Anomalous absorption of 1-μm laser radiation by laser-produced plasmas

We present the results of experiments where we have measured the absorption of laser light by plasmas generated by 20--120-psec duration pulses of Nd laser radiation incident upon solid targets at intensities exceeding 10¹⁴ W/cm². The results were obtained using box calorimetry, supported by ion calorimetry, ion probe data, and 2{ω} calorimetry. We have found a marked asymmetry in the absorption for 120-psec pulses with irradiation geometry with the absorption dropping by 60% when the beam diverged at the target surface. The results were analyzed using a simple rigid plasma model and show that resonance absorption is not the dominant mechanism, but the absorption is caused by the presence of a narrow region near the critical surface having a strongly enhanced collision frequency most probably due to the presence of long-wavelength ion acoustic waves in the plasma.