RESUMO
Hexagonal close-packed iron (hcp-Fe) is a main component of Earth's inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density-sound velocity data of Earth's core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (V P) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch's law for hcp-Fe, which gives us the V P of pure hcp-Fe up to core conditions. We find that Earth's inner core has a 4 to 5% smaller density and a 4 to 10% smaller V P than hcp-Fe. Our results demonstrate that components other than Fe in Earth's core are required to explain Earth's core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth's core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.
RESUMO
Multiphoton ionization and oxidation processes of ammoniated magnesium clusters are investigated by the multiphoton ionization method with an intense femtosecond laser. In the photoionization of mass-selected Mg+ (NH3)n, evaporation dominates at lower laser intensity, while the oxidation reaction to produce H-atom elimination products, MgNH2+ (NH3)m, becomes predominant at higher intensity. In addition to these fragment ions, doubly-charged ions are observed for n > or = 2 at the laser intensity higher than 10(12) W cm(-2). We also examined the femtosecond pump probe experiments for Mg+ (NH3)4 by monitoring these reaction products. The lifetime of the first excited state is determined as 0.8 ps from the temporal profile of MgNH2+ (NH3)m. On the other hand, the time profile of the evaporation products exhibits a bleaching of the absorption, which gives the recovery time of the initial state as 1.2 ps. Multiphoton excitation of Mg(NH3)n with the femtosecond laser at 800 nm gives doubly-charged ions with n > 3 in addition to singly-charged cluster ions and H-atom elimination products such as MgNH2+ (NH3)m. The absence of small doubly-charged ions is ascribed to a charge reduction reaction followed by Coulomb explosion. On the basis of these results, the dynamics of the solvation and oxidation reaction processes of Mg(NH3)n is discussed.
