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Physical Review A

Ion formation in laser-irradiated sodium vapor

M. Allegrini, W. P. Garver, Vikram S. Kushawaha, Jacob J. Leventhal

Published Jul 1, 1983• 19 citations
Publisher

Abstract

The formation of ${\mathrm{Na}}^{+}$ and ${\mathrm{Na}}_{2}^{+}$ in laser-irradiated sodium vapor has been studied using a nitrogen-laser-pumped dye laser. The effects of irradiation by this pulsed laser are substantially different from those reported previously using a cw device, but otherwise identical experimental conditions. In contrast to the earlier experiments, in which ions were observed only when the cw laser was tuned to a $D$ line, relatively large "background" ion signals are observed at all excitation wavelengths, a result of multiphoton processes involving the ${\mathrm{Na}}_{2}$ component of the vapor. It is shown that the laser bandwidth and pulse duration are extremely important parameters; a wide bandwidth effectively magnifying the importance of the dimer component of the vapor and a short pulse duration minimizing the effects of interactions between excited atoms and molecules.

DOI
10.1103/physreva.28.199

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Journal of the Optical Society of America B•Jan 1, 1995•11 citations
The formation of Na+{\mathrm{Na}}^{+}Na+ and Na2+{\mathrm{Na}}_{2}^{+}Na2+​ in laser-irradiated sodium vapor has been studied using a nitrogen-laser-pumped dye laser. The effects of irradiation by this pulsed laser are substantially different from those reported previously using a cw device, but otherwise identical experimental conditions. In contrast to the earlier experiments, in which ions were observed only when the cw laser was tuned to a DDD line, relatively large "background" ion signals are observed at all excitation wavelengths, a result of multiphoton processes involving the Na2{\mathrm{Na}}_{2}Na2​ component of the vapor. It is shown that the laser bandwidth and pulse duration are extremely important parameters; a wide bandwidth effectively magnifying the importance of the dimer component of the vapor and a short pulse duration minimizing the effects of interactions between excited atoms and molecules.
The production of Na+{\mathrm{Na}}^{+}Na+ and Na2{\mathrm{Na}}_{2}Na2​+{\mathrm{}}^{+}+ in sodium vapor irradiated by pulsed laser light in the D-line region of the spectrum has been studied by varying the laser power density and the wavelength. The data, which were acquired with an order-of-magnitude higher resolution than those in our previous experiments, support our earlier conclusion that ions formed at laser wavelengths close to, but not coincident with, the D lines result from three-photon resonance-enhanced ionization of Na2{\mathrm{Na}}_{2}Na2​ molecules present in the vapor.
We have observed ion formation (both Na+{\mathrm{Na}}^{+}Na+ and Na2+{\mathrm{Na}}_{2}^{+}Na2+​) and radiation from high-lying Na states when low-density vapor (\ensuremath{\sim}1013{10}^{13}1013 ${\mathrm{cm}}^{\ensuremath{-}3}$) is irradiated with a cw dye laser tuned to one of the DDD lines. The Na+{\mathrm{Na}}^{+}Na+ ions probably result from photodissociation of Na2+{\mathrm{Na}}_{2}^{+}Na2+​ formed by $\mathrm{Na}(3p)\ensuremath{-}\mathrm{Na}(3p)$ associative ionization, while the high-lying radiating states are formed in energy-pooling reactions.
The formation of Na (nl\mathrm{nl}nl) and Na2+\mathrm{Na}_{2}^{}{}_{}{}^{+}Na2​​+ in sodium vapor (\ensuremath{\sim}1013{10}^{13}1013 ${\mathrm{cm}}^{\ensuremath{-}3}$) irradiated by a single-frequency (20-MHz bandwidth) cw dye laser tuned to one of the DDD lines has been studied. The dependence of product signal on laser power density suggests that Na 3p3p3p-Na 3p3p3p energy pooling collisions are responsible for these species. Rate constants for specific reaction channels and temperature dependences of rate constants are also reported.
Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.
The steady-state absorption of a beam of sodium atoms irradiated by a train of pulses from a mode-locked dye laser has been measured. The sodium atoms were prepared as two-level systems by optical pumping with a two-mode laser. Absorption spectra on the 589.0-nm D2 line were recorded over 20 GHz with a frequency-stabilized, tunable, mode-locked dye laser having a bandwidth of 3.7 GHz and a mode spacing of 82 MHz. The laser-pulse profile was determined from the spectral profile assuming transform-limited pulses and was incorporated in theoretical calculations of excitation spectra obtained by numerical integration of the optical Bloch equations. By comparison of measured absorption spectra and theoretical excitation spectra the validity of the weak-field solution as well as the predicted saturation behavior were fully verified by the experimental results. For the highest level of irradiation applied, the laser pulses were identified as π pulses.