Instrumental Width Research Articles

High resolution absorption cross-section measurements of ozone at 195 K in the wavelength region 240–350 nm

Cross-sections of the Hartley-Huggins bands of 03 at the temperature 195 K have been obtained from photoabsorption measurements at column densities in the range 2 × 1017−1 × 1021 cm−2 throughout the wavelength region 240–350 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mm−1 grating and operated at an instrumental width (FWHM) of 0.003 nm. The assumptions made in putting the measured relative cross-sections on an absolute basis are discussed. Fine structure in the cross-section observed in the Huggins bands is illustrated in the region 323–327 nm where shallow features of width 0.01–0.02 nm occur superposed on a stronger apparent continuum exhibiting broader wavy structure.

High resolution absorption cross section measurements and band oscillator strengths of the (1, 0)−(12, 0) Schumann-Runge bands of O 2

Abstract Cross sections of O 2 at 300 K have been obtained from photoabsorption measurements at various pressures throughout the wavelength region 179.3–201.5 nm with a 6.65 m photoelectric scanning spectrometer equipped with a 2400 lines mm −1 grating and having an instrumental width (FWHM) of 0.0013 nm. The measured absorption cross sections of the Schumann-Runge bands (12, 0) through (1, 0) in this wavelength region are absolute, i.e., independent of the instrumental width, a result not achieved previously. The measured cross sections are presented graphically and are available at wavenumber intervals of > sim; 0.1 cm −1 as numerical complications stored on magnetic tape from the National Space Science Data Center, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A. Band oscillator strengths of the (12, 0) through (1, 0) bands have been determined by direct numerical integration of the measured cross sections.

Magnetic excitations in amorphous ferromagnets

The magnon excitations near the first $\mathcal{Y}(Q)$ maximum in amorphous ferromagnets have been reexamined by the use of a triple-axis polarized-beam spectrometer. This technique permits us to separate inelastic magnetic cross sections from phonons and all elastic components. We point out that this technique is not useful once the magnon excitation energy becomes smaller than the instrumental width. We have examined three different Fe-based alloy compositions at room temperature. In each case our results are consistent with a mechanism involving "umklapp scattered" long-wavelength spin waves with no energy gap. We compare our results on ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$ with those of Mook and Tsuei who interpreted their results in terms of a 19-meV energy gap in ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$.

Neutron scattering study of the tetragonal-to-incommensurate ferroelastic transition in barium sodium niobate

The static and dynamic characteristics of the incommensurate phases of barium sodium niobate have been investigated by means of elastic and inelastic neutron scattering between room temperature and 660\ifmmode^\circ\else\textdegree\fi{}C. The observation of two incommensurate phases on heating from room temperature is consistent with previous x-ray results. Phase II, which is stable up to ${T}_{\mathrm{II}}=250\ifmmode^\circ\else\textdegree\fi{}$C, is nearly commensurate, the modulation vector being $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}=[({\stackrel{\ensuremath{\rightarrow}}{\mathrm{a}}}^{*}+{\stackrel{\ensuremath{\rightarrow}}{\mathrm{b}}}^{*})\frac{(1+\ensuremath{\delta})}{4}+\frac{{\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}}^{*}}{2}]$, with $\ensuremath{\delta}\ensuremath{\approx}1$%. Phase I is incommensurate with the same direction of modulation, and $\ensuremath{\delta}$ varies linearly on heating from 8% at ${T}_{\mathrm{II}}$, up to 12.5% at ${T}_{\mathrm{I}}\ensuremath{\simeq}288\ifmmode^\circ\else\textdegree\fi{}$C. Above ${T}_{\mathrm{I}}$, crystalline phases with tetragonal symmetries are stable, ${T}_{\mathrm{II}}$ is discontinuous, and ${T}_{\mathrm{I}}$ continuous. On cooling down from ${T}_{\mathrm{I}}$ a large thermal hysteresis is noted in the modulation wavelength and the satellite intensities. The variations are smeared and no discontinuity similar to ${T}_{\mathrm{II}}$ is observed. The precursor effects of ${T}_{\mathrm{I}}$ consist, in the tetragonal phase, of a soft phonon and a central peak. The soft mode is underdamped only above \ensuremath{\sim}385\ifmmode^\circ\else\textdegree\fi{}C. The static susceptibility deduced from the total scattered intensity diverges at ${T}_{\mathrm{I}}$, with ${\ensuremath{\omega}}_{0}^{2}\ensuremath{\sim}\frac{1}{\ensuremath{\chi}}$, varying linearly between 300\ifmmode^\circ\else\textdegree\fi{}C and 360\ifmmode^\circ\else\textdegree\fi{}C. The central peak, which is observed even at 660\ifmmode^\circ\else\textdegree\fi{}C, diverges at ${T}_{\mathrm{I}}$ while its width remains always equal to the instrumental width. The dispersion surface of the soft mode in the ($\ensuremath{\eta}\ensuremath{\xi}\frac{1}{2}$) plane has a valley shape, the bottom of which is perpendicular to the modulation vector. It gives rise to diffused scattering forming rods along $[1\overline{1}0]$. At a microscopic level the modulation is due to displacements consisting of a collective shearing of the oxygen octahedra in the structure. The dynamics of this motion involve strong correlations in planes containing the modulation vector and the polar $c$ axis. The order parameter of the observed sequence of transitions has four components. This unusual dimension is responsible for the breaking of the macroscopic symmetry in the incommensurate phases and the onset of improper ferroelastic properties. However, the incommensurate behavior deriving from it is expected to be the same as that previously studied for two-component order parameters. Thus the corresponding extended Landau theory is not likely to account for the unusual hysteresis observed below ${T}_{\mathrm{I}}$, and for the persistence of a residual incommensurability below ${T}_{\mathrm{II}}$. These anomalous effects are attributed to the influence of defects having their origin in an off-stoichiometry or off-equilibrium distribution of the cations in the structure.

