共查询到20条相似文献,搜索用时 31 毫秒
1.
S.W.H. Eijt R. Currat J.E. Lorenzo P. Saint-Grégoire B. Hennion Y.M. Vysochanskii 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):169-178
The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn2P2S6 is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a*-c* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (Px), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn2P2Se6, a strong coupling between the TO(Px) and TA(uxz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(Px) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is
not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise
from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(Px) dispersion in the a*-b* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are
observed, revealing a significant interaction between the TA(uyx) and LA(uxx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting
acoustic slopes in the ferroelectric phase is discussed.
Received: 11 May 1998 / Revised and Accepted: 15 June 1998 相似文献
2.
Jaw-Luen Tang Chen-Wei Chen Jiunn-Yuan Lin Chia-Chen Hsu Tzer-Hsiang Huang 《Optics Communications》2006,266(2):669-675
Transient optical Kerr effect of liquids C2H4Cl2 and C2H4Br2 is investigated, for the first time to our knowledge, with a femtosecond (fs) probe laser delayed with respect to a coherent fs pump laser. Coherent coupling and electronic Kerr signals are observed around zero delay when pump and probe overlap. Persisting after the pump-probe overlap are Kerr signals arising from the torsional and other intramolecular vibrations of the trans and gauche conformations; Kerr signals arising from the intermolecular motion are also observed. Vibrational quantum interference is only observed in liquid C2H4Br2 and the related beats data are fitted with the torsional vibrations, 91 cm−1 (gauche) and 132 cm−1 (trans), and the CCBr angle-bending vibrations, 231 cm−1 (gauche) and 190 cm−1 (trans), with dephasing times, 0.45 ps, 0.45 ps, 2 ps, and 1.5 ps, respectively. These vibrational frequencies agree with those obtained in the frequency-domain. That no vibrational mode is observed for C2H4Cl2 might be attributed to ineffective Raman-pumping. Kerr signals observed after the pump-probe overlap are Fourier transformed to give the spectra of the intermolecular motion and the vibrational spectrum, which agrees with the one observed in the infrared absorption and/or Raman scattering heretofore. 相似文献
3.
S. R. John S. C. Langford J. T. Dickinson 《Applied Physics A: Materials Science & Processing》2008,92(4):981-985
We report time- and mass-resolved measurements on neutral particles emitted from polytetrafluoroethylene (PTFE) during exposure
to 157-nm laser radiation at fluences where etching is observed. By comparing the time-of-flight signals over a range of masses,
we conclude that (CF2)
N
fragments for N=1–6 are emitted directly from the surface in substantial quantities. In contrast, the monomer (N=2) is the principal product during irradiation at 248 nm, where thermal decomposition is important. The time-of-flight signals
of all the (CF2)
N
fragments show fast components with kinetic energies on the order of an electron volt. These high kinetic energies are consistent
with photochemical scission of the polymer backbone, where a fraction of the excitation energy is delivered to the fragment
as kinetic energy. Although clean etching is observed under these conditions, the great majority of the mass removed from
the target appears as much larger fragments that do not reach our detector. The nature of this material and its affect on
the velocity distribution of the observed (CF2)
N
fragments is discussed. 相似文献
4.
Raman spectra of phosphorous doped ZnSe are recorded from 9 to 300 K. In addition to TO mode at 208 cm−1 and LO mode at 253 cm−1, a new mode around 240 cm−1 is observed between 55 and 270 K. The spectra at lower and higher temperatures do not show the new mode. This new mode confirms that there is a reduction of (Se site) symmetry from Td to C3v when P substitutes for Se. This is due to Jahn Teller distortion. 相似文献
5.
Thermal grating and broadband degenerate four-wave mixing spectroscopy of OH in high-pressure flames
H. Latzel A. Dreizler T. Dreier J. Heinze M. Dillmann W. Stricker G.M. Lloyd P. Ewart 《Applied physics. B, Lasers and optics》1998,67(5):667-673
2 ∑–X2Π(0,0) band of OH has been studied in premixed methane/air flames using a cw Ar+ laser probe. Measurements of flame temperature and pressure were derived from fits of theoretical simulations to the observed
time variation of signals over a pressure range of 10 to 40 bar and for different stoichiometry that were in agreement with
independent measurements using N2 CARS and predictions of a one-dimensional flame calculation. Broadband DFWM spectra in the same band of OH were observed
up to a pressure of 9 bar, above which signals were obtained only from scattering from thermal gratings.
Received: 10 November 1997/Revised version: 28 May 1998 相似文献
6.
