A Theoretical Study of Photoabsorption Cross Sections of Na^＋ 2

In the framework of quantum defect theory, we calculate photoabsorption cross sections of Na^＋ 2. Based on our calculations, there is an absorption window in the photoabsorption cross sections of Na^＋ 2. and more than one bump above the absorption window. The calculated photoabsorption cross sections provide an explanation for the abnormal bump in the experimental measurements of Hudson, which is a long-standing experimental puzzle.     

Synthesis, photoluminescence and bioconjugation of rare-earth (Eu) complexes-embedded silica nanoparticles

Eu(DBM)₃Phen-embedded silica nanoparticles were synthesized in water-in-oil (W/O) microemulsion containing aqueous phase of Eu(DBM)₃Phen, surfactant Triton X-100, cosurfactant octanol and oil-phase cyclohexane. The size and morphology of the nanoparticles were characterized by transmission electron microscopy (TEM). The low-temperature time-resolved emission spectra indicate that the Eu complex in the silica nanoparticles have longer lifetime than that of the pure complex. Under 355 nm continuous excitation, the nanoparticles show high resistance to photobleaching. The free amino groups were attached to silica surfaces by copolymerization of 3-aminopropyl(triethoxy)silane. Preliminary results demonstrated that the silica-coated Eu complex nanoparticles can be a probe in the detection of biomolecular interactions.     

Middle-infrared luminescence of Nd ions in silver halide crystals

The optical absorption and the luminescence emission in the middle infrared (mid-IR) were investigated in AgCl_xBr_1−x crystals doped with Nd³⁺ ions. Strong luminescence emission, in the spectral range 4.5-5.8 μm, in mid-IR was observed for the first time in Nd³⁺-doped silver halide crystals. Various optical parameters were calculated for the Nd³⁺-doped crystals, using the Judd-Ofelt approximation. The measured results and the calculated parameters indicate that these doped crystals could be used for the development of mid-IR solid-state lasers or mid-IR fiber lasers.     

Quantitative analysis of dilute mixtures of SO<Subscript>2</Subscript> in N<Subscript>2</Subscript> at 7.4 μm by difference frequency spectroscopy

We describe a 7.4-μm source based on difference frequency generation with 6.5 mW of 1278-nm radiation from an extended cavity laser and 66 mW of 1544-nm radiation from another extended cavity laser, amplified in an erbium-doped fibre amplifier. Optimum focusing of the input beams in the 5×5×10-mm³ AgGaSe₂ crystal, and the spatial and temporal characteristics of the output beam, are determined. The source is used for accurate determination of line parameters for selected lines of the ν₃ band of SO₂, centred at 1361 cm^-1. Subsequently, these lines are used for performing quantitative analysis of gas mixtures containing SO₂ at concentration levels down to 4 ppm without relying on any calibration with certified gas mixtures. This demonstrates the potential of infrared spectroscopy as a primary method for low-concentration gas analysis. Received: 16 January 2003 / Revised version: 19 February 2003 / Published online: 9 April 2003 RID="*" ID="*"Corresponding author. Fax: +45-4593/1137, E-mail: jh@dfm.dtu.dk     

Theoretical Investigation of Femtosecond-Resolved Photoelectron Spectra of the Li2 Molecule

The time-dependent quantum wave packet method is used to investigate the dynamics for the Li2 molecule, and the time-resolved photoelectron spectra （TRPES） of the Li2 molecule are calculated. At the short delay time, the particular phenomenon of TRPES with four peaks is qualitatively interpreted in a dressed state picture by analyzing wave packet motion on light-induced potential （LIP）. The significant difference in the electronic structure of E1∑g^＋ between the inner and outer turning points has an impact on the TRPES. The control for the first excited state A1∑u^＋ of the initial wave packet is discussed.     

