Absolute Cross Section Measurements Research Articles

Measurements of Total Cross Sections for Positron Scattering from He under Magnetic-Field-Free Conditions Using an Electrostatic High-Brightness Slow Positron Beam System

Magnetic-field-free measurements of absolute total cross sections for positron scattering from helium have been performed using an electrostatic high-brightness slow positron beam apparatus. In the energy region above 20 eV, the present results are in good agreement with those of other studies. On the other hand, in the energy range of 4–20 eV, our values are higher than most of the other measurements except one study.

Photoionization measurements for the iron isonuclear sequence Fe3+, Fe5+, and Fe7+

Cross sections for single photoionization of Fe${}^{3+}$, Fe${}^{5+}$, and Fe${}^{7+}$ ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass- and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to $3p$--$\mathit{nd}$ transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe${}^{3+}$ and Fe${}^{5+}$ that are believed to result from unresolved fast super-Coster-Kronig transitions following $3p$--$3d$ excitation. Absolute photoionization cross-section measurements were also performed using ion beams containing undetermined fractions of ions in their ground and metastable states. A Rydberg series attributed to $3p$--$\mathit{nd}$ transitions from the ${}^{2}{G}_{1}$ metastable state in Fe${}^{3+}$ was identified. The data are compared with recently published measurements on Fe${}^{3+}$ and Fe${}^{5+}$ using a similar technique at lower spectral resolution.

Experimental photoionization cross-section measurements in the ground and metastable state threshold region of Se+

Absolute photoionization cross-section measurements are reported for Se+ in the photon energy range 18.0–31.0 eV, which spans the ionization thresholds of the 4S3/2 ground state and the low-lying 2P3/2, 1/2 and 2D5/2, 3/2 metastable states. The measurements were performed using the Advanced Light Source synchrotron radiation facility. Strong photoexcitation–autoionization resonances due to 4p → nd transitions are seen in the cross-section spectrum and identified with a quantum-defect analysis.

Electron interaction cross sections for a low-temperature ‘plasma-like’ gas mixture

Electron interaction cross sections for species found in low-temperature industrial plasma environments are a crucial component to allow for accurate modeling of those plasmas. However, such data are still rarely reported in the literature, due to the complexities of working with such highly reactive species in a laboratory. Here, absolute differential cross section measurements for a ‘plasma-like’ gas mixture, containing CF3I, CF3, I2, I and C2F6, are reported.

Aerosol Light Extinction Measurements by Cavity Attenuated Phase Shift (CAPS) Spectroscopy: Laboratory Validation and Field Deployment of a Compact Aerosol Particle Extinction Monitor

We present laboratory and field measurements of aerosol light extinction ( σep ) using an instrument that employs Cavity Attenuated Phase Shift (CAPS) spectroscopy. The CAPS extinction monitor comprises a light emitting diode (LED), an optical cavity that acts as the sample cell, and a vacuum photodiode for light detection. The particle σep is determined from changes in the phase shift of the distorted waveform of the square-wave modulated LED light that is transmitted through the optical cell. The 3-σ detection limit of the CAPS monitor under dry particle-free air is 3 Mm–1 for 1s integration time. Laboratory measurements of absolute particle extinction cross section ( σext ) using non-absorbing, monodisperse polystyrene latex (PSL) spheres are made with an average precision of ± 3% (2-σ) at both 445 and 632 nm. A comparison with Mie theory scattering calculations indicates that these results are accurate within the 10% uncertainty stated for the particle number density measurements. The CAPS extinction ...

Imaging and Absolute Extinction Cross-Section Measurements of Nanorods and Nanowires through Polarization Modulation Microscopy

This paper describes an experimental technique for measuring the extinction cross-section of anisotropic nanomaterials. The experiments are performed by focusing a laser beam to a diffraction-limited spot under an optical microscope, and using a photoelastic modulator (PEM) to rotate the polarization of the light beam. Monitoring the transmitted beam with a lock-in amplifier referenced to twice the resonant frequency of the PEM yields the difference in extinction for light polarized parallel and perpendicular to the optical axis of the nanostructure. These experiments are the single particle analog of polarization modulation microscopy (PMM). Experimental results for gold nanorods and CdSe nanowires are presented that demonstrate the sensitivity and properties of this technique. In particular, we show that by collecting images at two laser polarizations separated by 45° it is possible to construct an extinction cross-section image. The advantages and disadvantages of the PMM technique compared to existing ways of measuring the extinction of nanoparticles (photothermal heterodyne imaging and spatial modulation spectroscopy) are discussed. Results from experiments where we collected simultaneous absorption and emission images are also presented for different shaped CdSe nanowires. These measurements provide insight into how the structure of the nanowire affects its photophysics.

