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Journal of Physics B Atomic and Molecular Physics

Observation of laser-induced collisional two-photon ionisation in a strontium-caesium mixture

Catherine Bréchignac, Ph. Cahuzac, Anne Débarre

Published Jun 28, 1980• 15 citations
Publisher

Abstract

Laser-induced collisional ionisation of caesium is observed in a strontium-caesium vapour mixture. In their initial state both atomic species are in the ground level. The ionisation of caesium is achieved when the system is illuminated by an intense laser light, the frequency of which is tuned across the resonance line of strontium. The process involves photoabsorption during the collisional interaction. It is monitored by the observation of optical fluorescence after Cs + , e - recombination. The laser-induced collisional ionisation profile, i.e. the variation of the ionisation yield against the laser wavelength is recorded under various experimental conditions.

DOI
10.1088/0022-3700/13/12/006

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Journal of Physics B Atomic Molecular and Optical Physics•Feb 11, 2008•13 citations
Laser-induced collisional ionisation of caesium is observed in a strontium-caesium vapour mixture. In their initial state both atomic species are in the ground level. The ionisation of caesium is achieved when the system is illuminated by an intense laser light, the frequency of which is tuned across the resonance line of strontium. The process involves photoabsorption during the collisional interaction. It is monitored by the observation of optical fluorescence after Cs + , e - recombination. The laser-induced collisional ionisation profile, i.e. the variation of the ionisation yield against the laser wavelength is recorded under various experimental conditions.
Investigates theoretically a new aspect of radiative collisions on two-photon ionisation of colliding atoms of different species A and B. The authors derive the main characteristics of the resonance appearing in the two-photon ionisation cross section of atom B when the laser frequency is tuned on a resonance transition of atom A which cannot itself be ionised by two-photon absorption. Under some realistic conditions and for reasonable values of the light energy density flux F ( approximately 1 MW cm -2 ), the cross section of the process considered varies as F 2/5 while its width varies as F 1/2 . These predictions are in reasonable agreement with a recent observation on the mixture Sr(A)-Cs(B) by Brechignac et al. (1980).
The ionisation yields of the optically excited caesium atoms (9.4<or=n*<or approximately=29) are measured in a vapour cell at a caesium vapour density of about 10 14 cm -3 . Direct ionisation rate constants are given for Rydberg atoms with 9.4<or=n*<or=21.4. Collisions with caesium ground-state atoms and black-body radiation strongly modify the population of Rydberg levels with n*>or approximately=25 so the concept of direct collisional ionisation cannot be applied to those levels. The diffusional ionisation mechanism of Rydberg atoms in vapour cells, atomic beams or buffer gases is discussed.
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.
Photochemical formation of caesium hydride in a room temperature cell containing Cs vapour and H 2 under illumination of the mixture by laser radiation tuned to the D 2 atomic resonance line of Cs is reported. The chemical reaction is accompanied by dramatic changes in magnitude of the resonant absorption and fluorescence of Cs atoms in H 2 environment as compared with a pure Cs vapour case. A theoretical model, which takes into account all the optical and chemical processes involved at these densities, is developed. The effective rate of a two-step chemical reaction (1 ± 0.3) × 10 −10 cm 3 s −1 is determined from the fitting of experimental results to this model, exploiting the laser intensity dependence of the fluorescence signal. Continuous (several hours) illumination of the cell by radiation tuned to the Cs D 2 line results in decrease of the hydrogen pressure in the cell, which is an additional proof for the formation of CsH molecules. The proposed investigation technique is applicable for a large variety of resonant photochemical reactions. Possible application for isotope separation is discussed.
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.