A Novel Chemical Measurements Using A Laser Induced Ultrasonics

目前迫切需要开发一种新的技术来评价各种新开发的化学材料。从上述观点出发，我们尝试对其应用激光诱导超声。也就是说，使用相同频率的两束激光，通过两束激光的相交在材料内部产生干涉图样。在干涉区内，材料的光学性质（即折射率、光吸收强度）也通过光与材料的相互作用而发生空间调制。在吸光材料的情况下，由激发态的非辐射松弛过程产生的热产生了相同的模式。热量的空间调制产生热光栅，也通过加热区域的膨胀和收缩产生反传播声波。通过另一探针束的布拉格衍射检测材料内部产生的衍射光栅。利用上述激光诱导热光栅的实验结果表明，该技术可以成功地应用于材料的痕量分析和超声光谱分析。即利用热光栅对<H_2O>中的<KMnO_4>溶液进行了定量分析，直至<10^(-7)> M浓度量级。在不限制与样品接触的情况下，可以在大约MH到100 MHz的范围内产生和检测超声波，并在液体样品中测量其超声波速度。

A novel technique for evaluating a variety of newly developed chemical materials has been required to develope nowadays. From the above-mentioned view point, we have tried to apply a laser -induced ultrasonic to them. That is, using two laser beams of the same frequencies,interference pattern is produced inside materials by intersecting the two beams. In the interference region optical properties ( ie, refractive index, light absorption strength ) of the material are also spatially modulated through the interaction between light and material. In the case of light absorbing materials, the same pattern arises from the heat generated by nonradiative relaxation processes of the excited states. The spatial modulation of the heat gives rise to thermal grating and also counterpropagating acoustic waves through expansion and contraction of the heated region. A diffraction grating generated inside the materials is detected through the Bragg diffraction of another probe beam. Using the above-mentioned laser-induced thermal grating, we demonstrated some experimental results that this technique can be successfully applied to trace analysis of materials and ultrasonic spectroscopy.That is, using thermal grating, quantitative analysis of <KMnO_4> solutions in <H_2O> was carried out down to the order of <10^(-7)> M concentration. Ultrasinc waves could be generated and detected over the ranges from some MH to 100 MHz without restriction of any contact with samples and its ultrasoniz velocity was measured in liquid samples.