1-萘酚的紫外共振双光子电离光谱

UV Resonant Two-Photon Ionization Spectrum of 1-Naphthol

利用脉冲分子束技术,在305-322nm范围内研究了1-萘酚(1NP)的共振双光子电离(R2PI)光谱.1NP分子存在cis和trans两种旋转异构体,但实验中仅观测到trans异构体的电子振动跃迁光谱,其S1←S0跃迁的(0-0)带头出现在317.90nm(即31456cm-1)位置.利用光谱选律及ab initio和密度泛函(DFT)计算,对trans异构体在S1态的振动模进行标识,得出主要对应于对称性为a′的平面内振动模.计算显示,cis异构体在电子基态S0的能量较trans异构体高出439cm-1,而第一激发能却比trans异构体的低1216cm-1,与之相应的实验值分别是220和274cm-1.计算数值与实验结果在能量变化趋势上完全一致.共振双光子电离谱中没有观测到cis异构体的光谱信号,其原因可归结为分子束的有效冷却效应使得处于基态的cis异构体的布居数密度相对trans异构体极低,导致cis光谱信号太小而未能被探测到.

The resonant two-photon ionization（R2PI） spectrum of 1-naphthol（1NP） was studied from 305 to 322 nm using supersonic molecular beam technique.1NP has two rotamers：cis and trans.The R2PI spectrum was used to characterize the S1←S0 transition of the trans species and its original band appeared at 317.8 nm（31456 cm^-1）.The vibrational bands of the trans species from the R2PI spectrum can be assigned to in-plane vibrational modes with a′ symmetry.Ab initio and density functional theory（DFT） calculations indicate that the cis rotamer has higher energy than the trans rotamer by 439 cm-1 in the ground state but it has a lower exciting energy than the trans rotamer by 1216 cm-1 for the S1 ←S0 transition.These calculated results are in good agreement with the experimental trend where the difference between the cis and trans energies in the S0 state is 220 cm-1 and the difference between the cis and trans exciting energies is 274 cm-1.There are no spectral features from the cis rotamer in the R2PI spectrum and we attribute this to the efficient cooling conditions in the molecular beam,which leads to a very small cis rotamer population with the higher energy（220 cm^-1）.