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North China area is playing a significant role in grain production in China. Winter wheat-summer maize rotation is a major cropping system in this region. Based on the database of county cultivated land survey of soil fertility in Ministry of Agriculture of the People's Republic of China, total 23 862 samples were selected, 17 945 evaluation units were taken from the superposition of soil map, landuse map and administrative map. 11 evaluation indicators, including precipitation, accumulated temperature, top layer thickness, texture, organic matter, available P, available K, available Zn, salinity and irrigation capacity, were used to evaluate the gradation of the cultivated land fertility. The results showed that the fertility grades from first to sixth of the cultivated land for winter wheat-summer maize rotation system in North China accounted for 11.29%, 19.30%, 28.06%, 21.57%, 11.99% and 7.80%, respectively, and the overall cultivated land fertility upgraded in comparison with the second soil survey. The concentrations of soil organic matter, available P, available K were increasing significantly. In conclusion, this study will provide some basic information and scientific insights into specifically practical fertilization and soil improvement, scientific layout of crop production and adjustment of agricultural structure to promote cultivated land production capacity.

In the Sanjiang Plain, Northeast China, the natural wetland is undergoing a rapid conversion into agricultural land, which has resulted in drastic ecological changes in the region. To investigate the effects of different land uses on soil labile organic carbon, soils of Calamagrostis angustifolia wetland, Carex lasiocarpa wetland, dry farmland, paddy field, forest land and abandoned cultivated land were collected for measuring the contents of soil microbial biomass carbon (MBC), dissolved organic carbon (DOC), readily oxidizable carbon (ROC) and carbohydrate carbon (CHC). The results show that soil organic carbon contents follow the order: Carex lasiocarpa wetland>Calamagrostis angustifolia wetland>forest land>paddy field>dry farmland. The contents of MBC and DOC in Calamagrostis angustifolia and Carex lasiocarpa wetlands are significantly higher than those in other land use types. The contents of CHC and ROC are the highest in Calamagrostis angustifolia wetland and the lowest in dry farmland. The contents of all the labile organic carbon increase along with the years of abandonment of cultivated land. The ratios of MBC, DOC and ROC to SOC also follow the order: Carex lasiocarpa wetland>Calamagrostis angustifolia wetland>forest land>paddy field>dry farmland, while the ratio of CHC to SOC is paddy field>forest field>Carex lasiocarpa wetland>Carex lasiocarpa wetland>dry farmland. When natural wetlands were cultivated, the activity of soil organic carbon tends to reduce in some extent due to the disappearance of heterotrophic environment and the reduction of vegetation residue. Thus, the abandonment of cultivated land is an effective way for restoring soil organic carbon.

3-hydroxyflavone (3-HF) is an organic molecule with an excited-stated intramolecular proton transfer (ESIPT) effect. All-optical switchings and beam deflections of 3-HF in three kinds of solvents (cyclohexane, ethanol and dimethyl sulfoxide) have been investigated by using the third-harmonic generation (355nm) of a mode-locked Nd:YAG laser as a pump beam and a continuous-wave (cw) He-Ne laser (632.8 nm) as a probe beam. The nonlinear refractive indices of 3-HF in the three different solvents are determined by using the Z-scan technique under an ultraviolet (UV) pump beam at a wavelength of 355 nm. It has been found that the optical switching and beam deflection effects result from the change in refractive index of 3-HF under the irradiation of the pump beam. On the basis of the analyses of absorption spectra and fluorescence spectra, we conclude that the change in refractive index of 3-HF is due to not the thermal effect but the ESIPT effect of 3-HF under the pump beam. As the ESIPT is exceedingly fast, 3-HF might be an excellent candidate for high-speed optical switching.

The optical nonlinear properties of CdSeS/ZnS quantum dots (QDs) are investigated by Z-scan technique using fundamental harmonic generation (1064 nm) of mode-locked Nd:YAG laser for the first time. The experimental results show that two photon absorptions (TPA) occur at input intensity up to 12.5 GW/cm2. CdSeS/ZnS QDs have an average TPA cross section of 13710 GM and large nonlinear refractive index on order of 10-7 esu. The large optical nonlinearities perhaps allow the CdSeS/ZnS QDs to be one kind of candidate material for bioimaging and fluorescence label, optical limiting and all-optical switching.