P. Carlsson N.T. Son F. C. Beyer H. Pedersen J. Isoya N. Morishita T. Ohshima E. Janzén 《固体物理学:研究快报》2009,3(4):121-123
Deep levels introduced by low‐energy (200 keV) electron irradiation in n‐type 4H‐SiC epitaxial layers grown by chemical vapour deposition were studied by deep level transient spectroscopy (DLTS) and photoexcitation electron paramagnetic resonance (photo‐EPR). After irradiation, several DLTS levels, EH1, EH3, Z1/2, EH5 and EH6/7, often reported in irradiated 4H‐SiC, were observed. In irradiated freestanding films from the same wafer, the EPR signals of the carbon vacancy in the positive and negative charge states, VC+ and VC‐, respectively, can be observed simultaneously under illumination with light of certain photon energies. Comparing the ionization energies obtained from DLTS and photo‐EPR, we suggest that the EH6/7 (at ~EC – 1.6 eV) and EH5 (at ~EC – 1.0 eV) electron traps may be related to the single donor (+ | 0) and the double acceptor (1– | 2–) level of VC, respectively. Judging from the relative intensity of the DLTS signals, the EH6/7 level may also be contributed to by other unidentified defects. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
7.
W. Que Y. Zhou Y.L. Lam K. Pita Y.C. Chan C.H. Kam 《Applied Physics A: Materials Science & Processing》2001,73(2):209-213
Luminescence properties from erbium (III) oxide nanocrystals dispersed in titania/organically modified silane composite thin
films were studied. Erbium oxide nanocrystals were prepared by an inverse microemulsion technique. A strong room-temperature
photoluminescence was observed at 1.531 μm, with the full width at half maximum (FWHM) of 22 nm due to intra-atomic transitions
between 4
I
13/2 and 4
I
15/2 levels in the erbium (III) ion. The shape, peak position, and FWHM of the photoluminescence signals from the composite thin
films were quite comparable to those prepared by other methods. The photoluminescence peak of the composite thin films showed
a maximum intensity at the heat-treatment temperature of 300 °C. A room-temperature green up-conversion emission at 543 nm
(4
S
3/2?4
I
15/2) was observed for the composite thin films with different heat-treatment temperatures upon excitation at 993 nm. The up-conversion
emission mechanism was explained by means of an energy-level diagram and the lifetime of the visible up-conversion emission
was measured.
Received: 10 July 2000 / Accepted: 11 July 2000 / Published online: 5 October 2000 相似文献
8.
J. Wollschläger 《Applied Physics A: Materials Science & Processing》2002,75(1):155-166
The growth of CaF2 on vicinal Si (111) surfaces was studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) for the
temperature range from 300 to 750 °C. In the submonolayer range a transition from terrace to step nucleation is observed for
increasing temperature. The first monolayer grows in the step-flow growth mode since second layer islands are not nucleated
before completion of the wetting layer. For the subsequent growth of CaF2 on the CaF interface layer, substrate-induced steps do not act as preferential nucleation sites, but rather as growth barriers.
Thus CaF2 films grow very inhomogeneously at high temperatures. At lower deposition temperatures, the film homogeneity increases due
to the increased (homogeneous) nucleation rate on terraces. The lattice mismatch leads to (lateral) relaxation of thicker
CaF2 film close to substrate steps. In addition, CaF2 self-decoration effects are caused by the relaxed regions close to the film steps at temperatures above 500 °C.
Received: 7 August 2001 / Accepted: 23 October 2001 / Published online: 3 April 2002 相似文献
9.
We have studied the superconducting properties of MgB2 from first-principles under isotropic, uniaxial, and biaxial compressions. We find that the in-plane boron phonons near the zone-center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This mode is found to be the key to quantitatively explain the observed high Tc, the total isotope effect and the pressure dependence of Tc. We propose that a stringent test on the hole and phonon based theories of the superconductivity in MgB2 would be a measurement of the biaxial ab-compression dependence of Tc. 相似文献
10.
A. P. Voitovich Yu. V. Loiko A. V. Cherkasov V. V. Nevdakh O. L. Gaiko 《Journal of Applied Spectroscopy》1999,66(5):780-786
The influence of the modal composition (transverse modes of a single longitudinal mode) of the laser radiation on the oscillation
dynamics of a CO2-laser with a saturable absorber in the cavity is investigated. Passive Q-switching in initiation of the fundamental mode
TEM00 and the modes of the first family TEM01 and TEM10 is considered. It is established that in the case of multimode oscillations sequences of single pulses with an irregular
change in their amplitude can be observed. The multimode oscillations obtained are compared with similar single-mode oscillations,
and their special features and differences are discussed.
Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 5, pp. 681–687, September–October, 1999. 相似文献
11.
Raman spectra for non-site-selectively and site-selectively Zn-doped CuGaS2 layers grown by vapor phase epitaxy (VPE) were investigated. Although an appearance of characteristic Raman line(s) related with the doped Zn atom was not seen, an enhancement of the Raman intensity ratio of the highest LO mode to the A1 mode (ILO/IA1) was observed. The site-selectively Zn-doped layers with p-type conductivity exhibited larger ILO/IA1 ratio compared to those with n-type conductivity. The observed correlation between the ILO/IA1 ratio and the peak energy of the photoluminescence characteristic for Zn-doped p-type samples (L emission) suggests that the enhancement of ILO/IA1 is due to the increase of Zn atom substituting Ga site (ZnGa) which is acting as an acceptor. 相似文献
12.
Field-induced ionization and Coulomb explosion of nitrogen 总被引:1,自引:0,他引:1
C.Y. Wu H.Z. Ren T.T. Liu R. Ma H. Yang H.B. Jiang Q.H. Gong 《Applied physics. B, Lasers and optics》2002,75(1):91-96
Femtosecond-laser field-induced ionization and Coulomb explosion of diatomic nitrogen were systematically investigated using
time-of-flight mass and photoelectron spectrometry. Both linearly and circularly polarized femtosecond laser pulses were used
at intensities varying from 5×1013 to 2×1015 W/cm2. Strong N2
+, N2
2+, N+, N2+ and N3+ ion signals were observed for horizontally polarized pulses. Moreover, signals from the atomic ions exhibited a double-peak
structure. Suppression of ionization was observed for circularly polarized pulses, while for vertically polarized pulses,
only N2
+ and N2
2+ ions were observed. The angular distributions of the ions were measured under zero-field conditions in the ionization zone.
The atomic ions N+, N2+ and N3+ exhibited highly anisotropic distributions, with maxima along the laser polarization vector and zeroes normal to the laser
polarization vector. In contrast to the atomic ions, N2
+ exhibited a strong isotropic angular distribution. These observations indicate that dynamic alignment is responsible for
the observed anisotropic angular distribution of the atomic ions. The kinetic energy spectrum of the photoelectrons is featureless
and broad, extending above the ponderomotive potential of the laser pulse. The angular distribution is markedly anisotropic,
with a maximum along the laser polarization vector. These observations further support the notion that the field-ionization
mechanism is dominant under our experimental conditions.
Received: 29 January 2002 / Revised version: 15 March 2002 / Published online: 12 July 2002 相似文献
13.
V. Mohaček-Grošev D. Kirin 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,20(1):85-90
Methyl-mercury(II) halides CH3HgX (X = Cl, Br and I) were studied by means of temperature dependent Raman spectroscopy from 10 K to 410 K. In addition to
the previously reported soft phonons, new changes in the low frequency spectra were observed at T
≈ 70 K in CH3HgBr and at T
≈ 100 K in CH3HgI. The bandwidths of the two internal modes in CH3HgBr, the CH3 symmetric stretching band and the C-Hg-Br bending band, rise towards a local maximum at T
≈ 50 K as the temperature is raised from 10 K to 300 K. On the other hand the bandwidths of the two corresponding modes in CH3HgI crystals monotonously increase with temperature, obeying an Arrhenius law. Besides the three phonon modes present in the
Raman spectra of CH3HgCl at room temperature, the fourth phonon band that has been observed at temperatures below 245 K might correspond to the
freezing of methyl librations. The huge bandwidth of the C-Hg-Br bending mode could suggest the presence of additional weak
bonding of a mercury atom with bromine atoms from other molecules, thus inducing positional disorder.
Received 19 November 1999 and Received in final form 10 November 2000 相似文献
14.
Decay of time-dependent nutation signals from E′1 centers in crystalline quartz is investigated at room temperature. ESR-nutation signals were excited in the single-photon
mode and were shaped by Zeeman field pulses. It is found that the decay of this signal follows an exponential law with a rate
Λ substantially smaller than 1/T2 (T2 being the transverse relaxation time) and increases with the amplitude of the acting ultrahigh-frequency field B1. It is shown with the use of the Bloch equations that the intergral decay rate of nutations in an inhomogeneously broadened
two-level system depends on the ratio of B1 to the linewidth and increases nonlinearly with increasing B1. The observed quantitative disagreement between the experimentally observed nutation decay with Bloch's theory is explained
by the stochasticity of conditions of the resonant interaction of radiation with a system of spins.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 2, pp. 213–219, March–April, 1999. 相似文献
15.
