Collision Broadening in the Argon Spectrum

High resolution photographs of the near infra-red spectrum of an argon discharge at various pressures revealed that lines whose lower levels are connected by resonance transitions to the ground state suffered markedly greater collision broadening than those whose lower levels are metastable. The excessive broadening of the former group of lines is believed to be due to resonance collision broadening of their lower levels. The collision broadening of one line from each group, namely 1s 2 - 2p 2 , λ8264 Å and 1s 5 - 2p 2 , λ6965 Å, was measured. The half intensity collision width of λ8264 was 14.7 ± 1.0 × 10 -20 cm -1 per atom per cm 3 of argon, while that of λ6965 was 2.0 ± 0.3 × 10 -20 cm -1 per atom per cm 3 of argon. The ratio of collision width to collision shift (the latter being taken from Hindmarsh and Thomas 1959) was 3.1 ± 0.8. This is in agreement with the value 2.75 predicted by the Lindholm theory for interaction due to van der Waals forces between the atoms. The resonance broadening of λ8264 is related to the f -value of the argon resonance line 1p 0 - 1s 2 , λ1048, and this is found to be f =0.13 ± 0.03, in agreement with calculations quoted by Knox in 1958.