# Terahertz Raman Measurements Using a Spatial Heterodyne Raman Spectrometer
## 4 Results and Discussion
### THz Raman Spectra of Organic Acids  
Benzoic acid (BA) is a kind of organic acid. It is a flake- or needle-like crystalline powder with the smell of benzene or formaldehyde. It is directly connected with the carbon atom of a benzene ring by a carboxyl group. It can be used in medical treatments, fuel carriers, plasticizers, perfumes, and preservatives. The THz Raman spectrum of BA is shown in Figure 5a, where the Raman peaks at 29 cm\({}^{-1}\) (0.870 THz) and 93.17 cm\({}^{-1}\) (2.795 THz) belong to translational vibration of A\({}_{\rm g}\), the Raman peaks at 48.74 cm\({}^{-1}\) (1.462 THz) and  
Figure 4: (**a**) THz Raman spectra of sulfur at the same integration time and different powers. (**b**) SNR of sulfur under different laser powers and integration times.  
\(117.8\) cm\({}^{-1}\) (3.534 THz) belong to directional vibration of B\({}_{\text{g}}\), and the Raman peak at 73.4 cm\({}^{-1}\) (2.202 THz) belongs to directional vibration of A\({}_{\text{g}}\)[22].  
Salicylic acid (2-hydroxy BA) is a white crystalline powder that is similar to BA in structure. It can be used in perfumes, medicines, and pesticides. Figure 5b shows the THz Raman spectrum of 2-hydroxy BA, which is significantly different from Figure 5a, indicating that the measurement system performs well in distinguishing chemical structures of similar substances.  
Figure 6 is the spectrum of 2-hydroxy BA at different times (from 10:30 to 16:30 at 30 min intervals). Under the same power and integration time, the signal position and intensity of the Raman spectrum of 2-hydroxy BA obtained at different times have almost no change, which proves the stability of the system.  
In addition, gallic acid was placed in different containers (plastic bags, plastic bottles, and glass bottles) to detect the influence of the container on the measurement. The final results are shown in Figure 7, where the thickness of the plastic bags is 0.02 mm, the  
Figure 5: (a) THz Raman spectra of BA at a laser power of 50 mW with an integration time of 50 s. (b) THz Raman spectra of 2-hydroxy BA at a laser power of 50 mW with an integration time of 50 s.  
Figure 6: Spectra of 2-hydroxy BA at different times with a laser power of 50 mW and integration time of 20 s.  
thickness of the plastic bottles is 0.1 mm, and the thickness of the glass bottles is 0.2 mm. The Raman peak intensity of gallic acid in plastic bags is the highest, and the vibration spectrum can be clearly distinguished. The signal intensity of gallic acid in plastic bottles and glass bottles is weaker, and the peak intensity at 114.7 cm\({}^{-1}\) (3.441 THz) is obviously weaker. The results show that the reflection of laser from plastic bottle and glass bottle weakens the Raman signal and affects the measurement results. The influence of containers with different thickness on the measurement results is different. The results of measurement with thin plastic bags as containers are the best, indicating that the spatial heterodyne THz Raman spectroscopy system can be applied to the detection of thin plastic bag packaging samples. This widens the application prospects of THz Raman spectroscopy.