Raman spectroscopy using a gas laser J.P. Russell To cite this version: J.P. Russell. Raman spectroscopy using a gas laser. Journal de Physique, 1965, 26 (11), pp.620-626. 10.1051/jphys:019650026011062001. jpa-00206048 HAL Id: jpa-00206048 https://hal.archives-ouvertes.fr/jpa-00206048 Submitted on 1 Jan 1965 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. 620 dotted line the intensity ratio here is about 3.2 Discussion times too large. The intensity anomaly is even greater in the case of deuterium where in normal M. THEIMER. - Is the interpretation of the deuterium the observed ratio is as great as 9 times Raman spectrum of hydrogen in terms of exci- the value expected on the basis of isotropic scat- tons different from that in terms of lattice vibra- tering only. Up to the present no plausible expla- tions ? It appears that the assignment of the nation of this curious effect has been forthcoming. different peaks to different nearest neighbor ratios It might be noted that the anomaly is not present of ortho and para molecules is not quite consistent in the high pressure gas at 300 OK and 85 °K. with the concept of lattice vibrations. BIBLIOGRAPHY [1] CLOUTER (M.) and GUSH (H. P.), Phys. Rev. Letters [5] Mc KAGUE (A. H.), M. A. Thesis, University of Toronto (in press). 1965. [2] GUSH (H. P.), HARE (W. F. J.), ALLIN (E. J.) and [6] VAN KRANENDONK (J.), Physica, 1959, 25, 1080-1094. WELSH (H. L.), Can. J. Phys., 1960, 38, 176-193. [7] VAN KRANENDONK (J.), Can. J. Phys., 1960, 38, 240- [3] BHATNAGAR (S. S.), ALLIN (E. J.) and WELSH (H. L.), 261. Can. J. Phys., 1962, 40, 9-23. [8] STOICHEFF (B. P.), Can. J. Phys., 1957, 35, 730-741. [4] SOOTS (V.), Ph. D. Thesis, University of Toronto, 1963. [9] MAY (A. D.), VARGHESE (G.), STRYLAND (J. C.) and WELSH (H. L.), Can. J. Phys., 1964, 42,1058-1069. LE JOURNAL DE PHYSIQUE TOME 26, NOVEMBRE 1965, RAMAN SPECTROSCOPY USING A GAS LASER (1) By J. P. RUSSELL, The Royal Radar Establishment, Great Malvern, Worcs, England. Résumé. 2014 Le laser à hélium-néon présente, sur la lampe à vapeur de mercure généralement utilisée, l’avantage de donner un rayonnement plus intense, de longueur d’onde (6 328 Å) plus grande, à laquelle les milieux solides étudiés sont plus transparents. En outre, ce rayonnement est spontanément polarisé. Le montage expérimental indiqué, formé d’éléments existant dans le com- merce, permet d’observer la radiation diffusée à 90° et, légèrement modifié, la lumière diffusée vers l’arrière par le silicium, sous un angle de 45° environ. On a ainsi obtenu les spectres Raman du second ordre de divers matériaux solides, dont le tungstate de calcium, le fluorure de calcium, le phosphure de gallium et même, pour ce dernier, le spectre Raman à trois phonons. Deux tableaux résument les résultats expérimentaux. Abstract. - The helium-neon gas laser has many advantages over the mercury arc, previously used. It is more intense, its light has a longer wavelength (6 328 Å) and is plane polarised. The experimental arrangement is described : it is made of commercially available apparatus. The scattered radiation is normally observed at right angle to the direction of the laser beam, but little modification allow the observation of backward scattered radiation, the angle between the laser beam and the direction of observation beeing about 45°. Using this arrangement, the first and second order Raman spectra of several materials, as cal- cium tungstate, calcium fluoride and gallium phosphide and even, for GaP, three phonons Raman scattering, has been measured. Two numerical tables summarize the experimental results. 1. Introduction. - Since Raman scattering was taline solids. In a solid the scattering is due to first reported in 1928 (C. V. Raman, 1928) measu- the interaction of the light with the lattice vibra- rements of Raman spectra have been a valuable tions or phonons. The first order spectrum which tool in the investigation of vibrational and rota- is due to the creation or destruction of single tional energy levels of molecules and the deter- optic phonons at the center of the Brillouin zone mination of lattice vibration frequencies of crys- identifies the zone center phonons unambiguously. The second order is due to (1) British Crown Copyright. Reproduced with the permis- spectrum processes sion of the Controller, Her Britannic Majesty’s Stationery involving pairs of phonons of equal or equal and Office. opposite wave vector. It provides information Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019650026011062001 621 which is complementary to that obtained from two of the disadvantages of the old technique. It phonon infrared absorption and is directly related operates at a longer wavelength, it is intense, plane to the two phonon density of states. The comple- polarized and makes low temperature measu- mentary nature of the two measurements arises rements easy. from differences in selection rules for the two pro- In developing our technique for Raman spec- cesses. (This is particularly true in a lattice like troscopy we have made measurements on a number CaF2 which has inversion symmetry, then because of materials. We will illustrate the advantages of of parity conservation the Raman process can the laser by reference to each of these materials observe even parity phonons and phonon combi- and then describe some new results which have nations while the infrared absorption involves only just been obtained on GaP at 20 OK. odd parity phonons. In this case both sets of measurement are necessary for a complete picture 2. Experimental. - The equipment which we of the phonon spectrum.) have used is all commercially available. The laser Raman scattering probabilities are low, typical is a model 116 Spectra Physics helium neon gas values being 1 : 106 for first order scattering and laser, which we operate at 6 328 A where it gives 1 : 109 for second order scattering. This means a uniphase output of 25 to 30 milliwatts. The that to observe Raman scattering very intense spectra are measured using a Hilger and Watts sources of monochromatic radiation are required. Raman spectrograph with an aperture of approxi- It is usual to use one of the intense emission lines mately F /6 and dispersion of 40 A/mm at 6 328 A. obtained from a gas discharge as the exciting radia- The experimental arrangement is shown in tion. Generally a mercury discharge lamp is used, figure 1. The parallel beam output from the laser this has intense lines at 2 536 A, 4 047 Á, and is passed through a series of baffles to eliminate 4 358 as. The discharge lamp has proved useful as the fluorescent radiation from the collimated laser a source of exciting radiation, however it suffers beam. The small amount of fluorescence which from a number of disadvantages which have tended remains is used to calibrate the plates. The beam to restrict the full development of Raman spec- is passed vertically through the sample, it is focus- troscopy. sed by a short focal length lens so that inside the The laser has provided us with a new source of crystal the mean diameter is less than 100 microns. exciting radiation. It has many advantages over For measurements at room temperature the crystal the mercury lamp and in fact suffers from none rests on a mirror which reflects the laser beam FIG. 1. 622 back along the same path. At low temperatures Rayleigh scattered radiation (radiation which one face of the crystal is silvered and attached to a has been scattered without frequency change) can cool finger. This arrangement again reflects the be very troublesome in measuring Raman spectra. beam along the same path. In both cases an Using the laser there is not equivalent filter to the external resonator is formed with the output self absorption of Hg 2 536 by mercury vapour. mirror of the laser. This is not a true optical We have found that by placing a small absorbing resonator, however we estimate that as many as shield in the plate holder at the position corres- 5 passes occur. Allowing for the very lossy nature ponding to 6 328 auk most of the Rayleigh scattered of the system this effectively increases the useful radiation can be removed. This technique while beam intensity by a factor of two. effective does prevent small frequency shifts being The scattered radiation is normally observed measured. For shifts greater than 200 cm-1 a at right angles to the direction of the laser beam, multi layer dielectric filter can be used to remove see figure 2. However, in the case of silicon which Rayleigh scattered radiation. 3. Comparison between the laser and mercury discharge. - 3.1. WAVELENGTH. - The laser ope- rates at 6 328 A, which is considerably longer wave- length than the normal mercury lines. This extends the range of materials which can be inves- tigated as many materials are opaque at the shorter wavelengths. The best example is Gallium Phos- phide which is a III/V semiconductor with a band gap of 2.3 e. v., it is opaque at the mercury wave- lengths.