Backscattering mirrors for X-rays and Mössbauer radiation

Observation of exact backscattering of X-rays and studies of its energy and angular dependences; test of the validity of the dynamical theory of diffraction in the extreme case of exact backscattering; backscattering high-energy-resolution monochromators; backscattering interferometers, in particular of the Fabry–Pérot interferometer type; and precise, up to 5·10^—9 Å, measurements of crystal lattice parameters: these are central topics of the paper. Special attention is paid to the selection of crystals to be used as backscattering mirrors. Noncubic crystals like Al₂O₃, SiC, etc., allow backscattering for X-rays with practically any energy above 10 keV. Feasibility of backscattering mirrors for Mössbauer radiation of ⁵⁷Fe (14.4 keV), ¹⁵¹Eu (21.5 keV), ¹¹⁹Sn (23.9 keV), and ¹⁶¹Dy (25.6 keV) nuclei is demonstrated by Al₂O₃ crystals. A concrete design of a sapphire Fabry–Pérot–Bragg étalon is presented.

     

CARS investigation of collisional shift of the hydrogen Q‐branch transitions by water at high temperatures

A high‐resolution (∼0.1 cm⁻¹) spectroscopic method based on the application of a Fabry–Pérot interferometer to the spectral analysis of the coherent anti‐Stokes Raman scattering (CARS) signal from an individual Raman transition was used to obtain single‐shot spectra of hydrogen Q‐branch transitions directly in the flame of a pulsed, high‐pressure H₂/O₂ combustion chamber. Simultaneously with the Fabry–Pérot pattern, a broadband CARS spectrum of the complete H₂Q ‐branch structure was recorded in order to measure the temperature of the probe volume. During every cycle of the combustion chamber, a pressure pulse together with single‐shot CARS spectra, providing information on individual line shapes and medium temperature, was recorded. On the basis of the experimental data, the temperature dependences of lineshift coefficients for several Q‐branch lines of hydrogen molecules under collisions with water molecules were determined in the temperature range 2100 < T < 3500 K, and an empirical ‘fitting law’ for H₂ H₂O lineshift coefficients is proposed. Copyright © 2010 John Wiley & Sons, Ltd.     

Stark-Effekt-Verbreiterung der Neon I-Übergänge mit schwacher Feldstärkeabhängigkeit (2p 5 3s-2p 5 3p)

The half widths of Stark-broadened Ne I spectral lines (2p ⁵ 3s-2p ⁵ 3p) were measured in wall stabilized arc plasmas (pressurep=1 atm, andp=3 atm) consisting of several gas components (Ar 68%, Ne 27%, He 3%, H₂ 2%). The theoretical half widths of two spectral lines (λ=6,266.50 Å,λ=6,163.59 Å) with known calculated broadening parameters according to Griem differ by a factor 1.8 to 1.9 from the measured widths. Within the errors of Griem's line broadening calculations of about 20% there was agreement between experimental and theoretical half widths regarding two other spectral lines (λ=5,881.90 Å,λ=5,852.49 Å). Calculated adiabatic widths are smaller than the widths according to Griem. Therefore in some cases the deviation of the adiabatic widths from the measured widths is smaller than the difference between Griem widths and experimental ones. Static quadratic coefficients of NeI transitions 2p ⁵ 3s-2p ⁵ 3p calculated from measured Stark-broadened lines showed wavelength displacements in the order of 0.005 Å referring to an electrical field of 100 kV/cm. The determined coefficients are 10² times smaller than those values measurable by known methods of emission spectroscopy.     

Microbead dynamics in optical trap assisted nanopatterning

An optically pumped ZnO nanowire laser with a 10-period SiO₂/SiN _x distributed Bragg reflector (DBR) was demonstrated. Stimulated emissions with equally distributed Fabry–Pérot lasing modes were observed at pumping powers larger than 121 kW/cm². This result, when compared to nanowires of the same length and without a DBR structure, shows that a lower threshold of pumping power, higher quality factor, and larger cavity finesse can be achieved due to the high reflectivity of the DBR in the designed wavelength range. A coexistence of stimulated and spontaneous emissions was also observed above threshold and was attributed to partially confined waveguide modes in nanowires with diameters smaller than 100 nm.     

