We assume here that the normal laws of refraction also apply, thereby ignoring the detailed problems of the ' extraordinary rays.'

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The Raman and infrared spectra of gas phase Re(2)O(7) are reported. The experimental vibrational spectra of molecular Tc(2)O(7) and Re(2)O(7) are compared with calculated spectra. The results of these studies agree with a nonlinear M-O-M bridge for Tc(2)O(7) and Re(2)O(7). For infrared intensity calculations, the point charge approximation is used, while for the Raman calculations a combination of bond and atom polarizabilities is adopted. Pure Re(2)O(7) was prepared from rhenium wire, but attempts to prepare it from rhenium powder and oxygen always led to infrared spectra showing serious contamination from a species containing an -OH linkage. Detailed experiments identified this molecule as HReO(4), a unique transition metal analogue of the perhalic acids, and a partial infrared spectrum of this molecule is reported.

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Calibrated coherent anti-Stokes Raman (CARS) spectra have been recorded for the resolved Q branches of the vibrational fundamentals and first hot bands of I4N,, l4N''N and 15N, at a temperature of 1200 K. The spectra show a convolved linewidth of 0.13 cm-', and have been used to derive new values of spectroscopic constants, for the species observed, with specific reference to the position of the Q branch lines where J > 20 for u = 1-0, and J = 0-30 for u = 2-1. The critical overlap region of the fundamental and the hot band may now be computer simulated with confidence, which should be of considerable help in combustor thermometry using CARS.

The Raman and i.r. spectra of the title compounds are recorded in various phases. Using helium-neon laser excitation the Raman spectra of intensely coloured melts of Ga,l , and ln216 are recorded. Vibrational assignments are made for all compounds using a normal-co-ordinate treatment identical to that used previously for Ga, CI , .ON the basis of new experimental data coupled with normal-co-ordinate calculations, two of us recently made a major reassignment of the Ga,Cl, mo1ecule.l It was subsequently suggested to us that the treatment could not be extended to Ga,Br6. As we were interested in possible reasons for this behaviour we decided to reinvestigate a number of Group I11 trihalide molecules of the type M,X,. In particular we wished to examine M,I, species where the spectral data were very restricted. Raman and i.r. data on the title compounds are reported in Tables 1-5.The calculated values reported in these Tables were obtained by techniques identical to those developed in the earlier paper,l which should also be consulted for nomenclature. Our assignments frequently differ from those of earlier workers, largely owing to the fact that we have available the results of normal-co-ordinate analyses. However, the generally improved spectral data (attributable at least in part to excellent instrumentation) also helped, notably in the assignment of polarised bands.Aluminium Tribromide (Table 1) .-The most recent assignments for this molecule are due to Miller,2 whose calculated values include predictions for the i.r.-active bands. Our polarisation data unambiguously confirm

The chemical implications of single crystal Raman spectroscopy by means of laser excitation are briefly outlined. Unambiguous assignments are presented for PdCl 2- 4 , PtCl 2- 4 , PtCl 2- 6 , PdCl 2- 6 , GeCl 2- 6 and TeCl 2- 6 species. Detailed scattering equations are applied to mercurous chloride. The vibrational spectra of cassiterite, anataseand rutile are discussed in terms of the factor group. Gallium trichloride and aluminium tribromide spectra are discussed from the viewpoint of a perturbed D 2h Raman tensor. The complex spectrum of molybdenum trioxide is interpreted in terms of a site-factor group analysis. Problems of birefringence, crystal imperfection, dichroism, the resonance Raman effect, twinning, enantiomorphism, internal reflexion and dependence of relative intensity on physical state are noted. The methods discussed are applied to the first-order spectra of centrosymmetric crystals and rely on the validity of the Raman tensor predictions resulting from factor group analysis.