ABSTRACT
Two ices, O2 and a mixture of O2 and N2, are bombarded by 252Cf fission fragments (FF) (approximately 65 MeV at target surface); the emitted positive and negative secondary ions are analyzed by time-of-flight mass spectrometry (TOF-SIMS). These studies shall enlighten sputtering from planetary and interstellar ices. Three temperature regions in the 28-42-K range are analyzed: (1) before N2 sublimation, in which hybrid chemical species are formed, (2) before O2 sublimation, in which the TOF mass spectrum is dominated by low-mass (O2)p cluster ions and (3) after O2 sublimation, in which (N2)p or (O2)p cluster ions are practically inexistent. In the first region, four hybrid ion series are observed: NOn-1+, N2On-2(+/-), and N4On-4(-). In the second region, two positive and negative ion series are identified: (O2)pO(+/-) and (O2)pO2(+/-). Their yield distributions are fitted by the sum of two decreasing exponentials, whose decay constants are the same for all series. It is observed that the cluster ion desorption from solid oxygen is very similar to that of other frozen gases, but its yield distribution oscillates with a three- or six-atom periodicity, suggesting O3 or 3O2 units in the cluster structure, respectively.
ABSTRACT
We report on the current status and performance of the toroidal grating monochromator beamline at the Brazilian Synchrotron Light Laboratory (Laboratorio Nacional de Luz Sincrotron). This beamline provides photons in the vacuum ultraviolet and soft x-ray regions from 12 to 330 eV with three interchangeable gratings. We report on the improvement, which allows the possibility of choosing the light polarization degree from linear to almost circular. Here, we also describe the development of a new apparatus, namely, the mirror-inserted harmonic attenuator and calibrating-device with a long length (MIRHACLLE). All beamlines based on diffraction gratings suffer from the problem of high harmonics contaminations to some extent. The MIRHACLLE provides a way to efficiently suppress high harmonics from 25% to 1 ppm in a grazing incidence bending magnet beamline. Its principle of operation relays on the absorption of the high energy photons in a gas phase region. It allows negligible high harmonics contamination for photon energies ranging from 12 eV to the gas first ionization threshold, 21.6 eV, in the case of neon. We also demonstrate the possibility to use this device for energy calibration and resolution evaluation together with any experiment needing its filtering capabilities. The device is also very cost effective compared to other filters presented previously in the literature.
ABSTRACT
Condensed CO and CO2 are bombarded by approximately 65 MeV 252Cf fission fragments and the desorbed ions are analyzed by time-of-flight mass spectrometry as a function of target temperature, in the ranges 25-33 K and 75-91 K, respectively. Absolute desorption yields are measured up to complete ice sublimation. The mass spectra of both ice targets reveal the emission of: (1) low mass ions, produced by direct Coulomb interaction of the highly charged projectiles and delta-electrons with CO and CO2, and (2) pronounced series of cluster ions. The basic ice cluster structures (CO)n and (CO2)n are present in the emitted cluster series such as (CO)nCO+, (CO2)nCO2+, or (CO2)nCO3-. In the case of CO ice, however, the intense production of the series Cn+, Cn-, and (CO)mCn+ shows that Cn is the main cluster structure, consequence of a higher concentration of free carbon atoms in the nuclear track plasma of CO ice than in that of CO2 ice. Ion cluster abundance is observed to decrease exponentially with cluster mass. The decay constant is k(n) congruent with 0.13, about the same for series based on (CO)n and (CO2)n, but a factor 3.3 higher for the Cn series. The Cn clusters are formed by gas-phase condensation, but the (CO)n and (CO2)n clusters are produced by fracturing of the highly excited solid around the nuclear track. A dramatic reduction of the ion desorption yield is observed near T = 29 K for CO and near T = 85 K for CO2, when fast sublimation occurs and ice thickness vanishes. Close to sublimation temperature, the decay constant of the (CO)2Cn+ series increases due to a decreasing formation probability of large Cn clusters.
ABSTRACT
Threshold photoelectron spectrum of ozone is presented for the first time at a resolution of 21-38 meV using synchrotron radiation in the energy region of 12-21 eV. The ionization energies of the first ionized states were determined and an interpretation of the O3 spectrum with respect to its first three ionic states, 1 2A1, 1 2B2, and 1 2A2, is presented. Above 16 eV the enhancement of the intensities of the 2 2B1, 3 2A1, and 4 2B2 band systems due to the contribution of indirect processes was observed, not accessible by conventional photoelectron spectroscopy. It was also resolved and assigned the extensive vibrational structures of ozone. Between 15.5 and 18.5 eV the main band contours are similar to those found in conventional photoelectron spectroscopy, except that our threshold photoelectron spectrum reveals extensive additional vibrational structures. The band 2 2B1 was found to present an irregular vibrational spacing DeltaE, with a minimum value of 80 meV at approximately 16.47 eV.
ABSTRACT
We investigate the angular distribution of photoionization fragments at low photon energies (12-40 eV) in an open shell atom, by synchrotron radiation recoil ion momentum spectroscopy in a laser cooled and trapped sample. For cesium atoms, for which relativistic effects play an important role and the ion recoil is relatively small, we could determine large and rapid changes of the asymmetry parameter beta from two, observed for s electrons outside resonances and far from the Cooper minimum. They can be explained by relativistic effects and interchannel coupling arising from final state configuration mixing.