多聚甲醛和四聚乙醛的太赫兹光谱吸收特征测量

Absorptive feature testing measurements of paraformaldehyde and metaldehyde with the terahertz spectroscopy

利用太赫兹时域光谱技术在24℃、湿度小于10%RH的干燥空气条件下,对不同质量浓度的多聚甲醛和四聚乙醛与高纯聚乙烯混合物的压片样品进行测量,得到其在太赫兹频段的特征吸收光谱。在0.1~2.5 THz范围内,多聚甲醛出现了2个微弱的吸收峰,分别位于1.20 THz和1.66 THz处;四聚乙醛只出现了1个比较明显的吸收峰,位于1.83 THz处。两者的吸收系数变化特征存在显著差异,可将其作为太赫兹指纹谱进行物质成分鉴别的依据。另外,通过进一步试验得到了多聚甲醛和四聚乙醛样品的平均吸收系数与质量浓度的关系式。

The given paper intends to report its exploration and achievement of the characteristic absorption spectra of paraformaldehyde and metaldehyde in the terahertz frequency region and the mutual relationship between the average absorption coefficients and the mass concentrations by using the terahertz spectroscopy.In this paper,we have prepared the testing case study samples from the paraformaldehyde or metaldehyde powder mixed with highly pure polyethylene. At the same time,we have also conducted the measurement of the terahertz time-domain waveforms of the tablets with the different mass concentrations of the dry air at24 ℃ by using terahertz time-domain spectroscopy（ THz-TDS）.In order to reduce the absorption of terahertz wave by water vapor in the air and the background noise,we have managed to control the humidity within the range of 10% RH. For the aforementioned purpose,we have first of all tried to show the terahertz time-domain waveforms passing through the measured specimen and their respective reference background status-in-situ,for there does exist time delay and amplitude reduction in comparison with the reference signs. And,then,we have done an analysis of the terahertz frequency domain spectra of the samples and their reference signals from 0 to 2. 5 THz via Fourier Transform,in which the refractive index and the absorption coefficients of the two aldehydes have been worked out through formula models. For instance,from 0. 1 THz to 2. 5 THz,there seem to appear two weak absorption frequency peaks at 1. 20 THz and 1. 66 THz in the absorption spectra of paraformaldehyde,whereas only one distinct absorption peak emerges at 1. 83 THz for metaldehyde. This must imply that there exist some significant differences between the variation of the absorption coefficients of paraformaldehyde and those of metaldehyde,which can be taken as the terahertz＂fingerprint＂spectrum to identify such like substances. Furthermore,we have also investigated the relationship between the average absorption coefficients and the mass concentrations to disclose the average absorption coefficient-mass concentration diagram of paraformaldehyde and metaldehyde. In contrast and comparison between the two,we can find that,for the said paraformaldehyde,there exists a liner relationship between the average absorption coefficient and the natural logarithm of mass concentration,whereas,for metaldehyde,there occurs a simpler liner relationship between the average absorption coefficient and the mass concentration. It is just due to the aforementioned different characteristic features of the terahertz absorption between paraformaldehyde and metaldehyde that it would be possible as well as necessary to apply the terahertz time-domain technique to the qualitative and quantitative detection of the two aldehydes so as to reduce the unpredictable hazards likely to be brought about by the two substances.