Apparent Cross Section Research Articles

Cross sections for excitation of sodium by impact of H+, H2 +, H3 +, and H- ions.

Apparent cross sections are reported for the excitation of Na atoms from the ground 3s level to the 3d level by incident ${\mathrm{H}}^{+}$, ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$ ${\mathrm{H}}_{3}$${\mathrm{}}^{+}$, or ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions with velocities in the range 0.5\ifmmode\times\else\texttimes\fi{}${10}^{6}$ to 2.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ m/s. The apparent cross section for ${\mathrm{H}}^{+}$ ions has a maximum value of 4.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ ${\mathrm{cm}}^{2}$ at an incident ion velocity of 1.7\ifmmode\times\else\texttimes\fi{}${10}^{6}$ m/s. At velocities above 1.4\ifmmode\times\else\texttimes\fi{}${10}^{6}$ m/s the apparent cross section for Na(3d)-level excitation by ${\mathrm{H}}^{+}$ ions is the same function of the velocity as the apparent cross section for 3d-level excitation by electrons. The apparent cross section for excitation of Na(3d)-level atoms by incident ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions is about 45% below the apparent cross section for excitation by incident ${\mathrm{H}}^{+}$ ions at velocities above 1.7\ifmmode\times\else\texttimes\fi{}${10}^{6}$ m/s. Combination of the apparent Na(3d) cross sections with the apparent Na(3p) cross sections reported earlier gives the direct excitation cross sections of Na(3p) by ${\mathrm{H}}^{+}$, ${\mathrm{H}}_{2}$${\mathrm{}}^{+}$, ${\mathrm{H}}_{3}$${\mathrm{}}^{+}$, and ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ impact.

Double ionisation of Ar atoms by single electron impact

With the aid of translational energy spectroscopy the authors have studied the double ionisation of Ar by single electron impact at collision energies between threshold and 600 eV. On the one hand the authors have determined the total cross section for double ionisation, sigma t, and on the other hand they have measured, for the first time, apparent partial cross sections for production of Ar2+ in specific long-lived states: 3P, 1D2, 1S0, 5Ddegrees 4 and 3Fdegrees 4. The experimental setup, the principle of the applied method and the evaluation procedure are described. Where possible the results are compared with other experimental results, yielding rather good agreement.

The photodissociation spectrum of SO+2

The photodissociation spectrum of SO+2 corresponding to the process SO+2 +hν→SO++O has been measured on a triple quadrupole system for the wavelength ranges: 3000–3400 and 4400–5120 Å. The spectrum in the visible has been assigned to the C̃ 2B1←Ã 2A2 transition and vibrational structure analyzed to yield λ00=4187 Å, the harmonic vibrational frequencies ν̃′1=953 cm−1, ν̃′′2=499 cm−1, ν̃′1=767 cm−1, and ν̃2 =410 cm−1, and the anharmonicity constants X″11 =−6.3 cm−1, X′′12 =−6.1 cm−1, X″22 =−4.2 cm−1, X12 =−6.3 cm−1, and X′22 =−7.4 cm−1. Apparent photodissociation cross sections ranged from ∼1×10−20–1.6×10−19 cm2 in the visible spectrum. In the UV spectrum the highly congested vibrational structure could not be resolved sufficiently for analysis; photodissociation cross sections ranged from ∼2–4×10−19 cm2 with broad bands roughly corresponding to expected progression in ν1 and ν′2 within the C̃ 2B1←X̃ 2A1 electronic transition. The process SO+2 +hν→S++O2 showed an onset near 3108 Å and the corresponding photodissociation spectrum showed sufficient vibrational structure to yield ν̃′′2∼454 cm−1, ν̃′1 ∼955 cm−1, and ν̃2 ∼411 cm−1. This new process, which had σ≤3×10−20 cm2, was proposed to result from the D̃ 2B2←X̃ 2A1 transition. Several new photodissociation cross section measurements on N2O+ have also been made in the vicinities of 3381 and 4880 Å, taking advantage of the high sensitivity (→10−21 cm2) of the triple quadrupole system.

Electron excitation cross sections for the metastable and resonant levels of Ne(2p53s).

The electron excitation cross section both for the $^{3}\mathrm{P}_{2}$ and $^{3}\mathrm{P}_{0}$ metastable levels (1${s}_{5}$ and 1${s}_{3}$ in Paschen's notation) and for the mixed $^{3}\mathrm{P}_{1}$ and $^{1}\mathrm{P}_{1}$ resonant levels (1${s}_{4}$ and 1${s}_{2}$ in Paschen's notation) of the 2${p}^{5}$3s configuration of Ne have been measured. Apparent and direct excitation cross sections are reported for incident electron energies from threshold to 300 eV and are compared with results of different methods of measurements. Ground-state Ne atoms are excited by a monoenergetic electron beam to the 1s levels, and atoms in the 1s levels are pumped to the 2p levels of the 2${p}^{5}$3p configuration by means of a cw dye laser. The laser-induced fluorescence from the 2p level is utilized to determine the electron excitation cross section for the 1s level. Methods for absolute calibration of the cross sections are given.

