X-Ray and Extreme Ultraviolet Emission from Small-Sized Kr Clusters Irradiated by 150-fs Laser Pulses

X-ray and extreme ultraviolet (EUV) emission from Kr clusters irradiated by 150-fs laser pulses at the peak laser intensity of 5 x 10¹⁵W/cm² was experimentally investigated. Strong transitions (10 nm-13 nm) from Kr X and Kr IX were observed and some spectral lines from Kr XIII and Kr XIV, which have been predicted to be not produced by optical-field-ionization at the laser intensity used, also appeared. The laser energy absorption and the intensity of x-ray emission started to grow remarkably above the backing pressure of 0.5 MPa and to decrease at the backing pressure of 3 MPa. It is suggested that an optimum backing pressure may exist for Kr clusters heated by 150 fs laser pulses at a certain laser intensity to produce x-ray emission.