3620 Langmuir 1995,11, 3620-3622 Effects of Adsorption and Reaction on the Second Harmonic Generation of Langmuir-Blodgett Filmst Xinsheng Zhao,* Jianhua Xing, Peng Li, Xiaoming Xie, and Xiaohua Xia Department of Chemistry and Znstitute of Physical Chemistry, Peking University, Beijing 100871, China Hui Li and Chunhui Huang Research Center of Rare Earth Chemistry, Peking University, Beijing 100871, China Tiankai Li and Lingge Xu Znstitute of Photographic Chemistry, Academia Sinica, Beijing 10082, China Received November 29, 1994. In Final Form: August 2, 1995@ The effect of the adsorption and reaction of HC1 and N(CzH& on the second harmonic generation (SHG) of Langmuir-Blodgett (LB) films made of nonlinear optical materials was studied. It was found that introduction ofHCl or N(CzH& gases separately greatly reduced the SHG ofthe LB films. This is attributed as due to the modification of the film molecules by the acid or base. However, when the two of gases subsequently adsorbed, the SHG signal recovered, and in some cases it was several fold of magnitude higher than that from the original film. Both UV-vis spectroscopy and image of scanning electron microscopy supported the idea that a thin uniform layer of salt was formed on the matrix of the LB film molecules. Second order nonlinear optical susceptibilities of films are some of the important properties that have been studied extensively in recent years.l Among the methods of film formation, the Langmuir-Blodgett (LB) method is one ofthe convenient research tools to investigate second order nonlinear optical materials. In the literature, the second order nonlinear optical properties were all char- acterized in air. However, in applications the material may be put in various kinds of atmosphere. Recently, we reported the observation of variations of the second harmonic generation (SHG) with the chemical environ- ment of the LB films made of hemicyanide dyes2 In this Letter, we report a new phenomenon that the acid-base reaction on the LB film can enhance the SHG of the film and that the deposit of the salt product on the film is uniform, which may provide a new way for film formation or manipulation. Three film materials shown in Figure 1 with high molecular hyperpolarizabilities on the order of esu were chosen to prepare the LB films.3 The synthesis of the materials and the film formation followed the de- scription in ref 3. The apparatus is shown in Figure 2. SHG by 1.064 pm radiation from a Nd:YAG laser at monolayer LB film with an incident angle of 45" was measured with standard transmission mode4 except that the LB film was in a gas cell. The outcome beam after the fundamental frequency was filtered out was focused down onto the slit of a monochromator. The SHG signal was detected by a Hamamatsu R955 PMT and recorded and averaged by an EG&G 4100 boxcar averager or an HP * E-mail: zhaoxs@sun.ihep.ac.cn. Supported by NSFC and by the Fok Ying Tung Education @ Abstract published in Advance ACS Abstracts, September 1, (1) See, for example, Garito, A.; Shi, R. F.; Wu, M. Phys. Today 1994, (2) Zhao,X.; Xie,X.;Xia,X.; Li, H.; Wang, K.; Huang, C.;Xu, L.;Tian, (3) Li, H.; Huang, H.; Xu, G.; Xu, L.; Li, T.; Zhao, X.; Xie, X. Prog. Foundation. 1995. 47 (51, 51. K. Thin Solid Films, in press. Nut. Sei., in press. Li, T. Lengmuir 1994, 10, 1910. (4) Zhou, D.; Huang, C.; Wang, K.; Xu, G.; Zhao, X.; Xie, X.; Xu, L.; 0743-74631951241 1-3620$09.00/0 A: I M a m N = N--@N-Ci~H371 CiePMP: O N "=?- CH3 'C-C-C-Ci5H3r I1 - II 0 0 Figure 1. Structure of the film materials labeled A, B, and C in this paper. The arrow indicates the amino nitrogen which is most likely first protonated in acid environment. 1 16 Figure 2. Experimental setup: 1, laser; 2, prism; 3, photodiode; 4, polarization rotator; 5, Glan-Taylor prism; 6, RG850 filter; 7, reaction cell; 8, LB film; 9, Baratron; 10, total reflector of the fundamental wave; 11, BG40 filter; 12, focal lens; 13, neutral density filter; 14, monochromator; 15, PMT; 16, signal processor and PC computer; 17, valve; 18, needle valve; 19, sample flask. 54510A digitizing oscilloscope. A typical laser energy was 4 mJ/pulse with a pulse duration of 6 ns. The cell was 0 1995 American Chemical Society