Why heat a neon lamp before it will generate light

Why heat a neon lamp before it will generate light  

Neon lamps generate a pleasant pink-red glow. Gaseous neon within the tube (at low pressure) is subjected to a strong electric discharge. One electron per neon atom 

Generally, the flask holding the neon gas contains a small amount of sodium to catalyse ( kick start ) the ionization process - see p. 481. 

We ionized the neon atoms to form Ne+ cations, i.e. each bears a positive charge. (On p. 480 we discuss in detail the photochemical processes occurring at the heart of the neon lamp.) 

To remove any possible confusion, we further refine the definition of ionization energy, and say that 7 is the minimum energy required to ionize 1 mol of a gaseous element. The ionization energy 7 relates to process (2) process (1) is additional. 

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