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Japanese Journal of Applied Physics

Photoionization Characteristics of Sr into 5skl Continua through the Spin-Resolved Ion Detection by Laser-Induced Fluorescence

Yukari Matsuo, T. Kobayashi, Nobuaki Yonekura, Takashi Nakajima

Published Mar 1, 2007• 3 citations

Abstract

We study the characteristics of one-photon resonant two-photon ionization of Sr through the spin-resolved detection of photoions by laser-induced fluorescence. In particular, the dependence of spin polarization on the wavelength and polarization of the ionization laser is experimentally investigated. By changing the polarization of the ionization laser from parallel linear to perpendicular linear with respect to the quantization axis defined by the circularly polarized pump laser, a significant difference is found in the degree of spin polarization, whereas the dependence on the laser wavelength at three different wavelengths (308, 266, and 248 nm) turns out to be very small. To understand the experimental results, we have also undertaken a theoretical analysis, which predicts consistent spin polarization with our experimental results for both parallel and perpendicular linear polarization for a given ratio of bound-free transition moment into the 5 s k s and 5 s k d continua. The ratio of the bound-free transition probabilities from the bound 5 s 5 p 3 P 1 state to the 5 s k s and 5 s k d continua is evaluated.

DOI: 10.1143/jjap.46.1181

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We study the characteristics of one-photon resonant two-photon ionization of Sr through the spin-resolved detection of photoions by laser-induced fluorescence. In particular, the dependence of spin polarization on the wavelength and polarization of the ionization laser is experimentally investigated. By changing the polarization of the ionization laser from parallel linear to perpendicular linear with respect to the quantization axis defined by the circularly polarized pump laser, a significant difference is found in the degree of spin polarization, whereas the dependence on the laser wavelength at three different wavelengths (308, 266, and 248 nm) turns out to be very small. To understand the experimental results, we have also undertaken a theoretical analysis, which predicts consistent spin polarization with our experimental results for both parallel and perpendicular linear polarization for a given ratio of bound-free transition moment into the 5 s k s and 5 s k d continua. The ratio of the bound-free transition probabilities from the bound 5 s 5 p 3 P 1 state to the 5 s k s and 5 s k d continua is evaluated.

We report the detailed experimental study on the production of electron-spin-polarized Sr+ ions through one-photon resonant two-photon ionization via laser-excited 5s5p 3P1 (MJ = +1) of Sr atoms produced by laser-ablation. We have experimentally confirmed that the use of laser-ablation for the production of Sr atoms prior to photoionization does not affect the electron-spin polarization. We have found that the degree of electron-spin polarization is 64+/-9%, which is in good agreement with our recent theoretical prediction. As we discuss in detail, we infer, from a simple analysis, that photoelectrons, being the counterpart of electron-spin-polarized Sr+ ions, have approximately the same degree of electron-spin polarization. Our experimental results demonstrate that the combined use of laser-ablation technique and pulsed lasers for photoionization would be a compact and effective way to realize a pulsed source for spin-polarized ions and electrons for the studies of various spin-dependent dynamics in chemical physics.

The intensity and polarization dependence of the ion production due to three quantum ionization was investigated in potassium. The energy of two of the photons was tuned into resonance with n 2 D levels for 4 < n < 22. The ratio of the ion signals produced with circular polarization to that found with linear polarization was significantly less than that predicted by a single rate approximation. The discrepancy is explained by the increased linewidth of the resonant 2 D level due to the ionization process itself. Single quantum photoionization cross sections for the excited levels calculated by means of quantum defect theory are shown to be consistent with the experimental results.

An ``all-optical'' technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (\ensuremath{\sim}

{10}^{11}

--

{10}^{15}

${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558--590 nm at laser intensities \ensuremath{\sim}1--50 GW/

{\mathrm{cm}}^{2}

. Fine-structure-resolved emission of the

{\mathrm{Sr}}^{+}

(5

{p}^{2}

{P}_{3/2/,1/d}^{0}

) ions which are formed is observed at \ensuremath{\lambda}=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity \ensuremath{\sim}

{1:10}^{3}

with respect to the ionic-resonance emission.

Measurements are reported of the ratio of the ion count rates obtained in two-photon ionization of caesium atoms for linearly and circularly polarized light at the nine wavelengths of the argon-ion laser (514.5-454.5 nm). Comparison of this data with the recent theoretical values of Teague et al. (1976) and Flank et al. (1976) shows discrepancies at all photon energies except near the 7P resonance. In addition, upper limits are presented for the absolute two-photon rates in the rhodamine-6G wavelength region (563-635 nm).