Excited States Of Ca Research Articles

Theoretical investigation of the diatomic Van der Waals systems Ca+He and CaHe

An ab initio investigation has been performed for the ground and several excited states of Ca+He and CaHe systems. The potential energy curves, spectroscopic constants, vibrational levels energies and electric dipole moments have been calculated and analyzed. The ab initio method uses the pseudo-potentials technique, Core Polarization Potentials (CPP) and Full Configuration Interaction (FCI). The number of active electrons of Ca+He and CaHe are reduced to one and two active electrons, respectively. The examination of these numerous results shows interesting structures and behaviors resulting from a collision-induced process that involves calcium and helium atoms. An excellent agreement is found with the available results which confirms the reliability of our calculation.

Excitation–ionization of the calcium atom by electron impact

Double differential cross sections, measured using an electron–photon coincidence method, are reported for the 4p 2P3/2 excited state of Ca+ produced by electron interaction with the 4s2 1S ground state of the calcium atom. The cross section at an incident electron energy of 400 eV is dominated by indirect processes involving 3p excitation to states lying 31.4–31.6 eV above the atomic ground state. These then autoionize to the observed 4p 2P3/2 ion state. Other indirect processes involving states with binding energies in the range 27–41 eV, including triply excited states, are also observed. Preliminary measurements of the linear Stokes parameters at the peak of the main resonance give P1 = −0.09 ± 0.01 and P2 = 0.01 ± 0.01.

Study of non-linear optical phase conjugation in Ca by resonant degenerate four-wave mixing via bound excited states

We have studied the optical phase conjugation (OPC) effect in Ca vapour by using the resonant degenerate four-wave mixing (DFWM) technique. The effect was studied through the first excited state of Ca, 4s4p 1 P1. The depen- dence of the reflectivity on the vapour density, the buffer gas pressure and the pump beams intensity was investigated. The influence of the strong linear absorption was taken into account in a theoretical model that led to a satisfactory interpretation of the experimental results. A maximum reflectivity of 46% was measured under optimized conditions. The effect was also studied through the high-lying bound states 4p 23 P 0 , 1 , 2 . The maximum measured reflectivity was as high as for the first excited state. The effect of the oscillator strength on the reflectivity was demonstrated experimentally and interpreted with the help of a simple theoretical model.

Resonance structure in the electron-impact excitation of Ca+ below the 5s threshold.

Detailed calculations of energies and widths of Ca autoionizing states and the related resonance structure in electron-impact excitation of the 4{ital s}-4{ital p} transition in Ca{sup +} are performed using the diagonalization method. This method is divided into two steps: a three-state (4{ital s},3{ital d},4{ital p}) close-coupling calculation of cross sections and extensive configuration-interaction calculations of wave functions of autoionizing states. Both steps take into account the correlations related to one- and two-body core-polarization potentials. The results of calculations of energies, oscillator strengths, and autoionizing widths of doubly excited states of Ca are in better agreement with the experimental data than any other calculation. Rich resonance structure is predicted by theory at incident electron energies below the 5{ital s} threshold. The present results support the general pattern of the measurements of Frontov (Opt. Spectrosc. 9, 460 (1985)), although there are some discrepancies in the details, particularly the existence of the high and broad resonance at that threshold. A detailed comparison with the previous resonance calculations by Mitroy {ital et} {ital al}. (Phys. Rev. A 38, 3339 (1988)) is carried out.

Field-Emission Studies of Electronic Energy Levels of Adsorbed Atoms

The relative changes in the total energy distribution of field-emitted electrons upon adsorption of single atoms have been measured for adsorption of the alkaline-earth atoms (Ba, Sr, and Ca) on several crystal planes of tungsten. The expected perturbations of the energy distribution due to the tunneling resonance through an atomic "virtual level" at various positions relative to the Fermi surface and of various half-widths $\ensuremath{\Gamma}$ is demonstrated by a simple one-dimensional calculation. The measured energy-dependent structure in the current-enhancement factor due to the adsorbate has been interpreted in a tunneling-resonance model to yield the positions and shapes of the atomic "virtual levels." The ground-state $^{1}S$ $6{s}^{2}$ level of Ba is broadened to a half-width ${\ensuremath{\Gamma}}_{S}=0.75$ eV and shifted upward by $\ensuremath{\Delta}{E}^{S}=0.95$ eV. This causes it to overlap and mix with the first two excited states: a triplet $^{3}D$ $6s5d$ and a singlet $^{1}D$ $6s5d$. The observed $^{3}D$ and $^{1}D$ levels were not shifted and had a half-width ${\ensuremath{\Gamma}}_{D}=0.1$ eV. Similarly, the first excited state of Ca, a triplet $^{3}P$ $4s4p$, was shifted by $\ensuremath{\Delta}{E}^{3\mathrm{P}}\ensuremath{\approx}0.4$ eV with a width ${\ensuremath{\Gamma}}_{3\mathrm{P}}\ensuremath{\approx}0.3$ eV ($2\ensuremath{\Gamma}=\mathrm{FWHM}$). The effect of the band structure of the substrate on tunneling resonance is discussed.