DISTORTION OF ACTION SPECTRA IN PHOTOMORPHOGENESIS BY LIGHT GRADIENTS WITHIN THE PLANT TISSUE

Abstract—Recently a method for measuring optical properties of light scattering and absorbing tissues was discussed (Seyfried et al., 1983). Based on measurements of transmittance and reflectance of cotyledons of dark‐ and light‐grown Cucurbita pepo L, seedlings a method to compute and correct apparent photoconversion cross sections for various distributions of a sensory pigment is described. The distortion function may be used to calculate expected apparent (distorted) action spectra for a pigment in vivo when the spectral properties of the pigment in vitro are known. With phytochrome as an example for a sensory pigment of low concentration within the tissue it is shown that the low apparent photoconversion cross section in blue light observed in etiolated seedlings can be explained solely by optical distortions caused by light gradients. In green tissue on the other hand the ratio of the apparent photo conversion cross sections in red and blue light is almost identical with that observed in vitro, but the ratio in red and far‐red light depends strongly on the distribution of the relevant sensory pigment. An analysis of the rate of phytochrome cycling shows an additional peak in the far‐red if the pigment is localized in the lower part of the tissue only.

Laser-Induced Fluorescence Detection of Metastable Atom and Molecular Ions Produced by Controlled Electron Impact

The laser-induced fiourescence were observed for th6 metastable 23S level of He and the ground state of the molecular ion N2' produced by the controlled electron impact. The apparent electron excitation cross section of the 23S level of He was measured in the energy range of 15-95 eV. The rotational distribution of the ground state Ni ions was a Boltzmann distribution and the rotational temperature was 326K.

Phytochrome-mediated phototropism in maize mesocotyls. Relation between light and Pfr gradients, light growth response and phototropism

Unilateral irradiation of maize (Zea mays L.) seedlings results in a fluence-rate gradient, and hence below saturation, a gradient of the far-red-absorbing form of phytochrome (Pfr). The Pfr-gradients established by blue, red and far-red light were spectrophotometrically measured in the mesocotyl. Based on these Pfr-gradients and the fluence-response curves of phytochrome photoconversion the fluence-rate gradients were calculated. The fluence-rate gradient in the blue (460 nm) was steeper than that in the red (665 nm), which in turn was steeper than that in the far-red light (725 nm). The fluence-rate ratios front to rear were 1:0.06 (460 nm), 1:0.2 (665 nm), and 1:0.33 (725 nm). The assumption that phytochrome-mediated phototropism of maize mesocotyls is caused by local phytochrome-mediated growth inhibition was tested in the following manner. Firstly, the Pfr response curve for growth inhibition was calculated; these calculations were based on measurements of Pfr-gradients and data from red-light-induced phototropism. Secondly, the Pfr response curve for growth inhibition was used as a basis for calculating fluence-response curves for blue-and far-red-light-induced phototropism. Finally, these calculated results were compared with experimental data. It was concluded that the threshold for phytochrome-mediated phototropism of maize mesocotyls reflects the apparent photoconversion cross section of phytochrome whereas the maximal inducable curvature depends on the steepness of the light (Pfr) gradient across the mesocotyl.

Improvement in the estimation of dominate wavenumber and direction from spaceborne SAR image spectra when corrected for ocean surface movement

Because a synthetic aperture radar (SAR) is a Doppler device, ocean surface movement can degrade a SAR ocean image. Ocean surface movement causes the redistribution of apparent radar cross section in a SAR image to neighboring image elements. It is demonstrated in this paper that this aspect of SAR imaging can be treated as a simple resolution loss in a linear system. Such an approach explains the falloff in response at high azimuth wavenumbers experienced in SAR image spectra. More importantly, this approach allows for a straightforward method to alleviate the problem of ocean surface motion. Correction of SAR image spectra using a linear systems approach results in SAR image spectra in which the locations in wavenumber-space of surface wave spectral peaks are more consistent with the estimated location of storm sources than are the locations of peaks from uncorrected spectra.

Effect of inhomogeneous radiation fields on free-free scattering processes

The apparent collision cross section of an inverse-bremsstrahlung (free-free) collision process is investigated analytically for the case of laser-field inhomogeneity. Numerical computations are performed for typical experimental conditions involving high-intensity laser fields, and the results are presented in a table: significant effects due to the field inhomogeneity are found, and it is determined that free-free processes are less likely in the TEM(01) mode than in the TEM(00) mode for given values of the parameters p and n.

