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Optics and Spectroscopy

Photoionization of the 4d state of the sodium atom in the field of a nanosecond pulse of a Nd:YAG laser

E. V. Vsevolodskiĭ, А. А. Черненко

Published Nov 1, 2001• 5 citations
Publisher

Abstract

Results of experiments on photoionization of excited sodium atoms in the 4 d state in the field of nanosecond pulses of a Nd:YAG laser are reported. The dependence of the photoionization signal for the given atomic state on the ionizing pulse energy was obtained. The experimental results are compared with the model calculations of the saturation curve for the photoionization signal.

DOI
10.1134/1.1420846

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Results of experiments on photoionization of excited sodium atoms in the 4 d state in the field of nanosecond pulses of a Nd:YAG laser are reported. The dependence of the photoionization signal for the given atomic state on the ionizing pulse energy was obtained. The experimental results are compared with the model calculations of the saturation curve for the photoionization signal.
We report an experimental analysis of a structure induced by a Nd:YAG laser embedding the sodium excited state in the ionization continuum. The induced structure is probed from the ground state via a one-photon ionization. The experiment is performed on an atomic beam apparatus which uses a time-of-flight spectrometer for energy analysing the emitted photoelectrons. The selectivity of this technique provides the best measurement of a laser-induced resonance in the photoionization spectrum reported to date. A good agreement is found between the theory and experimental results.
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.
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.
Laser-induced photoionization and fluorescence signals were simultaneously observed in atomic uranium using a single Nd:YAG-pumped dye laser. These signals were recorded in two specific cases. In the first case, the dye laser was resonant to the first-step transition (0–16900.38 cm−1). In the second case, the laser was near-resonant to the first-step transition with a slight detuning (0.15 cm−1) so that it became two-photon resonant at 33801.06 cm−1. The uranium atoms in the ground state were ionized by a single-color, three-photon photoionization technique resulting in the photoionization signal, and the fluorescence signal was simultaneously obtained from the first excited state involved in the photoionization process. The photoionization and the fluorescence signals in the above-mentioned cases were also estimated theoretically for several values of the photoionization cross section for the transition between the second excited state at 33801.06 cm−1 and the autoionization state at 50701.59 cm−1 using density matrix formalism. From the comparison of theoretically calculated ratios of fluorescence signals in the two specific cases with the experimentally obtained values, the photoionization cross section for the 33801.06–50701.59 cm−1 transition has been obtained, which is found to be (5±1)×10−16 cm2.