Investigation of Pico-second Laser Heating Process of Polymer-Film through Thermo-Modulation Spectroscopy

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Thermo-modulation spectroscopy is applied for the investigation of the relaxation process in nano-seconds laser heating of a polystirene-film. A thin polystirene-film of a 1 micron thickness is pasted on a polished copper surface. The film is heated by a pulse beam of CO_2 laser (wavelength of 10.6 microns) ; where a CW laser beam is modulated by a chopper which is made of a stainless disc of 120 mm diameter and 0.2 mm thickness and is rotated by synchronous motor with 24000 rpm. Since there are180 slits on the circumference of the disc, when the width of the slit is 250 microns, the half-width of the pulse beam is about 2 micro-seconds and the interval of each pulse is 14 micro-seconds. With the cycle, the polystirene-film is heated and cooled, alternately. In relation to the pulse beam of CO_2 laser, the beam of He-Ne laser (wavelength of 3.39 microns) modulated by the same chopper is incident on the same point of the film. The reflected beam intensity of He-Ne laser is measured by P … More bS detector. In this case, the delay time between two pulse beams can be set up arbitrarily. That is, with increasing delay time the time history of the reflectivity of the film for the wavelength of 3.39 microns is obtained after the film is heated by one pulse of CO_2 laser.In experiment, since it is not clear when the two pulse beams are synchronized completely, the delay time is increased from an arbitrary time, i.e., zero second, to the time interval of 14 micro-seconds at an regular intervals of 300 nano-seconds. As a result, at a delay time the reflected beam intensity begins to increase drastically and then decreases sharply with increasing delay time. That is, only for about 2 micro-seconds the reflected beam intensity changes, which may mean that the two pulse beams are synchronized around the time. Consequently, it is disclosed that the thermo-modulation spectroscopy is successfully applied for the investigation of a high speed heating process by an infrared laser, with the time resolution of an order of a few hundred nano-seconds.The frequency of CO_2 laser is close to that of deforming vibration of C=C-H in the benzene nucleus in the polystirene, while the frequency of He-Ne laser is much the same as that of the stretching vibration of -CH_2-. Since the time resolution of the thermo-modulation spectroscopy is close to the relaxation time from a vibrational mode to other vibrational and/or rotational modes (<100 nano-seconds), the relaxation process in nano-seconds laser heating is expected to be captured through the infrared-ray thermo-modulation spectroscopy. Less