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Annual Meeting Optical Society of America

Fluorescence detection of Sr+ following Sr multiphoton Ionization

Andreas Othonos, Harold K. Haugen

Published Jan 1, 1987• 0 citations
Publisher

Abstract

The atomic beam studies of Feldmann, Welge, and co-workers,1 utilizing single-ion or electron detection, have indicated the probable importance of stepwise double ionization of Sr via Sr+ excited states. Recently, in a strong field regime (≳100 GW/cm2), Agostini and Petite2 found a broad low-energy peak which was associated with the near-energy-resonant autoionizing decay to Sr+(2P1/2,3/2).

DOI
10.1364/oam.1987.thpo20

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The atomic beam studies of Feldmann, Welge, and co-workers,1 utilizing single-ion or electron detection, have indicated the probable importance of stepwise double ionization of Sr via Sr+ excited states. Recently, in a strong field regime (≳100 GW/cm2), Agostini and Petite2 found a broad low-energy peak which was associated with the near-energy-resonant autoionizing decay to Sr+(2P1/2,3/2).
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+.
Multiphoton ionization of Ca and Sr was studied at a wavelength of 532 nm with optical pulses of 12-ns and 35-ps duration. Excess photon absorption followed by autoionizing decay leads to populations of excited states of singly charged ions that can be observed via their spontaneous emission. Relative measurements were performed on fluorescence from Ca+{\mathrm{Ca}}^{+}Ca+(4p), Ca+{\mathrm{Ca}}^{+}Ca+(5s), and Ca+{\mathrm{Ca}}^{+}Ca+(4d), and from Sr+{\mathrm{Sr}}^{+}Sr+(5p) and Sr+{\mathrm{Sr}}^{+}Sr+(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.
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}1011{10}^{11}1011--1015{10}^{15}1015 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) 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/cm2{\mathrm{cm}}^{2}cm2. Fine-structure-resolved emission of the Sr+{\mathrm{Sr}}^{+}Sr+ (5p2{p}^{2}p2P3/2/,1/d0{P}_{3/2/,1/d}^{0}P3/2/,1/d0​) 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}1:103{1:10}^{3}1:103 with respect to the ionic-resonance emission.
A model of multiple resonances between groups of discrete and autoionizing levels is proposed for the description of the process of multiphoton ionization. The phenomena of the extremely rapid broadening and growth of amplitudes of the multiphoton resonances, observed experimentally in the yield of Sr+ and Ba+ ions [ J. Phys. B.14, 2253– 2359 ( 1981);Elementary Processes in Laser Radiation Field ( 1988), pp. 122– 153)] are explained on the basis of this model. The strong-coupling method was used in numerical calculations for Sr and Ba atoms in a strong laser field.
We present experimental photoelectron energy spectra and angular distributions from multiphoton ionization of Sr by dye laser pulses of ∼ns duration in the wavelength range of 715-737 nm and relatively low intensity (≈4 × 10 11 W·cm -2 ). The selected wavelength range comprises the highly correlated 5p 2 1 S 0 autoionizing state, excited by four photons. Our results reveal the dominant (single and double) ionization pathways that were obscured in earlier experiments, based solely on the recording of ion and ionic-fluorescence yields.