Coaxial nanosecond flashlamp

We have developed a coaxial thyratron-gated flashlamp capable of being operated with high stability over a much wider range of conditions than previous flashlamps. Novel methods for continuously monitoring discharge stability are described. We present comprehensive data on the dependence of pulse width and intensity on operating parameters. Using the single photon detection technique an overall instrumental profile of 950 ps FWHM and 630 ps rise time has been obtained using hydrogen as a filler gas. An instrumental width of 1.2 ns FWHM can be routinely obtained in nitrogen even at a repetition rate of 100 kHz. Intensities of ≳109 photons per pulse in nitrogen and ≳108 in hydrogen can be obtained at the minimum pulse duration. The maximum intensity per pulse without added capacitance is ≳1010 photons. This performance is considerably better than that reported for conventional flashlamps.

High-resolution measurement of the helium1s2s2S2resonance profile

The 19.37-eV helium scattering resonance profile has been measured with an instrumental width less than the natural width and much less than that attained in any previously published study. The electron beam was produced by photoionization and scattered from a He beam from a supersonic nozzle. The instrumental width, including residual Doppler effects, was routinely around 5 meV and the current was 5 x 10/sup -12/ A. The profile was measured at 22/sup 0/, 90/sup 0/, and 135/sup 0/ scattering angles. From the 90/sup 0/ and 135/sup 0/ data the s- and p-wave phase shifts were found to be 1.813 +- 0.017 and 0.309 +- 0.013 radians, respectively. The natural width G of the resonance was inferred by two methods: nonlinear least-squares fitting, and use of the integral of the differential cross section over the resonance. Both methods yield 11.0 +- 0.5 meV, if it is assumed that there is no instrumental background in the scattered electron signal. Experimental evidence indicates that any such instrumental background must be less than 15% of the total signal, and is probably less than 5%. If an extreme allowance is made for the larger figure, a G as high as 13 meV could bemore » possible.« less

Deconvolution of Brillouin spectra

Abstract Brillouin spectra of amorphous quartz are analyzed using an iterative deconvolution procedure founded upon Bayes' theorem. It is shown that spectral line-shapes can be determined even when the scattering excitation has a linewidth as small as one-sixth the instrumental width.

Leed intensity calculations for finite instrumental transfer width caused by the angular aperture of the primary beam

Abstract The influence of the angular divergence of the primary beam on the dynamic intensities in LEED is investigated. It is shown that the corresponding finite instrument transfer width restricts the area of dynamic scattering within one layer. For usual angular apertures of about 1° reductions considerably below the area determined by inelastic processes result. This reduces the lattice sum to be executed in intralayer scattering. For a typical value of the imaginary part of the optical potential, v0i=4 eV, changes up to a few percent of the intensity result. Hence more realistic intensity spectra are calculated with a simultaneous considerable saving of computing time.

