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Journal of Physics B Atomic Molecular and Optical Physics

Three-photon resonances due to autoionizing states in calcium

Anna M. Zawadzka, Roman S. Dygdała, Andrzej Raczyński, Jarosław Zaremba, Jacek Kobus

Published Apr 11, 2002• 19 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/35/8/301

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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.
We present experimental results in AC-Stark splitting of an autoionizing state probed by a multiphoton process. In the experiment performed in calcium, the 3d4f 1 P 1 autoionizing state is single-photon coupled with the 4s6s 1 S 0 excited state. The splitting caused by this coupling is probed by three-photon excitation from the ground state. Detuning and laser intensity dependent measurements have been carried out in a collision-free environment. The experimental results can be interpreted and modelled in terms of a few levels system.
Multiphoton ionization of Ca and Sr was studied at a wavelength of 532 nm with optical pulses of 12-ns and 35-ps duration. Excess photon absorption followed by autoionizing decay leads to populations of excited states of singly charged ions that can be observed via their spontaneous emission. Relative measurements were performed on fluorescence from Ca+{\mathrm{Ca}}^{+}Ca+(4p), Ca+{\mathrm{Ca}}^{+}Ca+(5s), and Ca+{\mathrm{Ca}}^{+}Ca+(4d), and from Sr+{\mathrm{Sr}}^{+}Sr+(5p) and Sr+{\mathrm{Sr}}^{+}Sr+(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.
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.
Singly and doubly charged Ca ions have been produced by multiphoton excitation of Ca vapour with 5 ns, low intensity (⩽3 × 10 11 W cm −2 ) dye-laser pulses in the 675–685 nm wavelength range, including the four-photon 4s 2 1 S 0 → 4p 2 1 S 0 excitation. The intensity and wavelength dependence of the ion yields was recorded as well as the fluorescence emission from excited states of the Ca ion in an effort to identify the excitation pathways leading to single and double ionization. Unambiguous evidence for the absorption of at least two photons above the first ionization threshold was recorded, in agreement with earlier results for Mg and Sr obtained under similar conditions. However, certain characteristics of the process differ significantly from those of the earlier results, despite the apparent similarity in both the atomic structure and the excitation scheme. The most striking and unexpected finding is that for a certain wavelength, the Ca 2+ yield is observable well before the saturation intensity of Ca + and, moreover, it grows with intensity and saturates in parallel with the Ca + yield. Possible mechanisms behind this outcome are discussed in detail as well as their implications for the multiphoton multiple ionization of complex atoms. Our tentative conclusion is that the occurrence of doubly charged ion production before the singly charged ion saturation should not be considered as 'synonymous' with direct (or non-sequential) multiphoton double ionization.
We theoretically study multiphoton ionization of the Ca atom irradiated by the second (photon energy 3.13.13.1 eV) and third (photon energy 4.65 eV) harmonics of Ti:sapphire laser pulses (photon energy 1.551.551.55 eV). Because of the dense energy level structure the second and third harmonics of a Ti:sapphire laser are nearly single-photon resonant with the 4s4p4s4p4s4p 1Po{}^{1}{P}^{o}1Po and 4s5p4s5p4s5p 1Po{}^{1}{P}^{o}1Po states, respectively. Although two-photon ionization takes place through the near-resonant intermediate states with the same symmetry in both cases, it turns out that there are significant differences between them. The photoelectron energy spectra exhibit the absence or presence of substructures. More interestingly, the photoelectron angular distributions clearly show that the main contribution to the ionization processes by the third harmonic arises from the far-off-resonant 4s4p4s4p4s4p 1Po{}^{1}{P}^{o}1Po state rather than the near-resonant 4s5p4s5p4s5p 1Po{}^{1}{P}^{o}1Po state. These findings can be attributed to the fact that the dipole moment for the 4s24{s}^{2}4s2 ${}^{1}{S}^{e}\ensuremath{-}4s5p$ 1Po{}^{1}{P}^{o}1Po transition is much smaller than that for the 4s24{s}^{2}4s2 ${}^{1}{S}^{e}\ensuremath{-}4s4p$ 1Po{}^{1}{P}^{o}1Po transition.