To produce line emission spectra , the light must be viewed through a light spectrometer (diffraction grating experiment).
First step is excitation of gas molecules. The gas molecules are bombarded with accelerated electrons. Upon collision with electrons in the molecules the electron transit upwards to higher energy level. This depends on how much kinetic energy is transferred to the molecule. The downward transition of this electron emits photon of frequency corresponding the the energy difference between the two levels.
Excitation by photon absorption would require the photon of exactly the same energy as the difference in energy between 2 levels. Unlike collison where some or all kinetic energy can be transferred , in photon absorption , all the energy must be absorbed by the electron in the molecule.
