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Journal of Physics B Atomic and Molecular Physics

One- and two-photon production of very highly excited states of caesium

Anthony F. J. van Raan, G. Baum, Wilhelm Raith

Published May 11, 1976• 12 citations
Publisher

Abstract

Very highly excited p states of Cs with principal quantum numbers 30<or=n<or=85 were produced by one-step photoexcitation with frequency-doubled light from a flash-lamp-pumped dye laser. Field ionization was employed for selective detection. Ionization occurs at field strengths E>or=E min with a sharp threshold. The relation E min (n*) varies as (n*) -4 was verified over a wide range. Two-photon excitation to a very high state via a virtual intermediate state leads to immediate ionization of most of the excited atoms; in this case the ion signal as a function of the light intensity I is not proportional to I k , k>or=2, whereas a direct two-photon transition into the continuum yields proportionality with I 2 .

DOI
10.1088/0022-3700/9/7/002

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Very highly excited p states of Cs with principal quantum numbers 30<or=n<or=85 were produced by one-step photoexcitation with frequency-doubled light from a flash-lamp-pumped dye laser. Field ionization was employed for selective detection. Ionization occurs at field strengths E>or=E min with a sharp threshold. The relation E min (n*) varies as (n*) -4 was verified over a wide range. Two-photon excitation to a very high state via a virtual intermediate state leads to immediate ionization of most of the excited atoms; in this case the ion signal as a function of the light intensity I is not proportional to I k , k>or=2, whereas a direct two-photon transition into the continuum yields proportionality with I 2 .
The relative cross sections of the production of Cs + and Cs 2 + ions through the two-photon ionization of Cs 2 were measured in the 5300-6200 Å wavelength range. A linear polarized dye laser was operated in the CW mode as a light source. The results showed that (1) the cross sections for Cs + ions have a broad peak at around 5570 Å and the threshold at around 6010 Å, and (2) those for Cs 2 + ions have the largest peak at around 5940 Å and many structures in the wavelength range longer than it. Through consideration of the above results, we suggested that Cs + ions are produced not only by two-photon ionization of Cs 2 , but also by a participation of a third photon. The threshold for Cs + ions at around 6010 Å is concerned with the intermediate excited-state Cs 2 (C 1 ∏ u + (ν ′ )), which can be excited from the ground state X 1 Σ g + (ν ′′ =0).
The two-photon excitation of cesium to high-n Rydberg levels through resonant dissociative states of the dimer is investigated over the wavelength range 6000-5900A. Highly vibrationally excited sp1Sigmasbrmgsp+\\sp1\\Sigma\\sb{\\rm g}\\sp+sp1Sigmasbrmgsp+ ground state molecules are excited to the continuum of sp1Pisbrmu\\sp1\\Pi\\sb{\\rm u}sp1Pisbrmu bound states which dissociate to give 5D atoms. The 5D atoms are subsequently photoexcited to high-n Rydberg states which are detected by selective field ionization techniques. Series corresponding to both (5D to\\toto nF) and (5D to\\toto nP) transitions with components up to n = 150 were observed. The photoexcitation process appears to be very efficient and produces copious quantities of Cs Rydberg atoms.
The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-\AA{} wavelength region with a tunable-dye-laser source having a 0.06-0.08-\AA{} linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground 62S126^{2}S_{\frac{1}{2}}62S21​​ level to resonant n2Dn^{2}Dn2D and n2Sn^{2}Sn2S intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from n=9to13n=9\mathrm{to}13n=9to13 have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from n=11to14n=11\mathrm{to}14n=11to14 for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.
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.
Measurements are reported of two-photon ionization probabilities in atomic Cs at the nine wavelengths of the Ar ion laser (5145-4545 AA), the first step of the two-photon process occurring to intermediate levels in the vicinity of the 7 2 P 1/2,3/2 levels. Additional ionization due to molecular Cs was monitored by ion time-of-flight analysis and found to be negligible when choosing proper oven temperatures. An absolute scale for the two-photon rates is provided by normalization on a one-photon UV cross section. Comparison of our data with calculations based on quantum defect Coulomb wavefunctions shows discrepancies of up to four orders of magnitude.
This paper will review briefly a recent experiment which measured the photodetachment probability for I-ions due to simultaneous absorption of two ruby quanta. We have calculated the two-quantum photoionization rate for atomic cesium as a function of wavelength. The "transition" from the initial state to the bound intermediate state is expressed in terms of oscillator strengths. The second "transition," from these excited valence states into the coulomb continuum is expressed as a cross section. We have observed what appear to be genuine two-photon ionization signals in three cases: 1) near resonance in the10P_{3/2}state using the second-harmonic ofQ-switched ruby, 2) away from resonances, using the 5300 Å second-harmonic ofQ-switched neodymium laser, and 3) away from resonances using the first anti-Stokes line at 5750 Å from ruby-pumped D2gas. Present experiments aim at scanning the interesting range near resonance, e.g., by thermally tuning the ruby.