LEED intensity calculations for finite instrumental transfer width caused by the angular aperture of the primary beam

LEED intensity calculations for finite instrumental transfer width caused by the angular aperture of the primary beam

Reply to comments on "Spectroscopic investigation of plasma turbulence"

It is true that in measurements of the small effect of plasma turbulence on the spectral shape near allowed lines in the parent gas one must be on guard against spurious effects, impurities, etc. Furthermore, as noted in our paper (1), there is an added difficulty due to the fact that the turbulence in the field of view of the spectrometer is not very reproducible from shot to shot or from one run to the next. The feature we have identified as the forbidden line (F), due to low-frequency turbulence, is present in most of the runs in contrast to the satellites (S*) due to electron plasma oscillations, which are expected at a wavelength separation of f h2(27cc)-'a,, from the forbidden line, with a,, = ( t z e 2 / m ~ ~ L ' Z , the electron plasma frequency. Pie1 and Pinnekamp (2) suggest that the features we have interpreted in terms of plasma turbulence may instead be due to molecular impurities in the helium discharge. We do not find their arguments convincing and would like to deal with them in two parts. First their conclusion, labelled (I), that our observed profile for the allowed line is narrower than the theoretical profile leads them to conclude that the emissions observed actually originate near the wall. If true this would be serious since the plasma density at the wall is too low to allow interpretation of the satellites in terms of electron plasma oscillations. However, the conclusion (I) of Pie1 and Pinnekamp is incorrect for the following reason. The shape of the allowed line in the theoretical curve they have used bears no relation to our conditions. It includes Doppler broadening, with the emitting neutrals also assumed to be at 4 x lo4 K (see Pie1 and Pinnekamp ref. 4) and this is in fact the dominant broadening mechanism at the plasma density assumed, rather than Stark broadening. (Doppler broadening alone gives a halfwidth of 0.35 A, very close to the halfwidth of their theoretical profile; see also Barnard et al. (3) for corroboration of this statement.) Actually, in our experiment the neutrals entering the plasma from the walls are expected to be much colder than 4 x lo4 K, and to remain cold on the time scale of the experiment. At our high electron temperature, furthermore, Stark broadening will have even less influence (4) on the allowed line shape than in the conditions assumed by Pie1 and Pinnekamp. Consequently the allowed line width is not expected to be much larger than the instrumental width and cannot be used to provide information on the spatial origin of the emissions. (A detailed analysis of the observed shape of the allowed line shows that it is not Gaussian but that it can be explained in terms of two groups of neutral atoms -a cold group and a group created by charge exchange with the ions. The cold group predominates, however, and the line width at half maximum is largely determined by that group.) The remainder of the argument of Pie1 and Pinnekamp that we are observing emissions from impurities rather than structure due to turbulence rests on the circumstance of similarity between one of our spectra and one they have obtained from a hydrogen-contaminated helium discharge. We consider this evidence to be unconvincing for the following reasons: (a) The feature we have identified as S+ (first small peak from the left in the reproduction of our spectrum in ref. (2) is always observed to be smaller than S(third small peak) as it should be according to an interpretation in terms of plasma turbulence. This is not in agreement with the measurements from a hydrogen-contaminated discharge. (b) Our measurements in the vicinity of the 4471 A line show similar features S+ and F as well as similar estimates for fluctuating field strengths (see Table 2 of ref. 1) to those found from the 4922 A line. Furthermore, it is not evident from the work cited by Pie1 and Pinnekamp (their ref. 2) that these features could be due to molecular emissions. (c) It would be surprising if a significant level of hydrogen impurity were present in our experiment. It was more than a year since hydrogen had been used in the device and many thousands of shots in helium were made prior to the spectroscopic measurements reported. It is unfortunately not possible to verify the absence of significant amounts of hydrogen for the spectroscopic work since the system has been used recently in other experiments with hydrogen discharges.

ChemInform Abstract: A STUDY OF THE THERMAL POLYMERIZATION OF STYRENE BY DEPOLARIZED RAYLEIGH LIGHT SCATTERING SPECTROSCOPY

The technique of depolarized Rayleigh light scattering spectroscopy has been applied to follow the kinetics of the thermal polymerization of styrene. By measuring the spectrum of the depolarized scattered light we are able to separate the intensity of light scattered by the polymer being formed from the intensity of light scattered by the monomer. The spectrum consists of a wide Lorentzian due to the monomer and a central spike with instrumental width due to the polymer. This separation occurs because the polymer relaxation time which is determined by the longest Rouse–Zimm mode or by overall rotation is four orders of magnitude longer than the relaxation time of the monomer which is governed by overall rotation. Using the intensity data, the initial rate of the polymerization reaction is measured and is found to agree with the rate determined in previous experiments using classical techniques. After about one‐third of the reaction is complete, the rate increases by 50%. This increase in rate is interpret...

