The five lowest levels of a certain monatomic gas have the values E₁ = 0, E₂ = 3.80 eV, E₃ = 4.30 eV, E₄ = 7.2 eV, and E₅ = 7.5 eV.

(a) If the temperature is high enough that all levels are occupied and the gas is illuminated with light of wavelength 2400 nm, what transitions can occur?

(b) Which of those found in part (a) will still occur if the temperature is so low that only the state E₁ is occupied? 

(c) Repeat (a) and (b) for light of 250 nm wavelength.

(d) What wavelength of the incident light would stimulate emission from state E₄?