Chi Hong Chio

Micro-Raman studies of gypsum in the temperature range between 9 K and 373 K

Abstract Raman spectra were collected for synthetic gypsum (CaSO4⋅2H2O) powder between 9 and 373 K under atmospheric pressure with special emphasis on the temperature dependence of the OH-stretching modes. The stretching bands of the water molecules in gypsum were found to shift in opposite directions as a result of the different degree of intermolecular hydrogen-bonding between nonequivalent water H atoms and the O atoms of nearby SO4 ions. The anharmonic parameters of the OH-stretching modes are calculated using the temperature derivatives measured from the present investigation and existing pressure derivatives. These parameters are -4.7 × 10-6 K-1 and -0.6 × 10-6 K-1 for the 3407 and 3494 cm-1 bands, respectively. The dehydration of gypsum into γ-CaSO4 and the subsequent rehydration of γ-CaSO4 into hemihydrate are clearly identified in the Raman spectra by the observed variation in Raman shifts of the OH and ν1(SO4) bands. The latter increases as the mineral becomes increasingly anhydrous (1007 cm-1 in gypsum; 1014 cm-1 in hemihydrate; 1026 cm-1 in γ-CaSO4), which can be used as a fingerprint for the remote detection of these minerals on planetary surfaces.

Raman spectroscopic investigation on Jarosite-Yavapaiite stability.

The stability of synthetic Jarosite (KFe(3)(SO(4))(2)(OH)(6)) at low temperature and reduced atmospheric pressure has been studied by Raman spectroscopy. Jarosite remains stable between 8 and 295 K, provided that the sample is not exposed to reduced atmospheric pressure. When exposed to reduced atmospheric pressure (2.0x10(-2) Torr), however, the conversion of Jarosite into a different mineral is readily detected at room temperature by the appearance of a new Raman peak. The Raman shift of this peak (1032 cm(-1)) matches with that of Yavapaiite (KFe(SO(4))(2)), which can be obtained by thermal decomposition of Jarosite above 473 K. These studies provide a better understanding of the stability of Jarosite subjected to conditions similar to that on the surface of Mars.

Effects of Particle Size and Laser-Induced Heating on the Raman Spectra of Alpha Quartz Grains

Raman spectra of α-quartz (Qz) grains of various size (250 μm to <11 μm) and arrangement (individual and aggregated) have been investigated with a combination of confocal Raman and micro-Raman systems. Frequency downshift and line broadening of the 464 cm−1, vs(Si–O–Si) band are observed in the smallest size group (<11 μm, both individual grains and aggregates) because of laser-induced heating and are used to estimate the temperature of the sampled region. The intensity ratio of the anti-Stokes to Stokes Raman lines is also used to estimate the vibrational temperature of the samples under different excitation power. The degree of laser-induced heating is more noticeable in the aggregates than in the individual grains with the use of medium-level laser excitation (≤150 mW). Heating diminishes with increasing grain size, and it can only be detected in grain aggregates between 11 and 20 μm in diameter using 150 mW excitation. Intensity studies of the vs(Si–O–Si) band using individual grains show no noticeable signs of grain size effects. However, grain size effects become an important factor in the study of aggregates in which spectral intensity diminishes with respect to decreasing grain size.

Stand-off Raman instrument for detection of bulk organic and inorganic compounds

We have designed and tested a portable stand-off gated-Raman system that is capable of detecting organic and inorganic bulk chemicals at stand-off distances to 100 m during day and night time. Utilizing a single 532 nm laser pulse (~25 mJ/pulse), Raman spectra of several organic and inorganic compounds have been measured with the portable Raman instrument at a distance of 10 m in a well-illuminated laboratory. Raman spectra, obtained during a very short period of time (2 micro second), from organic compounds such as acetone, benzene, cyclohexane, 2-propanol, naphthalene, and inorganic nitrates, showed all major bands required for unambiguous chemical identification. We have also measured the Raman spectra of acetone, sulfuric acid, hydrogen peroxide (50%) aqueous solution, nitro-methane containing fuel, and nitrobenzene in glass containers with a 532 nm, 20 Hz pulsed laser excitation and accumulated the spectra with 200 to 600 laser shots (10 to 30 sec integration time) at 100 m with good signal-to-background ratio. The results of these investigations show that the stand-off Raman spectra to 100 m distance can be used to identify Raman fingerprints of both inorganic and organic compounds and could be useful for Homeland security and environmental monitoring.

Daytime rapid detection of minerals and organics from 50 and 100 m distances using a remote Raman system

We have developed a remote Raman system, using an 8-in telescope and a 532-nm pulse laser (20 Hz and 20 mJ/pulse), which is capable of operating in daylight. From distances of 50 and 100 m and with an integration time of just 1 second (equivalent to 20 laser pulses at 20 Hz), good quality Raman spectra with high signal-to-noise ratios were readily obtained. The Raman system was also tested using only single-laser-pulse excitation (8 ns pulse width) with an integration time of 2 μs. The spectra obtained from single-laser-pulse excitation also show clear Raman features and can be used for rapid, unambiguous identification of various chemical substances. We successfully identified a number of substances, including organic chemicals (acetone, naphthalene, nitro-methane, nitro-benzene and cyclohexane); inorganic chemicals and minerals (nitric acids, sulfuric acid, potassium perchlorate, gypsum, ammonium nitrate, epsomite, melanterite, calcite and sulfur); and amino acids. The remote Raman system has a range of applications, such as environmental monitoring (e.g., detection of hazardous chemicals and chemical spills from a safe distance in real time) or homeland security (e.g., rapid identification of chemicals on a conveyor belt or from a fast-moving object).

High pressure raman spectroscopic study of low-cristobalite GaPO4

Gallium orthophosphate of low-cristobalite structure has been studied by Raman spectroscopy up to 28.5 GPa. Three phase transitions have been identified: 1.6, 3.1 and ~7 GPa. Raman spectra (200 to 700 cm-1) acquired above 3 GPa show signs of six-coordinated gallium ions, in agreement with earlier studies that the orthorhombic C2221 structure transforms into the orthorhombic Cmcm structure. As pressure increases above 8 GPa, the Raman peaks associated with the low-cristobalite framework structure diminish gradually and replaced by peaks that are possibly related to isolated phosphate units. The spectra also show that the structure of the quenched sample retains that of the high-pressure phase prior to decompression.