Relaxationszeiten für die Umorientierung von Quecksilberatomen im metastabilen 63 P 2-Zustand durch Stöße

From the linewidth of the radiofrequency resonance between the Zeeman levels of the 6³ P ₂ state the lifetimeT _m of the metastable Hg atoms in a certain magnetic sublevel was derived as a function of the Hg vapour pressure. The lifetime of the metastable stateT(6³ P ₂) was determined under the same experimental conditions by optical absorption measurements. In the pressure range from 2·10^?3 to 2.4·10^?2 mm HgT(6³ P ₂) was longer thanT _m by one to two orders of magnitude. This result means thatT _m is only dependent on disorienting collisions and is practically not affected by the collisional destruction of the metastable state. ThusT _m is the relaxation time for the disorientation of the metastable atoms by collisions. The pressure dependence of the relaxation time indicates that the disorientation is achieved by two-body collisions with ground state Hg atoms. The disorientation cross section was found to be (2.1±0.2)·10^?14 cm².     

Influence of zeolite water on paramagnetic and ferromagnetic resonances in the Co2[Nb(CN)8] · 8H2O molecular magnet

The contributions of Co²⁺ and Nb⁴⁺ ions to the high-frequency dynamic magnetic susceptibility of the Co₂[Nb(CN)₈] · 8H₂O molecular magnet in the paramagnetic state at T > 12 K are separated. It is found that the ferromagnetic ordering, which leads to the reconstruction of the electron paramagnetic resonance spectrum into the ferromagnetic resonance spectrum, occurs at T < 12 K. The influence of zeolite water on the spectra of the paramagnetic and ferromagnetic resonances is found. Dehydration leads to a decrease in the time of the spin relaxation of the ferromagnetic system from 50 ps to 17 ps at T = 4 K and to the variation in the temperature dependences of the widths of the lines and g factors in the electron spin resonance spectra.     

Photon echoes in single- and trilayer semiconductor films of different nanosized thicknesses and their properties

A femtosecond stimulated photon echo is detected for the first time in thin semiconductor films with thicknesses of 100, 800, and 2400 nm. It is established that relaxation time T ₁ varies with ZnO film thickness, taking values of 0.96, 2.96, and 4.312 ps at thicknesses of 2400, 800, and 100 nm, respectively. For ZnO/Si⁺/Si^? trilayer films, the relaxation time is T ₁ = 12.73 ps.     

Anomalously slow relaxation of a nonwetting liquid in the disordered confinement of a nanoporous medium

The time evolution of the water–disordered nanoporous medium Libersorb 23 (L23) system has been studied after complete filling at elevated pressure followed by full release of overpressure. It is established that relaxation of the L23 rapidly flows out during the overpressure relief time, following the variation in pressure. At a temperature below that of the dispersion transition (T < T _d = 284 K), e.g., at T = 277 K, the degree of filling θ decreases from 1 to 0.8 within 10 s. The degree of filling varies with time according to the power law θ ~ t ^–α with the exponent α < 0.1 over a period of t ~ 10⁵ s. This process corresponds to slow relaxation of a metastable state of a nonwetting liquid in a porous medium. At times t > 10⁵ s, the metastable state exhibits decay, manifested as the transition to a power dependence of θ(t) with a larger exponent. The relaxation of the metastable state of nonwetting liquid in a disordered porous medium is described in the mean field approximation as a continuous sequence of metastable states with a barrier decreasing upon a decrease in the degree of filling. Using this approach, it is possible to qualitatively explain the observed relaxation process and crossover transition to the stage described by θ(t) with a larger exponent.     

Excitation processes leading to ruby emission

We excited the ²T₁-levels in ruby by tunable laser light, pumped bya pulsed YAG:Nd³⁺ laser, and observed the low temperature emissions from the 2$\text{A}$ and $\text{E}$ sublevels of ²E in the mild bottlenecking regime. The R₂-line decay time was about five-fold longer and the ratio of intensities R₂/R₁ higher (by up to a factor 10) than after pumping into ⁴T₂. It is suggested that when ⁴T₂ is pumped the relaxation is nonadiabatic, so that transitions between the sublevels occur while the ²E state relaxes to its normal (trigonal) configuration. The time-development of the relaxing state is studied. It is shown that its coherence between the 2$\text{A}$ and $\text{E}$ sublevels leads to a narrowing of the spectral width in transition between the sublevels.     

Ultrafast dynamics of intersubband relaxation in GaAs quantum wells: hot carrier and phonon populations effects

The dynamics of intersubband relaxation in GaAs quantum wells and the role of hot carriers and the phonon distributions have been investigated using two different optical techniques with femtosecond resolution: 1) time-resolved photoluminescence and 2) pump and probe experiments. The (2→1) intersubband relaxation times have been measured as functions of well widths (100Å < L_well < 220Å), under different experimental conditions (15K < T_lattice < 300K, and 1×10¹⁰ cm^-2 < excitation densities < 1×10¹² cm^-2). The electron intersubband relaxation time is deduced from the decay time of the n=2 well luminescence (or differential transmission) intensity. For thin wells (<150Å), a fast intersubband (2→1) relaxation time ≤ 3 ps has been measured. For thicker wells, the measured decay times are found to be critically dependent on the excitation conditions (vary from 5 ps to 40 ps). The well width dependence of the intersubband relaxation time does not show the strong dependence (2 orders of magnitude) predicted theoretically for electron-LO phonon scattering. Our results show that the hot phonon populations and the slow carrier cooling rate limit the observation of subpicosecond relaxation time. For thick well widths, our results also suggest that hot carriers effects play an important role in the intersubband relaxation mechanisms.     

