Ultrarot-Absorption und phononenfreie Übergänge in strahlungsverfärbtem LiF

Spectral positions, properties and behaviour of the lines and bands found in the infrared absorption of X-rayed LiF single crystals at 90°K are investigated. Thermal annealing and optical bleaching of the crystals give informations about the correlation of the lines and the bands. Both prove to be transitions in the same absorption centre. This agrees well with the fact that the lines are due to phonon-free transitions in colour centres, whilst the bands are broadened by multiphonon processes. For the thermal annealing process of the infrared absorption a characteristic temperature of 420°K and an activation energy of 0·44 eV for the recombination process can be calculated by analysing isochrone annealing curves. According to their similar behaviour the infrared bands are compared with theR-centre absorption.     

Coherent control of transient optical phenomena: Molecules,atoms, and nanostructures

The dynamic nature of the interaction between the electromagnetic field of radiation and a resonantly absorbing medium is manifested as Rabi oscillations and can substantially affect both stationary and transient optical processes. This effect was repeatedly demonstrated for nonstationary transient processes in gaseous media with homogeneously broadened absorption lines. A single approach to the coherent control of the transient processes in the media with essentially inhomogeneous broadening of the absorption line is proposed. The results of the study of the Doppler-broadened molecular transition are presented. The prospects for the study of alternative inhomogeneously broadened media (low-pressure atomic gas and the exciton transitions localized in quantum wells (nanosized semiconductor heterostructures)) are reviewed.     

Ti：Al2O3晶体的电子自旋共振谱研究

Ｔｉ：Ａｌ２Ｏ３晶体中的顺磁中心Ｔｉ＾３＋由于强烈的晶格自旋耦合而使其电子自旋共振吸收只有在液拟温度附近才能看到，本实验在液氮温度附近记录到由许多强的的吸收峰迭加于Ｔｉ＾３＋宽吸收线所组成的电子自旋共振谱，这些吸吸收峰被认为是Ａｌ２Ｏ３基质中的Ｆｅ＾３＋，Ｍｎ＾２＋，Ｃｒ＾３＋，Ｍｏ＾３＋，Ｆｅ＾２＋，Ｃｏ＾２＋等杂质的共振吸收及Ｔｉ＾２＋３Ａ２ｇ基态的双量子跃迁造成的。     

Über das Verhalten der 4f-Elektronen in einigen Metallen der Seltenen Erden unter der Einwirkung von Licht

The visible spectra of the rare-earth metals Pr, Nd, Sm, and Er were examined whether they contain absorption lines due to transitions within the 4f-shell. Optical measurements were made on vacuum evaporated transparent films at 90° K. No spectral lines were found. It was shown by complete oxydation of the metal films that eventual absorption lines of the metals must be at least 3.5 times weaker or broader than those of the oxydes. The missing of the lines is suggested to be caused by the conduction electrons: 1. As the crystalline field is largely screened by the conduction electrons the oscillator strength of the electric dipole radiation is rigorously reduced. 2. Lines may be broadened by exchange of excitation between 4f-electrons and conduction electrons.     

Far-infrared absorption studies of Be acceptors in δ-doped GaAs/AlAs multiple quantum wells

We report a far-infrared absorption study of internal transitions of shallow Be acceptors in both bulk GaAs and a series of δ-doped GaAs/AlAs multiple quantum well samples with well thicknesses of 20, 15 and 10 nm. Low temperature far-infrared absorption measurements clearly show three principal absorption lines due to transitions of Beacceptor states from the ground state to the first three odd-parity excited states, respectively. Using a variational principle, the 2p-1s transition energies of quantum confined Be acceptors are calculated as a function of the well width. It is found that the theoretical calculation of the 2p_z → 1s transitions is in good agreement with the D-like line experimental data.     

