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Acta Optica Sinica

Saturated three-photon ionization of Na atoms

F Hureng

Published Jan 1, 1985• 0 citations
OpenAlex

Abstract

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.

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Journal of Physics B Atomic Molecular and Optical Physics•Jan 28, 1993•2 citations
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.
Cross section σ4D→4F was measured for the collision transfer process of Na(4D) +Na(3S)→Na(4F)+Na(3S).The 4D state is excited by two-photon absorption using a tunable dye laser.Since the 4F→3D fluorescence can not be detected directly with the apparatus used the cascade 3D→3P fluorescence is monitored instead.Then the cross-section σ4D→4F=(1.3±0.3)×10^-14cm^2 is got from the relative intensity of the fluorescence.A theoretical evaluation of σ4D→4F is also given and compared with experimental results.
We measured the cross section for the collision transfer process Na(4D)+Na (3S)Na(4F)+Na(3S). The 4D state was excited by two photon absorption using a tunable dye laser. Since the direct 4F→3D fluorescence can not be detected with our apparatus, we monitored instead the cascade 3D→3P and 4D→3P fluorescence. Relative intensity of the fluorescence, combined with the measurement of sodium density, yielded cross section σ_(4D)→(4F)=1.3×10~(-14)±28％(cm~2). Our result is compared to other experiments.
In studying ionization reactions in a single dense effusive beam of laser-excited Na(3p) atoms, we observe production of both Na{sub 2}{sup +} and Na{sup +} ions. The Na{sup +} results from the familiar associative ionization reaction of two Na(3p) atoms and the Na{sup +} ions under our conditions result mostly from the collisional dissociation of the Na{sub 2}{sup +} by reaction with a third excited Na(3p) atom, rather than from photo- dissociation or photoionization. A CW, single-frequency dye laser tuned to 589 nm resonantly excites the Na(3s) atoms to the 3p state. The atomic beam Na(3s) density is determined from low-intensity 589 absorption measurements. Na{sup +} ion yield depends on the cube of the Na beam density, suggesting successive collisions of three Na(3p) atoms: we believe the Na{sub 2}{sup +} ions polarize the third Na(3p) atom, resulting in a R{sup -4} orbiting collision interaction. A large collisional dissociation cross-section of approximately (4{plus_minus}2)x10{sup -13} cm{sup 2} results, for oven temperatures in the 500-600K range.
The strong radiative single photon coupling between two excited states of sodium atoms (3p and 4d) has been studied at high intensities, exceeding 10 9 W cm -2 . These excited states are probed by Raman-like resonant five-photon ionization processes. Using two tunable lasers the authors have observed that the doubly resonant process does not occur. In the model picture of photon-dressed atomic levels, this situation can be described as an avoided crossing, with a large minimum separation of 30 cm -1 .