A single mode flashlamp-pumped dye laser

A flashlamp-pumped dye laser capable of single transverse and longitudinal mode operation is described. The maximum untuned power output is 2 MW cm^-2, TEM₀₀ mode. By employing three intracavity Fabry—Pérot etalons, a minimum spectral width of 4 MHz, stable to within 12 MHz, has been obtained.     

Some observations on the collisional broadening and shift of the Sr 4607.33 Å line in H2/O2/Ar flames

Spectral profiles of the 4607.33 Å Sr emission line $\text{(}{}^1\text{P}{}_1\text{?}{}^1\text{S}{}_0\text{transition}\text{)}$ were measured with a pressure-scanned Fabry-Pérot interferometer in a set of pre-mixed, laminar shielded H₂/O₂/Ar flames at 1 atm with temperatures ranging from 1765 to 2365 K. Some additional integrated line profile measurements were carried out.From the full width at one-half maximum (FWHM), δλ_T, and shift of the optically thin line profiles, adiabatic cross sections for broadening and for shift were determined for SrAr and SrH₂O.     

Extended measurements of isotope shifts in Mg I

The isotope shifts in the Mg I transitionsλ 2,852 Å (3s ^{2 1} S ₀-3s3p ¹ p ¹ P ₁) andλλ 3,829, 3,832 and 3,838 Å (3s3p ³ P-3s3d ³ D) have been measured for²⁴Mg,²⁵Mg and²⁶Mg. The measurements were carried out using a pressure scanned Fabry-Perot interferometer and separated isotopes excited in hollow cathode discharge tubes. The results are compared to theoretical predictions and an attempt is made to convert measured line shifts into level shifts.     

Rumpfpolarisation des 4d 9 5s 22 D-Zustandes von Ag I

The hyperfine structure of the transition Ag I, 4d ⁹ 5s ²² D _3/2-4d ¹⁰5p ² P _1/2, λ=19,372 Å has been investigated by means of a photoelectrically recording Fabry-Pérot interferometer using separated isotopes. Evaluating the hfs-pattern by a digital analysis the following values for the magnetic dipole interaction constants have been obtained: $$A_{3/2}^{(109)} (d^9 5s^2 ) = - 12.11(6)mK; A_{1/2}^{(109)} (d^{10} 5p) = - 6.72(10)mK.$$ Using the knownA-factor of the other level of the configurationd ⁹ s ² and the values of the Ms anomalies of the levelsd ⁹ s ²² D _5/2 andd ¹⁰ s ² S _1/2 together with our result, the influence of the core polarization on theA-factors of the configurationd ⁹ s ² is discussed. Assuming that the used relativistic correction factors are correct, the influence of the core polarization cannot be due only to the unpaireds-electrons of the core, but must be described by the three-parameter hfs-Hamiltonian given by Harvey. Consequences for calculating the quadrupole moments from hfs data are mentioned.     

Magnetic tuning of exciton–photon resonance in II–VI microcavities

We demonstrate the effectiveness of the giant Zeeman effect in II–VI semimagnetic semiconductors to tune the exciton resonance of quantum wells onto the Fabry–Pérot resonance of a microcavity. A large oscillator strength of 3 × 10¹³cm⁻²per quantum well is deduced from the measured 10.6 meV vacuum Rabi splitting.     

