Frequency Blue Shift Research Articles

Vibrational Sum-Frequency Spectrum of the Water Bend at the Air/Water Interface

We present the spectrum of the water bend vibrational mode (ν2) at the air/water interface measured using vibrational sum-frequency generation (SFG). The blue-shift of the ν2 frequency from the gas phase value reports on the hydrogen bonding in the interfacial region. The ν2 line shape of surface water is inhomogeneously broadened and structured. The dominant feature is the least blue-shifted and relatively narrow Lorenztian, tentatively assigned to water molecules straddling the interface, those with free OH bonds. This feature appears at different frequencies in the SFG spectra recorded using different polarization combinations (SSP and PPP for SFG-visible-IR), pointing to possible orientational inhomogeneity. Weaker features are observed at higher frequencies, tentatively assigned to fully H-bonded water molecules. Small but measurable changes of the line shape with temperature are observed in the 0 to +20 °C range.

Rattler model of the boson peak at silica surfaces

Recent experiments unveiled two new aspects of the low-energy excitation spectrum of silica glass--commonly termed as the "boson peak" region. The first is that at low temperature the silica surface exhibits a different, softer boson peak than the bulk. The second is a giant thermal blueshift of the surface boson peak frequency causing it to cross and overcome the bulk peak with increasing temperature. Here we present a simple lattice model that reproduces this behavior in all its aspects. Each site consists of rigid tetrahedral units softly connected so as to be able to rotate anharmonically as "rattlers" in their cages. As shown by simulations, the model dynamics exhibits a boson-like peak, which has lower frequency at the surface where rattlers have a weaker restoring force. Upon heating however the larger angular freedom of surface units allows them to rattle more than in the bulk, leading to a steeper frequency increase similar to experiment.

FTIR investigation of the O–H···Xe interaction in simple carboxylic acids in solid xenon

The O-H stretching region of the infrared spectra of a series of carboxylic acids in Xe matrices was investigated as a function of temperature. Upon increasing the temperature, the νO-H band site-components undergo reversible frequency blue-shifts, which are larger for the lowest-frequency components. This unprecedented observation indicates both that different types of O-H···Xe specific interactions occur, depending on different trapping sites, and the prevalence of stronger interactions of this type for molecules trapped in sites corresponding to lower frequency νO-H band site-components. These results are in agreement with previous investigations pointing to an increased stabilization and larger νO-H frequency red-shifts in carboxylic acid∕Xe complexes bearing a specific H-bond like O-H···Xe interaction. O-H···Xe interaction energies were obtained theoretically and also estimated from the spectroscopic data. Changes in the interaction energies upon temperature variation were also evaluated.

Superconducting fluctuations and anomalous phonon renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature Tc = 28.3 K, reveal that the Raman mode at ∼230 cm−1 shows a sharp jump in frequency by ∼2% and linewidth increases by ∼175% at To ∼ 60 K. Below To, anomalous softening of the mode frequency and a large decrease by ∼10 cm−1 in the linewidth are observed. These precursor effects at T0 (∼2Tc) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weak coupling between the well separated (∼15 Å) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K and 60 K (∼7%) indicates strong spin-phonon coupling in this superconductor.

A tunable hybrid metamaterial absorber based on vanadium oxide films

A tunable hybrid metamaterial absorber (MA) in the microwave band was designed, fabricated and characterized. The hybrid MA was realized by incorporating a VO2 film into the conventional resonant MA. By thermally triggering the insulator–metal phase transition of the VO2 film, the impedance match condition was broken and a deep amplitude modulation of about 63.3% to the electromagnetic wave absorption was achieved. A moderate blue-shift of the resonance frequency was observed which is promising for practical applications. This VO2-based MA exhibits many advantages such as strong tunability, frequency agility, simple fabrication and ease of scaling to the terahertz band.

Light-Controlled Plasmon Switching Using Hybrid Metal-Semiconductor Nanostructures

We present a proof of concept for the dynamic control over the plasmon resonance frequencies in a hybrid metal-semiconductor nanoshell structure with Ag core and TiO(2) coating. Our method relies on the temporary change of the dielectric function ε of TiO(2) achieved through temporarily generated electron-hole pairs by means of a pump laser pulse. This change in ε leads to a blue shift of the Ag surface plasmon frequency. We choose TiO(2) as the environment of the Ag core because the band gap energy of TiO(2) is larger than the Ag surface plasmon energy of our nanoparticles, which allows the surface plasmon being excited without generating electron-hole pairs in the environment at the same time. We calculate the magnitude of the plasmon resonance shift as a function of electron-hole pair density and obtain shifts up to 126 nm at wavelengths around 460 nm. Using our results, we develop the model of a light-controlled surface plasmon polariton switch.

