On the decrease in charge radii of multi-quasi particle isomers

We report changes in mean-square charge radii, δ〈r²〉$\delta 〈r^2〉$, magnetic moments and quadrupole moments for three multi-quasi particle isomers; ^97m2Y, ^176mYb and ^178m1Hf. All the isomers are observed to display a decrease in 〈r²〉$〈r^2〉$ compared to the lower-lying nuclear state on which the isomer is built. The decreases in 〈r²〉$〈r^2〉$ occur despite the isomers showing increases in quadrupole moment. Possible mechanisms for the effect, which is now seen for six multi-quasi particle isomers, are discussed.     

Isotonic and isobaric dependencies of nuclear charge radii for rare-earth nuclei

Isotonic and isobaric dependencies of nuclear charge radii in the region between Z=54 and Z=70 are obtained from the analysis of isotope shift data r² and muonic and electron scattering data on r². They are compared with the predictions of the droplet model and the Hartree-Fock calculations. The isobaric dependencies of r² have proven to be especially sensitive to the choice of an effective nucleon-nucleon interaction.     

Ultra-Stable Rubidium-Stabilized External-Cavity Diode Laser Based on the Modulation Transfer Spectroscopy Technique

We construct an ultra-stable external-cavity diode laser via modulation transfer spectroscopy referencing on a hyperfine component of the ST Rb D2 lines at 780 nm. The Doppler-free dispersion-like modulation transfer signal is obtained with high signal-to-noise-ratio. The instability of the laser frequency is measured by beating with an optical frequency comb which is phase-locked to an ultra-stable oven controlled crystal oscillator. The Allan deviation is 3.9 × 10-13 at I s averaging time and 9.8 ×10-14 at 90s averaging time.     

Broadband difference frequency generation around 4.2 μm at overlapped phase-match conditions

Broadband difference frequency generation is theoretically and experimentally confirmed. It is shown that a wide tunable range of greater than 220 nm (FWHM) around 4.2 μm can be obtained in a 40 mm long periodically poled lithium niobate crystal with a single quasi-phase-matched period at a certain temperature. The broad bandwidth can be explained by means of the group-velocity matching or phase-mismatch minimization at overlapped phase-match conditions. The result shows that the broadband mid-infrared (mid-IR) laser source may find profound applications in trace gases detection of multiple atmospheric species and high resolution spectroscopy.     

High-accuracy frequency atlas of 13C2H2 in the 1.5 μm region

A pair of 1.5 μm semiconductor laser frequency standards have been developed for optical telecommunications use, stabilised to Doppler-free transitions of the ν₁ + ν₃ and ν₁ + ν₂ + ν₄ + ν₅ combination bands of ¹³C₂H₂. The Allan deviation σ/f for a laser locked to line P(10) of the former band follows a slope of 1.6 × 10⁻¹²τ^−1/2, reaching a minimum of 5.7 × 10⁻¹⁴ at τ = 4000 s. The absolute frequencies of 61 lines of the ν₁ + ν₃ band and 43 lines of the ν₁ + ν₂ + ν₄ + ν₅ band, covering the spectral region 1520 nm to 1552 nm, have been measured by use of a combined frequency chain and femtosecond comb, together with a passive optical frequency comb generator. The mean uncertainties for the line frequencies within each band are 1.4 kHz for the ν₁ + ν₃ band and 1.9 kHz for the ν₁ + ν₂ + ν₄ + ν₅ band, representing improvements on the precision of previously published data by factors of 100 and 10⁴, respectively. Improved values of the rotational constant B″ and centrifugal distortion coefficients D″, H″ and L″ of the vibrational ground state are presented.This article is published with the permission of the Controller of HMSO and the Queen’s Printer of Scotland     

Broadly tunable difference-frequency spectrometer for trace gas detection with noncollinear critical phase-matching in LiNbO3

3 is reported. Using an external cavity diode laser, tunable from 795 to 825 nm, and a cw diode-pumped Nd:YAG ring laser at 1064 nm, we produced narrowband mid-ir laser light between 3.16 and 3.67 μm. This broad tuning range of 440 cm^-1 can be critically phase-matched by changing the external crystal angle within a range of only 0.4°. Even for a fixed crystal angle broadband phase-matching over 230 cm^-1 was achieved. No realignment was required when tuning the laser over the whole wavelength range, which enabled the use of a compact 36-m multipass cell as gas chamber. The conversion efficiency could be improved by almost a factor of 3 by applying noncollinear instead of collinear phase-matching, resulting in 30 nW of idler power with good beam quality. Spectra of methane were recorded in laboratory air, which demonstrate the rapid and continuous broad tunability at high sensitivity, enabling sub-ppm detection. Finally, wavelength modulation spectroscopy at high resolution was applied as a promising tool for further inproving the performance of this laser spectrometer. Received: 16 March 1998/Revised version: 1 July 1998     

Stabilization and Shift of Frequency in an External Cavity Diode Laser with Solenoid-Assisted Saturated Absorption

A simple method to realize both stabilization and shift of the frequency in an external cavity diode laser （ECDL） is reported. Due to the Zeeman effect, the saturated absorption spectrum of Rb atoms in a magnetic field is shifted. This shift can be used to detune the frequency of the ECDL, which is locked to the saturated absorption spectrum. The frequency shift amount can be controlled by changing the magnetic field for a specific polarization state of the laser beam. The advantages of this tunable frequency lock include low laser power requirement, without additional power loss, cheapness, and so on.     

