Wavelengths from thorium-halide lamps

The prese nt system. of in te rnationa l secondary standa rds of wavelen gth for sp ectroscopic meas urements is based on in terfcrometri c det erminations of wavelengths emitted at atmospheri c p.r es~ure .by an electri.c a rc between iron electrod es. Because of the poor qu ali ty a nd uneven d lstnbutiOn of these Iron standards t hey are not s uitable for accurate m ea surement of .wavelengths in the sp ectra of h eavi er elements, most of which are more complex a nd co nsist of much sha rper hn es t ha n t he standa rds. Quar tz-tube lamps containing a small quantity of a t horium halide, when excited b y microwaves, emit thousands of uniformlv s ha rp a nd e."enly distributed lin es whose wavelen gths, or positions in a s pectrum , can b e dete rmllled with about one-te nth t he error of locating iron-a rc lin es. Prelimin ar y values of 222 vac uum wavelengths emi tted b y a t horium-iodide lamp have been measured relative to 5462.2705 and 4047.7144 A e mi tted by a s imilar lamp containin g mercury-198. Fabry-Perot !I1 terferom eters wi t h l)l ate sepa ratiO ns of 25, 4,0, or 50 m illi m eters were used with a stigm atic gratin g spectrograph m m a km g t hese m easurements . The thor ium wavelengths ra nge from ::\288.7356 to 6991.5839 A in vacuum a nd from 3287. 7885 to 6989.6562 A in stand ard air. The acc uracy in r elative value of 27 claRs ifi ed thoriu m lines is tested b y m ea ns of t he combin ation prin ciple, which indicates t h at t he average error is less than 1 p ar t in 20 m illi o n.