Raman Scattering from Surface Adsorbed Molecules Excited by Ultra-violet Laser Light

为了扩大拉曼光谱研究固体表面吸附分子的能力，我们建立了一套脉冲紫外光激发拉曼光谱系统。通过使用UV激发，我们利用了众所周知的光散射中的omegas^4效应，以及由于许多有机分子在UV范围内的电子跃迁而产生的共振增强。作为第一个实验，我们测量了硝基苯在Ni(111)面上的吸附光谱。当266 nm处的平均入射功率为-0.6 mW时，我们可以探测到NO2对称伸展模(1346 cm~(-1)~(-1))的拉曼信号。(平均入射功率密度为-20 mW/cm2，在样品表面0.03 cm2的面积上采集到拉曼散射光。)当入射波长为266 nm时，拉曼散射截面比入射波长为488 nm时的拉曼散射截面提高了5×10^3。这种增强部分归因于众所周知的omega^4因子，部分归因于在250 nm左右的紫外光范围内的电子共振。灵敏度的限制源于这样一个事实，即至少使用脉冲激光，入射激光的功率密度不能在不对样品表面造成损害的情况下增加到我们所使用的水平之外。

For the purpose of extending the capability of Raman spectroscopy in investigating adsorbed molecules on solid surfaces, we have constructed a Raman spectroscopy system with a pulsed ultraviolet (UV) excitation source. By using the UV excitation we take advantage of the well-known omegaS^4 effect in light scattering and of resonant enhancement due to electronic transitions of many organic molecules in the UV range. As a first test we have measured the spectra of nitrobenzene adsorbed on Ni (111). We could detect the Raman signal from the NO_2 symmetric stretchig mode (1346 cm^<-1>) of two monolayrs of nitrobenzene, when the average incident power was -0.6mW at 266nm. (The average power density of incidence was -20mW/cm^2, and Raman scattered light was collected from an area of 0.03cm^2 at the sample surface.) The Raman scattering cross-section for the incident wavelength of 266 nm is enhanced by a factor of 5*10^3 relative to that for the incident wavelength at 488 nm. This enhancement is due partly to the well-known omega^4-factor and partly to an electronic resonance in the UV range around 250 nm. The limitation in the sensitivity arises from the fact that the power density of the incident laser cannot be increased, at least using a pulsed laser, beyond the level we have used without causing damage to the sample surface.