massive jaws 12 of tensioning device 14 for a typical aluminum sample of 1 millimeter width w and 50 millimeters length l and 15 millimeters thickness d the characteristic time for heat conduction to jaws 12 is approximately one second this is long compared to a laser pulse duration 100 nanoseconds and to the time required to diffuse heat across the sample 1 millisecond the peak sample relaxation is an accurate measure of the total laser energy absorbed the ratio of absorbed to intercepted incident energy is the coupling coefficient u03b1 this method has been tested on both painted and unpainted surfaces coupling coefficients measured by this method for unpainted samples of bare aluminum are in good agreement with calorimetric values previously reported fig3 shows the energy absorbed by painted and unpainted samples of aluminum as a function of incident energy the slope is the coupling coefficient u03b1 for the unpainted sample the slope increases dramatically from 3 to 25 at an incident energy of approximately 17 millijoules this energy is also the threshold for initiation of a plasma consisting of ionized gasses formed by evaporation from the surface of the sample during irradiation coupling above threshold is dominated by the laser induced plasma since the plasma produced by the first part of the arriving laser pulse absorbs the remainder of the pulse more strongly than does the reflective bare metal for pulse energies not far above threshold thermal contact is maintained between plasma and sample and more net energy is coupled into the sample than would be the case were the plasma not present this accounts for the abrupt increase in slope the plasma.