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UDC 543.423.8 :535.374 Laser Raman Microprobe Technique; A New Analytical Tool in the Study of Iron and Steel* By Kimitaka SATO** Synopsis It may be said that such background has naturally In recentyears, the laser Raman microprobetechnique has attracted the led to the development of a new microbeam analysis attention of many researchers as a new analytical techniquefor local ap- method, called the laser Raman microprobe,s-11) to plication. This paper introducesthe principle, function andfeatures of the obtain the information about the chemical bonds or technique and discusses its applicability to the study of iron and steel, conformation of molecules in local areas. Attempts citing several examples of application. This technique is based on phe- were made to apply the laser Raman microprobe to nomenon,called the Roman scattering, which is caused when a sample is the analysis of defects in materials, inclusions in min- irradiated with a laser light focussed to the utmost. The advantage of erals, catalysts, samples for environmental control this techniqueover conventionalones is that it is capable of obtaining the information about chemical species and atomic groups of moleculesin local and biological samples. In 1978, the 6th Interna- areas (~ rcm) through point, line or image (Raman image of specific tional Conference on Raman Spectroscopy was held wavenumber) analysis. Although this technique has notyet been applied in Bangalore, India, a place noted in connection with widely in the study of iron and steel, the scope of its application will be Raman, in commemoration of the 50th anniversary greatly widenedas a powerful tool in the elucidation of local defects, such of the discovery of the Raman effect.12) At this con- as blisters,precibitates, inclusion'sand corrosionproducts. ference, the laser Raman microprobe was taken up as one of the subjects for discussion, stimulating inter- I. Introduction est among researchers. In 1979, a symposium on the A recent trend in materials science is the shift of application of laser Raman microprobe was held in interest from the macroscopic to microscopic field. France. 13) Particularly, requirements for information on local It is not very long since the laser Raman micro- areas or surface (or interface) have become stronger. probe was developed. As this technique is still in the In the background of such change is the recent re- development stage, there are not many examples of markable progress of the so-called microbeam analysis its application. It is, however, expected that the techniques employing laser, electron or ion, as is well scope of this spectroscopy will be widened, including known. 1,2) application to the study of iron and steel materials. In the study of iron and steel materials, electron This paper describes the principle and function of probe X-ray microanalysis (hereinafter referred to as the laser Raman microprobe, introducing examples the " EPMA ") is making a particularly great con- of its application to the study of iron and steel. tribution. It is considered that the excellent results achieved by the EPMA have led to wide recognition II. Principle and Function of Laser Raman Mi- of the importance of microbeam analysis. Recent croprobe remarkable progress of electronic and vacuum tech- The laser Raman microprobe technique is an ana- niques has made it easier to measure the spectra of lytical method for obtaining the Raman scattered light Auger electron, secondary ion, photoelectron, etc. and the Raman image of specific wavenumber by ir- For this reason, microbeam analysis has been applied radiating a sample with an extremely focussed laser positively as an effective analytical tool in the study light. The principle and function of this method, of iron and steel materials."3) However, these tech- apparatus used, and the characteristics of laser as a niques are basically designed for analysis of the com- light source will be described hereinbelow. position of elements. It may not be too much to say that electron diffraction has been the only method 1. Principle that can obtain the information resulting from the so- The principle of the laser Raman microprobe is called material structure (for example, molecular best explained if it is compared with that of the species, atomic group, crystal structure, etc.). EPMA.14) While the EPMA uses an electron beam It was in 1928 that Raman discovered the Raman as an exciting source, the laser Raman microprobe scattering with the sunlight as a light source.4) Ini- employs a laser light. As shown in Fig. 1, X-ray, tially, mercury-arc lamps were used as a light source back-scattered electron, secondary electron, cathodo for measurement of the Raman effect. As the mer- luminescence, etc., are emitted when a sample is ex- cury-arc lamp lacked brightness, it was replaced by cited by an electron beam. When a sample