Production of metastable 23S1 helium in a laser produced plasma at low pressures

We used cavity ringdown spectroscopy to study how the laser ablation of various materials (graphite, aluminum and iron) influences production of 23S1 metastable helium in the regime of low pressure helium gas. Plasma was produced by nanosecond Nd:YAG laser at 1064 nm with irradiance from 0.8 to 10.9 × 109 W/cm2. Helium pressure ranged from 0.03 to 0.2 mbar. It was found that at higher irradiances, metastable production for all materials reaches saturation due to plasma shielding and is more pronounced for graphite. The highest yield of metastables was found in the case of aluminum target (0.05%). For metals, production rate is discussed in terms of work function and ionization potential.