Relaxation Of HF Research Articles

Quantum study of the rovibrational relaxation of HF by collision with 4He on a new potential energy surface.

The HF molecule is considered the main reservoir of fluorine in the interstellar medium (ISM). Also, the interactions of this molecule with the most common atoms and molecules in the ISM have attracted great interest from the astrochemical community. Collisions between HF and helium have recently caused controversy following a study using a two-dimensional SAPT potential energy surface (PES) that exhibited large discrepancies with previous scattering calculations based on more recent ab initio potentials. To address this issue, our current work aims to develop the most precise three-dimensional PES for the HF+He system. We employ the size-consistent CCSD(T) method in conjunction with the aug-cc-pV6Z basis set. The main features of the new PES as well as the bound states of the He-HF complex are compared to the existing data. The new PES is then utilised to conduct close coupling calculations that demonstrate He-HF as a good instance of vibration-rotation near resonant energy transfer. The novel rate coefficients will be accessible via the BASECOL database, and the use of the new PES is advised when describing HF in helium droplets.

Thermal behavior and dielectric properties of Fe-doped Bi2CaNb2O9

Solid solutions of Bi2CaNb2–xFexO9-δ were obtained by the method of solid phase synthesis. The thermal analysis of the samples revealed diffuse low- and high-temperature exo-effects in the DSC curves in the temperature ranges of 493–673 K and 1123–1223 K. According to the high-temperature X-ray diffraction results, the exothermic effect in the range of 1123–1223 K is related to ferroelectric phase transition from polar (A21am) to nonpolar phase (I4/mmm). The high-temperature X-ray diffraction detected no reconstructive phase transitions in calcium bismuth niobate or its solid solution in the low temperature range of 493–673 K. The study of the electrical properties of Bi2CaNb2–xFexO9-δ samples (x = 0.06 and 0.10) showed that the substitution of niobium with iron led to the increase in the direct current electrical conductivity of the sample. In this case, the dielectric loss decreased and the dielectric permittivity increased. The studied solid solutions were characterized by the low-frequency ion-migration polarization. The radical change in the electrical characteristics including the almost double activation energy growth, the maxima of the LF and HF relaxation times and the maximum of polarization inhomogeneity were observed at the temperatures of 573–598 K. Which was probably caused by the phase transition accompanied by activation of oxygen ions. Thus, the impedance spectroscopy and thermal analysis data correlate well pointing at reproducibility of the low-temperature effect recorded by the thermal analysis methods.

Spontaneous polarization, tilt angle, hysteresis and low-frequency relaxations in hydrogen bonded chiral liquid crystal dimer: M*SA:9OBA

Influence of low-frequency (2-Hz-10MHz) AC field on LC phases exhibited by chiral hydrogen bonded liquid crystal dimer, viz., M*SA:9OBA is investigated. Phase transition temperatures Tc determined by capacitance C(T) and loss factor TanΔ(T) agree with the earlier report. Induced surface polarization reflected through capacitance C(E) exhibited hysteresis in behavior SmC* phase. Growth of primary order parameter in SmC* phase investigated by tilt angle θ(T) inferred critical field exponent β1. Secondary order parameter investigated as spontaneous polarization Ps in SmC* inferred Mean field exponent β2. Dipole moment (μ) is resolved as longitudinal (μl) and transverse (μt) components. Reorientation of μ to the field analyzed. Relaxation studied at different temperatures in SmC*; SmBcryst and SmG phases inferred a low-frequency (LF fR∼100Hz) and another high frequency (HF fR∼100kHz) reorientation processes. LF relaxation in SmC* phase is influenced by dc bias field to suggest polarization helix formed by coupling of field μt. Threshold field (Vth) for polarization switching estimated from the data of field variation of loss εmax″(E). LF relaxation in SmC* is identified with Goldstone mode (GM) manifested as the suppressed ε″max(E) with field. HF relaxation in SmC* is identified with Soft Mode (SM) by its decreasing trend of fR towards TIC*. Activation energies (Ea) estimated from Arrhenius behavior of fR(T) agree with reports. Field influence on GM manifests through shift of loss ε″max(E) and fR(E). Activation energy Ea is estimated from the shift of fR(E) in terms of equivalent temperature plots. Dispersion, ε″ versus ε′ in LC phases infers a non-Debye distribution of relaxation times. Cole-Cole plots inferred large shift of static permittivity εo. Data of dielectric relaxation parameters, i.e., relaxation frequency fR, relaxation time τR, activation energy Ea, loss maxima ε″max, static permittivity (εo), high frequency permittivity ε∞, dielectric strength Δε and distribution (α) parameter are presented. Dielectric strength analyzed for SM in SmC* phase confirms Ferro-Electric Curie-Weiss behavior.