On the branching ratios of several pairs of He(II) lines

Branching ratios of He(II) lines have been measured for the principal levels 5–10. The observations have been made on an He plasma produced by a hollow cathode-arc discharge, having an electron density of 3×10¹⁹m^-3 and an electron temperature of 2×10⁵ K on the axis of the discharge.The observed branching ratios differ by about 30% from the ones following from theoretical transition probabilities, if the sublevels of the main levels were populated according to their statistical weights. The shape of the He(II) 4686Åemission line observed off the axis of the discharge using a Fabry-Pérot interferometer gave evidence of a non-statistical population distribution for the sublevels.     

Random lasing in an organic light‐emitting crystal and its interplay with vertical cavity feedback

The simultaneous vertical‐cavity and random lasing emission properties of a blue‐emitting molecular crystal are investigated. The 1,1,4,4‐tetraphenyl‐1,3‐butadiene samples, grown by physical vapour transport, feature room‐temperature stimulated emission peaked at about 430 nm. Fabry‐Pérot and random resonances are primed by the interfaces of the crystal with external media and by defect scatterers, respectively. The analysis of the resulting lasing spectra evidences the existence of narrow peaks due to both the built‐in vertical Fabry‐Pérot cavity and random lasing in a novel, surface‐emitting configuration and threshold around 500 μJ cm⁻². The anti‐correlation between different modes is also highlighted, due to competition for gain. Molecular crystals with optical gain candidate as promising photonic media inherently supporting multiple lasing mechanisms.     

Experimental determination of isotope shifts in Mg I

The isotope shift in the Mg l transitionsλλ 5167, 5172 and 5183Å (3s3p ³ P-3s4s ³ S) andλ 5528 Å (3s3p ¹ P 3s4d ¹ D) has been measured for the isotopes²⁴Mg,²⁵Mg and²⁶Mg. The hyperfine structure of theλ 5167 Å line has been studied and theA andB factors for the 3s4s ³ S ¹ level in²⁵Mg determined. The measurements were made with separated isotopes using a scanning Fabry-Perot interferometer.     

Measurement of number density of lead and thallium see-through hollow cathode discharges with a high resolution Fabry–Pérot spectrometer and by conventional atomic absorption

A see-through hollow cathode lamp, or galvatron, is investigated. A novel method is presented for the measurement of an atomic absorption profile using a quasi-continuum source created by the combination of two line sources and a high-resolution Fabry–Pérot interferometer coupled to a spectrometer. Number densities are calculated from the resulting absorption profiles by the peak absorption coefficient relationship and compare well with results obtained from high-resolution emission measurements. Number densities are also determined for the lead ³ P ₁ metastable state and thallium ² P _1/2 ^o ground state by conventional atomic absorption. A hollow cathode lamp is used as an emission source and is set at a relatively low current to approximate as a line source relative to the galvatron. Due to the relative line widths of the source and absorber, only the lead metastable state results compare to results obtained by saturated fluorescence. PACS 52.25.Tx; 32.70.Jz; 07.60.Ly     

Determination of the spectral width of Raman gain regions for different far infrared laser gases using a metal mesh Fabry Pérot interferometer

In this paper we report on the determination of the spectral width of Raman gain regions for different far infrared laser gases using a metal mesh Fabry Pérot interferometer. By use of broadband pump radiation emitted from a high pressure CO₂ laser we generated far infrared radiation within the Raman gain regions via stimulated Raman scattering. The spectral width of the far infrared radiation was determined using a Fabry Pérot interferometer in high interference order. We applied the method to study stimulated Raman scattering in D₂O and methylfluoride at different gas pressures.     

Dependence of the spectral width of the self-absorbed Sr 4607.33Åresonance line on Sr atomic density in a flame

Using a pressure-scanned Fabry-Pérot interferometer, we measured spectral profiles of the self-absorbed Sr resonance line at 4607.33Åemitted in a pre-mixed, laminar, shielded air-C₂H₂ flame at 1 atm for various Sr-solution concentrations. The dependence of the full width at one-half maximum (FWHM) on Sr atomic density in the flame was found to agree with a calculation based on the Voigt expression and Beer's law. It appears that one can derive the FWHM under optically thin conditions (i.e. at vanishing Sr atomic density) by linear extrapolation from measurements made at higher concentrations where the signal-to-noise ratio is good, but broadening due to self-absorption plays a role.     

