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Journal of Physics B Atomic Molecular and Optical Physics

Multiphoton single and double ionisation of strontium in the range 532-541 nm

Pierre Camus, Μιχάλης Κομπίτσας, S. Cohen, C Nicolaides, M Aymar, Michele Crance, P. Pillet

Published Feb 14, 1989• 26 citations

Abstract

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.

DOI: 10.1088/0953-4075/22/3/010

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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.

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.

We present experimental photoelectron energy spectra and angular distributions from multiphoton ionization of Sr by dye laser pulses of ∼ns duration in the wavelength range of 715-737 nm and relatively low intensity (≈4 × 10 11 W·cm -2 ). The selected wavelength range comprises the highly correlated 5p 2 1 S 0 autoionizing state, excited by four photons. Our results reveal the dominant (single and double) ionization pathways that were obscured in earlier experiments, based solely on the recording of ion and ionic-fluorescence yields.

We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.

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}}^{+}

.