Development of Molecular Beam Double Resonance Spectroscopy by Use of Evanescent Wave and Study of Molecule-Solid Surface Interactions

我们利用分子束光谱和倏逝波光谱研究了气相中固体表面与分子的相互作用，并将超声分子束技术与红外双共振光谱技术相结合，测量了分子特定振转态之间碰撞诱导跃迁的几率。首先，用分子束双共振方法测量了NH_3分子与非极性靶分子偶极-四极相互作用的态-态转动跃迁截面。我们的实验结果与碰撞理论的非经验计算结果符合得很好，证明了我们的实验方法是正确的。然后，我们用这种方法观察了NH_3在固体表面（如玻璃、Al和Au）上散射的转动态的碰撞诱导布居数变化。结果表明，偶极跃迁中的选择定则对玻璃表面的碰撞诱导跃迁起主导作用。同时，我们研制了一台消逝波激光光谱仪，并在NaCl表面附近探测到了NH_3分子的旋振光谱。NH_3在表面附近的谱线比气相中的谱线宽约50%。线宽与表面温度的平方根成正比。这可能是由于与固体表面相互作用所引起的相位弛豫所导致的，今后我们将联合收割机与倏逝波光谱学相结合，进一步研究固体表面的散射过程。

We have carried out molecular beam spectroscopy and evanescent wave spectroscopy to study interactions between solid surface and molecules in the gas phase.Combination of the supersonic molecular beam technique and the infrared double resonance spectroscopy enables measurements of the probability of collision induced transitions between specific vibration-rotation states of molecules. First, the molecular beam double resonance method was tested In the measurement of the state-to-state cross section of the rotational transitions in NH_3 caused by the dipole-quadrupole interaction with nonpolar target molecules. Our experimental procedure has been justified because the obtained cross sections agree quite well with those nonempirically calculated from a collision theory.Then, we applied the method to observe collision-induced population change in the rotational states of NH_3 scattered from solid surfaces, such as glass, Al, and Au. It has been found that the selection rule in the dipole transition dominates the collision induced transition on glass surface. On the other hand, the molecule makes a hard collision with metal surfaces, and there is no definite selection rule.Simultaneously, we have developed a evanescent-wave laser spectrometer, and detected rotation-vibration spectra of NH_3 molecules close to NaCl surface. The spectral line of NH_3 near the surface was broadened approximately by 50 %, compared with that in the gas phase. The line width was proportional to the square root of the surface temperature. This may be attributed to the phase relaxation induced by the interaction with the solid surface.In the future, we are going to combine the molecular beam method with the evanescent wave spectroscopy for further research of the scattering process on solid surface.