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Physica Scripta

Ionization of sodium vapour by nanosecond resonant laser pulses tuned to 3S→ 4Ptransition

T. Stacewicz, W Latek

Published Dec 1, 1990• 14 citations
Publisher

Abstract

We present a model of dense sodium vapour ionization induced by nanosecond resonant laser pulses exciting the 3 S → 4 P transition. The results of calculations are compared with those obtained by means of a similar model for the case of 3 S → 3 P excitation. The possibility of population inversion formation between excited sodium levels is also discussed.

DOI
10.1088/0031-8949/42/6/005

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Annual Meeting Optical Society of America•Jan 1, 1985•0 citations
We present a model of dense sodium vapour ionization induced by nanosecond resonant laser pulses exciting the 3 S → 4 P transition. The results of calculations are compared with those obtained by means of a similar model for the case of 3 S → 3 P excitation. The possibility of population inversion formation between excited sodium levels is also discussed.
The authors report direct measurements of the photodissociation of Na 2 + ions created when a Na vapour in the density range 10 12 -10 13 cm -3 is irradiated by a pulsed laser with intensities in the 10 5 -10 6 W cm -2 range. The Na 2 + ions, accelerated by a weak electric field, are photodissociated in a second laser beam tuned to 6040 AA. In a preliminary experiment the Na 2 + ions are formed in the lowest vibrational levels and the measured photodissociation cross section is compared with existing calculations. When the Na vapour is irradiated at one of the 3S 1/2 -3P 1/2,3/2 resonances the photodissociation measurements show the existence of a distribution of rovibrational excited levels of the Na 2 + ions. A discussion is given in terms of associative ionisation in collisions of two Na(3p) atoms. The origin of the Na + ions produced simultaneously with the Na 2 + ions, in the vapour irradiated at resonance, is also discussed.
i n Ùnal for m Marc h 1, 2001) W e rep ort results of investigation on the Ûuorescence induced in sodium v ap our by pulses of laser light tuned near the 3 S ! 4 P transition (330 nm ).T he Ûuorescence w as observed in perp endi cular and parallel direction s in resp ect to the laser b eam.A depletion of Ûuorescence intensity corresp ondin g to the 5 D ! 3 P transition (498 nm) was observed along the laser beam.W e explain this e˜ect as a result of reabsorption that is caused by intense p opulati on of the 3 P level due to cascade stimulated pro cesses.T he most intense depletio n occurs ab out 20 ns after the laser pulse and it corresp onds to the moment of maximal populati on of the 3 P level.
levels of atomic sodium using ultrashort laser pulses in a stepwise excitation scheme. Two broadband femtosecond laser pulses are used to excite the 6S level from the 3S ground level in sodium vapor. The first one is simultaneously resonant with the D1 and D2 lines of the 3S→3P transition, and the second one with the 3P→6S transition. The delay between the two pulses is scanned in order to probe the dephasing between the two excited intermediate levels. Quantum beating is observed by monitoring the fluorescence signals at 330 nm and 515 nm, namely transition 4P→3S and 6S→3P, respectively. Signals are compared to a semi-classical model that reproduces the oscillations with a very good agreement.
The recently observed efficient ionization of sodium vapor by a laser tuned to the sodium resonance line can be explained in terms of the large population of resonance-state atoms created by laser-induced radiative equilibrium. This large population of excited atoms represents (a) a reservoir of energy that can rapidly be transferred to the free electrons via superelastic collisions and (b) a source of pseudo-ground- state atoms possessing a much reduced ionization energy. Three-photon ionization is proposed as the mechanism for creation of the primary pool of free electrons.
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.