Emission and absorption experiments in CaF2(57Fe): After-effects and relaxation

Two long-lived excited level contributions are evidenced in the emission spectra of substitutional⁵⁷Fe²⁺ ions in CaF₂. One originates from the Γ₄ triplet of the⁵E state, whose relaxation properties are analysed by absorption spectroscopy and the other is probably emitted by the lowest Γ₅ triplet of the⁵T₂ state. Other after-effects, including a Fe⁺ state, are observed.     

电子回旋共振氩等离子体中亚稳态粒子数密度及电子温度的测量

测量了电子回旋共振（ECR）氩等离子体中Ar的1s₅亚稳态粒子数密度，在气压 为02—0 8 Pa、功率为500—700W的范围内，利用吸收光谱法测量了Ar原子8115 nm谱线的吸收强 度，得到1s₅亚稳态粒子数密度为1×10¹⁵—4×10¹⁵ m ^-3.本文综合考 虑基态和1s₅亚稳态粒子的激发对Ar发射谱线强度的贡献后，用两条发射谱线强 度之比得 到电子温度.结果表明，计入了1s₅亚稳态激发的贡献后，所得到的电子温度与 只考虑基态的贡献得到的电子温度相比存在较大的差别. 关键词： 光谱法 亚稳态粒子数密度 电子温度 ECR等离子体源     

飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置,并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲“抽运-探测”方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm^-1的光谱分辨率,光谱检测范围为300—4000cm^-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明,β-胡萝卜素被激发到S₂态后,经由寿命约为0.3ps的中间态S_X态实 关键词： 飞秒时间分辨拉曼光谱 β-胡萝卜素 激发态内转换 振动弛豫     

Biexponential intersubband relaxation in n-modulation-doped quantum-well structures

Intersubband scattering in an n-modulation-doped GaAs/Al_0.31Ga_0.69As quantum-well structure is systematically investigated as a function of temperature and pump intensity. For the first time biexponential relaxation is observed during infrared bleaching experiments. The fast component with a time constant of ∼1 ps, which represents the depopulation of the first excited well subband, is found to dominate the signal more and more with decreasing temperature and pump intensity. The decay time of the slower component rises with decreasing temperature from τ₂=8 ps atT=300 K to τ₂=23 ps atT=10 K. This component is believed to be connected with a carrier transfer to the potential minima in the barrier layers. The intensity dependent excitation mechanism and the relaxation processes are discussed with the help of detailed numerical simulations.     

Theoretical analysis of the acoustic phonon limited energy loss rate and relaxation time of hot 2D-excitons in a GaAs-square quantum well (SQW)

The average energy loss rate and relaxation time of non-degenerate 2D-excitons interacting with the deformation- and piezoelectric potential of 3D acoustic bulk phonons are calculated perturbation theoretically as a function of the exciton temperature using the matrix elements previously derived in [1]. The energy loss rate limited by the acoustic deformation potential increases proportional toT ₃ ^7/2 (T ₃ ^3/2 ) if the phonon energy is much larger (smaller) than the thermal energy of the excitons having the temperatureT _e . It is shown, that the phonon wavevector componentq _z perpendicular to the interface of the QW must be taken into account in the calculation of the total excitonic loss rate in order to obtain the energy relaxation time value of 30 ps recently estimated in [2] from photoluminescence intensity measurements.     

The mean life of the second 0+ state in 42Ca

The mean life of the second excited state in ⁴²Ca (1.84 MeV, O⁺) was measured by a direct electronic timing technique to be 558 ± 8 ps. This result compares with a previous measurement of 480 ± 30 ps which yields a high B(E2) value. The present result gives a B(E2; 0₂⁺ 1.84 MeV → 2₁⁺ 1.52 MeV) value of 484 e² · fm⁴ or 55 single-particle units; the theoretical implications of this are discussed.The accurate mean life was extracted using a convoluted function to describe both the inherent time resolution of the experimental system and the exponential decay of the state of interest.     

Theoretical study of the 5p56s – 5p6 spectra of neutral xenon

Recent high precision measurements on the lifetime of the metastable 6s[3/2]₂ state of atomic xenon display a difference with previous predictions by a factor of 2–3. In the present work, a systematic relaxation and correlation approach, which has been developed on the basis of a widely used multi-configuration Dirac-Fock method, is applied to study the electric dipole allowed E1 and forbidden M1, E2 and M2 transitions between the 5p⁵ 6s and 5p⁶ configurations. We systematically include the correlation effects which arise from all the single and double excitations from the occupied shells into the (n=6–10) active sets and the relaxation effects caused by change of the electron density between the radiative initial- and final-states. This study not only reduces greatly the existing discrepancy in the lifetime of the 6s[3/2]₂ state, but also presents rather consistent results for both the lifetime of the metastable 6s'[1/2]₀ state and the oscillator strength of the 5p⁵ 6s - 5p⁶ E1 resonant transitions.     

