This thesis is devoted to experimental study of the interaction of femtosecond laser pulse at terawatt power level with various forms of matter. The soft X-ray emission form laser-irradiated gases, solids, and gaseous clusters has been investigated by means of time-integrated spectroscopy in the wavelength region 40-500 Å. Spectroscopic study of optical-field-ionized Ar, Kr, Xe, and oxygen plasmas is presented. By focusing 20-fs laser pulse at intensity of $\sim7\times10^16W/cm^2$ into high-pressure gas jet and/or low-density static gas cell, emission on lines originating from ions up to Ar IX, Kr IX, and Xe IX was observed. The feasibility of using the powerful, femtosecond Ti:Sapphire laser system developed at KAIST for pumping of collisional-excitation and recombination soft X-ray lasers is investigated. Spectra from highly charged ions of up to F VII, Al XI, Ti XII, Fe XVI, and Cu XIX are presented, and the effect of femtosecond prepulse on the X-ray emission from solid target is examined. The interaction of intense, 25-100 fs pulses with Ar and Ne clusters from a cryogenically-cooled gas jet target is explored. New spectral lines from charge states as high as Ar XI and Ne VIII appeared with decreasing temperature, and significant enhancement of X-ray line emission was observed with cooling, demonstrating efficient collisional heating of near-solid-density expanding clusters by 25-100 fs laser pulses.