We investigate the two-photon absorption and nonlinear refractive index properties of a quantum dot material based on ZnS nanocrystals doped with Mn isoelectronic impurities, using the Z-scan technique with 532 nm picosecond laser pulses. The Mn-doped ZnS quantum dots have an average two-photon absorption cross section as high as 13600 Goeppert–Mayer units, which turn it into a very promising material for fluorescent label and imaging in biological samples. In addition, we also found that the two-photon absorption coefficient initially increases and then decreases with increasing pulse irradiance, which demonstrates the presence of the higher-order nonlinearity under the strong excitation.

CaS : Eu2+ materials were prepared by using sulfur-flux method and analyzed with X-ray diffraction technology. The photoluminescence spectra of the sample excited by 532 nm pico-second laser pulse were investigated. The luminescence of Eu3+ as well as Eu2+ was observed. It revealed that some Eu2+ ions could be ionized to become Eu3+ ions when the sample was excited by the pico-second laser pulse. The dependence of the luminescence intensity of Eu3+ and that of Eu2+ on the concentration of Eu2+ was also observed. As luminescence peaks of Eu3+ were just in the region of Eu2+ absorption, the energy transfer from Eu3+ to Eu2+ would take place.

When CaS:Sm3+, Eu2+ is excited at 476.5 nm (Ar+), the emission spectra taken at room temperature and at 77 K are different, indicating that there are two competitive energy transfer processes-Sm3+ --> Eu2+ and Eu2+ --> Sm3+ with phonon participation. So, the luminescence intensity of Sm3+ increases first, and then decreases as the concentration of Eu2+ is increasing. (C) 2001 Elsevier Science Ltd. All rights reserved.

The dynamic process of 7-hydroxyquinoline with excited state proton transfer has been studied by the pump–probe method in which a three-dimensional degenerate four-wave mixing technique is employed for the probe. The molecular hyperpolarizability and lifetime of the excited state involved in the excited state proton transfer process of 7-hydroxyquinoline are obtained in our experiment. The decay time of the cation excited state is 35 ps and the decay time of the anion excited state is 250 ps. The molecular hyperpolarizability of 7-hydroxyquinoline in the enol form ground state, cation excited state and anion excited state are 1.3±0.2×10 −31 , 8.0±1.0×10 −31 and 5.8±0.8×10 −30 esu, respectively.

The linear and nonlinear optical properties of nanometer-sized In2O3 organosol were investigated by using TEM, ESR, UV-visible absorption spectrum, photoluminescence spectra and Z-scan technique. It was found that the prepared In2O3 organosol showed a series of new optical properties, which can be explained by its special structure. These new optical properties made its application in information optics enlarged.

In this paper we report the x-ray photoemission characteristics of CuO nanoparticles with surface modifications. We found the enhancement of interatomic resonant charge transfer and multi-electron relaxation processes in the coated CuO nanoparticles, which were in agreement with the conclusions of polaron-related structural transition appearing under quantum confinement and dielectric confinement effects, as observed in the optical absorption and infrared vibration spectra of this system.

Nonlinear transmission property of 7-hydroxyquinoline with excited state proton transfer effect was investigated at 355 and 532 nm using picosecond laser pulses. The experiment result shows that the transmissivity of 7-hydroxyquinoline is nonlinear at 355 nm but it is linear at 532 nm. The nonlinear transmissivity of 7-hydroxyquinoline originates from the larger nonlinearity of anion and cation formed in excited state proton transfer process. Therefore, 7-hydroxyquinoline might be used as ultrafast optical amplitude limit material.

The nonlinear optical properties of In2O3 nanoparticles coated by a layer of organic molecules (DBS) in toluene sol and bare In2O3 nanoparticles in hydrosol have been studied in the transparency region by the Z-scan technique.It was found that the surface modification can enhance nonlinear response of In2O3 nanoparticles.The influences of pole effect resulting from surface modification on the optical nonlinearities of In2O3 nanoparticles were also investigated.