Absolute Dissociative Electron Attachment Cross Sections in Acetylene and its Deuterated Analogue

An instrument for the measurement of absolute dissociative electron attachment cross sections was constructed and applied to two prototype molecules, acetylene (C2H2) and deuterated acetylene (C2D2). The isotope effect at the first dissociative electron attachment band at 3 eV was determined--the cross section for C2D-/C2D2 is 14.4 times smaller than that for C2H-/C2H2. The results play a central role in validating recent theoretical work of Chourou and Orel. The dominant fragments at the second dissociative attachment band around 8 eV are the light fragments H- or D-. These bands exhibit weaker isotope effects.

Absolute cross-sections for dissociative photoionization of some small esters

The first measurements of absolute cross-sections for near-threshold molecular and dissociative photoionization are presented for 11 small esters (methyl formate, ethyl formate, vinyl acetate, methyl propanoate, ethyl propanoate, methyl butanoate, methyl isobutanoate, methyl propenoate, ethyl propenoate, methyl crotonate, and methyl methacrylate). Photoionization mass spectrometry (PIMS) is employed with an energy resolution of 40 meV (fwhm) using a monochromated VUV synchrotron light source. An extensive literature exploring isomerization/dissociation mechanisms is available for six of these molecules. There are, however, no previous observations of dissociative ionization for four simple monounsaturated esters (methyl propenoate, ethyl propenoate, methyl crotonate, and methyl methacrylate), and dissociative ionization of ethyl propanoate has received scant attention. Appearance energies for dissociative photofragment ions of these five molecules are presented.

Total electron scattering cross sections for methanol and ethanol at intermediate energies

Absolute total cross section (TCS) measurements of electron scattering from gaseous methanol and ethanol molecules are reported for impact energies from 60 to 500 eV, using the linear transmission method. The attenuation of intensity of a collimated electron beam through the target volume is used to determine the absolute TCS for a given impact energy, using the Beer–Lambert law to first approximation. Besides these experimental measurements, we have also determined TCS using the additivity rule.

Experimental and theoretical photoionization cross sections for Se+ from 18 eV to 31 eV

Absolute Se photoionization cross-section measurements and Dirac-Coulomb R-matrix calculations are reported for the photon energy range 18.0 eV – 31.0 eV, which spans the ionization thresholds of the 4S03/2 ground state and the low-lying 2D03/2,5/2 and 2P01/2,3/2 metastable states. The determination of the photoionization and recombination properties of n-capture element ions is motivated by their astrophysical detection and the importance of their elemental abundances in testing theories of nucleosynthesis and stellar structure.

Valence-shell photoionization of the Cℓ-like ion Ar+: Experiment and theory

Absolute photoionization cross-section measurements for Ar+ ions and theoretical calculations performed within the semi-relativistic Breit-Pauli R-matrix approximation are reported for the photon energy range 27.4 eV to 60.0 eV. The resonance features are assigned spectroscopically, and their energies and quantum defects tabulated with excellent agreement being obtained between experiment and theory.

Isotope effect in dissociative electron attachment cross sections in acetylene

We present absolute cross section measurement of dissociative electron attachment to C2H2 and C2D2. The C2H−/ C2D− band at 3 eV shows pronounced isotope effect with the cross section for C2H2 being 14.7 times larger than that for C2D2. The light fragments H− and D− dominate the second dissociative electron attachment band around 8 eV. These bands exhibit much weaker isotope effects which is in agreement with their assignment to Feshbach resonances.

Estimation of peak Raman gain coefficients for Barium-Bismuth-Tellurite glasses from spontaneous Raman cross-section experiments

In this paper we explore the TeO(2)-Bi(2)O(3)-BaO glass family with varied TeO(2) concentration for Raman gain applications, and we report, for the first time, the peak Raman gain coefficients of glasses within this glass family extrapolated from non-resonant absolute Raman cross-section measurements at 785 nm. Estimated Raman gain coefficients show peak values of up to 40 times higher than silica for the main TeO(2) bands. Other optical properties, including index dispersion from the visible to the long wave Infrared (LWIR) are also summarized in this paper.

Surface Plasmon Resonance of Single Gold Nanodimers near the Conductive Contact Limit

We have studied the spectral response of individual pairs of gold nanoparticles chemically synthesized and stabilized on a transparent thin film. Very thin interspacing distances are explored by taking advantage of a residual layer of surfactant molecules at the metal surface. Absolute extinction cross section measurements by the spatial modulation spectroscopy technique are correlated to the corresponding high magnification images independently obtained from transmission electron microscopy. The signature of a transition from isolated to conductively coupled particles is evidenced in the optical spectra and is discussed in relation with their morphology. The strength of the electromagnetic coupling is controlled not only by their spacing but also by the shape of their opposite surfaces in the gap region.