Remeasurement of the lifetime of the 4 1 + -state of156Gd and recalculation of itsg-factor

The half-life of the 4 ₁ ⁺ -state of¹⁵⁶Gd has been remeasured by the delayed coincidence technique. The excellent time resolution which can now be achieved by use of small BaF₂ detectors allows a more reliable determination than with previously applied methods. The resultT _1/2(¹⁵⁶Gd,4 ₁ ⁺ )=108(2) ps is smaller than previously published data, but it fits well into the systematics of theB(E2) values of the rotational transitions of this nucleus. A recalculation of the previously measuredg-factor of the same state givesg(¹⁵⁶Gd,4 ₁ ⁺ ) =0.327(19). This value is still smaller than theg-factor of the 2 ₁ ⁺ state, but the magnitude of the reduction can now easier be interpreted by nuclear structure calculations.     

Pulsed NMRON measurements on insulators

Selective single and double quantum excitation pulsed NMRON has been utilized to obtain rotation patterns, free induction decays, and spin echoes in antiferromagnetic⁵⁴Mn−MnCl₂·4H₂O and the quasi-2 dimensional ferromagnet⁵⁴Mn−Mn(COOCH₃)₂·4H₂O. The pulsed technique is well suited to observing fast spin-lattice relaxation and T₁ values down to 100 ms have been measured in these systems. These short times may make magnetic insulators viable hosts for on-line experiments. The dependence on field and temperature of the⁵⁴Mn spin-lattice relaxation time T₁ has been investigated and a T₁ minimum at high fields observed in both systems. A spin-spin relaxation time T₂≈50 μs is measured in⁵⁴Mn−MnCl₂·4H₂O. The observation of NMRON in the paramagnetic phase of MnCl₂·4H₂O allows the resonant frequencies in the antiferromagnetic and paramagnetic phases to be compared, yielding a value for the zero point spin deviation in the former phase.     

Transient differential reflectivity of ferromagnetic and paramagnetic phases in the bilayered manganite La1.24Sr1.76Mn2O7

Photoinduced effects in a single crystal of bilayered manganites, La_2−2xSr_1+2xMn₂O₇ (x=0.38), were investigated in a wide range of temperatures by pump-probe measurement at a photon energy of 1.6 eV. In a ferromagnetic metallic state, significant enhancement of positive rise in differential reflectivity with a slow relaxing time of 100 ps was observed just below T_C=127 K, indicating that the reflectivity change with the slow relaxation time constant is induced by laser heating. We have also observed an unconventional fast relaxing component that has a time constant of the order of 10 ps. This fast relaxing component, whose absolute value has an asymmetric peak at T_C, is presumably due to short-range correlation of Jahn-Teller distortion.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Metastable chaotic state in K2ZnCl4

The ³⁵Cl NQR frequencies of K₂ZnCl₄ have been measured close to the incommensurate-commensurate transition. A calculation of the temperature dependence of the soliton density n_s from the NQR data showed that in K₂ZnCl₄ close to T_c exists a metastable chaotic state.     

Dynamics of first- and second-order phase transitions in amorphous magnetooptic TbFeCo films

The dynamics of phase transformations in thin amorphous TbFeCo films under the action of ～ 1 ps laser pulses is investigated. The films are heated to the Curie temperature in the amorphous state (T _C1), to the crystallization temperature (T _ac), and to the Curie temperature in the crystalline phase (T _C2). The change in magnetization is detected by Faraday magnetooptic effect during and after the action of the heating pulse. A static external magnetic fieldH～1?12 kOe, whose flux lines are directed perpendicular to the plane of the film, is used in the experiments. Amorphous TbFeCo films possess a perpendicular magnetic anisotropy, which on crystallization becomes reoriented in the plane of the film. It is observed that crystallization and magnetization reorientation occur during the heating pulse (within ～ 1 ps). The spin subsystem is heated to the Curie temperature several picoseconds after the end of the laser pulse. The characteristic spin relaxation time is ～ 10 ps. A model of the dynamics of the electronic, spin, and phonon subsystems that makes it possible to explain the experimental results is proposed on the basis of the data obtained.     

Influence of OH- impurities on the relaxation of F centers studied with picosecond optical pulses

Abstract

The relaxation of the two configurations of the F_H(OH^?) center in KBr is studied with a time-resolved induced transparency technique. Multiple decay components are observed. The slowest of these is interpreted as conversion in the ground state between the red and blue configuration of the F_H center, on a longer than nanosecond time scale. A contribution of the order of 100 ps might be related to non-radiative electronic relaxation, but cannot be identified with certainty. An unresolved transient, faster than 10 ps, might be related to a cross-over process.