Laboratory production of magnesium sulfide grains and their characteristic infrared spectra due to shape

¹ Department of Physics, Ritsumeikan University, Kusatsu-shi, Shiga 525-8577, Japan e-mail: rp092955@se.ritsumei.ac.jp
² Laboratory of Physics, Kyoto Pharmaceutical University, Misasagi, Yamashina, Kyoto 607-8414, Japan
³ Department of Earth and Space Science, Osaka University, 1-1, Machikaneyama, Toyonaka, Osaka 560-0043, Japan

Received: 24 January 2005
Accepted: 13 May 2005

Abstract

Nanosized MgS grains, which have been considered the origin of the 30 μm emission feature of carbon-rich evolved objects, were produced from the gas phase using an advanced gas evaporation method. The far-infrared spectrum of cubic MgS grains showed a characteristic absorption peak at 311 cm^-1 (32.1 μm) with three shoulders at 460, 400 and 262 cm^-1 (21.7, 25.0 and 38.2 μm). On the other hand, when the grains were roundish or network-like, the absorption peak at 250 cm^-1 became predominant. The cubic MgS grains were produced by direct nucleation from the gas phase. In the case of production via a gas-solid reaction, the MgS grains were network-like. Therefore, the formation environments of MgS grains around carbon-rich evolved objects may be predicted from the intensity of 310 and 250 cm^-1 bands. We suggest that the origins of the absorption band at 310 and 250 cm^-1 are (100), (110) and/or (111) surfaces of MgS grains, respectively.