We report on the observation and absolute measurements of the population dynamics for several Sr i and Sr ii levels when the vapor is illuminated by a short laser pulse. When the laser beam excites the 5s5p
1P1 resonance level, we show that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s10s 7thinsp
;1S0 Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed.
We have observed ion formation (both
Na+ and
Na2+) and radiation from high-lying Na states when low-density vapor (\ensuremath{\sim}
1013 ${\mathrm{cm}}^{\ensuremath{-}3}$) is irradiated with a cw dye laser tuned to one of the
D lines. The
Na+ ions probably result from photodissociation of
Na2+ formed by $\mathrm{Na}(3p)\ensuremath{-}\mathrm{Na}(3p)$ associative ionization, while the high-lying radiating states are formed in energy-pooling reactions.
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.
This thesis presents the results of a series of experiments in which electrons are superelastically scattered from various laser excited states of sodium. The atoms, once in the optically prepared state, are forced to relax via the superelastic collision with an electron. The rate of detection of superelastically scattered electrons was measured as a function of the laser polarisation which enabled pseudo Stokes parameters to be determined. These pseudo Stokes parameters are functions of both optical pumping parameters and atomic collision parameters. The optical pumping parameters describe the laser-atom interaction and the atomic collision parameters describe the electron-atom collision process. Three different laser excitation mechanisms were used to optically pump the atoms into various excited states. The first of these used a single laser tuned to the 32S 112(F'=2 hyperfine state)-~32P312 transition. The excited atoms underwent a superelastic collision with an electron leaving the atom in the ground state and pseudo Stokes parameters were measured as a function of both scattering angle and incident electron energy. The second superelastic experiment, utilised a folded step excitation mechanism which employed two lasers tuned from the two hypethne states of the 32S112 ground state respectively to the 32P312 excited state. Power broadening effects in the single laser experiment cause the atoms to be optically pumped into the F= 1 hyperfine ground state. The laser powers used were not great enough to power broaden the hyperfine ground states and as such the F'= 1 sublevel effectively acted as a sink. The folded step excitation method enabled the excited state population to be increased so that data at larger scattering angles could be obtained. Stokes parameters from both of these experiments which had an incident energy range of 10eV to 30eV and an angular range of 5°-25° were compared to three current electron-atom scattering theories and previous experimental data. Overall, fair to good agreement was found between theory and experiments for the individual Stokes parameters. Losses of coherence was observed at small scattering angles (50.200) at 20eV and 25eV incident electron energies which were poorly modelled by the three different theories. The third superelastic experiment involved the use of two lasers of specified polarisation to stepwise excite the atoms to the 32D512 excited state. Superelastic collisions with incident electron energies of 20eV from the 32D512-*32P312~312 collision were studied at three different scattering angles and pseudo Stokes parameters for the case where the polarisations of the radiation from the lasers were parallel were measured. The single step and folded step laser-atom interactions for it excitation were modelled using a full quantum electrodynamical treatment so that the optical pumping parameters from the single and folded step experiments could be investigated. Equations of motion were derived in the Heisenberg picture and it is shown that for the single laser case 59 equations of motion are required to fully model the interaction and for the folded step ease 78 equations of motion are required. The results of calculations demonstrated that the optical pumping parameters were sensitive to laser intensity, laser detuning and the Doppler width of the atomic beam. The theoretical quantum electrodynamical calculation results were in good agreement with the experimental results.
Ion formation in sodium vapor (\ensuremath{\sim}
1011 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) containing Rydberg atoms (18\ensuremath{\le}n\ensuremath{\le}35) was investigated. Both atomic ions and diatomic molecular ions resulting from collisions between highly excited sodium,
Na∗∗, and Na(3s) were observed. However, the predominant production mechanism was found to be photoionization of
Na∗∗ by blackbody radiation, the rate of which was measured at 500 K and found to be \ensuremath{\simeq}5.7\ifmmode\times\else\texttimes\fi{}
103 ${\mathrm{sec}}^{\mathrm{\ensuremath{-}}1}$ for n=18, in good agreement with theory. The
Na+ signal from blackbody photoionization is observed to be independent of n, which we interpret as implying rapid and nearly complete self-l-mixing, mixing by collisions with Na(3s) atoms, for both s and d states. It is probable, however, that the s states are mixed to the adjacent, n-1, manifold of l states. It was also found that the heavy-body collision cross sections for
Na+ and
Na2+ formation, while roughly three orders of magnitude lower than our estimate of the self-l-mixing cross section, are appreciable, of order 100 A
˚2.