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

应用热调制光谱法研究了纳米秒激光加热聚苯乙烯薄膜的弛豫过程。一层厚度为1微米的聚苯乙烯薄膜被粘贴在抛光的铜表面。用波长为10.6微米的CO_2激光脉冲束加热薄膜；其中，连续波激光束由斩波器调制，斩波器由直径120毫米、厚度0.2毫米的不锈钢圆盘制成，由同步电机以24000转/分的速度旋转。由于圆盘的圆周上有180个狭缝，当狭缝的宽度为250微米时，脉冲光束的半宽约为2微秒，每个脉冲的间隔为14微秒。在循环过程中，聚苯乙烯薄膜交替加热和冷却。相对于CO_2激光的脉冲光束，经过相同斩波器调制的波长为3.39微米的He-Ne激光光束入射到薄膜的同一点上。用P…More bS探测器测量了He-Ne激光器的反射光束强度。在这种情况下，两个脉冲光束之间的延迟时间可以任意设置。也就是说，随着延迟时间的增加，在CO_2激光的一次脉冲加热下，薄膜在3.39 μ m波长处的反射率时程变化。在实验中，由于不清楚两束脉冲何时完全同步，延迟时间从任意时间，即0秒，以300纳秒的规则间隔增加到14微秒的时间间隔。因此，在延迟时间处，反射光束强度开始急剧增加，然后随着延迟时间的增加而急剧下降。也就是说，只有大约2微秒的反射光束强度变化，这可能意味着两个脉冲光束在时间上是同步的。因此，公开了热调制光谱成功地应用于红外激光高速加热过程的研究，其时间分辨率为几百纳秒数量级。CO_2激光的频率与聚苯乙烯中苯核中C=C- h的变形振动频率相近，He-Ne激光的频率与- ch_2 -的拉伸振动频率相近。由于热调制光谱的时间分辨率接近于从振动模式到其他振动和/或旋转模式的弛豫时间（<100纳秒），因此有望通过红外热调制光谱捕捉纳秒激光加热中的弛豫过程。少

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