Vibrational Relaxation of HF(v=3) by CO

Vibrational Relaxation of HF(v=3) by CO

AGEs breaking and antioxidant treatment improves endothelium-dependent dilation without effect on flow-mediated remodeling of resistance arteries in old Zucker diabetic rats

BackgroundA chronic increase in blood flow in resistance arteries is associated with increased lumen diameter (outward remodeling) and improved endothelium (NO)-mediated relaxation. Flow-mediated remodeling of resistance arteries is essential for revascularization in ischemic diseases. Nevertheless, it is impaired in 12 to 24-month old rats and in young Zucker Diabetic Fatty (ZDF) rats due to advanced glycation end products (AGEs) and oxidative stress. As type 2 diabetes occurs preferentially in older subjects we investigated flow-mediated remodeling and the effect of the AGEs breaker ALT-711 associated or not to the antioxidant TEMPOL in one-year old lean (LZ) and ZDF rats.MethodsMesenteric resistance arteries were exposed to high (HF) or normal blood flow (NF) in vivo. They were collected after 2 weeks for in vitro analysis.ResultsIn LZ rats, diameter expansion did not occur despite a significant increase in blood flow in HF arteries. Nevertheless, endothelium-mediated relaxation was higher in HF than in NF arteries. ALT-711, alone or in combination with TEMPOL, restored outward remodeling in HF arteries in association with AGEs reduction. TEMPOL alone had no effect. ALT-711, TEMPOL or the combination of the 2 drugs did not significantly affect endothelium-mediated relaxation in HF and NF arteries.In ZDF rats, diameter did not increase despite the increase in blood flow and endothelium-mediated relaxation was further decreased in HF arteries in association with AGEs accumulation and excessive oxidative stress. In both NF and HF arteries, endothelium-mediated relaxation was lower in ZDF than in LZ rats. ALT-711, TEMPOL or their combination did not improve remodeling (diameter equivalent in HF and NF arteries). In parallel, they did not reduce AGEs level and did not improve MMPs activity. Nevertheless, ALT-711 and TEMPOL partly improved endothelium-mediated relaxation through a reduction of oxidative stress and the association of ALT-711 and TEMPOL fully restored relaxation to the level found in LZ rats.ConclusionsALT-711 did not improve outward remodeling in mature ZDF rats but it reduced oxidative stress and consequently improved endothelium-dependent relaxation. In mature LZ rats, ALT-711 improved outward remodeling and reduced AGEs level. Consequently, AGEs breaking is differently useful in ageing whether it is associated with diabetes or not.

Rotational relaxation of HF by collision with ortho- and para-H2 molecules

The first quantum mechanical investigation of the rotational deactivation of HF induced by collisions with ortho- and para-H(2) molecules is reported. Ab initio potential energy calculations are carried out at the coupled cluster level with single and double excitations, using a quadruple-zeta basis set. The global rigid rotor four-dimensional potential energy surface is obtained by fitting ab initio points with a least squares procedure for the angular terms and interpolating the radial coefficients with cubic splines. It is shown that the equilibrium structure of the H(2)-HF complex is T-shaped and the well depth is found to be 359 cm(-1). Close coupling scattering calculations are performed at collision energy ranging from 10(-2) to 1600 cm(-1). A comparison of the rotational quenching of HF with para-H(2) and (4)He is used to validate our potential energy surface. The rotational quenching cross sections of HF by ortho- and para-H(2) are also compared and found to be very different. An explanation of these differences based on a resonance mechanism is proposed.