Periodic interactions between solitons and dispersive waves during the generation of non-coherent supercontinuum radiation

We present a numerical study of interactions between dispersive waves (DWs) and solitons during supercontinuum generation in photonic crystal fibers pumped with picosecond laser pulses. We show how the soliton-induced trapping potential evolves along the fiber and affects the dynamics of a DW-soliton pair. Individual frequency components of the DW periodically interact with the soliton resulting in stepwise frequency blue shifts. In contrast, the ensemble blue shifts of all frequency components in the DW appear to be quasi-continuous. The step size of frequency up-conversion and the temporal separation between subsequent soliton-DW interactions are governed by the potential well which confines the soliton-DW pair and which changes in time.

Covalently Bound Azido Groups Are Very Specific Water Sensors, Even in Hydrogen-Bonding Environments

Covalently bound azido groups are found in many commercially available biomolecular precursors and substrates, and the NNN asymmetric stretching band of these groups is a strong infrared absorber that appears in a spectral region clear of other signals. In order to evaluate comprehensively the solvatochromism of the asymmetric azido NNN stretching band for site-specific use in biomolecular contexts, infrared spectra of the model compounds 5-azido,1-pentanoic acid and 3-(p-azidophenyl),1-propanoic acid were acquired in a large variety of nonpolar, polar, and hydrogen-bond-donating solvents, as well as mixed aqueous-organic solvents. Spectra in pure solvents indicated that the aliphatic NNN stretching frequency maximum does not depend on solvent polarity, while the aromatic NNN frequency displays a weak but nonzero sensitivity to polarity. In both cases, the NNN frequency exhibits a blue-shift in H-bond-donating solvents, but the frequency in water is higher than in any other H-bond-donating solvent including solvents that are stronger H-bond donors. In nonfluorinated H-bond donor solvents, the frequency blue shift scales with the density of H-bond donors. This sensitivity to the presence of water was further explored in several mixed solvent environments, with the conclusion that this vibrational mode is a highly specific sensor of hydration, even in environments containing other H-bond donors like amides and alcohols, due to the very high local density of H-bond donors in water. The relatively uncomplicated (compared to nitriles, for example), water-specific response of this vibrational mode should lead to its adoption as a site-specific probe of hydration in many different possible systems in which the presence and role of molecular water is of primary interest.

Effects of Mode-Mode Coupling on Vibrational Frequency Blue Shift in Improper H-Bonded Systems

Bond shortening of NH/CH due to improper hydrogen bonding is always thought to be accompanied by a blue shift in its corresponding frequency.However,results here show that owing to anharmonic effects,especially the contribution of mode-mode coupling,the blue shift could be greatly decreased.The strong interaction may even,in some cases,cause the previously predicted blue-shifting frequencies by harmonic methods to be red-shifted instead in the gas phase.Though these results need to be clearly verified by further infrared(IR) spectrum experiments in the gas phase for the selected systems,comparisons with previous IR results obtained from matrix isolation experiments strongly support the results of the calculations.

Identification of Genistein and Biochanin A by THz (far-infrared) vibrational spectra

The temperature dependent vibrational spectra of Genistein and Biochanin A, major components in Mongolian medicines Agi, in the range of 0.5–4.5THz (16.7–150cm−1) are presented for the first time. Over the temperature range from 295 to 77K, 12 highly resolved spectral features for the Genistein and 13 features for Biochanin A were measured by THz-TDS and display strong linewidth narrowing and frequency blue-shift with cooling. Such narrow-line THz fingerprint spectra provide a rapid, nondestructive and reliable method for the identification of these Chinese traditional medicines.

Thermo-optical control of dielectric loaded plasmonic racetrack resonators

The optical properties of racetrack shaped dielectric loaded surface plasmon polariton resonators are studied experimentally for various radius, interaction lengths, and separation gaps between the resonator and the bus waveguide. Using radiation leakage microscopy, their power transmission spectra is recorded over the telecom frequency range and modeled by a notch filter made of a lossy bus waveguide coupled to a lossy resonator. For a typical separation gap around 250 nm, the optimum critical coupling condition is determined by a radius of 6 μm and an interaction length of 2.5 μm. Extinction ratios of the order of -30 dB are reported for resonators featuring quality factors that are found to be of the order of 110. The static thermo-optical control of such racetrack resonators produces a blueshift of the resonance frequency that can be as large as 4.5 nm for a temperature increase of 75 K. Extinction ratio of the order of 9 dB can be achieved between hot and cold states.

