Infrared spectroscopy of the solid phases of ammonia

Abstract

Thin films of solid ammonia (NH(3) and ND(3)) have been characterized using low temperature (25-110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the nu(2) and nu(3) regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time.

An infrared spectroscopy study of the phase transition in solid ammonia

“…Further, our experiments do not show the existence of a metastable phase. Based on our results, we suggest that the spectrum of a metastable phase claimed by Holt and coworkers [21] probably arises from the cubic phase. The authors mentioned that their spectra did not change with temperature as soon as the sample was prepared.…”

“…We would like to stress that the spectra between 58 K and 84 K are very diverse from the spectra recorded between 10 K and 56 K. This suggests that the phase of solid ammonia does not change in the temperature interval from 58 K to 84 K. However, can this phase be attributed to a metastable or the cubic crystalline phase? In order to answer that question, we kept solid ammonia at 84 K for 3 h. According to previous studies, solid ammonia undergoes a transition to the cubic crystalline phase at a temperature above 80 K [21]. We recooled the sample to 10 K at 1 K min À1 ; hereafter, we warmed it up back to 84 K. Again, no significant change has been found during the cooling and warming cycle from 84 K to 10 K and back from 10 K to 84 K. We also warmed up the solid ammonia sample to 100 K. The spectrum at 100 K is almost the same as those recorded at 84 K and 58 K -except that the intensity dropped slightly due to the sublimation at elevated temperatures.…”

“…However, the exact temperature of the transition from the amorphous structure to the cubic crystalline phase is still unknown. No reproducible spectrum of a potentially metastable ammonia phase can be found in the literature [17][18][19][20][21]. Based on the previous experience about water contamination [16], to minimize the impurities might be very crucial for addressing this problem.…”

OPTICAL CONSTANTS OF NH3AND NH3:N2AMORPHOUS ICES IN THE NEAR-INFRARED AND MID-INFRARED REGIONS

“…The first study was conducted by Reding & Hornig (1951), who recorded and assigned the MIR spectrum of crystalline ammonia at 83 K. In the 1970s and 1980s, other works reported IR spectra of the different phases of ammonia (Robertson et al 1975;Pipes et al 1978). Some discrepancies in the assignments of the IR spectra to different phases are still open to debate, as revealed in the works of Holt et al (2004), Zheng & Kaiser (2007), and Dawes et al (2007).…”

“…In this case, when the substrate temperature is below 50 K, an amorphous phase is formed, while a crystalline phase is obtained at higher temperatures (above 80 K). A metastable phase that grows between these temperatures has also been reported by Holt et al (2004). The IR spectrum of ammonia ice has been investigated by a number of research groups.…”