Wavelength dependences in generation of highly charged ions by intense nanosecond laser-cluster interaction

The highly stripped ions of C4+, N5+, and S6+ were produced when 1 064 nm laser with an intensity of 1011 W·cm-2 was used to irradiate the molecular beam of benzene, ammonia and sulfide hydrogen. The ratio of the produced multiply charged ions to the total ion signals decreased scarcely as the laser intensity decreased by almost two magnitudes. When 532 nm laser was used, the main ion products turned to be C2+, C3+, N2+, N3+, S4+, S3+ and S2+ with lower charge states. While when 266 nm laser was used, no multiply charged ions were detected. The highly charged ions are speculated to arise from the electron impact ionization process in the cluster. The initial electrons are produced by multiphoton ionization and then heated by inverse bremsstrahlung, where the heating rates scale with the