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Optics Letters

Associative ionization of laser-excited Rydberg states in strontium vapor

Earl F. Worden, J. G. Conway, Jeffrey A. Paisner

Published Oct 1, 1978• 22 citations
Publisher

Abstract

Using mass spectrometry we have confirmed that Sr(2)(+) ions are produced from laser-populated Rydberg states by the associative ionization process Sr(5snl) + Sr(5s(2)) ? Sr(2)(+) + e(-). Dimer ion signal was detected for the 5s6d(3)D(2) level, indicating that the binding energy of Sr(2)(+) exceeds 0.77 eV. We have observed that under certain conditions the Rydberg states are destroyed by electron-impact ionization before an associative ionization collision can occur.

DOI
10.1364/ol.3.000156

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Collisional and thermal ionization of sodium Rydberg atoms: I. Experiment fornS andnD atoms withn= 8–20

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Kinetics of Rb2+ and Rb+ formation in laser-excited rubidium vapor

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Open Physics•Apr 10, 2008•4 citations
Using mass spectrometry we have confirmed that Sr(2)(+) ions are produced from laser-populated Rydberg states by the associative ionization process Sr(5snl) + Sr(5s(2)) ? Sr(2)(+) + e(-). Dimer ion signal was detected for the 5s6d(3)D(2) level, indicating that the binding energy of Sr(2)(+) exceeds 0.77 eV. We have observed that under certain conditions the Rydberg states are destroyed by electron-impact ionization before an associative ionization collision can occur.
We report on the observation and absolute measurements of the population dynamics for several Sr i and Sr ii levels when the vapor is illuminated by a short laser pulse. When the laser beam excites the 5s5p 1{}^{1}1P1{P}_{1}P1​ resonance level, we show that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s10s 7thinsp;1{;}^{1};1S0{S}_{0}S0​ Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed.
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 Sr+(5p2P32,12){\mathrm{Sr}}^{+}(5p^{2}P_{\frac{3}{2},\frac{1}{2}})Sr+(5p2P23​,21​​) 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 Sr+(5p2P){\mathrm{Sr}}^{+}(5p^{2}P)Sr+(5p2P) in the intensity regime of $I\ensuremath{\sim}1\ensuremath{-}50$ GW/cm2{\mathrm{cm}}^{2}cm2, but no emission is detected from higher-lying states of Sr+{\mathrm{Sr}}^{+}Sr+.
Collisional and thermal ionization of sodium n S and n D Rydberg atoms with n = 8–20 has been studied. The experiments were performed using a two-step pulsed laser excitation in an effusive atomic beam at atom density of about 2 × 10 10 cm −3 . Molecular and atomic ions from associative, Penning and thermal ionization processes were detected. It was found that the atomic ions were created mainly due to photoionization of Rydberg atoms by photons of blackbody radiation at the ambient temperature of 300 K. Blackbody ionization rates and effective lifetimes of Rydberg states of interest were determined. The molecular ions were found to be from associative ionization in Na( nL ) + Na(3S) collisions. Rate constants of associative ionization have been measured using an original method based on relative measurements of Na + 2 and Na + ion signals.
Abstract We have studied the formation of the molecular ion Rb2+ and the atomic ion Rb+. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb+ and Rb2+. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×1013cm−3 and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb2+ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb+ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb+ and Rb2+ is obtained.