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Atoms

Three-Photon Ionization with One-Photon Resonance between Excited Levels

Aleksandr I. Gomonai, Eugene Yu. Remeta, A. N. Gomonaĭ

Published Sep 17, 2021• 0 citations
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Abstract

Within the framework of a three-level model, the process of three-photon ionization with one-photon resonance between two excited levels (with the lower one being initially unpopulated) is considered using the density matrix method. It is shown that such resonance can result in the appearance of a maximum in the three-photon ionization spectrum when detuning between the resonance wavenumber and the wavenumber of the transition responsible for the lower excited level being populated exceeds the laser radiation linewidth by more than three orders of magnitude.

DOI
10.3390/atoms9030068

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Within the framework of a three-level model, the process of three-photon ionization with one-photon resonance between two excited levels (with the lower one being initially unpopulated) is considered using the density matrix method. It is shown that such resonance can result in the appearance of a maximum in the three-photon ionization spectrum when detuning between the resonance wavenumber and the wavenumber of the transition responsible for the lower excited level being populated exceeds the laser radiation linewidth by more than three orders of magnitude.
The manifestation of double resonances upon the three-photon ionization of the samarium atom has been studied in a frequency range of 17200–18400 cm −1 . Thirty peaks caused by double resonances have been found in the dependence of the yield of Sm + ions on the laser radiation frequency. The majority of these peaks greatly exceed in amplitude the peaks associated with ordinary two-photon resonances. The influence of double resonances on the three-photon ionization has been theoretically analyzed using the density matrix approximation. The calculations show that the probability of the three-photon resonance ionization can increase by more than two orders of magnitude due to the additional one-photon population of an intermediate level. The double resonance slows down the rate of increase in the probability of ionization with an increase in the laser radiation intensity compared to ordinary two-photon resonance.
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.
The coherent excitation of a three level atom by one- and two-photon resonances coupled by a common level is treated by the method of averages. The equations for the density matrix of the three resonant levels are solved analytically for square pulses of duration shorter than relaxation times. Double free induction decay occuring when both exciting fields are switched out of resonance simultaneously is exhibited. The emitted radiation is also calculated for an experiment in which the field driving the one-photon resonance remains on after the other field ends, and it is shown that experiments at several detunings and amplitudes would allow determination of various combinations of relaxation rates. Numerical studies for nonsquare pulses are briefly described. The results show that the main qualitative features of the square-pulse case are retained. 15 references.
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.
We consider the possible physical processes that may arise in a three-level atom when only two of its levels interact with a strong electromagnetic field and when the atomic transition frequency is nearly equal to once and twice the frequency of the laser field, respectively. There have been found pronounced cooperative effects in the spectrum of the two-level system, which is in resonance with the laser field, arising from the presence of the third level. The excitation spectra describing the transitions from the first excited state into the second excited state and from that to the ground state consist, apart from the two central peaks, of two pairs of sidebands which are induced by the laser field of the neighbouring system. Detailed expressions of the spectral functions for the physical processes of one- and two-photon resonance fluorescence have been derived and discussed in the limit of high photon densities. The excitation spectrum of the low frequency modes has been considered and discussed in detail. It is found that quantum beats in spontaneous emission may appear in the spectra of the one- and two-photon resonance fluorescence arising from the interference between the two atomic transition frequencies and the frequency of the laser field. The importance of the low frequency modes that occur in the processes in question has been pointed out.
A semiclassical treatment of collision-induced three-photon ionization of atoms with a near two-photon resonance is presented. The system is described by an effective two-level system. The wavelength dependence of the ionization yield does not obey a power law. When the first excited state is far from resonance, and with a C6R6\frac{{C}_{6}}{{R}^{6}}R6C6​​ interatomic potential, a first-order correction to the expected $\ensuremath{-}\frac{3}{2}$ power law obeys a - \textonehalf{} power law. This three-photon process promises to be an extremely sensitive method for the study of collisional effects in the far wing of two-photon $S\ensuremath{-}S$ and $S\ensuremath{-}D$ transitions, and without the requirement of high densities.