Saturation spectroscopy of coherent raman scattering in molecular gases

We present the theory of two-photon Raman saturation of a two-level Raman transition studied by an independent CARS process. The main goal here is to probe the saturated homogeneous Raman line shape. It is shown that there appears a saturation dip with a width determined by the relaxation timeT ₁. In the case of Doppler-broadened line the coherent Raman saturation spectroscopy may be used to determine both theT ₁ andT ₂ relaxation times.     

Determination of T1-spin-lattice relaxation time in a two-level system by continuous wave multiquantum electron paramagnetic resonance spectroscopy in a presence of tetrachromatic microwave irradiation

Applicability of continuous wave multiquantum EPR methods to study relaxation times at X-band is examined. Multiquantum transitions excited in a two-level system by tetrachromatic irradiation are used for these studies. The Bloch equation model is applied to simulate lineshapes of the three quantum transitions as a function of frequency difference between exciting fields. The dependence of multiquantum transition signals on relaxation times and microwave amplitude is shown. On this basis a method of deducing relaxation times from these signals is formulated. The case of a homogeneously and inhomogeneously broadened resonance line is considered. Two experimental methods are used to verify the proposed hypothesis: the X-band continuous wave multiquantum EPR with four frequencies microwave field and saturation recovery EPR. The values of T₁ obtained from CW MQ EPR and SR EPR are compared.     

Changes in relaxation times and strength constant and its effect on four-wave mixing spectra

In the present work we have studied modifications in the four-wave mixing (FWM) signal with independent simultaneous changes in the ratio between the longitudinal and transversal relaxation times T₁/T₂ and with the strength constant of the crossed harmonic potential curves that describe the two-levels system employed in this study, which is measured by the quotient between the resonance frequencies of the harmonic curves . In the formalism employed, the permanent dipole moments of the states in the uncoupled basis have been included and the rotating wave approximation is neglected in order to observe the processes out of the resonance region. We have observed changes in the shape and intensity of the FWM signal spectra with changes in the ratio T₁/T₂ and changes in the intensity and positions of the lines by modifying the parameter δ.     

Dynamic light scattering in solid polymers

The viscosity of an amorphous polymeric solid above its glass transition [T _g (T,P)] increases as the temperature of the solid is decreased or the pressure is increased. Under changes in temperature or pressure, molecular subunits in the polymeric solid undergo configurational changes. Such changes or relaxations have a distribution of relaxation strengths and times. As the solid is cooled or as the hydrostatic pressure on the solid is increased, the relaxation strengths increase and the relaxation times increase. These changes in relaxation or dynamic properties are very dramatic as the empirical T _g is approached. Near T _g the polymeric solid is no longer in volume equilibrium; continued cooling or pressuring at a time rate faster than the average relaxation time will produce a polymeric glass. This glass is a nonequilibrium, amorphous solid. If the glass is held at a fixed temperature and pressure very close to, but below, T _g , the volume of the glass will be observed to relax to its equilibrium value. For temperatures and pressures well below T _g , equilibrium is a much more conjectural concept since the relaxation times become extremely long. It has been proposed^1,2 that there is a characteristic temperature T _g at which an amorphous polymer undergoes a second-order transition to an equilibrium glass with zero configurational entropy (i.e., a noncrystallizable solid).     

Electron paramagnetic relaxation in aqueous solutions of manganese (II) ions

The processes of the electron paramagnetic relaxation, molecular motions and structural changes in aqueous solutions of manganese nitrate have been investigated by direct measurement of spin-lattice (T ₁) and spin-spin (T ₂) relaxation times for a wide range of concentrations, temperatures and viscosities. T ₁ and T ₂ were measured by a non-resonance absorption method.

It was discovered that some structural regions exist at the different concentrations of Mn(II) ions in solution. So, the structure of highly concentrated solutions may be considered as one of the corresponding crystallohydrate. The structural microinhomogeneities were observed also in the intermediate concentration range at definite temperatures. It is shown that the relaxation mechanism proposed by Bloembergen and Morgan is not effective in the concentration range studied by us.

The analysis of relaxation times and E.P.R. spectra has shown the formation of ‘liquid microphases’ at the freezing point of the solution. Such microphases can exist at temperatures a few tenths of a degree below the solvent freezing point, and its composition considerably differs from the initial solution.

The correlation times for intramolecular and intermolecular electron relaxation mechanisms are evaluated and their nature is discussed.     