Observation of upconversion fluorescence and stimulated emission based on three-photon absorption

The observations of three-photon-induced frequency-upconversion fluorescence and the highly directional stimulated visible emission in two dyes, 4-[p-(dicyanoethylamino) styryl]-N-methylpyridinium iodide (abbreviated as CEASP) and the complex of CEASP and Ce(NO₃) (abbreviated as CEASP-Ce), are reported. The photographs of the forward amplified spontaneous emissions spots, pumped by an optical parametric oscillator idler with a pulse width of 8 ns and a wavelength of 1.3 μ m, are shown. The upconversion fluorescence produced both in dimethyl formamide solution and 2-hydroxyethyl methacrylate (HEMA) polymer spans from green to red, with a cubic dependence on the pump light intensity. The experimental results imply that the existence of the lanthanide ion Ce^{3 +} sensitizes the nonlinear absorption and emission.     

Collision induced and resonantly enhanced extra resonances in four-wave mixing in inhomogeneously broadened four level systems

Role of collisions in inducing and enhancing extra resonances in non-linear generation by four-wave mixing in inhomogeneously broadened four level systems is studied. It is shown that collision induced three and two-photon resonances can be enhanced further by tuning to exact one photon (non-absorbing) transitions between unpopulated upper levels. In addition we also find existence of collision induced interference dip at the three-photon resonance that arises due to complete cancellation of signal at the line center.     

High-power source of coherent picosecond light pulses tunable from 0.41 to 12.9 μm

We report a high-power source of coherent picosecond light pulses based on optical parametric generation and amplification in LiB₃O₅ and AgGaS₂ crystals. The spectral range of this continuously tunable source covers the visible, near-infrared and medium-infrared spectrum from 0.41 to 12.9 m. An optical parametric generator and amplifier, consisting of two type-I phase-matched LiB₃O₅ crystals and a diffraction grating, is pumped by the third harmonic of a picosecond Nd:YAG laser and provides spectrally narrow, high-power pulses from 0.41 to 2.4 m. Energy conversion efficiencies up to 16 percent are achieved. The pulse duration is about 14 ps, the bandwidth between 10 and 30 cm^–1. The tuning range is extended to 12.9 m by mixing the infrared output between 1.16 and 2.13 m with the fundamental of the Nd:YAG laser in type-I-phase-matched AgGaS₂ crystals. Up to 25 percent of the pulse energy at 1.064 m is converted into parametric infrared pulses. Bandwidths between 3 and 8 cm^–1 and a pulse duration of approximately 19 ps are measured for these pulses. We also observe a retracing behaviour in the tuning curve of AgGaS₂ not reported before.     

A tunable light source in the 370 nm range based on an optically stabilized,frequency-doubled semiconductor laser

A tunable harmonic output power of 18 W at a wavelength of =370 nm is obtained by resonance-enhanced frequency doubling of an optically-stabilized semiconductor laser. A commercially available AlGaAs laser diode which emits a maximum power of 10 mW at =740 nm is operated in an extended-cavity configuration. Dispersion prisms are used in the extended cavity to obtain longitudinal-mode selection with low loss of optical power. The output is focussed into an optically isolated high-finesse ring resonator which contains a LiIO₃ crystal for second-harmonic generation. One potential application of this laser source is the optical excitation and laser cooling of ytterbium in an ion trap. In a related demonstration experiment, the frequency-doubled diode laser is applied to excite the =369.5 nm ² S _1/2-² P _1/2 transition of ytterbium ions in a hollow-cathode discharge.     

Absolute frequency measurement of iodine lines with a femtosecond optical synthesizer

We have used a single laser femtosecond optical frequency synthesizer together with a widely tunable Nd:YAG laser to measure the absolute frequency of several absorption lines in molecular iodine around 532 nm. The use of two different repetition frequencies allows us to determine the number of modes used for the frequency measurement unambiguously. The lines also provide data for the determination of improved ro-vibrational constants of the iodine molecule. Received: 3 July 2001 / Published online: 19 September 2001     

