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Journal of Physics B Atomic and Molecular Physics

Saturation effects in resonant three-photon ionisation of sodium with a non-monochromatic field

Pierre Agostini, S E Wheatley, M. D. Levenson, A. T. Georges, P. Lambropoulos

Published May 28, 1978• 102 citations
Publisher

Abstract

Presents an experimental and theoretical analysis of two-photon-resonant three-photon ionisation of Na. The transition 3S to to 4D to e - is studied. The theoretical analysis takes into account saturation effects, the finite laser bandwidth and the expansion of the active interaction volume with laser intensity. Good agreement is obtained between theory and experiment.

DOI
10.1088/0022-3700/11/10/011

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Journal of Physics B Atomic and Molecular Physics•Apr 28, 1982•14 citations
Presents an experimental and theoretical analysis of two-photon-resonant three-photon ionisation of Na. The transition 3S to to 4D to e - is studied. The theoretical analysis takes into account saturation effects, the finite laser bandwidth and the expansion of the active interaction volume with laser intensity. Good agreement is obtained between theory and experiment.
Synchronized by being triggered with the same spark gap, two independently tunable flashlamp-pumped dye lasers with compound cavity structure were used to obtain saturated doubly resonant (3S-3P-4D) three-photon ionization of sodium. Eate equations of such a system under the condition of saturated transitions were analytically solved. Comparison of the calculated ionization curve with experimental results gave good correspondence. The ionization cross section of sodium 4D state has been measured from the curve at laser wavelength 6890A, σ4D(5890A)= (1.2±#0.4)×10-18cm2. It is in good agreement with the theoretical calculation, which is also given in this article.
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.
Presents a theoretical analysis of the effect of laser light statistics on two-photon resonant three-photon ionisation of sodium. The ionisation probability and the population of the atom in the resonant state are compared in the presence of an intensity stabilised laser with a diffusing phase (phase diffusion model) and light undergoing Gaussian amplitude fluctuations (chaotic field). In the limit of large bandwidth fields the resonant two-photon excitation is statistically decoupled from the following ionisation step. A decrease of the bandwidth results in a partial correlation between these two steps manifesting itself in an enhancement of the ionisation probability in the chaotic field. With increasing intensity and the onset of saturation of the bound-bound transition, the ionisation probability and resonant excited-state population in the chaotic field may become smaller than the one in the phase diffusing light. In the large-bandwidth limit the Stark shift in the chaotic field is enhanced, which has a pronounced effect on the effective power-law dependence of the ionisation probability.
An experimental investigation was made of the dispersion of the cubic susceptibility of sodium vapor in the region of two- and three-photon resonances with the 3s-4s and 3s-7p transitions. A compensation of the contributions to the nonlinear susceptibility of two- and three-photon resonances was observed. A theory was developed for the generation of the third harmonic in a nonmonochromatic field under resonance conditions and this theory was found to be in good agreement with the experimental results. A reduction in the effective susceptibility occurred in a strong field and this was due to saturation and Stark shift of the levels.
The authors report the observation of a two-step process in which Na*(4d) is first populated by a radiative collision between Na(3s) and Ba*(6p) in the presence of an intense optical field i.e. Na(3s)+Ba*(6s6p 1 P)+h(cross) omega to Na*(4d)+Ba(6s 2 1 S) followed by photoionisation of the excited sodium Na*(4d)+h(cross) omega to Na + +e. The peak ion intensity is observed at a laser frequency close to the energy difference between Na*(4d) and Ba*(6s6p 1 P), and the magnitude of the cross section for the first step agrees well with theory.
The time evolution of the ionisation probability of a sodium atom irradiated by linearly polarised light with frequency near the 3s-3p transition frequency is studied theoretically by the following method. The 3s and 3p states are strongly coupled using the rotating-wave approximation and then the resulting strongly coupled wavefunction is used as the lowest-order approximation in applying second-order perturbation theory to obtain the ionisation probability. The photoelectron energy spectrum and total ionisation probability are studied for various laser detunings and turn-on forms. Processes which are forbidden at long times by energy conservation are found to be important at short times (t<or approximately=10 -9 s for a laser intensity of order MW cm -2 ) . The ionisation probability is found to exhibit a plateau behaviour at times much less than the 3s-3p Rabi cycling time and subsequently to oscillate with the 3p population before reaching rate behaviour in the long-time limit.