Evidence for a contribution to the extrinsic photoconductive signal by hopping through excited states of the donors in silicon and CdTe

The intensities of the bound to bound transitions of the shallow donors in silicon and CdTe are found to be strongly electric field dependent when observed by photoconductivity techniques at low temperatures. Under these conditions significant differences can be detected between the photoconductive spectrum and the same transitions observed in absorption. The photoconductive peaks are shifted consistently to lower frequency compared with the absorption spectrum and additional peaks appear which are not significant features in absorption. These additional peaks are believed to result from the formation of molecular complexes of donors. The strong electric field dependence of the bound transitions observed in photoconductivity, the shift to lower frequency of the peaks and the enhancement of the additional lines can all be explained qualitatively on the assumption that the mechanism responsible for the generation of the photosignal involves hopping between the excited states of the molecular complexes     

A study of the methanol laser with a 500 MHz tunable CO2 pump laser

The absorption spectrum of CH₃OH is measured optoacoustically over a 500 MHz tuning range in 84 different CO₂ laser lines, and 19 assigned CO-stretch absorptions are used for establishing the linearity of the measurements over two decades of absorption coefficient. 40 new FIR laser lines originating from large offset absorption, are reported in the wavelength range between 40 and 400 m, and their wavelengths are given with a relative accuracy of 5×10^–5. 30 lines are assigned, involving CO-stretch transitions in torsionaln=0, 1, and 2 states, as well as transitions inn=3 of the vibrational ground state, pumped by transitions to Fermi interacting states.Work supported by the Danish National Science Research Council grant No. 11-5375     

Nonlinear resonances free of field and doppler broadenings

For the case where the Rabi frequencies of the guiding fields are much larger than the relaxation constants but much smaller than the Doppler broadening, it is shown that resonances which are neither field nor Doppler broadened can appear in the absorption (or gain) spectrum of the probe field. A classification of four-level systems according to the number of resonances is made for cases where two strong fields interact either with opposite or adjoining transitions. The conditions under which the number of resonances reaches eight, while for stationary atoms the maximum number is four, are found. A method is proposed for calculating the number of resonances in a multilevel system with several strong fields using analysis of the extremum points of the frequency branches in the velocity-frequency plane.     

Effects of averaging of spin packets of interacting resonances in Gd<Superscript>3+</Superscript> EPR spectra of scheelites

The EPR spectra of Gd³⁺ tetragonal centers in crystals with a scheelite structure are analyzed. It is found that the EPR spectra exhibit additional signals in the vicinity of coincidence of the resonance lines attributed to Gd³⁺ EPR transitions. It is shown that these signals are caused by averaging (due to relaxation spin-lattice transitions between the resonance doublets) of the internal part of the quasi-symmetric system of spin packets corresponding to the inhomogeneously broadened initial EPR lines. The quasi-symmetric arrangement of the spin packets is associated with the mosaic structure of the studied crystals.     

FIR Stark spectroscopy on optically pumped CH3OH laser

An extensive theoretical and experimental analysis of the absorption and emission spectrum of a CH₃OH FIR-laser excited by a conventional CO₂ laser is presented. Particular interest is devoted to the Stark shifts of the pump and lasing lines and to the electric field dependence of the Fir-laser output of the various lines. The offsets with respect to the exciting radiation and the Stark shifts of the IR absorption (pump) lines are measured by means of the transferred Lamb dip technique. The theoretical behaviours of the Stark patterns are calculated for several choices of the quantum numbers and selection rules involved in the transitions. A large variety of experimental results are reported and compared to theory. Non-linear Stark shifts have been observed for the 37.5m FIR laser line and for the IR-pump transitions excited by the 9-P(38) and 10-R(38) CO₂ laser Lines. Line assignments are proposed and new FIR laser lines are reported.     

Anomalous absorption line in the magneto-optical response of graphene

The intensity as well as position in energy of the absorption lines in the infrared conductivity of graphene, both exhibit features that are directly related to the Dirac nature of its quasiparticles. We show that the evolution of the pattern of absorption lines as the chemical potential is varied encodes the information about the presence of the anomalous lowest Landau level. The first absorption line related to this level always appears with full intensity or is entirely missing, while all other lines disappear in two steps. We demonstrate that if a gap develops, the main absorption line splits into two provided that the chemical potential is greater than or equal to the gap.     