Effect of oxygen on the NMR relaxation properties of crude oils

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We comparedT ₁ andT ₂ relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T _1,2 ^ox to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T _1,2 ^ox for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ms, no significant oxygen effect is observed.     

Wave packet dynamics in different electronic states investigated by femtosecond time-resolved four-wave-mixing spectroscopy

This paper reviews results on wave packet dynamics investigated by means of femtosecond time-resolved four-wave-mixing (FWM) spectroscopy. First, it is shown that by making use of the various degrees of freedom which are offered by this technique information about molecular dynamics on different potential-energy surfaces can be accessed and separated from each other. By varying the timing, polarization, and wavelengths of the laser pulses as well as the wavelength of the detection window for the FWM signal, different dynamics are coherently excited and probed by the nonlinear spectroscopy. As a model system we use iodine in the gas phase. These techniques are then applied to more-complex molecules (gas phase: benzene, toluene, a binary mixture of benzene and toluene; solid state: polymers of diacetylene matrix-isolated in single crystals of monomer molecules). Here, ground-state dynamics are investigated first without any involvement of electronically excited states and then in electronic resonance to an absorption transition in the investigated molecules. Signal modulations result which are due to wave packet motion as well as polarization beats between modes in different molecules. Phase and intensity changes yield information about intramolecular vibrational energy redistribution, population decay (T₁), phase relaxation (T₂), and coherence times. Received: 12 October 1999 / Published online: 13 July 2000     

The role of excited species in UV-laser materials ablation

The stability of a planar surface upon pulsed UV-laser irradiation is studied with special emphasis on polymer ablation. Here, we consider a two-level system in which the excitation energy is dissipated via stimulated emission, non-radiative transitions, and activated desorption of excited species. With thermal relaxation times t _T10^–10 s the ablation front turns out to become stable. This could explain the smooth surfaces obtained after pulsed UV-laser ablation of pure and stress free organic polymers. The situation is quite different for materials, for example metals, where fast thermal relaxation of the excitation energy within times, typically, t _T<10^–11 s, gives rise to instabilities which result in surface roughening.On leave from the Institute of General Physics, Russian Academy of Sciences, 117942 Moscow, RussiaOn leave from the Institute of Applied Physics, Russian Academy of Sciences, 603600 Nishnii Novgorod, Russia     

Numerical Determination of the Relaxation Parameters of NMR in Complex Heterogeneous Systems

The methods of mathematical processing of the envelopes of spin-echo signals have been considered within the framework of the multiphase relaxation theory. A mathematical model for separation of multiexponential relaxation curves into individual exponential components of spin-spin relaxation times T _2i and amplitudes I _i is described. The multiphase nature of the relaxation of protons in complex heterogeneous systems has been revealed, and the relaxation characteristics of individual components — spin-spin relaxation times and amplitudes — have been determined.     

The effects of thermal annealing on the relaxation of rubbing-induced birefringence in polystyrene

We have conducted a systematic study on the effects of post rubbing annealing on the relaxation of rubbing-induced birefringence of polystyrene. It is found that annealing at T₀ only affects the relaxation up to T₀ + T_Lag, where T_Lag is proportional to the logarithm of the annealing time t_A. A theoretical model based on the distribution of relaxation times due to the individual birefringence elements is proposed. To remove its contribution to the net birefringence each element must overcome an energy barrier E = (317 + 1.17ξ)×10³ J/mol, and therefore must have a characteristic relaxation time τ which depends on temperature T and a barrier height which ranges from 340.4 kJ/mol to 445.7 kJ/mol. The relaxation of birefringence is expressed by the equation NB(T, t) = N(ξ)e^-t/τ(T,ξ)dξ, in which both the relaxation time τ(T,ξ) and the distribution function N(ξ) can be extracted from experimental data. The predictions of the model agree well with all the experimental results presented in this work. The differences and similarities of the relaxation of birefringence with respect to the physical aging of quenched PS are discussed. In particular, similarities in terms of the general temperature lag phenomena are noted.     

Linewidth-Resolved N HSQC, a Simple 3D Method to Measure N Relaxation Times from T1 and T2 Linewidths

A three-dimensional approach for measuring ¹⁵N relaxation times is described. Instead of selecting particular values for the relaxation period, in the proposed method the relaxation period is incremented periodically in order to create a 3D spectrum. This additional frequency domain of the transformed spectrum contains the relaxation time information in the T₁ and T₂ linewidths, and thus the longitudinal and transverse ¹⁵N relaxation times can be measured without determination of 2D cross peak volumes/intensities and subsequent curve fitting procedures.     

