Research Article Vol. 29, No. 21/11

Research Article Vol. 29, No. 21/11 Oct 2021 / Optics Express 33240

OptiCS EXPRESS

299 К d

336 К X (a)


3315 3320 3325 3330 3335 3340 3345


Raman shift /cm’¹


Raman shift /cm’¹


Raman shift /cm’¹


Fig. 4. Detailed structure of mode ranging from 299 К to 760 К

increasing the temperature from 299 К to 760 K. To help the understanding of this shift, we referred to the infrared spectra of NH3 [28], and labeled the line position of transitions of (3,3) and (6,6) states in Fig. 4(a). The quantum numbers (J, K) of the initial states are labeled on the top, whereas the final states are on the bottom. Therefore, we attribute the blue shift of main Raman peaks to the increasing population of the states with higher rotational quantum number J. Under high temperature, the intensity of the two wings of the vi band also increase. Figure 4(b), shows the detailed structure of the left wing of yj mode under different temperatures. Because of the strongly blended lines, the peak structures resolved here are difficult to be assigned to specific transitions. According to the line list provided by TROVE program [29] and the hot ammonia infrared spectra in the BYTE database [30], the left side wing spectra overlap with branch transitions of high J states. The probable transitions with the wavelength near the peak structure are labeled on Fig. 4(b). The high temperature spectra indicate that, the Raman intensity of high J states increase with the temperature. As presented in Fig. 4(c), the right side wing also increases with the temperature. The relative intensity of the right wing is lower than the left side. The line blending is more serious, under high temperature, the transition peaks can barely be resolved. We cannot assign these transitions to the spectral peaks without the knowledge of the polarizability of NH3.

The detailed high temperature Raman spectra of the overtone of У4 mode are presented in Fig. 5(a). The strong lines of 2У4 mode are relatively sparse when compared to the yj mode. For the lack of available theoretical Raman spectra of the 2у4 band, it is hard to assign these peak structures in the 2У4 band to specific transitions. Nevertheless, the temperature behavior of the 2у₄ band can be resolved in Fig. 5(a). The Raman peaks labeled as "A" type are sensitive to the temperature and decay rapidly, whereas, the intensity of the "B" type peaks are insensitive