Laser-based VUV spectroscopy of doubly excited Ca I

Abstract

We describe a new study of the odd-parity doubly excited spectrum of Ca I, in which a tunable coherent vuv source was used to scan over an energy range previously investigated by classical spectroscopy using synchrotron radiation. A vast improvement in the quality of the data, especially as regards the available spectral resolution, results from the use of a four-wave mixing source and enables the comparison between theory and experiment to be pushed further than ever before for doubly excited states in alkaline-earth spectra. Some remaining limitations of the theoretical computations are brought to light and discussed.

Three-photon ionization of Ca

“…In the high-resolution single-photon spectra obtained by Farooqi et al (see figure 2 of [10]) in the region 159.224-159.696 nm, one-photon resonances could be compared with our three-photon resonances. For lower wavenumbers (δν * < 0) only two peaks belonging to the 3d17f 1 P 1 and 3d19p 1 P 1 resonances could be recognized [10,15]. In our measurements it is possible to find two additional resonances, namely 3d18f 3 P 1 and 3d19f 3 P 1 .…”

“…Additionally, we have to take into account that between the 4s and 3d thresholds, the doubly excited states are reached by an absorption of three photons and must be of odd parity, as in a single-photon absorption. Some of the states, mainly due to 3dnl, could be observed as resonances in single-photon absorption (see, e.g., [10,15]). Note that the 4s 2 1 S 0 ground state is composed of 92.0% 4s 2 , 7.6% 4p 2 and 0.3% 3d 2 [31] which gives information about the expected 3dnp, 3dnf, 3dnh and 4p5s configurations, which may give rise to resonances in the range of ±100 cm −1 around the ν M = 62 679.47 cm −1 wavenumber.…”

“…As for other elements possessing two ns valence electrons, the electron correlation in the valence shell is sufficiently strong to allow a reasonable probability of a simultaneous excitation of both valence electrons. Recently [10], attention has been concentrated on measurements using VUV lasers and on a comparison between theory and experiment, both for the cross sections and the profile parameters. The autoionizing states of Ca are not too far from the part of the continuum reached by the third photon [10,11] and the influence of those states could be significant.…”

“…Recently [10], attention has been concentrated on measurements using VUV lasers and on a comparison between theory and experiment, both for the cross sections and the profile parameters. The autoionizing states of Ca are not too far from the part of the continuum reached by the third photon [10,11] and the influence of those states could be significant. The autoionization spectrum of Ca has been studied by Gaardsted et al [12] using the fluorescence signal from the Ca + (4p) state.…”

“…Taking into account that the closest bound levels of Ca (this means 5d 3 D J , 5d 1 D 2 , 6s 3 S 1 and 1 S 0 , 1 D 2 states of the 4p 2 configuration) are located about 1000 cm −1 [14,15] from the state 4s6s 1 S 0 , we can say that in our experiment we perform two-photon resonant and near-resonant three-photon ionization. However, the influence of the autoionizing states of Ca in the part of continuum reached by the third photon [10,11,15] is significant. We observed three-photon resonances due to the autoionizing states 3d17f, .…”