Keywords: 
• 对甲氧基苯甲腈 /
• 共振多光子电离 /
• 振动模式 /
• 基频和泛频

Abstract: p-methoxybenzonitrile is an important chemical and industrial material which has been widely used in many fields, such as medicine, chemistry, photoelectron, etc. In this paper, we use the technologies of supersonic molecular beam, resonance enhanced multiphoton ionization and time-of-flight mass spectrometer to obtain the high-resolution one color resonance two-photon ionization spectra of p-methoxybenzonitrile in a vibrational wavenumber range of 0–2400 cm?1. In order to analyze the experimental results, the theoretical calculations are performed. The molecular structure, energy, and vibration frequencies at the electronic excited state S1 are computed with time-dependent density functional theory at the level of B3PW91/6-311 g++**. According to the calculated results, the observed bands are assigned by the method of Varsanyi and Szoke. The band origin of the S1 ← S0 electronic transition of p-methoxybenzonitrile is determined to be (35549 ± 2) cm?1. A lot of vibrational bands of the electronic excited state S1 are observed. The results show that the vibrational modes of 9b, 6b, 15 and 1 are very easy to activate in a wavenumber range of 0–800 cm?1. There are also a lot of intense bands in a wavenumber range of 800–1600 cm?1. In addition to the fundamental vibrations, many combined vibrations between breathing and other fundamental vibrations are found. Several vibrations in this range are located at OCH3 and CN group. Most of the bands in a range of 1600–2400 cm?1 correspond to ones in the range of 800–1600 cm?1. Except for the bands appearing at 1664 and 2156 cm?1, which are assigned to 15101310 andν(CN) (CN stretching) respectively, the remaining bands in the range of 1600–2400 cm?1 are assigned as the combined vibrations between the breathing and the corresponding modes in the range of 800–1600 cm?1, i.e., the combined vibrations between the breathing overtone and other fundamental modes. Our theoretical calculations show that except for CN stretching vibration at 2162 cm?1, there is no fundamental frequency in a range of 1600–3000 cm?1, which is consistent with our experimental result and assignment. The fundamental of the breathing vibration 11 and its second overtone vibration 12 are very strong. The third overtone frequency 13 can be identified unambiguously. This is an important characteristic of p-methoxybenzonitrile, which is different from that of the usual polyatomic molecule. These results provide important reference for future researches on Rydberg states, chemical kinetics and zero kinetic energy spectroscopy of p-methoxybenzonitrile.