Absorption and emission of thin solid films of octaethylporphin

Absorption and emission spectra at room temperature and emission spectra atliquid nitrogen temperature are reported for thin solid films of free base octaethylporphin. The room temperature visible absorption spectrum resembles that of the solution, except all four peaks show a red shift of ≈ 12 nm and all except the farthest red are broadened. The Soret band in the near UV is greatly broadened in the film, and its peak absorbance is substantially reduced. The room-temperature emission of the films is quite similar to that of solution fluorescence, except for a red-shift of ≈ 10 nm. But at low temperature two new peaks appear at 657 and 672 nm. Their intensity as a function of temperature can be explained by a simple exciton trapping model.     

Infrared laser stark shift spectroscopy in ammonia

The Stark effect in ammonia has been theoretically and experimentally analyzed using lead salt tunable diode laser absorption spectroscopy and CO₂ laser absorption spectroscopy of several absorption lines around 1050 cm^–1 applied to an all-optical sensor for measuring of electric field strength. Measurements of the Stark splitting effect of theaR(5,K) ammonia lines forK=1–5 as well as for the sR(3,K) lines forK=0–3 have been made at Doppler broadening pressures and for several different electric field strengths. Theoretical electric field dependent energy levels have been evaluated by diagonalization of a 6×6 energy matrix constructed using both electric field independent and dependent terms. From the theoretical analysis the resolution can be predicted and optimized both in the Doppler broadened and in the pressure broadened regimes. The predicted resolution is 0.5% at an electric field strength of 20 kV/cm. The theoretical calculations and the experimental data recorded with the tunable diode laser system were compared with independent measurements made with a CO₂ laser system. The agreement between experimentally recorded and theoretically calculated spectra is good which indicates that the theoretical model is satisfactory for our purposes. The contribution from the normally forbidden ssR(5, 3) ammonia line to the absorption at theP(12) CO₂ laser line in the 9 m band is briefly discussed.     

Indirect exciton luminescense and Raman scattering in CdI2

Intrinsic luminescence and Raman scattering in 4HCdI₂ have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.     

Sub-doppler absorption resonances induced in a gas cell by transverse optical pumping

New sub-Doppler resonances at central frequencies of atomic (molecular) transitions that appear in the spectrum of absorption of the probe optical radiation under the influence of optical pumping propagating in the orthogonal direction through a relatively narrow area of a cylindrical cell containing dilute gas medium are discovered and analyzed. These resonances are induced by specific optical pumping of atoms as they fly freely from the inner cell surface through the pumped region toward the probe optical beam. The obtained mathematical relations are used to investigate the dependence of the discussed resonances on the intensity and spatial distribution of the localized optical pumping. The proposed method could allow reducing the Doppler broadening of the detected spectral lines by the factor equal to the ratio of the effective width of the narrow pumped region to the cell radius. The obtained results may find application in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization by using the discovered sub- Doppler resonances.     

Absorption and emission lines of a mercury molecule in a mercury discharge lamp

Absorption lines are revealed in spectra of a mercury discharge lamp (Planck’s radiator). Some of the lines were identified by Lord Rayleigh as the absorption lines of a Hg₂ molecule. New absorption lines at 253.29, 253.53, 253.76, and 254.20 nm, as well as emission lines at 264.80, 254.96, 265.11, 279.89, 288.82, 301.50, 301.74, and 301.95 nm are discovered, which enabled the construction of a diagram of the lower excited electronic states of a mercury molecule. It is demonstrated that the dips in the intensity profiles of the atomic mercury lines at 253.65, 435.83, 404.65, and 546.07 nm arise from the radiation absorption in vibronic transitions of diatomic Hg₂ molecules. The concentration of mercury molecules in the plasma of mercury discharge lamps is determined to be of the order of 10⁸ cm^?3. However, their contribution to the formation of the emission line profiles is essential due to the multiple radiation passage through the emitting plasma.     