A computational study on the ring stretching modes of halogen‐substituted pyridine involved in H‐bonding

AbstractDensity functional method B3LYP plus the AUG‐cc‐pVDZ and AUG‐cc‐pVTZ basis sets is used to investigate ring normal modes of halogen‐substituted pyridines involved in the N···HX H‐bonds with HX (X = F, Cl). The results demonstrated that the formation of hydrogen bond leads to an increase in the frequencies of the ring breathing mode v1, the N‐para‐C stretching mode v6a and the meta‐CC stretching mode v8a, whereas there is no change in the triangle mode v12 for free pyridine and a smaller blue shift for substituted pyridines. There is a strong coupling between the CY stretching vibration and the triangle mode (ortho‐ and para‐substituted) or the breathing mode (meta‐substituted) in substituted pyridines, which leads to the frequency decrease in the triangle or breathing modes. The natural bond orbital analysis suggests that electrostatic interaction and charge transfer caused by the intermolecular and intramolecular hyperconjugations are the origin of the frequency blue shift in the ring stretching modes. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Existence of both blue-shifting hydrogen bond and Lewis acid–base interaction in the complexes of carbonyls and thiocarbonyls with carbon dioxide

In this study, 16 gas phase complexes of the pairs of XCHZ and CO(2) (X = F, Cl, Br; Z = O, S) have been identified. Interaction energies calculated at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level including both BSSE and ZPE corrections range from -5.6 to -10.5 kJ mol(-1) for XCHOCO(2) and from -5.7 to -9.1 kJ mol(-1) for XCHS···CO(2). Substitution of one H atom by one halogen in formaldehyde and thioformaldehyde reduces the interaction energy of XCHZ···CO(2), while a CH(3) substitution increases the interaction energy of both CH(3)CHO···CO(2) and CH(3)CHS···CO(2). NBO and AIM analyses also point out that the strength of Lewis acid-base interactions decreases going from >C1=S3···C6 to >C1=O3C6 and to >C1-X4···C6. This result suggests the higher capacity of solubility of thiocarbonyl compounds in scCO(2), providing an enormous potential application for designing CO(2)-philic materials based on the >C=S functional group in competition with >C=O. The Lewis acid-base interaction of the types >C=S···C, >C-Cl···C and >C-Br···C is demonstrated for the first time. The contribution of the hydrogen bonding interaction to the total interaction energy is larger for XCHS···CO(2) than for XCHO···CO(2). Upon complexation, a contraction of the C1-H2 bond length and a blue shift of its stretching frequency have been observed, as compared to the isolated monomer, indicating the existence of a blue-shifting hydrogen bond in all complexes examined. Calculated results also lend further support for the viewpoint that when acting as proton donor, a C-H bond having a weaker polarization will induce a stronger distance contraction and frequency blue shift upon complexation, and vice versa.

Dynamics of Cherenkov radiation trapped by a soliton in photonic-crystal fibers

The trapping of Cherenkov radiation by Raman solitons is an important process during supercontinuum generation and has been demonstrated as an effective way to extend the Cherenkov-radiation-based wavelength conversion toward the visible band. In this Letter we demonstrate that the existence of the self-steepening effect increases the energy of the Cherenkov radiation during the trap while reducing its frequency blueshift. The frequency and energy evolutions of Cherenkov radiation are analytically studied, and the predictions are consistent with the simulations based on the generalized nonlinear Schrödinger equation.

Nonlinear particle interaction effects on Ramsey interferometry in an atom–heteronuclear molecule conversion system

We investigate the Ramsey interferometry with a nonlinear atom–heteronuclear molecule conversion boson system, where the nonlinearity arises from the interaction between coherent atoms. In this coupled system, the nonlinear interaction between particles plays a significant role in a dynamic process. The nice Ramsey-type interference fringes in the time domain have been obtained when a sequence of two identical magnetic field pulses near the Feshbach resonance is applied. We show that a blue shift (or a red shift) of frequency of Ramsey-type fringes emerges due to the repulsive particle interaction (or the attractive particle interaction). In particular, we find that the visibility of Ramsey-type patterns can be strongly modified by the nonlinear particle interaction and a much higher visibility of Ramsey-type fringes can be accessed by tuning the Ramsey pulse duration appropriately.