We studied the photoluminescence (PL) and Raman properties of the ordered defect compound CuGa5Se8. Twelve peaks were detected from the room-temperature Raman spectra with the A1 mode around 160 cm−1. Due to the stress in the polycrystalline thin film the corresponding frequencies of the Raman modes of a CuGa5Se8 single crystal were slightly shifted. One broad asymmetric PL band at 1.788 and 1.765 eV was observed at 10 K in the PL spectra of CuGa5Se8 single crystal and polycrystalline layer, respectively. The temperature and laser power dependencies of the PL spectra were also studied. The shape and properties of the PL band assure the presence of potential fluctuations and the analyses of the PL data suggest that the emission is due to band-to-tail (BT) or band-to-impurity (BI) recombination. 相似文献
16.
J.F Zasadzinski L Ozyuzer N MiyakawaK.E Gray D.G HinksC Kendziora 《Journal of Physics and Chemistry of Solids》2002,63(12):2247-2251
Break-junction tunneling data are reported in Bi2Sr2CaCu2O8+δ over a wide range of hole concentration from underdoped to overdoped. The strong conductance peaks in the superconducting state reveal a single gap consistent with d-wave symmetry. In addition, sharp dips are observed at a voltage, Ω/e, measured with respect to the gap edge. These features are shown to be reproduced in other junction types from the literature including atomically resolved STM and c-axis mesas, establishing their intrinsic character. Trends are observed with doping and temperature which link the dip to the resonance spin excitation and indicate that the quasiparticles are strongly coupled to this mode. 相似文献
17.
In this paper, CO2 laser–metal active gas (MAG) hybrid welding technique is used to weld high strength steel and the optimized process parameters are obtained. Using LD Pumped laser with an emission wavelength of 532 nm to overcome the strong interference from the welding arc, a computer-based system is developed to collect and visualize the waveforms of the electrical welding parameters and metal transfer processes in laser–MAG. The welding electric signals of hybrid welding processes are quantitatively described and analyzed using the ANALYSATOR HANNOVER. The effect of distance between laser and arc (DLA) on weld bead geometry, forming process of weld shape, electric signals, arc characteristic and droplet transfer behavior is investigated. It is found that arc characteristic, droplet transfer mode and final weld bead geometry are strongly affected by the distance between laser and arc. The weld bead geometry is changed from “cocktail cup” to “cone-shaped” with the increasing DLA. The droplet transfer mode is changed from globular transfer to projected transfer with the increasing DLA. Projected transfer mode is an advantage for the stability of hybrid welding processes. 相似文献
18.
In this work, we present X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis of laser treated vanadium oxide sols. The films were also observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to reveal how the original xerogel structure changes into irregular shaped, layer structured V2O5 due to the laser radiation. XRD revealed that above 102 W/cm2 the original xerogel structure disappears and above 129 W/cm2 the films become totally polycrystalline with an orthorhombic structure. XPS spectra showed O/V ratio increment by using higher laser intensities. 相似文献
19.
Ph. Courteille L. S. Ma W. Neuhauser R. Blatt 《Applied physics. B, Lasers and optics》1994,59(2):187-193
Precision spectroscopy on molecular tellurium is performed by measuring the frequency difference between the observed lines and an eigenfrequency of a high-finesse cavity mode. The mode frequency is derived from a measurement of the cavity's free spectral range taking into account the cavity dispersion due to phase shifts in the dielectric mirror coatings. The experimental technique is based on dual frequency modulation and is applied to determine the transition wavenumbers of several lines in130Te2 near 467 nm with an uncertainty of 2 × 10–8. 相似文献
20.
A confocal Raman investigation of Pb1 − xLaxTi1 − x/4O3 (PLT) thin films grown by RF magnetron sputtering on PbOx/Pt/Ti/SiO2/Si substrates with an intermediate LaSrCoO3 (LSCO) layer was performed. The influence of the LaSrCoO3 buffer layer was analyzed taking advantage of the observed Raman spectral band variation, which varied according to different manufacturing procedures. In the presence of a LSCO layer, the A1(1TO) Raman mode, which was indicative of tetragonal distortion, was pronouncedly enhanced, and a slight deviation from the (0 0 1) plane of the film was observed from the angular dependence of the polarized Raman spectral intensity. Furthermore, the spectral band variation as well as the residual stress along the in-depth direction was measured in the film from cross-sectional spectral line scans. This latter measurement showed a relaxation of the lattice mismatch in the presence of LSCO and PbO layers. 相似文献