Collision Broadening and Shift in the Resonance Line of Calcium

Abstract

The collision shift and broadening of the line Ca 4227 A due to an external pressure of helium have been measured. The line was formed in absorption by passing white light through calcium vapour in the presence of helium at various pressures of less than 1 atmosphere. The spectral resolution was provided by a Babcock grating used in the new Oxford solar spectrograph. The half-intensity damping width of the line was found to be 1.72 ± 0.12 × 10 ⁻²⁰ cm ⁻¹ per atom per cc of helium, and the shift 0.05 ± 0.04 × 10 ⁻²⁰ cm ⁻¹ per atom per cc towards the violet. The ratio of broadening to shift is therefore ≽ 19, compared with the value of 2.76 predicted by the Lindholm theory. The shift is also predicted to be towards the red. Possible explanations of the discrepancy are discussed and it is shown that the short-range repulsive forces between calcium and helium atoms must be involved as well as the long-range van der Waals forces. With the use of Lindholm's theory, coefficients for both types of force are evaluated from the measurements. Finally it is shown that the solar damping coefficient for Ca 4227 A is < 2.2 × 10 ⁻³ cm ⁻¹ at τ = 0.1, and that the collision shift makes a negligible contribution to the solar red shift.