Theoretical study on the ring stretching modes of pyridine in the hydrogen bonding with H 2O, HCONH 2 and CH 3COOH

Density functional theory was applied to study the effects of H-bonds N⋯H–X between pyridine and H 2O, HCONH 2 and CH 3COOH on normal vibrational modes of pyridine at the B3LYP/AUG-cc-pVDZ and B3LYP/AUG-cc-pVTZ levels. The results show that the formation of H-bonds leads to an increase in frequencies of the ring breathing mode v 1, the N- para-C stretching mode v 6 a and the meta-CC stretching mode v 8 a of pyridine but there was no change in triangle mode v 12. The natural bond orbital analysis shows that the frequency blue shift in the ring stretching modes of pyridine is a corporate result of the intermolecular charge transfer caused by the intermolecular hyperconjugation n(N) → σ∗(HX) and the intramolecular charge redistribution caused by intramolecular hyperconjugation n(N) → σ∗( meta-CC) in the pyridine ring. We also found that the magnitude of the frequency blue shift increases with the strength of the hydrogen bonding.

Theoretical analysis of the (HNO)2, (HNO···HNS), and (HNS)2 dimers — A case of red and blue shifts of N–H stretching frequency

The hydrogen-bonded interactions in the simple (HNZ)2 dimers, with Z = O and S, were investigated using quantum chemical calculations with the second-order Møller–Plesset perturbation (MP2), coupled-cluster with single, double (CCSD), and triple excitations (CCSD(T)) methods in conjunction with the 6-311++G(2d,2p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets. Six-membered cyclic structures were found to be stable complexes for the dimers (HNO)2, (HNS)2, and (HNO–HNS). The pair (HNS)2 has the largest complexation energy (–11 kJ/mol), and (HNO)2 the smallest one (–9 kJ/mol). A bond length contraction and a frequency blue shift of the N–H bond simultaneously occur upon hydrogen bond formation of the N–H···S type, which has rarely been observed before. The stronger the intramolecular hyperconjugation and the lower the polarization of the X–H bond involved as proton donor in the hydrogen bond, the more predominant is the formation of a blue-shifting hydrogen bond.

Theoretical study of H bonds of HArF and HF with isoelectronic systems N2, CO, and BF

The H bonds of HArF and HF with N2, CO, and BF were studied at the MP2(full)/6-311++G(2d, 2p) level. The results show that only the complexes WY···HArF (WY = N2, OC) and WY···HF (WY = N2, OC, FB) are stable, the H-bonding WY···HArF leads to contraction of the HAr bond with a concomitant frequency blue shift, but the H-bonding WY···HF causes the HF bond to elongate with a frequency red shift. A quantity P is defined to measure polarization of the HX bond; the H bonding causes the P value of the HX bond (X = Ar, F) to increase. The HX bond length change and frequency shift in the H-bonding WY···HArF and WY···HF are mainly caused by intermolecular hyperconjugation, n(Y) → σ*(HX) (X = Ar, F), where electrostatic interaction has only a small contribution. In HArF, the strong intramolecular hyperconjugation, n(F) → σ*(HAr), can adjust electron density on σ*(HAr); upon formation of H bonding, the HAr stretching frequency blue shift is caused by a decrease of intramolecular hyperconjugation and an increase of the s character of the Ar hybrid in the HAr bond, induced by the intermolecular hyperconjugation. In the H bonds of HF without intramolecular hyperconjugation, the intermolecular hyperconjugation, n(Y) → σ*(HF), leads to a red shift of the HF bond, although there is also large rehybridization.

Spin torque driven excitations in a synthetic antiferromagnet

Spin polarized current induced self oscillations have been investigated in both the free layer and the synthetic antiferromagnetic (SAF) pinned layer of spin valve nanopillars. Compared with free layer excitations, the acoustic type SAF excitations are characterized by high emitted power and much narrower linewidth. Furthermore, in contrast to free layer excitations, the SAF in-plane precession mode exhibits an unexpected crossover from redshift (df/dI<0) to blueshift (df/dI>0) in frequency f versus current I as the in-plane magnetic field is increased. From simulations we identify this crossover as a signature of large amplitude spin current induced precessional dynamics in the SAF.

Mechanically tunable surface plasmon resonance based on gold nanoparticles and elastic membrane polydimethylsiloxane composite

Surface plasmon in nanoscaled materials has recently attracted a great deal of attention due to its possibility in a wide range of application. From a practical standpoint, it is desirable for the devices having a tunability of surface plasmon frequency. To achieve this goal, in this study, a composite consisting of two-dimensional gold nanoparticles array embedded in elastic polydimethylsiloxane (PDMS) membrane has been synthesized. Because the elastic PDMS membrane has a high malleability, with an external stress, it is very easy to regulate the interparticle distance in the gold nanoparticle array. The change in the distance between each nanoparticle will alter the surface plasmon interaction, and hence surface plasmon frequency can be manipulated. It is found that when the interparticle distance increases, the enhanced surface plasma mutual coupling will cause the blueshift of surface plasmon resonance frequency. The observed result satisfies the forecast based on electromagnetic theory.