## 1 Introduction  
Due to the high-speed economic development and energy shortage, petroleum gradually occupies an important role in modern society. The total output of petroleum is about 4.45 billion tons across the world nowadays, and it has come to be used for more and more fields [1, 2]. In the process of exploitation, transportation, and production of petroleum, a large amount of petroleum will be spilled into the surrounding soils. Petroleum in soils has a significant impact on health through the bioaccumulation in the food chain [3]. This problem of environmental pollution brings huge challenges to the sustainable development of our economy. Therefore the study of rapid and in-site detection technique of petroleum pollutions in soils has become crucial to environmental monitoring. Molecules are useful platforms to detect phenomena at quantum-projection-limited precision [4]. In the molecular techniques, laser induced fluorescence (LIF) spectroscopy technique is a rapid and sensitive method for the analysis of organic materials based on their characteristics of fluorescence spectra emitted by fluorophores [5]. The macromolecular organic compounds in petroleum hydrocarbons, benzene series in polycyclic aromatic hydrocarbons and organophosphorus in pesticides can emit fluorescence under ultraviolet pulse laser irradiation [6]. The fluorescence spectra can be used to analyze the structure and characteristics of materials. This technique has been widely applied in a variety of fields in recent years, such as biological detection, environmental detection, energy exploration, petrochemical production, etc. [7; 8; 9]. Recently, many researchers have focused on the application of LIF in organic constituents and quantitative analysis [10; 11; 12]. [PERSON] et al. [13] used an airborne LIF instrument for measuring the gas-phase formaldehyde, and the accuracy of quantitative analysis for reporting mixing ratios was raised to \(\pm 10\%\). [PERSON] et al. [14] used LIF technique to detect coral bleaching by researching the variations of laser induced fluorescence spectral signatures in coral and proved that the average fluorescence intensity ratio values were different at different temperature. [PERSON] et al. [15] utilized LIF technique as a tool for imaging the flame of sulfur dioxide and visualized the development of the early flame kernel by the combination of SO2- and OH-PLIF (Planar Laser Induced Fluorescence) setup. [PERSON] et al. [16] focused on the use of LIF technique to quantitative analyze the preferential evaporation of fuel at engine relevant conditions. However, due to the complicated matrixes of soils, it is hard to use the same calibration curve to predict the concentrations of diesel oil in soils. The soils near the gas stations or on both sides of the roads are most vulnerable to be polluted by petroleum in their transportation or on traffic accidents. The physical characteristics of the above-mentioned soils are quite unified and the contents of soil moisture are relatively low, the diffusion of petroleum in soils can be assumed as symmetrical, therefore the content distribution of petroleum can be calculated by the fluorescence detection of the surface and one vertical section. To date, the LIF technique has already been used as a measuring tool for content distributions of materials [17; 18]. [PERSON] et al. [19] used LIF to analyze the distribution law of polynuclear aromatic hydrocarbons in radial direction from the central axis at the processes of soot formation. [PERSON] et al. [20] applied LIF to investigate the combustion efficiency and combustion phasing of fuel in different zones of an engine. The objective of this paper is to obtain the diffusion regularity of diesel oil in soils, we report the horizontal and vertical diffusion laws of diesel oil in different humidity soils by the application of LIF technique. The effects of soil moisture are discussed.