Kinetics of N 2+ and N + reactions with Mo AT ⩽ 100 eV impact energies

The kinetics of nitridation of polycrystalline Mo by N 2 + and N + have been investigated in the 7.5–100 eV impact energy range. Auger electron spectroscopy has been used to monitor the nitride growth as a function of ion fluence and energy for both N 2 + and N +. Bi-exponential nitride concentration versus dose curves are typical of this system. These data are interpreted within a proposed model which employs both surface and subsurface nitrogen binding sites and incorpo- rates sputtering and bombardment induced transport between the two layers. Fitting this model to the data yields apparent reaction cross sections σ and the extent of nitridation Δ N. Two measures of initial capture probability, (1) the experimental initial slope and (2) the product σΔ N, are used to characterize reactivity.

Kinetics of N 2+ and N + reactions with Mo at ⩽ 100 eV impact energies

Abstract The kinetics of nitridation of polycrystalline Mo by N2+ and N+ have been investigated in the 7.5–100 eV impact energy range. Auger electron spectroscopy has been used to monitor the nitride growth as a function of ion fluence and energy for both N2+ and N+. Bi-exponential nitride concentration versus dose curves are typical of this system. These data are interpreted within a proposed model which employs both surface and subsurface nitrogen binding sites and incorpo- rates sputtering and bombardment induced transport between the two layers. Fitting this model to the data yields apparent reaction cross sections σ and the extent of nitridation ΔN. Two measures of initial capture probability, (1) the experimental initial slope and (2) the product σΔN, are used to characterize reactivity.

Electric field effects on the absorption spectra ofH2near the ionization limit

The absorption cross section of ${\mathrm{H}}_{2}$ has been measured in the region between 77.5 and 82.4 nm and the effects of electric fields up to 22 kV/cm on the cross sections have been investigated. The apparent cross section is found to be increased in the neighborhood of optically allowed transitions to $\mathrm{np}\ensuremath{\sigma}{^{1}\ensuremath{\Sigma}_{u}}^{+}$ and $\mathrm{np}\ensuremath{\pi}^{1}\ensuremath{\Pi}_{u}$ states with $n=4\ensuremath{-}7$. This increase in absorption is attributed to field mixing with nearby optically forbidden states.

Physical properties of the deep X electron trap responsible for long time persistent photocapacitance in CdS

We describe the main physical properties of a very deep electron trap which is responsible for the long time persistent photocapacitance of Schottky diodes on undoped CdS single crystals. TSCAP and DLTS have been used to determine the apparent thermal activation energy Ea = 0.925 eV and the apparent electron capture cross section σ∞ = 5.5. 10-15cm2 of these centers. Photocapacitance measurements have been performed at various temperatures to allow the determination of the spectral dependence of the photoionisation cross section. The deduced optical threshold En° of the “bound state to the free conduction band state” transition is 2.5 eV. This transition gives a long tail absorption near the fundamental edge which is unambigously identified by the thermal emptying of the X centres. The large difference between Ea = 0.925 eV and En° = 2.5 eV is explained in the large lattice relaxation model. These values are consistent with the results of experiments which locate the X centres, under quasi-equilibrium conditions, at 0.87 eV below the conduction band. They also agree with the results of direct measurement of the thermal activation energy of the electron capture cross section EB = 0.2 eV. The low temperature quenching of both electron emission and capture is responsible for very large persistent photocapacitance effects which suggest that these X centres are acceptor like and probably correlated with cadmium vacancies.

Transient capacitance spectroscopy in polycrystalline silicon

We have performed deep level transient spectroscopy (DLTS) on 5 Ω cm p-type polycrystalline silicon. Typical DLTS spectra exhibit three peaks centered at 250, 300, and 340 K. The density of states which is estimated from these spectra is found to extend from 0.18 eV of the valence band with a density ∼ 2×1015 eV−1 cm−2. Measurements of capture rates have shown a characteristic trap filling time of ∼10−3 s corresponding to an apparent capture cross section of the order of 10−21 cm2. Studies of the passivation (in a deuterium plasma) of these boundary states have also been made. In that case, the DLTS spectra present only one significant peak at 245 K; the density of states of the unpassivated material is reduced to localized states centered at 0.32 eV from the valence band with a concentration of 5.6×1014 cm−2. The capture cross section of these states is 3.7×10−20 cm2.