Role of van der Waals resonances in the vibrational relaxation of HF by collisions with H atoms

Vibrational relaxation of HF(v) in collisions with H atoms can occur by three pathways: inelastic scattering with and without H atom exchange, and, for v>or=3, the HF+H-->F+H2 reaction. Fully quantum, reactive scattering calculations on the Stark-Werner FH2 potential energy surface reveal narrow peaks in the energy dependence of the integral cross sections for each of these processes. By means of an adiabatic-bender analysis, we show that each of these peaks corresponds to the position of quasibound HF-H vibrational states trapped in the weak van der Waals well. The width of these resonances indicates that the lifetime of the quasibound states is up to 30 periods of the HF-H van der Waals vibration.

Cole-Cole Analysis of a New Lead Free NaNbO3-Sr0.5NbO3-LiNbO3 Ferroelectric Relaxor

Dielectric response of (NaNbO3)0.65-(Sr0.5NbO3)0.325-(LiNbO3)0.025 lead-free relaxor ceramic has been studied as a function of frequency over a wide range of temperatures. The relaxor behaviour was evident from the diffused and frequency-dependent dielectric permittivity maximum together with Vogel-Fulcher-like frequency shift of permittivity maximum temperature (T m ). Analysis of the Cole-Cole plots as function of the temperature allowed us to elucidate two different dielectric relaxations and the appearance of a dc conductivity contribution above 100°C. The LF relaxation and dc conductivity have the same activation energy. The HF relaxation is characterised by a distributed relaxation time spectrum described by the well-known Cole and Cole expression. On cooling from 170°C a critical exponent α exhibits a pronounced increase from almost 0 (a Debye-like relaxation) to 0.8 in the vicinity of the Vogel-Fulcher temperature and saturates at lower temperatures. Such behaviour of α corresponds to the broadening of the relaxation time spectrum on cooling, which is typical for relaxor ferroelectrics.

Hierarchical Dynamics of Nano-Particles in Lyotropic Lamellar Phase

ABSTRACT Dynamics of nano-sized colloidal particles dispersed in a dilute lyotropic lamellar phase of a nonionic surfactant has been studied experimentally by ac electrophoretic light scattering and direct tracking under a microscope. The obtained frequency spectrum of complex electrophoretic mobility shows two relaxation processes at about 1 kHz (HF relaxation) and a few Hz (LF relaxation). These relaxations are due to the hindrance of free diffusion of particles by the hierarchical local static and dynamical structures of lamellar phase. From the direct tracking of fluorescent-labeled particles under a microscope, we find that particles show jump from sites to sites where they stay for a long time. This trap-jump process extremely decreases their mobility at low frequencies.

Resonance and reversibility of vibrational relaxation of HF in high temperature Ar bath gas

The Boltzmann averaged rate constants for total vibrational relaxation of HF(v=1) in collisions with Ar are computed in the range of temperatures between 100 and 1500 K. The computed rate constants overestimate the experimental measurements at high temperatures by a large factor. It is concluded that the deviation between theory and experiment cannot be explained by inaccuracy of the PES or dynamical approximations made. It is shown that increasing initial rotational energy enhances a resonant character of the vibrational energy transfer to a great extent. An assumption is made that total vibrational relaxation of HF(v=1) at high temperatures is determined by competition between vibrational relaxation to a resonant level (v=0,jres), vibrational excitation from the resonant level, and purely rotational relaxation of HF(v=0,jres). It is demonstrated that at high temperatures the latter process can be significantly slower than vibrationally inelastic transitions and rotational relaxation of HF(v=0,jres) may in fact be a rate-limiting stage of vibrational relaxation.

