A procedure is described for calculating rigorous upper and lower bounds to dipole oscillator strengths and is illustrated numerically with various simple applications to the hydrogen molecular ion. The procedure appears to give a good account of the actual errors which may arise in the conventional method based on energy minimization of the two states. In addition, specific regions of configuration space which are important for the oscillator strength, but perhaps not for the energy, are singled out and emphasized automatically.

Calculation of Upper and Lower Bounds to Oscillator Strengths

Publisher Summary This chapter focuses on the lifetime measurements of free atoms, molecules, and ions because it is well-known that atoms, ions, and molecules in excited electronic levels tend to decay spontaneously to low energy levels of their respective systems by the emission of electric dipole radiation. The simplest experiment that can be imagined to measure lifetimes would consist of a sample chamber in which the atoms to be studied are excited to the desired upper level by electron bombardment. In most multichannel systems, the lifetime resolution is limited by the combined effects of the finite time taken for the gun pulse cutoff and the finite transit time of electrons across the source. The numerical methods discussed in this chapter can be used to resolve the observed decay curves into a sum of exponential components. However, considerable care is required in analyzing and interpreting the results obtained from cascade decay curves, and the possibility exists of making quite serious errors. The lifetime has also been obtained indirectly by the combined studies of the intensities of absorption and emission spectra.

The Measurement of Lifetimes of Free Atoms, Molecules, and Ions

Atomic polarization can have a profound effect on the generation of the high-order harmonics during the interaction of the polarized atom with intense field. In this work, the atomic polarization effect is investigated for the specific case of the $2\phantom{\rule{0.2em}{0ex}}^{1}P$ helium subject to infrared radiation. The strong coupling between the $2\phantom{\rule{0.2em}{0ex}}^{1}P$ and $2\phantom{\rule{0.2em}{0ex}}^{1}S$ states of helium is taken into account using a multichannel version of the strong-field approximation of Lewenstein and co-workers [Lewenstein et al., Phys. Rev. A 49, 2117 (1994); Antoine et al., Phys. Rev. A 53, 1725 (1996)]. The analysis reveals a strong dependence of the high-order harmonic generation efficiency on the atomic polarization. While the occurrence of the predicted effect can be explained in terms of the recollision model, its magnitude is deeply affected by the fast population oscillations between the excited bound states. Moreover, the induced bound-bound transitions bring about the interesting double-maximum structure in the ellipticity dependences of the emitted harmonics, both maxima occurring for nonzero field ellipticities. This unusual structure shows up even for the high-energy harmonics generated solely by the bound-continuum mechanism.

High-order harmonic generation by excited helium: The atomic polarization effect

Absolute Generalized Oscillator Strength of 11S→21P Transition in Helium

Lifetime of the2P11and3P11States of Atomic Helium

The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the $2^{1}P$ state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of $2^{1}S$ metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the $2^{1}S$ atoms, which are excited to the $2^{1}P$ state. This state then primarily decays to the ground $1^{1}S$ state with the emission of resonance (584 \AA{}) radiation. The lifetime of the $2^{1}P$ state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.

W.L. Williams

Papers

HANLE EFFECT IN 2$sup 1$P HELIUM.