The role of metastable atoms in atomic excitation process of magnesium in microwave-assisted laser plasma

Study on the excitation process of magnesium atom has been conducted in microwave-assisted laser plasma. Experimentally, an Nd:YAG laser (532 nm, 5 mJ) was directed into the magnesium oxide (MgO) sample in various gases of helium, argon, and air at a reduced pressure to induce a luminous plasma. The plasma emission was significantly enhanced when a microwave (2.45 GHz, 400 W) was introduced into the laser plasma. Results revealed that the emission intensity of the ionic magnesium lines that have relatively low excitation energy (around 4 eV) is high only in helium and argon ambient gases. On the contrary, the intensity of ionic magnesium lines having high excitation energy (8-11 eV) is high only for helium gas. The results suggested that a particular excitation process takes place in the microwave-assisted laser plasma induced in helium and argon gases, namely the excitation mechanism proceeds through metastable atoms overwhelmingly produced in the microwave-assisted laser-induced plasma.