511 research outputs found
The ground-state spectroscopic constants of Be_2 revisited
Extensive ab initio calibration calculations combined with extrapolations
towards the infinite-basis limit lead to a ground-state dissociation energy of
Be_2, D_e=944 \pm 25 1/cm, substantially higher than the accepted experimental
value, and confirming recent theoretical findings. Our best computed
spectroscopic observables (expt. values in parameters) are G(1)-G(0)=223.7
(223.8), G(2)-G(1)=173.8 (169 \pm 3), G(3)-G(2)=125.4 (122 \pm 3), and
B_0=0.6086 (0.609) 1/cm; revised spectroscopic constants are proposed.
Multireference calculations based on a full valence CAS(4/8) reference suffer
from an unbalanced description of angular correlation; for the utmost accuracy,
a CAS(4/16) reference including the orbitals is required, while for
less accurate work a CAS(4/4) reference is recommended. The quality of computed
coupled cluster results depends crucially on the description of connected
triple excitations; the CC5SD(T) method yields unusually good results because
of an error compensation.Comment: Chem. Phys. Lett., in pres
Study of coupled states for the (4s^{2})^{1}S + (4s4p)^{3}P asymptote of Ca_{2}
The coupled states A^{1}\Sigma_{u}^{+} (^{1}D +}1}S), c^{3}\Pi_{u} (^{3}P +
^{1}S) and a^{3}\Sigma_{u}^{+} (^{3}P +}1}S) of the calcium dimer are
investigated in a laser induced fluorescence experiment combined with
high-resolution Fourier-transform spectroscopy. A global deperturbation
analysis of the observed levels, considering a model, which is complete within
the subspace of relevant neighboring states, is performed using the Fourier
Grid Hamiltonian method. We determine the potential energy curve of the
A^{1}\Sigma_{u}^{+} and c^{3}\Pi_{u} states and the strengths of the couplings
between them. The c^{3}\Pi_{u} and \as states are of particular importance for
the description of collisional processes between calcium atoms in the ground
state ^{1}S_{0} and excited state ^{3}P_{1} applied in studies for establishing
an optical frequency standard with Ca.Comment: 15 pages, 12 figure
The contribution of tunnelling to the diffusion of protons and deuterons in rare gas solids
The stability and diffusion of protons and deuterons in rare gas matrices are reexamined. These are known to be stabilized in rare gas matrices in the form of linear, centrosymmetric Rg₂H⁺ cations. The elementary step in their diffusion, displacement from one Rg–Rg bond to a neighboring one, can be modelled as an isomerization of the triangular Rg₃H⁺ cation. Using an analytic approximation for thermally averaged transmission coefficients for tunnelling through and reflection by a truncated parabolic potential barrier (R.T. Skodje and D.G. Truhlar,J. Phys. Chem. 85, 624 (1981)) we calculate the rate constants for this elementary diffusion step. The calculated rate constants are consistent with all experimental observations, and confirm that tunnelling makes the dominant contribution to the diffusion of protons and deuterons in rare gas solids. Deuteration reduces the tunnelling rates by 5 to 8 orders of magnitude, which agrees with the observation that D⁺ in rare gas solids is signficantly more stable than H⁺
Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals
Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy electrons
are studied with the focus on the role of diffusion controlled atom-atom recombination reaction
of oxygen in the relaxation cascades. The results of correlated in real time measurements of
thermally stimulated phenomena are presented. The experiments have been performed using activation
spectroscopy methods — thermally stimulated exoelectron emission and spectrally resolved
thermally stimulated luminescence. Solid evidence of the radiative mechanism of electron detrapping
triggering the relaxation cascades is obtained
Thermally stimulated exoelectron emission from solid Xe
Thermally-stimulated emission of exoelectrons and photons from solid Xe pre-irradiated by low-energy
electrons were studied. A high sensitivity of thermally-stimulated luminescence (TSL) and thermally-stimulated
exoelectron emission (TSEE) to sample prehistory was demonstrated. It was shown that electron traps
in unannealed samples are characterized by a much broader distribution of trap levels in comparison with annealed
samples and their concentration exceeds in number that in annealed samples. Both phenomena, TSL
and TSEE, were found to be triggered by release of electrons from the same kind of traps. The data obtained
suggest a competition between two relaxation channels: charge recombination and electron transport terminated
by TSL and TSEE. It was found that TSEE predominates at low temperatures while at higher temperatures
TSL prevails. An additional relaxation channel, a photon-stimulated exoelectron emission from pre-irradiated
solid Xe, was revealed
Stimulated by laser light exoelectron emission from solid Ar pre-irradiated by an electron beam
Spatially separated stable charge centers, self-trapped holes and trapped electrons, were generated in Ar
cryocrystals by a low-energy electron beam. A combination of the cathodoluminescence (CL) and photon-
stimulated exoelectron emission (PSEE) methods was used to monitor center formation and selected relaxation
channel – exoelectron emission. It was found that photon-promoted electron current decreased exponentially
under irradiation with the laser operating in the visible range. Influence of the laser parameters
(power and wavelength) on the characteristic lifetime of exoelectron emission is discussed. Effective
bleaching of the low-temperature peaks of thermally stimulated exoelectron emission by the laser light in a
visible range was observed
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