Aniline Resonant Two-photon Ionization Spectroscopy And Calculation

Matter is composed of molecules and atoms, which are also the transition bridge of material structure from the micro to the macro. Molecules and atoms are all closely related to information, materials, life sciences and medicine, and play an important role in these applications, so it is significant to research the material structure and spectral characteristics at the microscopic level. Researching on spectroscopy is one of the important ways to understand the structure of the molecule. Laser multi-photon ionization technique is a relatively simple method to obtain molecular structure and research the interaction between laser and the matter. The process of research to the matter is usually carried out from simple to complex. Aniline is the simplest kind of aromatic amines. Studying the molecular structure of aniline and its spectrum can systematically master the method of researching material structure, and have great significance to those more complex matters.In this paper, pulsed molecular beam technology combined with multi-photon ionization technique has been carried out to get the resonant two-photon ionization spectrum of aniline from experiment. Combined with theoretical calculations, the structural information of aniline molecule on the ground state and the first electronic excited state are analyzed.(1) In this calculation, the information about the molecular structure of aniline is obtained, and helped to analyze spectral data coming from experiments. Ab initio calculations and density functional theory (DFT) method are performed using Gaussian03 program. The ground state and excited states of aniline molecule structure are optimized, and get its stable structure and frequency analysis. Calculated excited energy, ionization energy, vibrational frequencies and other relevant data, and analyzed molecular orbital and natural bond orbital. Theoretically discussed the electron transfer process of aniline that from the ground state to the first electronic excited state, and the result indicates that the exciting of one electron from the lone pair orbital of nitrogen atom to the anti-bondingπorbital of the aromatic ring corresponding to the S1←S0 transition.(2) The first electronic excited state of the majority of aromatic molecules in the ultraviolet region, so use in 200-400nm range of UV laser can achieve molecular resonance excitation and ionization. First, the molecule absorbing a single photon has been excited to the electronic resonance excited state, then molecule absorbing another photon and is ionized. The resonant two-photon ionization (R2PI) spectrum of aniline has been measured by supersonic molecular technique with time-of-flight mass spectrometer in the laser wavelength range of 287 to 296 nm. The original band for S1←S0 transition appears at 293.86 nm (34029 cm-1) and some vibrational modes of the S1 state are observed. For giving an assignment to the vibrational modes in S1 state, ab initio calculations at HF/6-31+G(d, p) and CIS/6-31+G(d, p) basis set level were carried out for the geometric structure optimization and vibrational frequency analysis in S0 state and S1 state, respectively. Combined of calculation and experiment data, most bands appearing in the spectrum are assigned and given a description of the vibration modes.