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Physical Review A

Multiphoton excitation and ionization of atomic cesium with a tunable dye laser

D. Popescu, C. B. Collins, B. W. Johnson, Iovitzu Popescu

Published Mar 1, 1974• 70 citations
Publisher

Abstract

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 $6^{2}S_{\frac{1}{2}}$ level to resonant $n^{2}D$ and $n^{2}S$ intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from $n=9\mathrm{to}13$ have been recorded for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}$ series and from $n=11\mathrm{to}14$ for the $6^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}$ series.

DOI
10.1103/physreva.9.1182

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Journal of the Optical Society of America B•Feb 1, 1985•37 citations
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 62S12\ensuremath−n2D52,326^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}D_{\frac{5}{2},\frac{3}{2}}62S21​​\ensuremath−n2D25​,23​​ series and from n=11to14n=11\mathrm{to}14n=11to14 for the 62S12\ensuremath−n2S126^{2}S_{\frac{1}{2}}\ensuremath{-}n^{2}S_{\frac{1}{2}}62S21​​\ensuremath−n2S21​​ series.
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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.
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.
The use of in situ ion detection in alkali vapors coupled with multiphoton ionization provides a technique of highest sensitivity for the investigation of the spectroscopy of excimer states involving alkali atoms. This technique has been applied in this work to the molecular components of the spectrum of the $6S\ensuremath{-}7P$ absorption doublet of cesium. The dispersion curve was obtained with a tunable dye laser for the two-photon ionization of cesium over the 4500-4600-\AA{} wavelength range as a function of vapor pressure and source intensity. Strongly developed satellite bands were observed to be associated with the atomic lines $6^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}7^{2}P_{\frac{1}{2}}$ and $6^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}7^{2}P_{\frac{3}{2}}$, and were found to have well-defined band heads at 29.4 and 31.4 ${\mathrm{cm}}^{\ensuremath{-}1}$ to longer wavelengths, respectively. Sensitivity was observed to be an order of magnitude in excess of that obtained from classical absorption techniques, and the observation of a quadratic dependence of the molecular components confirmed the multiphoton nature of the detection sequence.
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).
Two independently tunable, linearly polarized dye lasers have been used to study multiphoton ionization of gaseous xenon. One laser is tuned near three-photon resonance with the allowed 6s[3/2]J=1° resonance, which is not observed with one laser at these pressures because of the destructive interference between the three-photon-excitation and third-harmonic-excitation pathways to the 6s[3/2]J=1° state. The second laser is tuned to resonance with higher allowed np and np′ states of xenon. The missing ionization signal is seen to be dramatically restored by the addition of the second laser beam; however, the signal again disappears as the pressure is increased. These results are discussed in relation to recent experimental and theoretical studies of cancellation effects between three-photon excitation and third-harmonic generation. Spectroscopic information obtained from tuning each dye laser, together with data on the angular distributions resulting from rotating the plane of polarization of one laser relative to that of the second laser, clearly shows that two different ionization mechanisms are operative. In addition, these experiments unambiguously illustrate the importance of dissociative ionization of quasi-dimers of xenon over a rather broad frequency range.