No abstract

The population inversion in a three-level atomic system, interacting with two coherent laser pulses, occurs from the first level to the third level at the optimal detunings of the laser frequencies near the two-photon resonance. An examination is performed to see whether the energy density and time-delay of the photoionizing laser can significantly influence the ion yield for the case where the atom interacts with two coherent pulses and one photoionizing pulse incoherently coupled. The well-known three-colour, three-step, three-photon ionization scheme, which is available for the selective photoionization of 168Yb, is chosen for use in this research with the Bloch equation. It is revealed that the proper time-delay and energy density enable us to achieve higher efficiency for the selective photoionization of 168Yb, regardless of the Doppler-broadening, though the efficiency and selectivity of coherent excitation are prominently enhanced by applying the optimal detunings.

A principle for the e%cient excitation of a three-level medium in a selective photoionization method is examined. It is shown that with special detunings for a given set of the laser-atom interaction the third-level population can be maximized. The expression of the optimal detunings is derived near the two-photon resonance region. It is shown that if the optimal detuning method is applied to three-level systems of Yb and Ca, the first-level populations are inverted to their corresponding third levels by about 100% and 86%, respectively. Application of this method to a Doppler-broadened medium is discussed and it is found that the optimal detuning method with counterpropagating waves will be very effective for selective ionization of wanted atoms.PACS number(s): 32.80.Fb, 32.80.8x

three-step, three-photon scheme of 6'So (0 cm-I) -+ 6 3 P ~ (17992.008 cm-') --c (;, :)z4f1'6s26p (35 196.98 cm-') + autoionization state (52353 cm-') is chosen to examine the selective photoionization of 168yb. The Maxwell-Bloch equation is numerically solved, where the interacting laser pulses me supposed to be mllinear and resonam with the transition lines of '"Yb, and the effects of Doppler broadening and laser bandwidth are incorporated. Firstly. the thin medium dynamics are examined to determine the optimal energy of laser pulses for the selective photoionization of layb. and to determine Yb isotopes which have a large influence on the excitation of I6%.Secondly, the thick medium dynamics are investigated, resulting in the fact that the laser pulse for tke first transition is prominently deformed due to the near-resonant interaction with '"Yb and "%, and t h e ion yield along the propagatian of the laser beam is strongly dependent on the initial energy of pulse for the first h'ansition and the time-delay between input pulses.

The effects of laser bandwidth and Doppler broadening on the population dynamics of the ladder configuration which consists of three levels of Gd such as 9D50 (999.121 cm-1), 5d6s6p 9F4 (17973.611 cm-1) and 5d6s7s 9D40 (33534.095 cm-1) are studied numerically using the Bloch equation. In particular, for the examination of the bandwidth effect we choose a finite-bandwidth quasi-monochromatic laser and describe its field as a set of phase-independent modes. The remarkable fact is noted that the laser bandwidth of 1 GHz causes radiation-atom interactions to be very strongly incoherent, so the three-level populations are the same throughout almost all the interaction-time region.