Vibrational and rotational energy transfer in collisions of vibrationally excited HF molecules with Ar atoms

This work presents converged vibrational close coupling-rotational coupled states calculations of cross sections and rate constants for rotational and vibrational transitions in collisions of vibrationally excited HF molecules with Ar atoms. Reduced channel basis sets assuming both a lower and an upper cutoff in vibrational quantum number are used for calculations at high internal energies of the diatomic molecule. The most recent potential energy surface is employed for the calculations and the correspondence of the results to the previous investigation of rovibrational dynamics in collisions of HF(v=1) with Ar is examined. It is shown that initial vibrational excitation stimulates vibrationally inelastic transitions to a great extent while the rotational energy transfer is essentially unaffected by initial v-number. The cross sections for vibrational relaxation of different vibrational levels of HF are shown to exhibit a strong dependence on initial rotational energy which is, however, of different magnitude for different vibrational states. The dependence of the vibrational relaxation of HF(v=1,3,6) on the translational energy of the atomic collision partner is different by an energy independent factor for different vibrational levels in the high energy limit and shows minima at low collision energies.

On the role of interaction anisotropy in vibrational relaxation of HF and HCl by Ar

A coupled states quantum scattering study of vibrational relaxation in Ar + HF and Ar + HCl collisions is presented. The energy transfer in these systems exhibits a quasiresonant character resulting in a strongly inverted rotational population of the diatomic molecules after collision. The high anisotropy of interaction is shown to be determining the quasiresonant vibrational energy transfer by favoring the population of high rotational energy levels. The role of interaction anisotropy in atom-diatomic molecule collisions is discussed.

Quantum-mechanical study of vibrational relaxation of HF in collisions with Ar atoms

Vibrational relaxation cross sections and rate constants of HF(v=1) by Ar are calculated on a recent semiempirical potential energy surface (PES) [J. Chem. Phys. 111, 2470 (1999)] using the quantum-mechanical coupled states approach. Accurate theoretical estimations of rate coefficients for vibrational relaxation of HF(v=1) at temperatures between 100 and 350 K are obtained. The vibrational relaxation is shown to be of a quasiresonant character and occur mostly to two nearest rotational levels of the ground vibrational state. The weak isotope effect after substitution of HF by DF is investigated and explained. The cross sections for vibrational relaxation of HF(v,j=0), where v=1,2,3,4, are calculated and shown to increase significantly as v increases. In the same calculations we observe a dramatic increase of multiple quantum vibrational transitions as the difference between the initial and final states falls in close resonance with the collision energy. A comparison of the cross sections obtained from the coupled states calculations with those performed with rotational infinite-order-sudden approximation proves a crucial role of molecular rotations for vibrational relaxation. Finally, we describe the close coupling coupled states calculations for relaxation and rotational excitation of HF(v=1, j=0) with a reduced number of open channels in the basis set and show that it is possible to obtain converged results for rotationally inelastic transitions between the various levels of v=1 neglecting all states below v=1, j=0.

Rates of processes initiated by pulsed laser production of F atoms in the presence of HCl, CH4, and CF3H

AbstractTime‐resolved vibrational chemiluminescence from HF has been recorded following the production of F atoms by the pulsed laser photolysis (λ = 266 nm) of F2 in the presence of HCl, CH4, and CF3H. In the first two cases, experiments have been conducted by observing emission from HF(ν = 3) at four temperatures from 295 to 139 K. Rate constants have been determined over this range of temperature for the reactions of F atoms with HCl and CH4 and of CH3 radicals with F2, and for the relaxation of HF(ν = 3) by HCl and CH4. The reaction of F atoms with CF3H is slower than those with HCl and CH4 and measurements on the emission from HF(ν = 2) have been used to infer rate constants for reaction and relaxation only at 295 K. © 1994 John Wiley & Sons, Inc.

Vibrational relaxation rate constants for HF(v=1–4) by CO, CO2, and HCN with product identification by infrared emission

