Temporal and space resolved detection of laser-induced crystallization process

为了阐明固体中有机晶体成核过程的分子动力学，我们研究了模型体系、异戊烷和甲基环己烷混合溶剂低温玻璃基质中的芘分子。在一系列的实验中，我们证明，在355 nm的数千纳秒激光脉冲照射到模型系统诱导的溶质在固体基质中的有效聚集，即使在77 K。在连续的紫外照射诱导的结晶过程中，可以归因于芘的准分子的发射峰的强度随着紫外脉冲的发射次数的增加而增加，而没有检测到准分子发射。准分子发射甚至出现在紫外脉冲的单次照射下，这表明单次照射的激光脉冲使芘分子通过平移/旋转扩散粘附在一起。这可以解释由激光照射引起的玻璃基质的瞬时熔化。从ps到ns的时间分辨荧光测量表明，溶质分子在紫外脉冲的时间宽度内运动并形成准分子，准分子在解离前被冻结，这是由于准分子的瞬时熔化时间短，与紫外脉冲的熔化时间几乎相同，而芘准分子的寿命较长（~ 100 ns量级）。这种不同的时间尺度之间的聚集和解离可以解释的有效聚集的溶质通过紫外线照射在该系统中。

With a view to elucidating molecular dynamics in the nucleation process of organic crystals in solids, we have studied model systems, pyrene molecules in low temperature glassy matrices of mixture solvent consisting of isopentane and methylcyclohexane. In the series of experiments we demonstrated that irradiating several thousands of nanosecond laser pulses at 355 nm to the model system induced the efficient aggregation of the solute in the solid matrices even at 77 K. During the crystallization induced by successive UV irradiation, an emission peak that could be attributed to the excimer of pyrene increased in intensity with an increase in the shot-number of the UV pulse, while the excimer emission was not detected without laser irradiation. The excimer emission appeared even by single shot irradition of the UV pulse, which indicates that the single shot of the laser pulse enabled pyrene molecules to stick together by translational/rotational diffusion. This can be explained by transient melting of the glassy matrices induced by the laser irradiation. Time-resolved fluorescence measurements from ps to ns region revealed that the solute molecules moved and formed excimer within the time-width of the UV pulse; the excimers were frozen before dissociation because of short duration of the transient melting almost same as that of the UV pulse and longer lifetime of pyrene excimer (〜 100 ns order). This different time-scale between aggregation and dissociation can explain the efficient aggregation of the solute by UV irradiation in this system.