Characteristics of Raman spectrum from stand-off detection

Abstract

For developing a method to detect unknown or hazardous materials beyond safe distances, an experimental standoff detection system with using Raman scattering is established in laboratory. It consists of a pulsed laser with a wavelength of 532 nm as an excitation source, an optical assembly for light collecting and focusing with a 25 mm entrance aperture, a grating monochromator for dispersing scattering light, and a photomultiplier connected to an oscillograph for signal monitoring. The angle between the direction of incident laser beam and that of the scattering light collecting assembly is less than 2°. Raman signal intensities of ammonium nitrate, potassium nitrate and sodium nitrate in solid samples in a distance range from 2 m to 10 m are measured. The results are supposed to be comparable to those obtained in a distance range from 20 m to 100 m if a telescope of 250 mm diameter is used instead to collect Raman scattering light as in a usual standoff detection system. Some characteristics of Raman spectra are investigated, such as the spectrum features, the relationships between the amplitude of the highest Raman peak of ammonium nitrate and the intensity of the excitation light, the detection distance, the concentration of the sample and the normal direction of the sample surface. The Raman spectra of ammonium nitrate, potassium nitrate and sodium nitrate look similar: each of them has a highest peak in the vicinity of 1050 cm-1, small difference can be observed, and it can serve as a "signature" for discriminating between them. The experimental results demonstrate that the intensity of the characteristic Raman spectrum of ammonium nitrate is proportional to the excitation power, with approximate quadratic relationship, and tends to be inversely proportional to the square of the detection distance except that the detection distance is too short to ignore the influence of the focal length of light collecting optics on image size. In addition, the intensity of the characteristic Raman spectrum of ammonium nitrate decays approximately at an exponential rate with the decrease of its concentration. Finally, the intensity of the Raman signal of ammonium nitrate is approximately proportional to the cosine of the angle between the direction of the incident light and the surface normal. This relationship is similar to Lambert's cosine law that the radiant intensity observed from an ideal diffusely reflecting surface is directly proportional to the cosine of the angle. The last two phenomena imply that it may be particularly difficult to detect the substances of interest in a mixture on horizontal ground surface for Raman standoff detection system.