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From a study of the rotational Raman spectra of disilane and hexadeutero-disilane the following rotational constants were determined: for SizH6, Bo=0.16998+0.00003 cm-l; for SizD6, BIJ =0.13796 0.00004 cm-l. The rotational constants were used to provide values for the internuclear distances in these modecules.

A servocontrolled 1-m plane-parallel Fabry-Perot interferometer has been developed at NPL for the precise intercomparison of laser wavelengths. This instrument has been used to measure the wavelength ratio of a 679-nm radiation and that from a 633-nm iodine-stabilized He-Ne laser, achieving an accuracy of 2.9 parts in 10(11). The 679-nm light was derived from a stabilized CO(2) laser radiation by upconversion, and the wavelength of this 9.3-microm laser radiation can be calculated from the visible wavelength result. Frequency measurements on the same CO(2) laser radiation have already been made in this laboratory, so that the experiment reported here leads to a precise value for the speed of light in vacuum and to the value of 473, 612, 380.5 +/- 0.3 MHz for the absolute frequency of the visible radiation from a He-Ne laser stabilized to component d of (127)I(2).

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