The 39K transitions 42P1/2 ← 42S1/2 and 52P3/2 ← 42S1/2 have been observed by using two type of extended cavity laser diodes: the Toptica DL-100 Series and the Sacher Lasertechnik Lynx Series working around 769.9 nm (12985.169 cm-1) and 404.4 nm (24720.139 cm-1), respectively.
The level position schematic for the potassium absorptions in object is shown in Fig. 1.
Fig. 1. Potassium resonance levels object of this spectroscopy. The wavelength are given in vacuum. {Ref.: U. Gustafsson, J. Alnis, and S. Svanberg, Am. J. Phys. 68(7), 660-664 (2000). See also H.M. Concannon Ph.D. Dissertation by the Duke University: Two-photon Raman gain in a laser driven potassium vapor}.
Fig. 2. Sketch of the experimental apparatus. TDL: diode laser; B.S.: beam-splitter; D: detector; F.P.: Fabry-Perót interferometer; PC: personal computer; M: monochromator; O.I.: optical insulator.
The experimental apparatus uses a extended cavity diode laser as a source. Its injection current is driven by a stabilized low-noise current generator, which permits also the scan of the emission wavelength by mixing to the driving current an attenuated low frequency (~1 Hz) sawtooth signal. The DL is temperature regulated by a Peltier junction driven by a high stability temperature controller.
In front of the DL an optical insulator prevents the back-scattered light to influence the stability of the laser itself. A confocal 5 cm Fabry-Perót interferometer is adopted to mark the frequency scan. A monochromator is employed for the rough wavelength reading.
By using the mirror at the end of the measurement cell we can let the light come back through the very same path in order to get the "saturation" and consequently the Doppler-free spectroscopy.
Fig. 3. Saturated absorption features of potassium at 769.90 nm (42P½ ← 42S½). In this case the inner walls of the sample cell were coated by polydimethylsiloxane (PDMS) to obtain enough vapor pressure at lower temperature.
The sub-Doppler configuration permitted to observe resonance lines as narrow as 20÷25 MHz, as shown in Fig. 3. In this figure the 39K D1 hyperfine transitions are shown identified along with the crossover.
By applying the wavelength modulation technique we revealed the 2nd harmonic signal of the saturated absorption, as shown in Fig. 4.
Fig. 4. 2ƒ signal of the crossover of Fig. 3. {T = 33°C only, because the LIAD effect has been utilized [See S. Gozzini, A. Lucchesini, "Light induced potassium desorption from polydimethylsiloxane film", Eur. Phys. J. D 28(2), 157 (2004)]}
By using the appropriate fit procedure we measured the splitting of the crossover resonance with good accuracy. This gives directly the magnetic dipole coupling constant A = (28.7 ± 0.3) MHz, to be compared to the literature:At lower wavelength, higher energy, the saturation of the absorption at 404.4 nm is more difficult, therefore higher laser power and higher potassium concentrations are needed. The 20 mW cw Sacher blue laser has been used and a preliminary measurement result is shown in Fig. 5.
Fig. 5. 39K Saturated absorption signal at 404.41 nm (52P3/2 ← 42S1/2).
Fig. 6. 39Potassium Density & Pressure vs Temperature. [after C.B. Alcock, V.P. Itkin, M.K. Horrigan, "Vapour pressure equations for the metallic elements: 298-2500K", Canadian Metallurgical Quarterly 23(3), 309-313 (1984)]