9.9 Problems 9.1 What is the probability that a photon, emitted from a Ca atom at the centre of a spherical cloud with radius R = 1 cm on the intercombination line 3 P₁ → ¹So at 657 nm with the natural line-width Av = 3 kHz is reabsorbed by another 9.9 Problems 587 Ca atom, when the absorption cross section σ (wo) at the line centre is σ (wo) = 10-17 cm², the density of Ca atoms is n = 1017 cm³ and the collision cross section 10-16 cm²? How large is the recoil splitting? σcoll=