13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 micrometers

A portable modular gas sensor for measuring the ¹³C/¹²C isotopic ratio in CO₂ with a precision of 0.8‰(±1σ) was developed for volcanic gas emission studies. This sensor employed a difference frequency generation (DFG)-based spectroscopic source operating at 4.35 μm (∼2300 cm^-1) in combination with a dual-chamber gas absorption cell. Direct absorption spectroscopy using this specially designed cell permitted rapid comparisons of isotopic ratios of a gas sample and a reference standard for appropriately selected CO₂ absorption lines. Special attention was given to minimizing undesirable precision degrading effects, in particular temperature and pressure fluctuations. Received: 16 April 2002 / Revised version: 28 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-713/5245237, E-mail: fkt@rice.edu     

Simultaneous detection of methane, oxygen and water vapour utilising near-infrared diode lasers in conjunction with difference-frequency generation

An all-diode-laser-based spectrometer is used for the simultaneous detection of methane, oxygen and water vapour. This is accomplished using a 760-nm diode laser and a 980-nm diode laser in conjunction with difference-frequency generation to 3.4 μm in a periodically poled lithium niobate crystal. Each of the output wavelengths is resonant with one of the molecular species. Simultaneous recordings over a 15-m open path of laboratory air are demonstrated. The recording scheme shows the wide applicability of a diode-laser-based difference-frequency spectrometer for the detection of molecular species in different wavelength ranges. By increasing the frequency of the 760-nm diode laser and decreasing the frequency of the 980-nm diode laser, a maximum continuous tuning range in the mid infrared of 3.6 cm^-1 is achieved. This enables the recording of several methane lines at atmospheric pressure. Pressure-dependence studies of methane lineshapes are also performed in an absorption cell. An indoor-air methane background level of 3 ppm is measured. The signal-to-noise ratio in the recorded methane spectra indicates that sub-ppm detection of methane at atmospheric pressure is feasible. Received: 6 March 2000 / Revised version: 19 June 2000 / Published online: 11 October 2000     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

The use of zinc and iron emission lines in the depth profile analysis of zinc-coated steel

In this study we report on the results of experiments devoted to the depth profile analysis of zinc-coated steel samples using the laser-induced breakdown spectroscopy (LIBS) technique. The dependence of zinc and iron emissions in three ablation atmospheres (air, argon, helium) was measured using the fundamental wavelength (1064 nm) of the Nd:YAG laser. The highest possible depth resolution was achieved by optimizing the experimental parameters, such as the delay time (which affects the tailing of the zinc emission signal), focusing conditions, energy delivered to the sample, and choice of buffer gases. Current research indicates that there is a constant need to optimize these parameters so that reliable depth-profiling analysis can be performed.     

Sum-frequency generation at electrochemical interfaces: Cyanide vibrations on Pt(111) and Pt(110)

We use optical sum-frequency generation to investigate the stretching vibrations of cyanide (CN^–) molecules chemisorbed from aqueous electrolytes on single-crystalline Pt(111)- and Pt(110)-electrode surfaces. For clean and well-ordered Pt(111) electrodes, a single vibrational band between 2080 and 2150 cm^–1 with a nonlinear frequency dependence on the potential is observed and assigned to the CN stretching vibration of chemisorbed cyanide. A second band between 2145 and 2150 cm^–1 with very weak potential dependence appears on a surface which was subjected to oxidation-reduction cycles and is attributed to cyanide associated with a microscopically disordered surface. This assignment is supported by preliminary results for a Pt(110) single-crystal electrode. On a well-ordered (110) surface a single and potential-dependent cyanide vibration between 2070 and 2112 cm^–1 is observed. After oxidation of the cyanide and readsorption, this band is replaced by a higher frequency band at 2144 cm^–1 which is essentially not potential-dependent. Occasionally, additional vibrational bands at lower frequencies not reported in corresponding IR studies are observed on Pt(111).Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Measurements of g-Factor of Rotational Band States in 82Sr

The g-factors of the positive parity rotational states up to spin I = 8^＋ for the ground state band in even-even nuclei S2Sr have been measured by a transient-magnetic-field ion implantation perturbed angular distribution method. The experimentally measured 9-factors increase with the increasing spin along the band and show that the g9/2 proton aligns only and the alignment starts from I =6^＋. The measured g-factors also indicate that the nuclei ^82Sr gain their spins by the quasi-proton alignment at higher spin.     

Nuclear charge distribution of heavy fission fragments from thermal-neutron-induced fission of235U

Fragments from thermal-neutron induced fission of²³⁵U have been separated by a mass spectrometer with respect to their masses and kinetic energies within 1 μsec. The separation principles are briefly described. For masses 130 to 139 amu the charge distributions have been determined by counting the number of beta tracks emitted from the individual mass selected fission fragments in a nuclear photographic emulsion. In another method, the average number of beta particles for each fragment mass is determined by use of a 4π-proportional counter. The mean nuclear charge as a function of mass is compared with other experimental results and theoretical curves. Contradictory to the radiochemical results, this experiment yields a dip in the mean nuclear charge versus mass curve at mass 132 amu corresponding to the doubly magic nucleus (N=82,Z=50)¹³²Sn. Recent theoretical calculations of Nörenberg are in agreement with this finding.