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We report on the improved measurement of the hyperfine structure of $4f^{12}(^3 H_6)5d_{5/2}6s^2$ $(J=9/2)$ excited state in Tm-169 which is involved in the second-stage laser cooling of Tm. To measure the absolute value of the hyperfine splitting interval we used Doppler-free frequency modulation saturated absorption spectroscopy of Tm atoms in a vapor cell. The sign of the hyperfine constant was determined independently by spectroscopy of laser cooled Tm atoms. The hyperfine constant of the level equals $A_J=-422.112(32)$ MHz that corresponds to the energy difference between two hyperfine sublevels of $-2110.56(16)$~MHz. In relation to the saturated absorption measurement we quantitatively treat contributions of various mechanisms into the line broadening and shift. We consider power broadening in the case when Zeeman sublevels of atomic levels are taken into account. We also discuss the line broadening due to frequency modulation and relative intensities of transitions in saturated-absorption experiments.

Arrays of diamond photoemitters with silicon-vacancy (SiV) photoluminescent (PL) centers have been produced by epitaxy of CVD diamond inside laser-ablated channels in a-Si mask on single crystal or polycrystalline diamond substrates, the mask also serving as Si-doping source. Strong PL emission from the SiV centers with zero-phonon line at 738.6 nm wavelength (6 nm width, 0.8 ns decay time), localized within the photoemitters, has been measured.

We present an extensive analysis of the self-consistent extended Huckel theory (SC-EHT) and discuss the possibilities of constructing accurate and efficient semiempirical methods on its basis. We describe the mapping approach to derive a self-consistency correction to the effective 1-electron Hamiltonian (Fock) operator that is utilized in electronic structure calculations and that variationally minimizes the total energy in the SC-EHT method. We show that the SC-EHT Hamiltonian can play the role of the 1-electron operator by definition, in which case no self-consistency correction is needed. Then, the SC-EHT method can be derived from the Hartree–Fock theory by approximation of the Fock matrix. Therefore, the SC-EHT based methods have rigorous foundations that may be utilized to develop a family of successively accurate model Hamiltonians. We analyze the underlying approximation and discuss it in the light of existing formulations of the EHT method. We indicate two major deficiencies of the existing formulations of the EHT method—neglect of exchange integrals and incorrect asymptotic behavior of the Coulomb integrals. The SC-EHT is compared to the charge equilibration scheme and to the DFTB family of approximations. We show that an improved version of the SC-EHT method can be connected to both of them, indicating relation of the SC-EHT derived approximations to the fundamental DFT origins and their potential for efficient computations on large-scale systems.

The results of frequency-modulation (FM) spec- troscopy of coherent dark resonances from the Zeeman sub- levels of the transition F = 2 ↔ F = 1o fD1 line in absorp- tion of 87 Rb atoms are presented and discussed in detail. By contrast with the conventional spectroscopy of coherent dark resonances employing two laser beams, relative frequency of which can be varied, these data has been obtained with the help of a single frequency-modulated laser field. Varia- tion of the modulation frequency plays then a similar role as the variation of the relative frequency in conventional spec- troscopy. Experimental data are fit to the theoretical calcu- lations, which are based on the theory of FM spectroscopy of coherent dark resonances recently developed by us. Fea- sibility of using such experimental technique for accurate measurements of magnetic fields is also discussed.

A technique for creating the source of a bichromatic laser field with a stable mean value of the difference frequency is realized. The spectral characteristics of a femtosecond passive mode-locked laser are used in this technique. Such a source operating in the region of 780 nm is created. The possibility of using the phenomenon of coherent population trapping (CPT) in three-level systems of the rare-earth atoms for the development of secondary frequency standards in the optical range is considered. The contributions of the processes affecting the width and contrast of the CPT resonances and defining their attainable Q factors are studied. The prospects of using the experimental technique developed for the spectroscopy of the CPT resonances in samarium and thulium vapors are discussed.

Dark resonances in the 154Sm Λ-system 4f66s2(7F0) ↔ 4f66s6p(9F10) ↔ 4f6s2(7F1) are observed alongside the velocity selective optical pumping. The shape of the resulting spectra strongly depended on the buffer gas (He, Ar) pressure due to velocity-changing collisions (VCC): the sign of the effect could be reversed from the “dark” to the “bright” resonance. The observed spectra are interpreted within the framework of the hard-sphere collision model. The role of VCC in the formation of the “dark state” in the Λ-system is discussed.