Vibrational relaxation of HF(v=1–4) by CO, CO2, and HCN has been studied by infrared emission in a flow reactor. The vibrationally excited HF molecules were generated by the reactions of F atoms with CH4, CH3Cl, H2S, and HBr in 0.8 Torr of Ar carrier gas. By monitoring the rotational distribution in the lower v levels, the fraction of the relaxation proceeding by Δv=−1 and −2, V→R transfer was determined. Numerical integration of the rate equations and modified Stern–Volmer analysis of the HF(v) distribution vs reagent concentration were used to determine the relaxation rate constants and the mechanisms. The relaxation rate constants increase with v, but the magnitude of the rate constants depend upon the collision partner; the HF(v=3) values (in cm3 s−1) are 2.5×10−12 for CO, 1.1×10−11 for CO2, and 1.0×10−10 for HCN. The HF(v) relaxation mechanism by CO is dominated by V→R transfer (≥70%), giving HF(v−1, J=10–14). Relaxation by CO2 has significant Δv=−2 and −3, V→R (≊30%) and Δv=−1, V→R (∼20%) pathways, as well as a V–V pathway. Comparing the HCN(ν3) emission intensity to the loss in HF(v) emission intensity gave the efficiency of V→V transfer from HF(v) to HCN(ν3) as 55±15%; some Δv=−1, V→R transfer also was identified. The HF(v, J≥8) rotational relaxation rate constants increase in the CO, CO2, HCN series. R–V transfer from HF(J≥16) to the CO2 bending mode was identified as the fastest rotational relaxation process.

Influence of diluents on the pulse characteristics of an F2–H2laser under conditions of high-power initiation by a relativistic electron beam

An experimental study was made of the influence of various diluents on the output characteristics of an F2–H2 laser. Under high-power electron-beam initiation, the acceleration of the rotational relaxation of HF* molecules and the parameters of the laser pulse could be influenced to some extent by selecting the pressure and type of diluent.

Time‐resolved vibrational chemiluminescence: Rate constants for the reactions of F atoms with H2O and HCN, and for the relaxation of HF (v = 1) by H2O and HCN

AbstractRate constants have been determined at (298 ± 4) K for the reactions: equation image and the relaxation processes: equation image Time‐resolved HF(1,0) emission was observed following the photolysis of F2 with pulses from an excimer laser operating on XeCl (λ = 308 nm). Analysis of the emission traces gave first‐order constants for reaction and relaxation, and their dependence on [H2O] and [HCN] yielded:

Observation of rapid E-to-V energy transfer in the 2.3 μm laser-induced fluorescence of PuF6 containing HF

Following excitation of PuF6 at 800 nm, laser-induced fluorescence (LIF) of PuF6 is observed near 2.3 μm; HF v=1 to 0 emission, arising from near resonant electronic-to-vibrational (E→V) energy transfer, is also observed with the same temporal decay as the PuF6 fluorescence. From the rise and decay times of the PuF6 and HF fluorescence and their relative intensities, all the rate constants involved in this relaxation are determined. The E→V rate constant is found to be (7.5±2.5)×103 Torr−1 s−1, and that for the relaxation of HF (v=1) by PuF6 is found to be (5.7±0.7)×103 Torr−1 s−1.

Kinetics of the H2–NF2 system

The reaction of hydrogen atoms with NF2 was studied in a flow tube using pulsed KrF laser initiation of H2/NF2/Ar mixtures at 440 K. The quantum yield for NF(a) production from the 249 nm photolysis of NF2 was determined to be 0.10±0.05. Several important reactions in this system were investigated, and rate coefficients were determined at 440 K. A value of (1.03±0.20)×10−30 cm6/molecule2 s was obtained for the three-body recombination of F atoms with NF2 and Ar. Rate constants for the relaxation of HF(v=2) and HF(v=3) by NF2 are (9.7±1.0)×10−14 and (2.5±0.5)×10−13 cm3/molecule s, respectively. Study of the quenching of NF(a) by NF2 yields a rate coefficient of (2.7±1.0)×10−16 cm3/molecule s. Evidence is presented that indicates the disproportionation of NF(a) is three orders of magnitude slower than that of ground state NF. Modeling results are presented which agree well with experimental data.

Collisional energy transfer via vibrational predissociation: Relaxation of HF(v=1) by HF dimers

Overtone vibration–laser double resonance measurements determine the vibrational relaxation rate of HF(v=1) by HF dimers. Vibration-to-vibration energy transfer from the excited monomer to the dimer followed by vibrational predissociation of the dimer provides an efficient pathway for vibration-to-translation energy transfer that deexcites the monomer at 40% of the gas kinetic collision rate. Analysis of the pressure dependence of the observed decay constants using a simple kinetic model establishes a rough upper limit of 10 ns on the predissociation lifetime of the collisionally excited dimer.