Coherent control of optical transient processes in a gas

The possibility of controlling the processes of the free-polarization decay and photon echo in a gas by means of resonant CW radiation has been shown. The photon echo and free-polarization decay are formed using the method of the Stark switching of levels in a low-intensity radiation field in the presence of an orthogonally polarized strong field. The experiments in the gas at the R(4, 3) transition in the vibrational band 0 ? 1 v₃ ¹³CH₃F in the presence of radiation from a CW CO₂ laser and the calculations by the evolution operator method show a strong effect of the high-intensity radiation field on the photon echo and free-polarization decay to its complete suppression.     

甲醛的态选择性多光子电离研究

我们在XeCl准分子激光多光子电离甲醛分子的研究中,获得了甲醛经过2+1和2+2光子共振激发过程而产生的母体离子和高能离解通道的离子CO⁺。在电子轰击和单光子电离中占优势的低能阈离解通道离子HCO⁺,由于态选择性激发的结果而未出现。 关键词：     

Resonant Raman scattering of CdCr2Se4

The scattering cross section of the Raman-active phonons at 156 cm^?1 (Eg) and 169 cm^?1 (F2g) in the ferromagnetic semiconductor CdCr₂Se₄ (T_c=130 K) has been measured as a function of incident photon energy between 1.55 and 2.81 eV, both in the ferromagnetic and paramagnetic phases. The resonance curve peaks sharply near 2 eV and shows a broadening for temperatures below the Curie point. The relative line intensities change significantly with photon energy. The results show that the concept of spin-dependent Raman scattering in the ferromagnetic spinels has to be revised in terms of exchange-splitting-induced resonant Raman scattering.     

Resonant inelastic x‐ray scattering from highly correlated dysprosium compounds

Dy₂O₃ and Dy metal's resonant inelastic x‐ray scattering (RIXS) spectra were measured in the Beijing Synchrotron Radiation Facility. As a bulk sensitive probe and two‐photon process, RIXS provides more information on the electronic structure of matter. In this full RIXS experiment, the 2p⁶4fⁿ→ 2p⁵4fⁿ5d¹ (2p⁵4f^{n + 1}5d⁰) → 2p⁶3d⁹4fⁿ5d¹ (2p⁶3d⁹4f^{n + 1}5d⁰) channel of two samples (Dy₂O₃ and Dy metal) was studied. Further comparison shows that there are many differences in the RIXS spectra. Dy metal has only a single resonance and its 5d band is broader than that of Dy₂O₃. In the resonant regime, it has a lower final state energy, whereas in the non‐resonant regime it exceeds Dy₂O₃. This causes a broader bandwidth of the main final state B and a narrower bandwidth in the resonant and non‐resonant regime. The pre‐edge structure in Dy L₃ absorption spectra was also resolved using RIXS, which cannot be seen in conventional XAS owing to 2p core hole lifetime broadening. Copyright © 2005 John Wiley & Sons, Ltd.     

Intensity and resonance effects on the three-photon resonant third-harmonic generation in hg

The possible mechanism for the generation of near three photon resonant third-harmonic in the low energy side of the 6² P ₁ state of Hg is described. It is shown that the existence of strong ac Stark effect, due to the presence of the additional resonance 6¹ P ₁–6¹ D ₂ via the fundamental photon, results in a spatially dependent phase mismatch, which can lead to nonzero radiation in the k>0 region. Additionally, the influence of the ac Stark effect as a saturation mechanism with increased incoming radiation power is described.     

Absolute measurement of an atomic cascade rate using a two photon coincidence technique. Application to the 4p21S0 -4s4p 1P1 -4s21S0 cascade of calcium excited by a two photon absorption

The two photon coincidence technique yields an absolute measurement of a cascade decay rate without knowing the detection efficiency of each detector. This method is applied to calcium atoms in an atomic beam excited to the 4p²¹S₀ state, and decaying via the resonant ¹P₁ state; it yields the excitation rate of the upper 4p²¹S₀ level. Since the excitation process (two photon absorption) is controlled, one can compute the excitation rate of the upper level as a function of the transition probabilities in the cascade. The lower transition is well known and we can therefore deduce the transition probability for the 4p²¹S₀ -4s4p ¹P₁, transition which previously had not been accurately determined. The preliminary measurements yield a line strength s = 0.98 ± 0.3 au in good agreement with a recent measurement of the lifetime of the 4p²¹S₀ state.     

