Raman Spectroscopic Investigation of Solid Samples Using a Low-Repetition-Rate Pulsed Nd:YAG Laser as the Excitation Source

Abstract

We tried to investigate the possibility of using a low-repetition-rate pulsed Nd:YAG laser as an excitation source in Raman measurements for solid samples. Based on the results from the Raman spectra excited by continuous wave (CW) 532 and 325 nm lasers, we studied the influence of laser energy and irradiation time of 532 and 355 nm pulsed Nd:YAG lasers (10 Hz repetition rate) on the thermal stability of (NH₄)₆Mo₇O₂₄·4H₂O, NH₄VO₃, and Ce(NO₃)₃·6H₂O samples, which usually decompose at relatively low temperatures. It is observed that the heating temperature estimated at these samples caused by the irradiation of 532 nm pulsed laser with 22 mJ is no higher than 100 °C even for 60 min exposure. The 355 nm pulsed laser with energies below 8.0 mJ hardly causes thermal damage to hydrated (NH₄)₆Mo₇O₂₄ and hydrated Ce(NO₃)₃ samples. However, a 355 nm pulsed laser with only 2.2mJ causes heating temperatures as high as 200 °C in the NH₄VO₃ sample. These great differences should be attributed to the electronic absorbance of the above three samples at 355 nm. We also found that a 532 nm pulsed laser with even 22 mJ and a 355 nm pulsed laser with even 8.0 mJ do not cause the phase transition of TiO₂ and ZrO₂, whose phase transformation easily takes place at elevated temperatures, but pulsed lasers could remove some oxygen atoms from these samples. In addition, for L-alanine and DL-β-phenylalanine biological samples, it is surprisingly found that they are not damaged by the 355 nm pulsed laser even when the laser energy is increased to 8.0 mJ, possibly because they do not absorb the 355 nm laser. Based on these results, it is demonstrated that low-repetition-rate pulsed lasers with appropriate wavelength and energy can be employed as the excitation sources of Raman spectroscopy for characterizing some solid samples, even the thermally unstable samples.