Abstract
The total cross section for excitation of ground state mercury atoms to the 63P1 state by 50 eV electrons was experimentally determined to be 0.434 ± 0.04 × 10−16 cm2. The apparatus used a pair of single-photon detectors to observe correlated optical photons in the 7 1S0 → 6 3P1 → 6 1S0 and 7 3S1 → 6 3P1 → 6 1S0 cascades. The product of the number of singles counts in the two detectors divided by the number of time-correlated cascade events was used along with angular distribution and correlation factors to determine N, the total number of atoms excited to the 6 3P1 state in the interaction region. The correct number N was arrived at in this way without making a separate determination of the overall photon detection efficiency. No standard lamp was needed for calibration nor was reference made to another standard cross section. The apparatus was self-calibrating. The absolute excitation cross section was determined from N and other parameters of the bombardment apparatus.
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