Two-photon fluorescence spectrum in an atom + dielectric microsphere system

A strong resonant interaction of a two-level atom with a dielectric microsphere is studied on the basis of quantum electrodynamics. The initial condition considered is one in which the atom is initially excited and the resonant mode of the microsphere has been excited by a single photon. The spectrum of two emitted photons depends strongly on the method used to excite the microsphere, i.e., on the spatial distribution of the photon energy. The most characteristic feature of the two-photon fluorescence spectrum is a strong energy correlation of the emitted photons. This correlation is expressed in the fact that the energies of the emitted photons are related by the equation of an ellipse (ω+ω ₂−2ω _vA )²+3(ω ₁−ω ₂)²= 4Ω _Rabi ² . The relation between the results obtained and the predictions of the theory of dressed states is discussed. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 192–197 (10 August 1999)     

Hg蒸汽中通过双光子共振四波差频产生调谐真空紫外辐射

利用Hg原子6¹S₀—6¹D₂和6¹S₀—6³D₂双光子共振的四波差频(2ω₁—ω₃),获得了真空紫外相干辐射(ω₄)。当入射光之一(ω₃)波长调谐时,产生的真空紫外光也是调谐的,调谐范围为184.9-187.5nm.本文对非线性频率变换过程的机 关键词：     

Enhanced dissociation of H2+ into highly excited states by UV pulses

We theoretically study the dissociation of H₂⁺ by UV laser pulses as a function of the photon energy ω of the pulse. Our results show that pronounced enhancements of the dissociation into highly excited electronic states can be achieved at some critical ω. This is found to be attributed to a consecutively resonant excitation mechanism where the population is first transferred to the first excited state by absorbing one photon and then to higher states by absorbing another one or more photons at the same internuclear distance. This study indicates that the strong coupling between the lowest two states of H₂⁺ can significantly affect the dissociation through higher lying states.     

Conventional and collision photon echo in ytterbium vapor

The polarization properties of the photon echo generated by two linearly polarized pulses of resonant radiation at the (6s6p)³ P ₁ ? (6s ²)¹ S ₀ transition of ¹⁷⁴Yb are investigated. A complicated polarization behavior of the photon echo versus an angle between the polarization vectors of the excitation pulses is revealed in a mixture of ytterbium vapor with inert gas. For the angles ranging from 0° to 75°, a conventional echo with its linear polarization coinciding with the second excitation pulse dominates and the echo amplitude decreases with an increasing angle. For the angles ranging from 75° to 89°, the photon echo is elliptically polarized. Finally, for an angle of 90°, the conventional echo disappears and the collision echo becomes linearly polarized along the first excitation pulse.     

Resonant axion generation in a strong magnetic field as a mechanism of forming a cold latent mass

It is demonstrated that considering vacuum polarization in superstrong magnetic field B ≫B₀ = m² / e = 4.41∙10¹³ G leads to giant (by 13 orders of magnitude) intensification of axion generation processes. In this case, for example, the probability of resonant axion radiation is about 10^-12 (B / B₀ ) of the probability of photon radiation, which is by many orders of magnitude greater than usually. In this regard, it is assumed that this mechanism plays the main role in the formation of an axion component of cold latent mass in the Universe.     

Role of stray light in the formation of high-resolution resonant photoelectron spectra: an experimental and theoretical study of N2

We show that the undular stray light, diffusely scattered by the optical system of a synchrotron beamline, can play an important role in the formation of high-resolution resonant photoelectron (RPE) spectra. The influence of the stray light is mediated through the Stokes doubling effect, with the Lorentzian tail of the spectral function being replaced by a more complicated form. This effect is shown to appear in the high-resolution resonant photoelectron spectrum of the N₂ molecule in which the spectral shape of the “non-Raman (NR) bands” differs qualitatively for the A²Π_u and X²Σ⁺_g final states. A particularly large enhancement of the non-Raman Stokes line is observed for the A-state while the picture is inverted for the X-state where the non-Raman band is suppressed. It is shown that the resonant photoemission profile is affected by two qualitatively different detunings, the detuning of the monochromatized line relative to the photoabsorption line and the detuning of the undulator harmonic relative to the same reference line. The experimental data show that the relative intensity of the non-Raman line strongly depends on the tuning of the undulator harmonic with respect to the selected monochromator bandpass, leading to a strong decrease of the Stokes line intensity for certain undulator detunings. A clear red-shift asymmetry for the decrease in the Stokes line intensity is observed when the monochromator line is detuned towards negative photon frequencies, whereas the picture is reverted for the situation of a positively detuned monochromator line. The results show the necessity to control the stray light and to investigate both the Raman and non-Raman contributions to the spectral profiles in order to avoid misinterpretation and in order to make full use of the information available in resonant photoemission spectra of molecules.     

The total photon deuteron hadronic cross section in the energy range 0.265–4.215 GeV

The total cross section for photoproduction of hadrons on the deutron, σ_T^d, has been measured for photon energies in the range 0.265–40215 GeV. From this, using results for the photon total cross section, obtained previously with the same apparatus, the neutron total cross section has been determined in the resonance region. The resonant structure is found to be quite different from that for the proton. Thereafter the neutron cross section falls off steadily with energy, and the values obtained are consistently lower than those for the proton. Forward scattering amplitudes have been evaluated for the deuteron.     

