2 and 2E″ states of NO3 by ultraviolet photoelectron spectroscopy of NO−3

The photoelectron spectrum of the NO−3 anion has been obtained at 266 and at 213 nm. The 266 nm spectrum probes the 2A′2 ground state of NO3. The 213 nm spectrum represents the first observation of the 2E″ lowest-lying excited state of NO3. The 2A2 band shows vibrational progressions in the ν1 symmetric stretch and the ν4 degenerate in-plane bend of NO3. Our analysis of this band indicates that the NO3 ground state has a D3h equilibrium geometry and is vibronically coupled to the 2E′ second excited state via the ν4 mode. We also obtain the electron affinity of NO3, 3.937±0.014 eV, and the heat of formation of NO3 at 298 K, 0.777±0.027 eV (17.9±0.6 kcal/mol). The 2E″ state of NO3 lies 0.868±0.014 eV above the ground state. The 2E″ band shows complex and extensive vibrational structure. Several possible assignments of this structure are discussed.

Counting vibrational quanta with a diode laser probe: Bending and stretching excitation in CO2 caused by collisions with hot atoms from excimer laser photolysis

A high resolution (10−3 cm−1) cw diode laser probe technique has been developed and used to determine the number of CO2 vibrational quanta of each kind (ν3 antisymmetric stretch, ν2 bend, and ν1 symmetric stretch) produced as a result of collisions between translationally hot hydrogen atoms and CO2 molecules. The experimental method relies on probes of the CO2 vibrational transitions mnlp → mnl( p+1) all of which ‘‘ride’’ the large oscillator strength of the fundamental 0000 → 0001 antisymmetric stretching transition. Transitions with different values of m, n, l, and p are easily separated due to the narrow spectral characteristics of the diode laser and the small anharmonicities associated with different vibrational quantum numbers. The probability for excitation of a CO2 ν3 quantum by collisions with hot hydrogen atoms produced by 193 nm excimer laser photolysis of H2S is about 1% per gas kinetic collision. Bending (ν2) quanta are produced about 5–6 times more efficiently than (ν3) antisymmetric stretching quanta. A precise value for the excitation probability of symmetric stretching (ν1) quanta cannot be obtained due to rapid equilibration between ν1 and 2ν2, but the number of ν1 quanta is found to be roughly one third the number of ν3 quanta. The thermalization rate for cooling hot hydrogen atoms below threshold for excitation of a CO2 ν3 quantum corresponds to two H atom/H2S collisions or 16 H atom/CO2 collisions.

Photoacoustic Raman spectroscopy (PARS) using cw laser sources

We have devised a new nonlinear spectroscopic technique, photoacoustic Raman spectroscopy (PARS), which uses acoustic methods to detect energy deposited in a molecular gas by the process of stimulated Raman scattering. The technique was demonstrated by stimulated Raman excitation of the 2916.7-cm−1 ν1 symmetric stretch vibrational mode of CH4 using two cw laser sources (an argon ion laser at 514.5 nm and a tunable dye laser near 605.4 nm). Excellent agreement between calculated and experimentally observed PARS signals was obtained for CH4.

Infrared and Raman spectra and structures of matrix isolated magnesium dihalides: MgF2, MgCl2, MgBr2, and MgI2

Infrared and Raman spectra are presented for the magnesium dihalides isolated in matrices. Of particular interest is the Ramen observation of the ν1 symmetric stretch in argon matrices at 550.0, 326.5, 197.9, and 147.6 cm−1 for MgF2, MgCl2, MgBr2, and MgI2, respectively. For MgF2, ν1 had previously been assigned to a strong infrared feature near 480 cm−1, and this activity (along with isotopic frequency shifts) has been cited as evidence for a nonlinear structure for MgF2. The present results undermine this argument and a linear structure is favored, a result consistent with the failure to detect a dipole moment in gas phase electric deflection measurements. Force constants calculated for a linear structure are compared with values obtained from several theoretical models and the results favor an ion–ion interaction model in which the bonding is essentially ionic. Finally, features due to MgX2 dimers are assigned on the basis of a bridged D2h model, and some features attributed to weak complexes between MgX2 and impurities in the matrix are discussed.

Vibrational relaxation of the N2O ν1 mode by Ar, N2, H2O, and NO

The vibrational relaxation of the N2O ν1 symmetric stretch mode and the ν2 bending mode due to collisions with Ar, N2, and H2O were studied at room temperature using a laser fluorescence technique. The V–V equilibration time constants between the ν1 and ν2 modes were measured to be 0.38, 0.26, and 3.0×10−3 μsec⋅atm with Ar, N2, and H2O, respectively, as the collision partner. The ν2 V–T relaxation time constants of 12.1, 1.75, and 9.4×10−3 μsec⋅atm were measured, respectively, for collisions with Ar, N2, and H2O. In both relaxation processes H2O was found to be an extremely efficient relaxer. The effects of N2O–NO collisions were studied. The result suggested a fast V–V equilibration possibly due to the near resonance between the (11 1O) level of N2O and the first excited vibrational level of NO.

Vibrational relaxation of the N2O ν1 mode

The vibrational relaxation of the ν1 symmetric stretch mode of N2O was studied at room temperature following an overtone excitation via a DF laser. The ν1 and ν3 (asymmetric stretch) fluorescence at 7.8 and 4.5 μ were, respectively, monitored as a function of time. A V–V equilibration time constant of 0.062 μsec⋅atm between the ν1 and ν2 (bending) modes was measured. The vibrational relaxations out of the ν2 and ν3 modes were measured to be 1.3 and 2.1 μsec⋅atm, respectively, with good agreement compared to previous measurements reported in the literature.

Assignment of the Infra-red and Raman Bands of Liquid Water

THE assignment of the fundamental vibrational frequencies of water vapour is well established1: 3,651.7 cm−1 ν1 symmetric stretch, 1,595.0 cm−1 ν2 bending and 3,755.8 cm−1 ν3 antisymmetric stretch, of symmetry species a1, a1 and b1 respectively for molecular symmetry C2V.