Graduate Thesis Or Dissertation
 

Time-of-flight studies of high Rydberg states in atomic lithium excited by electron impact

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  • A thermal beam of lithium atoms excited to high Rydberg states by pulsed electron impact has been studied by time-of-flight analysis. Near the end of a 35-cm field-free flight path, the excited atoms pass through a pair of electric field plates before entering an excited-atom detector. Electric field ionization was used for detection as well as state selection. Time-of-flight spectra obtained with applied electric fields up to 5 kV /cm are similar to the transit -tune distribution for a non-decaying thermal beam, and yield lifetimes which are consistent with the presence of high angular momentum states in the beam. Angular momentum- mixing cross sections were estimated to be of the order 10⁻¹³ cm², by fitting observed transit-time distributions in the presence of introduced target gases with a model in which all angular momentum substates are equally likely, and in which arbitrary angular momentum changes are allowed. With He, Ne, Ar, H₂, and N₂ introduced as target gases at pressures up to 4 x 10⁻⁴ Torr, cross sections for the scattering of the Rydberg atoms were obtained. These cross sections were typically 10 ⁻¹⁴ cm² , apparently independent of the Rydberg electron state, and inversely proportional to the excited-atom velocity. These observations are in agreement with calculations based on a free core-ion deflection model, for which this study appears to be the first experimental verification. Measured cross sections for Rydberg atom interactions with NH₃ were found to be about 10⁻¹² cm² and consistent with previous measurements and a theory in which the Rydberg atom is ionized after gaining energy from thermally excited rotational levels of the target molecule. The experimental technique of this study suggests a new method for slow ion investigations.
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