Excitation of Kr and Xe in collisions with Ar +

The excitation of Kr and Xe in with Ar + ions has been studied by monitoring the resulting luminescence. Energetic collisions of Ar + with Kr and Xe excite the 116.5 and 123.6 nm Kr(I) lines and the 129.6 and 147.0 nm Xe(I) lines respectively. Apparent cross sections and kinetic energy onsets for collisional excitalition in bimolecular ion -atom collisions in the laboratory energy range of 5–100 eV are reported. The results are compared with those reported for other symmetric and asymmetric ion-atom collisions among the rare gases. Possible reaction mechanism and the criteria that may be applied to predict the relative importance of reaction pathways in ion-atom collisions are also discussed.

ChemInform Abstract: STATE‐TO‐STATE DYNAMICS OF IR MULTIPHOTON ABSORPTION IN THE A1A2 STATE OF THIOPHOSGENE: ABSORPTION CROSS SECTIONS

Infrared multiphoton absorption (IRMPA) in the first excited singlet state of Cl2CS has been studied. Absolute cross sections for absorption to a fixed energy level, the nonradiative limit, from initial levels of increasing energy have been obtained. Although intuition suggests that as the energy required to reach the nonradiative limit decreases the apparent cross section should increase, this is not observed. Instead, the relative probability of absorption from all initial states is nearly the same as that from the ground vibrational level. Because the density of states in Cl2CS (? 1A2) is higher at 3000 cm−1 than in SF6, where intermode coupling is postulated to be important in the dynamics of absorption, the results suggest that at energies ?2500 cm−1 intramolecular relaxation is competitive with absorption. This interpretation is in agreement with predictions from the current theory of IRMPA.

Electron excitation cross sections for the1s2and1s4levels in Ne

The apparent and direct electron excitation cross sections of both the $1{s}_{2}$ and $1{s}_{4}$ levels of Ne are measured using laser-induced fluorescence. The experiment utilizes radiation trapping to maintain appreciable level populations. The direct excitation cross section for the $1{s}_{2}$ level has a peak value of 1.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}17}$ ${\mathrm{cm}}^{2}$ at 65 eV; the direct excitation cross section for the $1{s}_{4}$ level has a peak value of 8.8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$ at 65 eV.

Radar backscattering from a rough rotating triaxial ellipsoid with applications to the geodesy of small asteroids

Radar geodesy of the asteroid 433 Eros was reported by Jurgens and Goldstein (1976). Their measurements were based on the spectral properties of a rough rotating triaxial ellipsoid. This paper presents the theory by which theoretical spectra based on a backscattering model of the form of the nth power of the cosine of theta, or any reasonable backscattering model, can be computed. Some general properties of these spectra are demonstrated, and simple measurements of the apparent radar cross section, center frequency, and effective bandwidth as a function of time are shown to be useful in determining the sizes of the semi-major axes, the exponent n in the scattering model, the backscatter efficiency of the surface material, the rotation period, and the declination of the observer above or below the rotation equator. Some of these parameters are correlated, and multiple estimation may be difficult unless the observations span a period sufficiently long so as to present a wide range of declinations of the observer. However, the departure from a spherical shape aids in separating the scattering properties from the target dimensions and facilitates the simultaneous estimation of all free parameters.

State-to-state dynamics of IR multiphoton absorption in the Ã1A2 state of thiophosgene: Absorption cross sections

Infrared multiphoton absorption (IRMPA) in the first excited singlet state of Cl2CS has been studied. Absolute cross sections for absorption to a fixed energy level, the nonradiative limit, from initial levels of increasing energy have been obtained. Although intuition suggests that as the energy required to reach the nonradiative limit decreases the apparent cross section should increase, this is not observed. Instead, the relative probability of absorption from all initial states is nearly the same as that from the ground vibrational level. Because the density of states in Cl2CS (? 1A2) is higher at 3000 cm−1 than in SF6, where intermode coupling is postulated to be important in the dynamics of absorption, the results suggest that at energies ?2500 cm−1 intramolecular relaxation is competitive with absorption. This interpretation is in agreement with predictions from the current theory of IRMPA.

Electron impact ionization cross section of metastable N2(A Σ+u)

The nondissociative electron impact ionization cross section of the metastable N2(A 3Σ+u) state is measured from threshold to 240 eV. Rising from an appearance potential of 10.1±0.6 eV, the cross section peaks at about 40 eV with a value of (1.14±0.36)×10−16 cm2. These values are extracted from the apparent ionization cross section of an N2 beam composed of ∼50% N2(A 3Σ+u) and ∼50% N2(X 1Σ+g). This mixed state beam is formed by charge transfer neutralization of a 1 keV N+2 beam with NO. The N2(A 3Σ+u) cross section is normalized by measuring it relative to the ionization of a ground state N2 beam formed by charge transfer neutralization of N+2 with N2.