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Physical Review A

Fluorescence studies of multiphoton ionization of Ca and Sr: Emission from highly excited ionic states

Harold K. Haugen, Henrik Stapelfeldt

Published Feb 1, 1992• 12 citations
Publisher

Abstract

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 ${\mathrm{Ca}}^{+}$(4p), ${\mathrm{Ca}}^{+}$(5s), and ${\mathrm{Ca}}^{+}$(4d), and from ${\mathrm{Sr}}^{+}$(5p) and ${\mathrm{Sr}}^{+}$(6s). The contribution of the highly excited states was found to be favored under short-pulse (35-ps) irradiation.

DOI
10.1103/physreva.45.1847

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Anna M. Zawadzka, Roman S. Dygdała, Andrzej Raczyński +2 more

In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.

Journal of Physics B Atomic Molecular and Optical Physics•Apr 11, 2002•19 citations
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.
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+.
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.
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.
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.
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.
In the present study we have investigated three-photon ionization in Ca in which autoionizing states are engaged. The two-photon resonant process (from the Ca ground state 4s 2 1 S 0 ) occurred through or at least in the vicinity of one of the following states: 4s4d 1 D 2 , 4p 2 3 P 2 , 4s6s 1 S 0 , 4p 2 1 D 2 and 4p 2 1 S 0 , with the third photon either reaching the continuum directly or one of the autoionizing states. The three-photon resonant transitions to 3d m p, m f: 1 P 1 , 3 P 1 and 3 D 1 autoionizing states for m up to 21 have been observed. Some of the autoionizing resonances which we have found had not been observed before in a high-resolution one-photon absorption experiment (for J = 1) and in multiphoton experiments (for J = 3). We have compared the ionization signal as a function of the laser detuning and the laser intensity with theoretical curves obtained within a simple model (three-level atom + one-mode laser field). This gives information about the order of magnitude of the three-photon ionization probability through autoionizing states.