A study of the thermal polymerization of styrene by depolarized Rayleigh light scattering spectroscopy

The technique of depolarized Rayleigh light scattering spectroscopy has been applied to follow the kinetics of the thermal polymerization of styrene. By measuring the spectrum of the depolarized scattered light we are able to separate the intensity of light scattered by the polymer being formed from the intensity of light scattered by the monomer. The spectrum consists of a wide Lorentzian due to the monomer and a central spike with instrumental width due to the polymer. This separation occurs because the polymer relaxation time which is determined by the longest Rouse–Zimm mode or by overall rotation is four orders of magnitude longer than the relaxation time of the monomer which is governed by overall rotation. Using the intensity data, the initial rate of the polymerization reaction is measured and is found to agree with the rate determined in previous experiments using classical techniques. After about one-third of the reaction is complete, the rate increases by 50%. This increase in rate is interpreted as resulting from the large increase in viscosity preventing the termination of the reaction by coupling of polymer radicals. Also the rotational relaxation time of the monomer is determined for the first 80% of the reaction and is found to increase linearly with time. However the value of the monomer rotational relaxation time at 80% of reaction is only 1.5 times the inital value. This indicates the monomer rotational motion is not determined by the large macroscopic viscosity, but by a local viscosity in the region of the monomer.

Raman scattering from impurities in nonmetals. I. Phonon effects

The influence on the impurity Raman spectrum owing to the variation of phonon frequencies with the occupancy of the impurity state is studied. For a Hamiltonian which simulates the situation of change in phonon frequencies, the Raman spectrum function, which yields information about the total scattered intensity per unit time per unit solid angle as well as the line shape, is expressed exactly as a triple integral. It is shown that when the impurity state interacts predominantly with a single phonon mode, a set of lines appear which are displaced at equal frequency interval from the (zero-phonon) Raman lines. Under the condition when they are unresolvable owing to instrumental width or when the impurity is coupled with the continuum phonon spectrum, these additional contributions asymmetrically broaden the zero-phonon Raman lines.

Analytical description of a Fabry-Perot spectrometer 4: Signal noise limitations in data retrieval; winds, temperature, and emission rate

A quantitative investigation of the effects of noise inherent in the signal received by the detector of a Fabry-Perot spectrometer has been carried out in detail. The study also includes the effects of the instrumental profile width and the presence of unwanted continuum radiation in the signal, such as background radiation and detector dark current. The results show the spectrum of the noise in the transform plane to have a quantitative description in terms of the instrumental and line shapes as well as the signal strength, or emission rate, of both the line profile and the unwanted radiation. The magnitude and shape of this noise spectrum not only limits in a known quantitative manner the amount of useful information available from a given measured profile, but it also provides information on the profile itself, thus allowing for more information to be obtained from the measurement than would otherwise be possible. A detailed example is given for the retrieval of winds, temperature, and emission rate from line profile measurements, and it is quantitatively shown how the (noise) limited information available from a measured profile appears as uncertainties of determination. These uncertainties are shown to be dependent on the signal strength, more correctly the noise inherent in this signal, as well as the shapes and relative widths of the line and instrumental profiles, with the temperature uncertainty showing the most sensitivity to these factors. Although in some cases the results are qualitatively obvious, their quantitative nature allows the design of experiments for least uncertainty and/or define the limitations inherent in such experiments.

Observation of Correlation Effects in the Argon L2,3-Auger Spectrum

The L2,3-Auger spectrum of Argon was excited by 100 to 500 keV H+, 300 to 500 keV He+ and 2.5 keV e-. Auger electrons from 145 to 210 eV were measured with an instrumental width of 160 meV. Lines due to the decay of singly ionized initial states L2 and L3 were found by examination of the line intensities as a function of the projectile velocity. This group includes diagram lines as well as satellites produced by final-state configuration interaction (CI) with diagram lines. The degree of mixing is determined for several configurations by multi-configuration HartreeFock (MCHF) calculations. Calculated term energies are consistent with observed levels. Some important levels are assigned.

Measurements of the dynamic structure factor near the lambda temperature in liquid helium

