A special interferometric technique with high sensitivity has been devised on the basis of rainbow refractometry without the use of an additional and delicate amplitude-splitting setup. This new technique was used for the characterization of shock wave plasma induced by a Q-switched Nd:YAG laser on various metal samples under reduced surrounding gas pressures. An unmistakable signal of the density jump was detected simultaneously with the observation of the emission front signal. It proved that the emission front and the front of the blast wave coincided and moved together with time at the initial stage of the secondary plasma expansion. However, at a later stage, the emission front began to separate from and left behind the blast wave front propagating in the surrounding gas at low pressures. With the use of Cu and Zn samples, the experimental results showed that the separation of the emission front and blast wave front took place at about 5 mm above sample surface for laser energy of 140 mJ.

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