The nonlinear optical properties of Fe2O3 nanoparticles coated with a layer of organic molecules(DBS) in toluene and bare nanoparticles in hydrosol have been studied in the transparency region by Z scan technique.The key parameters such as nonlinear refractive index γ,two-photon absorption coefficient β and the variation of refractive index per unit photoexcited carrier density σr as well as free-carrier absorption cross section σab were determined.The influences of free-carrier effects on optical nonlinearities of Fe2O3 nanoparticles were discussed.

The properties of the surface phonon modes in spherical SnO2 nanometer particles were studied by the method of infrared spectroscopy. The sum total of Frohlich modes of SnO2 nanometer particles were reported in our experiments. The theoretical results of surface mode frequency of SnO2 nanometer particles were presented, which agreed well with those of experiments. Finally, the effects of quantum confinement on the infrared properties of SnO2 nanometer particles were discussed.

The temporal behavior of DFWM and holography in four-level saturable and reverse saturable absorbers was analysed theoretically. It was demostrated that the signal response time, intensity and decay were dependent on laser exciting intensities, sample saturable intensity and triplet lifetime. An experimental verification was performed in molecule ethyl orange.

The dynamical behavior of nonlinear absorption in photoisomeric molecular with two ismoers was discribed theoretically. And the various factors influencing the dynamic nonlinear absorption properties were analyzed and discussed.

In this paper the theoretical and experimental studies on the properties of phase conjugate (PC) wave and the gratings formed as a result of degenerate four-wave mixing (DFWM) interference field of Ar+ laser in bacteriorhodopsin-polyvinyl alcohol (BR-PVA) solid film are reported. The diffraction efficiency temporary response and saturation property of the PC wave are described by useing a simplified double state model and the method of rate equation combined with a diffraction theory of holograph grating. The mechanism of forming grating in the film and the dependence of the properties mentioned above on polarization states, intensities are analysed. The time of arriving at the sample of three input beams and initial concentration of grating formed in the film are discussed. The theoretical results are in good agreement with the experimental observations.

The energy levels of Pr￣(3+) in SBN crystal are determined by means of absorptionspectra and fluorescence spectra. The fluorescence band exhibits a double peak dueto Pr￣(3+) ions at different sites in SBN crystal.This shows that the multiphoton rel-axation of ￣3P_0→￣1D_2 plays a dominant role in the radiationless transition of ￣3P_0 stateof Pr￣(3+) ion by analysing the relation between the fluorescencc lifetime and the sa-mple temperature.

This paper reports the anomalous infrared spectra of nanometer-sized Fe2O3 particles under certain conditions: some IR absorption peaks obviously shift, theirrelative intensities are changed; all peaks are remarkably broadened; and a broadbackground-absorption band appears. It is suggested that these anomalous IR chac-teristics indicate thata structural phase transition occurs, which results from the polaronic effect due to the increasing of electron-phonon coupling. This effect not only contains the contribution of LO phonons, but also the contribution of softmodes produced by interfacial atoms or molecules.

In this work, we carried out some experimental characterizations on the electronic structure of Fe2O3 nanometer particles, discussed the relationship between the electronic state variance under the quantum confinement, dielectric confinement and interfacial chemical bonding conditions and their optical properties, The experimental results are in good agreement with the recent theoretical considerations qualitatively.

The absorption and fluorescence spectra of Tb3+ and Eu3+ in LnP5O14 crystals at room temperture were observed. Based on absorption spectra and Judd-Ofelt theory, experimental and theoretical oscillator strengths were calculated. Series phenomenolo-gical intensity parameters were determined according to a least-square fitting. Then a calculation of the radiative transition probabilities and lifetimes of 5D3→7F5, 5D4→7F4 and 5D4→7F6 in TbP5O14, as well as 5D0→7F2 and 5D0→7F4 in EuP5O14 was carried out. We also obtain corresponding fluorescence lifetime by using time-resolved spectroscopy at various temperatures. The experimental results are in agreement with theoretical results. Our results confirmed that the lifetime of Tb3+ 5D3 depends mainly on the electric dipole-dipole cross relaxation between 5D3→5D4 and 7F6 →7F0.

Eu3+ ions at different crystal-field sites in SBN crystal are selectively pumped. From the time-resolved fluorescence spectra of 5D0→7F1, the energy transfer rates between ions in non-equivalent sites are evaluated. The dependence of fluorescence peak-frequency on the excitation wavenumber indicates the continuous distortion of local crystal field.