Spectrum fluorescence, intermodulated

Fig. 2.13 Intermodulated fluorescence method for saturation spectroscopy at small densities of the sample molecules (a) experimental arrangement (b) hyperfine spectrum of the v" = 1,...

The experimental performance is depicted in Fig. 2.18. The modulation frequency = 2nf is tripled by forming rectangular pulses, where the third harmonic is filtered and fed into the reference input of a lock-in amplifier that is tuned to 3f2. Figure 2.19 illustrates this technique by the third-derivative spectrum of the same hfs components of I2 as obtained with the intermodulated fluorescence technique in Fig. 2.13. 

V" = 1, J" = 98) (v = 58, J = 99) in the iodine molecule I2 [10.34]. The two laser beams were chopped by a rotating disc with two rows of a different number of holes which interrupted the beams at f = 600 s" and f2 = 900 s". The upper spectrum was monitored at the frequency f at which the pump beam was chopped while the probe was not modulated. The Doppler-broadened background caused by the linear terms in (10.25) and the Lamb-dips both show a modulation at the frequency f and are therefore recorded simultaneously. The center frequencies of the hfs components, however, can be obtained more accurately from the intermodulated fluorescence spectrum (lower spectrum) which was monitored at the sum frequency + f2) where the linear background is suppressed. 

Such power levels can be quite adequate for Doppler-free saturation spectroscopy, as demonstrated in a series of recent studies of ultraviolet transitions of neutral helium in our laboratory at Stanford. (35-37) Fig. 5 shows as an example a spectrum of the 2 S - 5 P transition of He near 294.5 nm, recorded by intermodulated fluorescence spectroscopy. The ultraviolet radiation was generated by a yellow cw ring cavity dye laser with cavity-enhanced external ADA (ammonium dihydrogen arsenate) frequency doubler. The absorbing metastable He atoms were produced by electron impact excitation of He gas at about 0.04 torr. The spectrum shows a cluster of resolved line components which could be assigned after the fine and hyperfine Hamiltonian had been diagonalized in an uncoupled representation. We were surprised to learn that the hyperfine structure of the 5 P state of this simple 3-body system had been neither measured nor calculated before. 

Fig. 5. Doppler-free intermodulated fluorescence spectrum of the 2 S - 5 P transition of He near 296 nm.

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