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Journal of Physics B Atomic Molecular and Optical Physics

Multiphoton Ca2+production occurring before the onset of Ca+saturation: is it a fingerprint of direct double ionization?

Ioannis Liontos, S. Cohen, A. Lyras

Published Apr 13, 2010• 8 citations

Abstract

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.

DOI: 10.1088/0953-4075/43/9/095602

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Physical Review A•Apr 1, 1989•11 citations

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.

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

{\mathrm{Ca}}^{+}

(4p),

{\mathrm{Ca}}^{+}

(5s), and

{\mathrm{Ca}}^{+}

(4d), and from

{\mathrm{Sr}}^{+}

(5p) and

{\mathrm{Sr}}^{+}

(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.

We report on the single and double multiphoton ionization of ground state Sr atoms observed in an atomic beam experiment with laser pulses of ∼5 ns duration, maximum intensity ∼4 × 10 11 W cm −2 and within the 710–740 nm wavelength range. The Sr + spectrum consists of two strong lines originating from three-photon resonant four-photon ionization of bound states, a number of weak autoionizing resonances and a broad line due to four-photon excitation of the doubly excited 5p 2 1 S 0 state. The latter, along with a strong, broad and structured spectral feature, is also evident in the wavelength dependence of the doubly charged Sr 2+ ion. A weakly evident but reproducible inflection point ('knee' structure) appears in the intensity dependence of the Sr 2+ yield at the location of the 5p 2 1 S 0 resonance. A complementary fluorescence experiment revealed the accumulation of population in the 5p 1/2 , 3/2 , 6s 1/2 and 5d 3/2 , 5/2 excited Sr + states during the laser pulse. All fluorescence signals depend on laser wavelength in a manner similar to the recorded Sr + spectrum. The population accumulation in the 5p 1/2 , 3/2 ionic states unambiguously proves the absorption of two photons above the first 5s 1/2 atomic threshold while that of the 5d 3/2 , 5/2 and 6s 1/2 ones suggest the absorption of at least two more. Since under our laser pulse duration and intensity the absorption of such a number of photons in an unstructured continuum is highly improbable, it may be concluded that the process is mediated by dense manifolds of near- resonant doubly excited states, their role in the dynamics of laser–atom interaction under our conditions being far more crucial than in studies conducted using intense ultrashort pulses.

Multiphoton ionization of strontium in the wavelength range of \AA{}558-590 nm is studied via the fluorescence from excited

{\mathrm{Sr}}^{+}(5p^{2}P_{\frac{3}{2},\frac{1}{2}})

ions. The fine-structure sublevels in the singly charged ion are fully resolved and are found to be nonstatistically populated. Four- and five- (or more) photon processes are observed to populate

{\mathrm{Sr}}^{+}(5p^{2}P)

in the intensity regime of $I\ensuremath{\sim}1\ensuremath{-}50$ GW/

{\mathrm{cm}}^{2}

, but no emission is detected from higher-lying states of

{\mathrm{Sr}}^{+}

.

In an effort to gain more insight into the process of multiphoton multiple ionization, several experiments have recently been carried out to determine the wavelength dependence of the double ionization of Sr and Ba. 1-3 These studies were not as conclusive as one might hope, in that the observed resonances in the production of Sr++ and Ba++ did not quite match the energy levels of Sr+ and Ba+. Here we report experiments in which we have compared the spectra for the production of Ba by an intense laser with and without the population of specific states of Ba by modest secondary lasers. The results of these experiments show clearly that Ba++ is produced by first producing real intermediate states of Ba+ which are multiphoton ionized through intermediate resonances in Ba+.

By focusing an excimer pumped dye laser on an atomic beam, the authors have observed multiphoton multiple ionisation of the neutral strontium atom in the tuned wavelength range 532-41 nm. Sr + and Sr ++ yields were measured with a time of flight spectrometer. At least three photons are required to produce Sr + and eight photons to produce Sr ++ . As a function of the wavelength, the Sr + yield exhibits resonances which correspond to two-photon excitation of bound states (5s 2 -5p 2 1 D 2 and 5s 2 -5p 2 1 S 0 ) and three-photon excitation of autoionising states 4d5f J=1 and J=3 of the neutral atom. The Sr ++ yield exhibits four broad smooth resonances that they interpret, from known spectroscopic data, as two-photon (5p 1/2 -4f), three-photon (5s-6p 1/2, 3/2 ) and four-photon (5s-7d) resonances in the singly charged ion. The latter transitions start from a singly charged ion in the 5s or 5p 1/2 state reached after three- or four-photon ionisation of the neutral ground state. The variation of the Sr ++ pattern as a function of the dye laser beam intensity shows that resonances are shifted and broadened when the intensity is increased. These features have been interpreted by a perturbative treatment of Sr + ionisation based on the use of an effective Hamiltonian which takes into account higher-order terms involved in the calculation of resonance effects. Calculated multiphoton ionisation probabilities are in good agreement with experimental data.

An ``all-optical'' technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (\ensuremath{\sim}

{10}^{11}

--

{10}^{15}

${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558--590 nm at laser intensities \ensuremath{\sim}1--50 GW/

{\mathrm{cm}}^{2}

. Fine-structure-resolved emission of the

{\mathrm{Sr}}^{+}

(5

{p}^{2}

{P}_{3/2/,1/d}^{0}

) ions which are formed is observed at \ensuremath{\lambda}=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity \ensuremath{\sim}

{1:10}^{3}

with respect to the ionic-resonance emission.