Absolute photodetachment cross section measurements of the O− and OH− anion

Absolute total photodetachment cross sections of O(-) and OH(-) anions stored in a multipole radio frequency trap have been measured using a novel laser depletion tomography method. For OH(-) the total cross sections of 8.5(1)(stat)(3)(syst) and 8.1(1)(stat)(7)(syst)x10(-18) cm(2), measured at 662 and 632 nm, respectively, were found constant in the temperature range of 8-300 K. The O(-) cross sections 5.9(1)(stat)(2)(syst) and 6.3(1)(stat)(2)(syst)x10(-18) cm(2) measured at 170 K at 662 and 532 nm, respectively, agree within error estimations with preceding experiments and increase the accuracy of the widely used calibration standard for relative photodetachment measurements of diverse atomic and molecular species.

Absolute absorption spectroscopy based on molecule interferometry

We propose a method to measure the absolute photon absorption cross section of neutral molecules in a molecular beam. It is independent of our knowledge of the particle beam density, nor does it rely on photoinduced fragmentation or ionization. The method is based on resolving the recoil resulting from photon absorption by means of near-field matter-wave interference, and it thus applies even to very dilute beams with low optical densities. Our discussion includes the possibility of internal state conversion as well as fluorescence. We assess the influence of various experimental uncertainties and show that the measurement of absolute absorption cross sections is conceivable with high precision and using existing technologies.

Electron-impact ionization of the SiCl 3 radical

We describe improvements to the fast-beam apparatus that has been used extensively for electron-impact ionization cross section measurements for atoms, molecules, and free radicals in our group for the past 15 years. A high-intensity, dispenser-type electron emitter capable of producing an electron beam of more than 2 mA at electron energies above 50 eV is used instead of a conventional indirectly heated, oxide-coated electron source. We also replaced the channel electron multiplier by a position-sensitive, triple multi-channel plate ion detector. Experiments using well-established ionization cross sections in conjunction with extensive ion trajectory simulations were carried out to verify the performance of the modified fast-neutral-beam apparatus. This apparatus was subsequently employed in the measurement of absolute partial cross sections for the formation of various singly charged positive ions produced by electron impact on SiCl 3 for impact energies from threshold to 200 eV. A comparison with calculations and with the previously reported ionization cross section for SiCl 4, SiCl 2, and SiCl is also made.

Theoretical photoionization spectra in the UV photon energy range for a Mg-like Al+ ion

In the present work, we report the photoionization cross sections of the Al+ ion calculated for the photon energy range 20–26 eV and 30–50 eV. We have expanded our previous calculation (2007 J. Phys. Soc. Japan 76 014302) with an optimized admixture of the initial ground state 3s21S and exited states 3s3p1,3P, 3s3d1,3D and 3s4s1,3S, and obtained significantly improved predictions for the main background and autoionizing resonance structures of the reported experimental spectra. The absolute measurements of the photoionization cross sections of the Al+ ion in these energy ranges have been performed by West et al (2001 Phys. Rev. A 63 052719), and they reported that the prominent peaks around 21 eV were attributed to the effects of the significant influence of the small fraction of the fourth-order radiation with energies around 84 eV from the synchrotron source. In our previous work, the main shape for these cross sections was calculated assuming an admixture of initial 3s21S and 3s3p3P states, only with a rough overall estimate for the experimental spectra in the photon energy range 20–26 eV, and without these peaks around 21 eV. The report of the experimental assignment attributes these peaks to the excitation of a 2p electron from the core. However, our present results with the new admixture reveal similar peaks without considering the possibility of the core excitation.

Electronic states of neutral and ionized tetrahydrofuran studied by VUV spectroscopy and ab initio calculations

The electronic spectroscopy of isolated tetrahydrofuran (THF) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 5.8–10.6 eV with absolute cross-section measurements derived. In addition, an electron energy loss spectrum was recorded at 100 eV and 10° over the 5–11.4 eV range. The He(I) photoelectron spectrum was also collected to quantify ionisation energies in the 9–16.1 eV spectral region. These experiments are supported by the first high-level ab initio calculations performed on the excited states of the neutral molecule and on the ground state of the positive ion. The excellent agreement between the theoretical results and the measurements allows us to solve several discrepancies concerning the electronic state spectroscopy of THF. The present work reconsiders the question of the lowest energy conformers of the molecule and its population distribution at room temperature.

Absolute dissociative electron attachment cross-sections of chloro- and bromo-ethylenes

Resonant electron capture by C2Cl4, C2HCl3, cis-, trans- and geminal-C2H2Cl2, C2HBr3, cis- and trans-C2H2Br2 and C2H3Br has been studied. A new method for the measurements of absolute electron attachment cross-sections based on the interface of a gas-chromatography and a reflectron-time-of-flight-mass spectrometer has been developed and the method was applied for these compounds.