is irra- lasers in 1962.5) The use of lasers as the light source diated with a laser light, it becomes possible to observe has greatly widened the scope of Raman spectroscopy, strong elastic (Rayleigh) scattered light and weak including application to the analysis of iron and steel. Raman scattered light in its neighborhood. In short, * Originally published in Bulletin of Th e Japan Institute of 1i'Ietals, 19 (1980), 200, in Japanese. Englis h version received July 29, 1980. ** Fundamental Research Laboratories, Nippon Steel Corporation, Ida, Nakahara-ku, Kawasaki 211, (370) Report Transactions ISIJ, Vol. 21, 1981 (371) Fig. 1, Basic principles of electron microprobe and Raman microprobe. the principle of the laser Raman microprobe is under- stood by replacing the incident electron beam in the EPMA by a laser beam, and the characteristic X-ray Fig. 2. Schematic representation of methods for obtaining. to be detected, by the Raman light, respectively. images of samples with characteristic Raman fre- With the use of laser lights, the laser Raman micro- quencies. (Delhaye and Dhamelincourt14~) probe permits three scanning modes as does the EPMA using electron beams, i.e., point analysis, line analysis and image analysis.14~ These three modes are illustrated in Fig. 2. For details of the modes, reference should be made to theses by Delhaye and Dhamelincourt14~ and Dhamelincourt et a1.'0' as well as the explanation by Furuya.9j 2. Measurement The basis for research on the laser Raman micro- probe was studied by Delhaye and Migeon in 1966.15> Based on the results of their study, the measurement of Raman spectra for gaseous and solid samples, even liquid samples in an amount of i1 and nl was tried.ll,ls~ In 1973, Hirschfeld succeeded in the mea- surement of the Raman spectra for polystyrene par- ticles of 0.5 pm in diameter and for particles of thorium dioxide of 0.3 ,um in diameter.17~ The meth- od used by him attracted wide attention as a new Fig . 3. Schematic diagram of micro-Raman microprobe in method of microbeam analysis for obtaining informa- National Bureau of Standards. (Blaha et al.2s~) tion on chemical species, molecular structure, etc. The original form of the laser Raman microprobe was independently published by Rosasco et a1.'8 and Delhaye and Dhamelincourt14~ in 1975. Since then, these two groups have been playing the leading role in research on the laser Raman microprobe. The laser Raman microprobe developed by Rosasco et al. is constructed as shown in Fig. 3. This is an im- provement of commercially available laser Raman spectrometer, and is designed to measure the Raman spectrum only in local areas by focussing the incident laser light to the utmost and detecting the scattered light effectively. The laser Raman microprobe developed by Del- haye and Dhamelincourt14} is called the " molecular microprobe optics laser examiner (MOLE) ". Based on the characteristics of laser lights and multi-channel spectroscopy, this microprobe is capable of scanning the spectrum in less than 10-9 sec, and of obtaining a distribution image, which is the Raman image, with a space resolution of about 1 pm by taking the line of specific wavenumber out of the Raman scattered lines. Figure 4 shows the structure of the laser Raman micro- Fig. 4. Optical scheme of Raman microprob e in Universite probe developed by Delhaye et al. Basically, this de Lille. (Dhamelincourt et al.10)) ( 3?2 ) Transactions ISIJ, Vol. 21, 1981 probe consists of a monochromator, including an opti- 155 cm~1. The Raman spectrum can be displayed cal microscope, an optical filter, etc., and a detecting on an oscilloscope. Photograph 1(g) shows, in real system. Various modes can be set by combining the time, the spectrum at wavenumbers from 140 to 240 irradiation of the sample and the detecting system.'°) cm-1 out of the spectrum shown on Photo. 1(b). Photograph 1 shows typical models of various modes. The samples used were 100 X 50 ~cm HgBr2 3. Characteristicsof Lasers As a Light Source and 200 X 30 im Mo03. Photograph 1(a) shows an The use of lasers as a source of light was an epoch- optical microphotograph of these samples, while making event, owing to which Raman spectroscopy Photo. 1(b) shows the Raman spectrum obtained has made remarkable progress. In this sense, the use across the entire field of vision of the microscope which of lasers as a light source is called the " Renaissance includes the spectra of both HgBr2 and Mo03. Photo- of Raman spectroscopy ".19) The characteristics of graph 1(c) shows the Raman spectrum of HgBr2 only, lasers as a light source are briefly described below. while Photo. 1(d) shows that of Mo03 only. Photo- Generally, photon probes are used for a wide range graph 1(e) shows the characteristic Raman image of of wavelength, such as infrared ray, ultraviolet ray HgBr2 obtained at a wavenumber of 188 cm', while and X-ray.

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