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

为了扩展拉曼光谱在固体表面吸附分子研究中的应用能力，我们建立了一套脉冲紫外（UV）激发的拉曼光谱系统。通过使用紫外光激发，我们利用了光散射中众所周知的ω S ^4效应，以及许多有机分子在紫外光范围内的电子跃迁引起的共振增强效应。作为第一个实验，我们测量了硝基苯在Ni（111）上的吸附光谱。在<-1>266 nm波长下，当平均入射功率为-0.6mW时，我们可以检测到硝基苯分子两个单分子膜的NO_2对称伸缩模（1346 cm ~ 2）的拉曼信号。(The平均入射功率密度为-20 mW/cm ^2，从样品表面0.03 cm ^2的面积收集拉曼散射光。）相对于488 nm的入射波长，266 nm的入射波长的拉曼散射截面增强了5*10^3倍。这种增强部分是由于众所周知的ω ^4因子，部分是由于在250 nm左右的紫外范围内的电子共振。灵敏度的限制来自于这样一个事实，即入射激光的功率密度不能增加，至少使用脉冲激光，超过我们所使用的水平，而不会对样品表面造成损害。

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.