O<Subscript>2</Subscript> photodesorption from AuO<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> and Au<Subscript>2</Subscript>O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack>

Using time-resolved photoelectron spectroscopy, the decay channels of AuO₂⁻ and Au₂O₂⁻ following photoexcitation with 3.1-eV photons have been studied. For AuO₂⁻, a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption can be excluded. For Au₂O₂⁻, the spectra indicate O₂ desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au _n O₂⁻ with analogous data on Ag _n O₂⁻ clusters, a discernible pattern emerges: for dissociatively bound O₂(AuO₂⁻, Ag₃O₂⁻), there are long-living excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au₂O₂⁻, Ag₂O₂⁻, Ag₄O₂⁻, Ag₈O₂⁻), the 3.1-eV photoexcitation triggers fast O₂ desorption with a high quantum yield.     

Prospects for precision measurements of atomic helium using direct frequency comb spectroscopy

We analyze several possibilities for precisely measuring electronic transitions in atomic helium by the direct use of phase-stabilized femtosecond frequency combs. Because the comb is self-calibrating and can be shifted into the ultraviolet spectral region via harmonic generation, it offers the prospect of greatly improved accuracy for UV and far-UV transitions. To take advantage of this accuracy an ultracold helium sample is needed. For measurements of the triplet spectrum a magneto-optical trap (MOT) can be used to cool and trap metastable 2³S state atoms. We analyze schemes for measuring the two-photon 2³S →4³S interval, and for resonant two-photon excitation to high Rydberg states, 2³S →3³P →n³S, D. We also analyze experiments on the singlet-state spectrum. To accomplish this we propose schemes for producing and trapping ultracold helium in the 1¹S or 2¹S state via intercombination transitions. A particularly intriguing scenario is the possibility of measuring the 1¹S →2¹S transition with extremely high accuracy by use of two-photon excitation in a magic wavelength trap that operates identically for both states. We predict a “triple magic wavelength” at 412 nm that could facilitate numerous experiments on trapped helium atoms, because here the polarizabilities of the 1¹S, 2¹S and 2³S states are all similar, small, and positive.     

Potential and restriction of high resolution,high energy photoemission in 400–8,000 eV for studying strongly correlated electron systems

InSe and InSe:Er single crystals were grown by using the Bridgman–Stockbarger method. The absorption measurements were carried out for voltage U=0 and U=30 V states of InSe and InSe:Er samples in the temperature range of 10–320 K with a step of 10 K. The absorption edge shifted towards longer wavelengths and the intensity of the absorption spectra decreased under a 5.90 kV/cm electric field. The same binding energy values for InSe and InSe:Er were calculated as 22.2 and 14.2 meV at U=0 and U=30 V, respectively. The steepness parameters and Urbach energies for InSe and InSe:Er samples increased with increasing sample temperature in the range of 10–320 K. An applied electric field caused a shift and a decrease of the intensity of the absorption spectra and an increase in the Urbach energy and steepness parameters. The shift of the absorption edge can be explained on the basis of the Franz–Keldysh effect or thermal heating of the sample under the electric field. PACS 71.20.Nr     

Room temperature magnetization measurements of some substituted rare earth iron garnets

The saturation magnetization (M _s) and the initial magnetic susceptibility (χ _in) of substituted yttrium iron garnet (YIG) Ho_3−x Y _x Fe₅O₁₂ where (x=0,0.3,1.5), pure terbium iron garnet Tb₃Fe₅O₁₂, and substituted aluminum iron garnet (AlIG) Ho₃Fe_5−x Al _x O₁₂ where (x=0.05,0.7) at room temperature are reported. It has been observed that M _s and χ _in decrease in a linear manner as Ho³⁺ replaces Y³⁺ in yttrium iron garnet. For substituted (AlIG), a drastic decrease in both M _s and χ _in as Al³⁺ ions replace Fe³⁺ ions. The rate of decrease of M _s and χ _in with the increase of Al³⁺ for substituted (AlIG) is greater than that for substituted (YIG). That is attributed to the decrease in the superexchange interaction (self and mutual iron interactions) with Al substitution.