NMR and quantum chemistry study of mesoscopic structuring in 3-methylpyridine/water/NaBr mixture in the vicinity of the phase separation boundary

²³Na and ⁸¹Br NMR spin-lattice relaxation times and signal half widths (Δ_1/2) have been measured in 3-methylpyridine (3MP)/H₂O/NaBr mixture along T?=?294 and 301 K isotherms gradually increasing the mass fractions of salt (X) up to the phase separation boundary. The extreme narrowing condition and thus 1/T ₁?=?1/T ₂?=?πΔ _1/2 was found to be valid in all cases. Discontinuous changes in slope of 1/T _1,2?=?f(X) were detected, and then corresponding points on the phase diagram (X, T) were attributed to the borderline between the molecular-ionic solution and the area of enhanced mesoscopic structuring. A very strong relaxation effect was observed for ⁸¹Br nuclei reaching relaxation rates of 14,000 s^–1. ²³Na and ⁸¹Br NMR relaxation data together with calculations of quantum chemistry model of electrical field gradient tensor evidence the migration of 3-methylpyridinium at increasing X from the anions hydration shells towards cations. An interchange from migration to steady distribution regimes is observed for anions and vice versa for cations at the borderline of the structured phase.     

Single-Pulse Echo and Its Secondary Signals in Two-Level Spin Systems

The dependences of the amplitudes of single- and two-pulse spin echoes and their secondary signals in NMR (protons of glycerin in an inhomogeneous magnetic field) in the exciting-pulse repetition period T _r have been compared. The difference in origin of the primary and secondary signals of a single-pulse echo in a two-level spin system has been confirmed. It is shown that only a primary single-pulse echo observed when T _r > T ₁ (T ₁ is the spin lattice relaxation time) results from single-pulse excitation. The secondary single-pulse echo signals are observed for T _r < T ₁ and are due to the multiphase formation mechanism. The results obtained for magnetically ordered substances are analyzed. Based on these data, it was inferred earlier that primary and secondary single-pulse echo signals were formed by one and the same multiphase mechanism.     

The kinetics of the structural relaxation process in PHEMA-silica nanocomposites based on an equation for the configurational entropy

The enthalpy relaxation of polymer-silica nanocomposites prepared by simultaneous polymerization of poly(2-hydroxyethyl methacrylate) (PHEMA) and tetraethyloxysilane, TEOS, a silica precursor, is investigated. Both the glass transition temperature, T_g, and the temperature interval of the glass transition, ΔT _g , increase as the silica content in the sample does. Structural relaxation experiments show that the temperature interval in which conformational motions take place broadens as the silica content in the hybrid increases. A phenomenological model based on the evolution of the configurational entropy during the structural relaxation process, the SC model, has been used for determining the temperature dependence of the relaxation times during the process. The results show an increase of the fragility of the polymer as the silica content increases, a feature that can be related to the broadening of the distribution of relaxation times characterized by the β parameter of the stretched exponential distribution. On another hand the silica content increase produces a significant change of the relaxation times in the glassy state.     

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically T _g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T _g/T). The glass cross-over temperature T_g related to correlations among solitons is equal to the average energy barrier and scales like T _g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap. Received 12 December 2001     

14N spin relaxation study in the hexagonal phase of a lyotropic liquid crystal

¹⁴N quadrupolar splitting and spin-lattice relaxation times T _IZ and T _IQ were measured in the hexagonal phase of a binary mixture of dodecyltrimethylammonium chloride and water. The derived spectral densities of motion at seven temperatures together with their corresponding quadrupolar splittings were analysed based on a simple model of ‘classical’ aggregates. Both fast local motion and slower surface self-diffusion about the cylindrical aggregate axis were required to account for the ¹⁴N spin relaxation in this phase. The azimuthal correlation time was found to be tens of nanoseconds.     

Spin-lattice relaxation and mobility of protons in the lattice of the TiV<Subscript>0.8</Subscript>Cr<Subscript>1.2</Subscript> alloy

Titanium-vanadium-chromium alloys are promising materials for hydrogen storage. They can absorb up to 3.8 wt % of hydrogen with a variable (depending on the composition) temperature of hydrogen release in a convenient range. This paper reports on the results of investigations of the TiV_0.80Cr_1.20H_5.29 hydride by continuous-wave (cw) and pulsed ¹H nuclear magnetic resonance spectroscopy. It has been revealed that the hydrogen atoms occupy tetrahedral positions of the face-centered cubic lattice. A model that takes into account the exchange between two states of hydrogen, i.e., mobile hydrogen and hydrogen bound to the lattice, has been proposed for interpreting the temperature dependences of the relaxation times T ₁ and T ₂ of ¹H nuclei. The assumption that the exchange occurs in these alloys has made it possible, in particular, to explain the strong difference between the relaxation times T ₁ and T ₂ in the high-temperature range.     

