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Journal of the Optical Society of America B

Observation of autoionizing states of beryllium by resonance-ionization mass spectrometry

Charles W. Clark, W. W. Smith, Jack D. Fassett, Thomas B. Lucatorto, Larry J. Moore

Published Jun 1, 1985• 56 citations
Publisher

Abstract

We have made the first reported observations of the Be 2p21S state, and of high-lying members of the Rydberg series 2pnd1P° (n⩽16), by multiphoton resonance-ionization mass spectrometry (RIMS). The energy of the 1S state is compared with a number of theoretical predictions, which differ from one another over a range of about 0.75 eV. Good agreement is found when corrections are made for intershell electron correlations. These results show that precision spectroscopy can be performed by RIMS with samples of a few hundred atoms and that direct multiphoton excitation of autoionizing states may be a useful new addition to the existing catalog of resonance-ionization schemes.

DOI
10.1364/josab.2.000891

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We have made the first reported observations of the Be 2p21S state, and of high-lying members of the Rydberg series 2pnd1P° (n⩽16), by multiphoton resonance-ionization mass spectrometry (RIMS). The energy of the 1S state is compared with a number of theoretical predictions, which differ from one another over a range of about 0.75 eV. Good agreement is found when corrections are made for intershell electron correlations. These results show that precision spectroscopy can be performed by RIMS with samples of a few hundred atoms and that direct multiphoton excitation of autoionizing states may be a useful new addition to the existing catalog of resonance-ionization schemes.
Single-colour resonant multiphoton ionization transitions of atomic gadolinium have been investigated by resonance ionization mass spectrometry. A total of 72 transitions have been found in the wavelength range of 530 - 635 nm. The wavelengths, relative intensities, linewidths and isotope shifts have been obtained for most of these transitions. In addition, the initial state for each transition has also been assigned employing the multicolour pump - probe technique. From the analysis of these transitions, it is found that most of the observed transitions arise from the three-photon process via the resonant first or second state and their isotope shifts correspond well with those of the resonant states. Based on these results, we have proposed new high-lying states and have determined isotope shifts of several excited states.
We measure photoion spectra and angular distributions of photoelectrons from the two-color, two-photon resonant ionization of barium atoms in order to probe the effect of the photoionization pathway on autoionization processes. The (6s6p)1{)}^{1})1P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ and the (5d6p)1{)}^{1})1P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{1}$ states are used as intermediates in the photoexcitation of barium to energies in the range of a series of 5dnd autoionizing states. Dramatic differences in the profiles of autoionizing lines are observed for excitation pathways involving different intermediate states. Angular distributions of photoelectrons collected across the range of autoionizing resonances demonstrate strong variations that follow constraints imposed by the angular momentum of the autoionizing peaks and also suggest significant spin-orbit interactions in the photoionization step.
Using the R -matrix Floquet theory we have carried out non-perturbative, ab initio one- and two-colour calculations of the multiphoton ionization of magnesium with the laser frequencies chosen such that the initial state of the atom is resonantly coupled with autoionizing resonances of the atom. Good agreement is obtained with previous calculations in the low-intensity regimes. The single-photon ionization from the excited state of magnesium has been studied in the vicinity of the autoionizing resonance at non-perturbative laser intensities. Laser-induced degenerate states (LIDS) are observed for modest laser intensities. By adding a second laser which resonantly couples the and autoionizing levels, we show that, due to the small width of the state, LIDS occur between this state and the state at intensities of the first laser below . We next investigate the case in which the first laser induces a resonant two-photon coupling between the ground state and the autoionizing state, while the second laser again resonantly couples the respective and autoionizing states. At weak intensities, our calculations compare favourably with recent experimental data and calculations. We show that when the intensity of the first laser is increased, the effect of an additional autoionizing state, the state, becomes significant. This state is coupled to the autoionizing level by one photon, inducing a triply resonant processes. We show that LIDS occur among the three autoionizing levels and we discuss their effect on the decay rate of the ground state. We consider dressed two- and three-level atoms which can be used to model the results of our calculations.
In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.