A HETERODYNE LASER SPECTROMETER FOR PRECISION-MEASUREMENTS OF LARGE LINE SPLITTINGS

A computer-controlled heterodyne laser spectrometer has been developed for precision measurements of spectral line spacings in the microwave regime. The instrument consists of two cw single-mode ring dye lasers which are mixed on an avalanche photodiode. Beat frequencies of up to 12.5 GHz are measured by a quartz-stabilized rf counter. Hyperfine structure lines of molecular iodine are used as reference transitions to frequency-lock the lasers and bridge large frequency distances. To demonstrate the performance, we measured the 1s2p 3P fine structure splittings of 70-150 GHz in the spectrum of helium-like Li-7(+). Applying Lamb dip spectroscopy and eliminating the Doppler background by a specific data acquisition technique, the splitting frequencies were obtained with a precision of approximately 2 x 10(-5).