Pulsed laser Raman spectroscopy in the laser-heated diamond anvil cell

Abstract

We describe the design and operation of a spatially-filtered Raman/fluorescence spectrometer that incorporates a pulsed 532 nm laser excitation source and a synchronized and electronically gated CCD detector. This system permits the suppression of undesired continuous radiation from various sources by a factor of up to 50,000 providing the possibility of acquiring Raman signals at temperatures exceeding 5,000 K.We present performance comparisons of this system with that of a state-of-the-art conventional CW system using a 458 nm excitation source. We also demonstrate that the pulsed system is capable of suppressing an impurity-induced (single nitrogen defects) fluorescence in diamond, and further suggest that this capability can be used to suppress the stress-induced fluorescence in diamond that may appear at pressures near or above 150 GPa.Raman spectroscopy in combination with the DAC is an extremely versatile experimental tool. It is used to investigate the behavior under extreme pressure of many different material properties including vibrational, structural, electronic and elastic [1][2][3][4][5][6][7]. The results of these studies find application in various diverse disciplines including the geoand planetary sciences [1,2,4,5], fundamental physics and chemistry [2,6], and material science [7,8]. Under simultaneous conditions of extreme temperature further information concerning phase changes, chemical kinetics, superconductivity etc. may be obtained. Since the processes described above have very different characteristic time-scales, there is the possibility to separate them using various gated techniques. A technique that has previously been used includes the use of a pair of rotating chopper wheels synchronized to occlude short-lived radiation including the excitation laser light and short-lived fluorescence. This approach has been used to suppress diamond fluorescence in order to permit measurements of ruby fluorescence (the basis of the common pressure scale) above 100 GPa [13][14][15]. The latter process can be separated from the stressinduced diamond fluorescence because it is longer lived (5-20 ms). In another technique the detector is gated to collect the desired signal only when the excitation is present. It has been used for high-temperature Raman measurements at ambient pressure to suppress unwanted thermal radiation. Different types of apparatus have been used including mechanical choppers [16], pulsed lasers and a single-channel gated detector [17]. More recently, pulsed-laser excitation and gated CCD array detectors have been used for in situ Raman monitoring of diamond film growth [18]. Very recently, a pulsed-laser Raman system, which employs either a gated CCD detector or a Pockels switch has been developed, and Raman spectra in the 2000 K range have been reported [19]. However, in the case of the LHDAC even CW Raman measurements are very rare [20][21][22], while none at all in the time-resolved regime have, to our knowledge, been reported. We present here our pulsed Raman system and compar...

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