Spectral characteristic study of Raman transition interaction in optically pumped FIR laser

The spectral characteristic of an optically pumped NH3 molecules FIR laser has been studied by a F-P interferometer and a Michelson interferometer. It was verified that when NH3 molecules were pumped by CO2-9R(16) line, a emission by Raman transitions of two photons was produced. When the gas pressure of NH3 increased, the interaction of the Raman transitions rose, then the emission was enhanced and the width of spectral lines were broadened. The experimental results are in good agreement with the theoretical calculations.Authors gratifully acknowledge to Mrs. Chen Baoqiong and Prof. Qiu Bingsheng for their technical assistance and helpful discussions.     

Books received

The inhomogeneous broadening of optical transitions in solids is of fundamental importance in understanding the interaction of optical centres with their environment. In perfect crystals optical linewidths are determined by excited state lifetime and other d-phasing mechanisms such as the electron-phonon interaction. However, in real crystals random differences of impurities in crystallographically equivalent sites result in overlapping transitions and inhomogeneously broadened Gaussian lines. The suppression of inhomogeneous broadening of solid-state spectra using optical hole-burning and fluorescence line-narrowing techniques is discussed in terms of the determination of the homogeneous widths, multi-site geometries and the distributions of spectroscopic parameters that accompany disorder in a number of laser crystals and glasses.     

The far-wing broadening of the Na D lines by K,Rb and Cs and the electrostatic interaction potentials of the NaK,NaRb and NaCs molecules

The far wings of the NaD lines broadened by K, Rb and Cs have been measured in absorption by scanning a single-mode dye laser across the lines and detecting the flourescence signal. The relative absorption coefficients for the NaRb and NaCs are normalized using the impact widths recently measured by Kamke et al. The found red asymmetries of the lines are surprising results when only the theoretical van der Waals interactions are taken into account. It is demonstrated that the addition of dipole-quadrupole forces is necessary to explain the experimental findings. Using the extended electrostatic interaction potentials the theoretical impact widths of the NaD lines by Rb were found to be in agreement with the experimental data of Kamke et al.     

噪声免疫腔增强光外差分子饱和光谱线型的理论分析

噪声免疫腔增强光外差分子光谱(NICE-OHMS)作为世界上最灵敏的光谱技术可以被应用到痕量气体检测、频率标准、原子分子光谱以及超灵敏引力波测量等领域中,高精细度谐振腔吸收池的使用在增长激光与腔内物质相互作用路径的同时,极大的提高了腔内激光功率,这就会饱和低气压下的气态样品吸收线从而获得亚多普勒光谱结构,因此NICE-OHMS技术不仅具有高灵敏、还具有超高分辨的优点。该研究基于光与二能级分子相互作用的密度矩阵理论对NICE-OHMS技术中包含亚多普勒的多普勒展宽光谱线型进行了理论推导,获得了光谱线型的表达式,同时以该表达式对光谱线型进行了数值模拟,其中调制频率、饱和参量、频率调制系数分别设置为384 MHz,10和0.2。由模拟结果可见吸收光谱由两个边带的吸收信号构成,在包络上存在四个亚多普勒饱和结构;色散光谱由载频以及边带的色散三者决定,并在包络上存在五个亚多普勒饱和结构,获得了与已有实验一致的结果。最后重点分析了不同探测相位、不同饱和参量下的NICE-OHMS光谱线型尤其是亚多普勒结构的变化,由于饱和参量按照调制系数分配给载频和边带,因此虽然载频饱和参量很大,但NICE-OHMS吸收光谱幅度变化不大,主要是由于该光谱信号只与边带饱和参量有关,可以看出NICE-OHMS多普勒展宽信号具有饱和效应免疫的特性,与已有实验结果也符合较好。为更进一步的实验研究提供了必要的理论基础。