The cyano group is a typical electron withdrawing group, which has attracted the interest of related researchers. Many papers reported the dispersed fluorescence spectra of o-hydroxybenzonitrile, its dimers, and complexes with small molecules, the main purpose of which is to study the intermolecule hydrogen bond and the vibration features of the electronic ground state. There are also reports using fluorescence excitation spectra to study excited state vibrations, but it is a lack of related work to systematically analyze the vibration features of excited state spectra. Compared with fluorescence spectroscopy, resonance enhanced multiphoton ionization (REMPI) spectroscopy detects ions to obtain excited state energy level information, which has mass resolution capability, and eliminates the interference of impurities with different charge-to-mass ratios. The strong electron-withdrawing ability of cyano group results in higher ionization energy for molecules containing cyano groups. Many REMPI experiments of benzonitrile derivatives require two-color lasers. In this paper, two-color resonance enhanced two-photon ionization experiment was performed on a home-made linear time-of-flight mass spectrometer, and the vibration-resolved REMPI spectrum of o-hydroxybenzonitrile was obtained for the first time. Combined with high-precision density functional theory calculations and Franck-Condon spectral simulations, the spectral characteristics were analyzed in detail, and a large number of fundamental, overtone and combined vibrations were found. The spectral assignment was carried out as accurately as possible. Most of the fundamental vibrations located at ring are assigned to the in-plane distortion or swing of the ring, which is related to the expansion of the ring during the molecular excitation. Theoretical and experimental results show that the low-frequency signal of REMPI spectrum is strong, the background is low, the band is less, and the resolution is good. As the vibration frequency increases, the signal changes in the worse direction. This is because the low-frequency spectrum mainly comes from the low-frequency fundamental vibrations and the contribution of a small amount of overtones. As the vibration frequency increases, the quantity of overtone and combined vibrations gradually increases, resulting in dense bands and low resolution. Theoretical calculations show that the high-order vibration and combination of multi-mode vibration usually have a lower Franck-Condon factor, so the signal gradually becomes weak as the frequency increases, and the signal-to-noise ratio becomes worse.
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