Collisional effects on metastable atom population in vapour generated by electron beam heating

Abstract

The metastable atom population distribution in a free expanding uranium vapour generated by electron beam (e-beam) heating is expected to depart from its original value near the source due to atom–atom collisions and interaction with electrons of the e-beam generated plasma co-expanding with the vapour. To investigate the dynamics of the electron–atom and atom–atom interactions at different e-beam powers (or source temperatures), probing of the atomic population in ground (0 cm−1) and 620 cm−1 metastable states of uranium was carried out by the absorption technique using a hollow cathode discharge lamp. The excitation temperature of vapour at a distance ∼30 cm from the source was calculated on the basis of the measured ratio of populations in 620 to 0 cm−1 states and it was found to be much lower than both the source temperature and estimated translational temperature of the vapour that is cooled by adiabatic free expansion. This indicated relaxation of the metastable atoms by collisions with low energy plasma electrons was so significant that it brings the excitation temperature below the translational temperature of the vapour. So, with increase in e-beam power and hence atom density, frequent atom–atom collisions are expected to establish equilibrium between the excitation and translational temperatures, resulting in an increase in the excitation temperature (i.e. heating of vapour). This has been confirmed by analysing the experimentally observed growth pattern of the curve for excitation temperature with e-beam power. From the observed excitation temperature at low e-beam power when atom–atom collisions can be neglected, the total de-excitation cross section for relaxation of the 620 cm−1 state by interaction with low energy electrons was estimated and was found to be ∼10−14 cm2. Finally using this value of cross section, the extent of excitational cooling and heating by electron–atom and atom–atom collisions are described at higher e-beam powers.

Systems considerations for Resonance-Ionization Mass Spectroscopy in the application of Nuclear Forensic Analysis by Ray Mariella Jr

“…Engineering Considerations for STEP 3 Minimize non-specific photoionization, excitation 19 or charge-changing collisions, etc., all of which give can spurious and possibly confounding signals. To maximize throughput and yield, it is important to match excitation/ionization volume to the extraction aperture of the mass spectrometer.…”

“…The presence of a resonant, autoionizing state significantly reduces the power requirements for the photoionization laser and the reduced power reduces the very undesirable, non-specific 19 photoionization. The kinetics of the coupled steps has been discussed by B. Isselhardt in his thesis 10 , but, in short, if the rate of photoionization exceeds the duration of the excitation and photoionization pulses, and the Rabi frequency of the excitation process between the ground state and the upper excited state is rapid enough to replenish the upper state population that is depleted by the photoionization, then most, if not all, of the ground-state atoms that are illuminated by the laser beams can be ionized for extraction into the mass spectrometer.…”