The dynamic structure factor $S(q,\ensuremath{\omega})$ was measured by Brillouin scattering in liquid $^{4}\mathrm{He}$ at $q=1.79\ifmmode\times\else\texttimes\fi{}{10}^{5}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. Results were obtained at two densities: $\ensuremath{\rho}=0.175$ g/${\mathrm{cm}}^{3}$ for which ${P}_{\ensuremath{\lambda}}=23.1$ bars and $\ensuremath{\rho}=0.179$ g/${\mathrm{cm}}^{3}$ for which ${P}_{\ensuremath{\lambda}}=28.5$ bars. Values of the reduced temperature $\ensuremath{\epsilon}\ensuremath{\equiv}\frac{(T\ensuremath{-}{T}_{\ensuremath{\lambda}})}{{T}_{\ensuremath{\lambda}}}$ varied from -5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ to -5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ and from 1.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ to 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$. This range involves both the hydrodynamic and the critical regions; values of $q\ensuremath{\xi}$ fall between 5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ and 21 below ${T}_{\ensuremath{\lambda}}$ and between 4.5 and 2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ above ${T}_{\ensuremath{\lambda}}$. This article presents the data on the critical mode, which is second sound below the transition and heat diffusion above. The most striking result is that the damping of the critical mode is essentially independent of $\ensuremath{\epsilon}$ over the entire range of temperatures studied. This behavior is qualitatively different from the strong temperature dependence, consistent with dynamic scaling, which is observed at low frequencies. On the other hand, any dispersion in the velocity of high frequency (\ensuremath{\sim} several MHz) second sound, if present, is less than 2%. $S(q,\ensuremath{\omega})$ retains a two-peaked structure well into the critical region below ${T}_{\ensuremath{\lambda}}$; it still appears in the measured spectra (which contain the instrumental width as well) at $q\ensuremath{\xi}\ensuremath{\sim}4$. At ${T}_{\ensuremath{\lambda}}$ and in the critical region just above, there is evidence for a non-Lorentzian shape of $S(q,\ensuremath{\omega})$. The critical mode contribution to $S(q,\ensuremath{\omega})$ is compared with a theoretical calculation of Hohenberg, Siggia, and Halperin based on the planar-spin model of $^{4}\mathrm{He}$. The theory matches both the overall width and the general features of the shape of $S(q,\ensuremath{\omega})$ to within our spectral resolution at ${T}_{\ensuremath{\lambda}}$. However, closer to $q\ensuremath{\xi}=1$ in the critical region, and in the hydrodynamic regions both above and below ${T}_{\ensuremath{\lambda}}$, the theory predicts linewidths which are too small.

Correction of Brillouin linewidths measured by multipass Fabry-Perot spectroscopy

The sharper instrumental profile of the multipassed Fabry-Perot etalon introduces distortions to Brillouin spectra which may differ considerably from those introduced by the single-pass etalon. The half-width at half-height of the observed profile is calculated for five-pass and three-pass spectrometers. Phonon spectral widths may be extracted from the graphs and appropriate relationships are presented. These calculations are valid for well aligned etalons when the phonon lineshape is Lorentzian and cover a wide range of instrumental and phonon spectral widths.

The rotation and gravitational redshift of white dwarfs

Study of 14 hydrogen-line (DA) degenerate stars at coude' dispersion shows that relatively sharp cores can be detected in most at Ha and sometimes at Hfl. These are produced by non-local- thermodynamic equilibrium effects, at effective temperatures from 14,000 to 25,000 K. Eight of the DA's have cores not much broader than expected from the thermal, slit, and instrumental widths. Projected rotational velocities are certainly less than 40 km s ', and possibly less than 20 or even 10 km I Four are appreciably broader, but rotation is still below km i One cool DAs star apparently shows chromospheric emission, possibly caused by convection. The coude' image tube and Image Photon Counting System permit relatively accurate velocity determinations. The sharp cores do not originate in shells. Their mean apparent gravitational redshift is K = +44.6 + 5.7 km 1 (from 13 stars). The mean mass determined from the massradius relation is in the range 0. .75 me. The major implication for stellar evolution is that degenerate stars have low speciflc angnlar momentum. Since they have also lost most of their mass, transport of angular momentum across molecular-weight barriers must be an efficient process. Subject headings: line profiles - relativity - stars: rotation - stars: white dwarfs

Hypersonic Velocity, Absorption and Relaxation in Molten CSNO3

Abstract Brillouin spectra of molten CSNO3 were investigated for scattering angles between 40 and 140° and in a temperature interval of 420-520 °C. An Ar+ singlemode laser was used for excitation and the total instrumental width was ~265 MHz. The measured frequency shifts and linewidths of the Brillouin components were used to determine velocities and attenuations of thermal sound waves in the frequency range 2.3-7.0 GHz. A dispersion of 4-5% was found between the present hyper­ sonic velocities and reported ultrasonic velocities. A considerable decrease in attenuation with frequency was observed in the investigated frequency range, with the value at high frequency ap­ proaching the classical attenuation. The results are in good agreement with Mountain's theory of a single relaxation time. The relaxation time of the bulk viscosity coefficient was calculated to 1.2×10-10 S.

Measurement of Lorentzian linewidths: Numerical evaluation of the Voigt integral

Abstract We discuss various methods for numerical evaluation of the Voigt integral and suggest alternatives to Gauss-Hermite quadrature which are valid for all values of Lorentzian and instrumental widths.