Quantum-interference effects for gamma radiation under crossing-anticrossing conditions for nuclear levels in an RF field

A new approach to observing the effect of electromagnetically induced transparency in gamma optics is proposed. The propagation of a resonant photon in a ⁵⁷Fe magnetic medium in an applied rf field is considered for this purpose. It is shown that, under crossing-anticrossing conditions, a resonant rf field substantially changes the gamma-optical properties of the medium, which become dependent on the parameters of the field. This opens the possibility for exercising a coherent control of the photon group velocity and a controllable filtration of unpolarized gamma radiation in a sample.     

锰酸钇薄膜中Mn3+离子d-d跃迁的超快光谱学研究

本文利用时间分辨光谱技术,系统研究了飞秒激光诱导YMnO₃薄膜中Mn³⁺离子3d轨道跃迁的 载流子动力学过程.当抽运光子能量为1.7 eV,对应于Mn³⁺离子的3d轨道跃迁, 抽运-探测零延迟时间处的透射率变化随着温度的降低逐渐减小. 这起源于低温下短程反铁磁有序诱导Mn³⁺离子d-d能级发生"蓝移". 载流子弛豫过程由快、慢两个过程组成,分别对应于电子-声子相互作用和自旋-声子相互作用. 实验发现,当温度低于80 K,电子-声子热化时间显著增加,表明低温下电子-声子的 耦合强度受长程反铁磁有序的影响.     

KLL Auger resonant Raman transition in metallic Cu and Ni

High energy resolution KL₂₃L₂₃ Auger spectra of polycrystalline Cu and Ni were measured using photon energies up to about 50 eV above the K-absorption edge and down to 5 eV (Cu KLL) and 4 eV (Ni KLL) below threshold. The spectra show strong satellite structures varying considerably as a function of the photon energy. In the sub-threshold region the linear dispersion of the diagram line energy positions and a distortion of the line shape as a function of photon energy, attributable to the Auger resonant Raman process, is clearly observed, indicating the one-step nature of the Auger emission. These changes in the resonant spectra are interpreted using a simple model based on resonant scattering theory in combination with partial density of states obtained from cluster molecular orbital (DV-Xα) calculations.     

Raman and infrared spectra of CdIn2S4 and ZnIn2S4

Four one-phonon Raman lines have been found in CdIn₂S₄ (ZnIn₂S₄) spinels at 92 (72) cm^-1, 186 (184) cm^-1, 246 (253) cm^-1, and 367 (372) cm^-1 for incident photon energies well below the energy gap E_G ～ 2.4 (2.2) eV at 300 K. For photon energies close to E_G, the 367 cm^-1 mode underwent a resonant enhancement in CdIn₂S₄ and four infrared active but Raman forbidden F_1u modes appeared in the CdIn₂S₄ and ZnIn₂S₄ Raman spectra: TO modes at 226 (221) cm^-1 and 309 (312) cm^-1, and LO modes at 274 (272) cm^-1 and 340 (342) cm^-1.     

Resonant Raman studies of surface composition and structural ordering in laser-irradiated Hg1-xCdxTe

1-x Cd_xTe for x=0.165 and x=0.18. The annealing was performed in air with a Nd:YAG laser tuned to the first harmonic. A pulse duration of 72 ns and irradiation energy densities of 270 and 400 mJ/cm² were used. Information on both the structural ordering and the alloy composition in irradiated Hg_1-xCd_xTe was obtained by resonance Raman spectroscopy with laser photon energies of between 2.41 and 2.54 eV. The presented Raman spectra from annealed samples indicate structural disorder and a decrease in the degree of alloying as a result of Hg outdiffusion and segregation caused by laser-induced nonequilibrium melting and solidification. Moreover, a shift of the maximum resonant enhancement for the TO₂, LO₂, A, and LO₁&TO₁ modes in the case of a Hg_0.82Cd_0.18Te sample, annealed by 400 mJ/cm² laser pulse, to higher photon energy points to an increase in the composition from x=0.18 to x?0.3. Received: 14 July 1997/Accepted: 10 October 1997     

Experimental evidence for molecular ultrafast dissociation in O<Subscript>2</Subscript> clusters

Resonant Auger spectra of O₂ clusters excited at the O1s edge are reported. After excitation to the repulsive 1s^-13σ^* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular ultrafast dissociation processes within the clusters or on their surface.     

Wavelength and concentration dependence of Raman scattering from CdS1−xSex

We present measurement of the resonant Raman effect in the alloy system CdS_1?xSe_x for laser photon energies both above and below the concentration-dependent energy gap. Our data show that the Raman intensities are not simply functions of the difference between the photon energy and the gap energy. These results may be partially understood in terms of a simple theoretical model which elucidates the role of local electric fields in determining Raman intensities in alloys.