When an electric potential is applied across cathode and anode, some atoms of the filled gas get ionized.the positively charged ions collide with the negatively charged cathode and dislodge metal ions due to sputtering. sputtered metal ions are further excited to emission thro impact with the filled gas.HCL has a finite life time.the sputtering removes some of the metal atoms from the cathode,which gets deposited elsewhere. filled gas is absorbed in the sputtered metal, on the glass walls and also absorbed into the glass from bombardment.to be continued.........