Relative line intensity measurement in absorption spectra using a tunable diode laser at 1.6 μm: application to the determination of 13CO2/12CO2 isotope ratio

The measurement of relative intensities in CO₂ combination bands spectrum is performed using wavelength modulation spectroscopy (WMS) and a DFB (distributed feedback) diode laser operating at 1.6 μm. The diode laser is stabilized with an external Fabry–Pérot interferometer and absorption spectroscopy is performed in a multipass gas cell. A spectrum containing spectral lines of both ¹³CO₂ and ¹²CO₂ isotopic species is recorded. The variation of laser power during frequency scanning and the line shape are taken into account to accurately extract line intensities from experimental data. The isotopic concentration ratio is deduced from the intensity ratio. Both ratios are measured with an accuracy of about 0.5% in pure CO₂. Received: 9 June 2000 / Published online: 8 November 2000     

Comparison of emission and absorption core profiles of Sr and Ba resonance lines in an air-acetylene flame

Under optically thin conditions, the core of emission-line profiles of the resonance lines (¹P₁-¹S₀ transition) of Sr and Ba have been measured with a pressure-scanned Fabry-Pe´rot interferometer in a laminar, pre-mixed, shielded air-acetylene flame at 1 atm (T = 2275 K). Under identical experimental conditions, where Doppler and collisional broadening are almost equally important, these profiles are compared with Zeeman-scanned absorption profiles of the same resonance lines. The values for the full width at one-half maximum (FWHM) and the shift of the emission and absorption core profiles for Sr and Ba proved to be identical within an experimental scatter of 5 and 20%, respectively. Obviously the radiation emitted by the Sr and Ba atoms in the flame is thermal, which means that the spectral shape of the line is consistent with Maxwell-Boltzmann equilibrium.     

Measurement of the lifetime of the3 P 2 and4 P 5/2 states in Ar16+ and Ar15+

Promptly decaying levels in Li- and Be-like Ar could be identified in the delayed x-ray and electron spectrum in addition to the four known metastable (1s2s)¹ S ₀, (1s2s)³ S ₁, (1s2p)³ P ₂, and (1s2s2p)⁴ P _5/2 states in He- and Li-like systems. The states having lifetimes of about 10^?14s still contribute to the spectrum observed 10^?9s after the ions have passed a foil. This observation is explained by cascading processes from states of high principal quantum numbern≧14. The measured lifetime of the⁴ P _5/2 state subtracting the contribution of cascades is (0.594±0.016) ns and slightly larger than the theoretically predicted value of 0.563 ns. The³ P ₂ state is depopulated at short distances behind the foil by cascading processes. Its measured lifetime of (1.44±0.08) ns compares to a theoretical value of 1.48 ns.     

Tunable diode laser measurements of spectral parameters of HCN at room temperature

A tunable infrared diode laser was used to record 17 fully resolved vibration-rotation transitions in the v₁ fundamental band of HCN at 3μ. The experiments were conducted in an absorption cell on room temperature mixtures of HCN diluted by N₂ and Ar. The v₁ fundamental band strength of HCN was determined to be 267±8 cm^-2 atm^-1 at 273.2 K. Small but significant reductions in the residual errors were obtained by using the Galatry profile rather than the Voigt profile to fit the experimentally recorded line shapes. Collisional broadening and narrowing parameters were determined simultaneously from Galatry profile fits to the data. The collision-broadened linewidths of HCN lines in N₂ and Ar were determined as a function of rotational quantum number of transitions ranging from P(14) to R(14) (3268.22-3353.29 cm^-1). The optical diffusion coefficients of HCN in N₂ and Ar at 300 K were determined from the collisional narrowing parameters and were 0.074±0.01 and 0.016±0.03 cm²s^-1 respectively.