NMR Solvent Spin‐Lattice Relaxation Rate in Colostrum

Abstract

In this study, by using a FT‐NMR spectrometer operating at 60 MHz for proton, the solvent spin‐lattice relaxation times (T ₁) in colostrum were measured versus the days of lactation, whereas the T ₁ values in dehydrated colostrum were determined versus concentration of its hydrating solid. Data show that the spin‐lattice relaxation rate (1/T ₁ or R ₁) in colostrum is linearly dependent upon the inverse of time (1/days), and the R ₁ in dehydrated colostrum increases linearly with increasing concentration of its hydrating solid content (C). From data, the total paramagnetic contribution of ions in colostrum to the R ₁ was found to be negligible. The dehydrated colostrum data indicates that the R ₁ in colostrum is linearly dependent upon its hydrating solid content. Therefore, the R ₁ changes in colostrum were analyzed in terms of the relaxivities (increase in relaxation rate per unit concentration of solid) and the concentrations of milk constituents. Such an analysis provides a relation similar to that of the R ₁ in dehydrated colostrum. The current data imply that the relaxation changes in colostrum by days may be explained through changes in the concentrations of milk constituents. Also, the data suggest that the relaxation mechanism in colostrum can be explained in terms of fast chemical exchange of protons between free water and water bound to milk constituents.     

Saturation recovery electron spin–lattice relaxation studies on benzene anion radical and its derivatives: application of Kivelson–Orbach mechanism of electron spin relaxation

The role of low-lying excited states on the spin–lattice relaxation times (T₁) of organic radicals has been investigated. To test the applicability of Kivelson's electric field fluctuation model (D. Kivelson, J. Chem. Phys. 45, 1324 (1966)), based on the Orbach mechanism of spin relaxation, the T₁s of the anion radicals of benzene, benzene-1-d, toluene, ethyl benzene, isopropyl benzene, t-butyl benzene, p-xylene, 1,2,4-trimethyl benzene and 1,3,5-trimethyl benzene in liquid solutions, with potassium cation as the counter ion, have been measured by the pulse saturation recovery technique. The energy gap between the ground and the first excited electronic states changed with the substitutions to different extent. The spin–lattice relaxation rates showed correlation with this energy gap. Anion radicals of benzene and benzene-1-d showed the shortest T₁ among the radicals studied here. A small but measurable energy splitting due to the deuterium substitution in benzene-1-d radical was obtained from the temperature dependence of T₁. Spin–lattice relaxation times of benzene anion measured here decreased monotonically in the range of ?60 to ?125 °C, in contrast to some reported claims of very unusual temperature dependence, based on the continuous wave microwave power saturation studies. Our results also showed that the ion pairing between benzene anion and potassium cation did not significantly influence the spin–lattice relaxation times.     

Onset of the nonlinear dielectric response of glasses in the two-level system model

We have calculated the real part of the nonlinear dielectric susceptibility of amorphous insulators in the kHz range, by using the two-level system model and a nonperturbative numerical quantum approach. At low temperature T, it is first shown that the standard two-level model should lead to a decrease of when the measuring field E is raised, since raising E increases the population of the upper level and induces Rabi oscillations cancelling the ones induced from the ground level. This predicted E-induced decrease of is at odds with experiments. However, a better, though still not perfect, agreement with low-frequency experimental nonlinear data is recovered if, in our fully quantum simulations, interactions between defects are taken into account by a new relaxation rate whose efficiency increases as , as was proposed recently by Burin et al. [Phys. Rev. Lett. 86, 5616 (2001)]. In this approach, the behavior of at low T is mainly explained by the efficiency of this new relaxation channel. Since a quantitative understanding of glasses is still missing, we finally discuss experiments whose results should yield a refined understanding of this new relaxation mechanism: i) a completely new nonlinear behavior should be found for samples whose thickness is ≃ 10 nm; ii) a decrease of nonequilibrium effects should be found when E is increased. Received 19 September 2002 / Received in final form 4 December 2002 Published online 14 March 2003