Optical second harmonic generation (SHG) signals from a cyanine dye during compression of a monolayer in a Langmuir-Blodgett trough were measured to investigate the mean tilt angle of the dye corresponding to the formation of an H-band. The pressure-surface area (π-A) isotherm for 3,3'-dioctadecyloxacarbocyanine molecules spread on an aqueous subphase showed a plateau region after a kink point in the isotherm characteristic, which suggests a phase transition from the liquid expanded (LE) phase to the liquid condensed (LC) phase. SHG signals from the monolayer were obtained as the surface area decreased for the input and output polarization combinations PP, MS, SP, PS, and SS, where the first and second letters denote the input and output polarizations, respectively, and M refers to the polarization midway between S and P. For polarization combination PP, the SHG signal increased steeply during compression, whereas for polarization combination MS, the signal increased gently. The SH field from the monolayer for polarization combination SP in the LC phase was almost inverted to the field in the LE phase. We estimated the mean polar angles from the SHG responses during compression. The angle decreased continuously by almost 5° from 28° with the growth of domains in the LC phase. Visible absorption spectra of monolayers transferred to glass substrates were also obtained. The area intensity of the peak in the spectra attributed to H-aggregates increased compared with that of the monomer as the surface area decreased in the LC phase. The results were consistent with the polar angle reduction in the LC phase observed by in situ SHG. The decrease of the angle in the LC phase probably allowed the cyanine dyes to pack together tightly and align side by side, promoting H-aggregation.

测量在Langmuir-Blodgett槽中单层压缩期间来自花青染料的光学二次谐波生成（SHG）信号，以研究该染料对应于H带形成的平均倾斜角。在水等温相上散布的3,3'-二十八烷基氧杂碳菁花青素分子的压力表面积（π-A）等温线在等温线特征的纽结点后显示出平稳区域，这表明从液相膨胀（LE）相发生了相变到液相冷凝（LC）相。输入和输出偏振组合PP，MS，SP，PS和SS的表面积减小时，从单层获得SHG信号，其中第一个和第二个字母分别表示输入和输出偏振，M表示S和P之间的极化。对于偏振组合PP，SHG信号在压缩过程中急剧增加，而对于偏振组合MS，信号则逐渐增加。LC相中来自偏振组合SP单层的SH场几乎反转为LE相中的场。我们从压缩过程中的SHG响应估计了平均极角。随着LC相域的增长，该角度从28°连续减小了近5°。还获得了转移到玻璃基板上的单层的可见吸收光谱。与LC相比，归因于H聚集体的光谱中峰的面积强度与单体的强度相比有所增加，这是因为LC相中的表面积减小了。结果与通过原位SHG观察到的LC相中的极角减小是一致的。