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The coherent Dicke narrowing (CDN) of the absorption spectrum in an Rb, Cs, K, or Na atomic vapor column of thickness L = (2n + 1)λ/2 (when n is an integer) and its collapse (absence of narrowing), which occurs at an atomic vapor column thickness L = nλ (where λ is the wavelength of the laser radiation in resonance with an atomic transition), are experimentally and theoretically studied. The application of second-derivative (SD) processing is shown give additional important information about CDN and its collapse. Optical nanocells with a wedge gap thickness of 30—3000 nm between the windows inner surfaces are used in experiments. At thicknesses λ/2, 1.5λ, and 2.5λ, all atomic transitions, which are even masked by a large Doppler broadening of 1000–1500 MHz, are fully spectrally resolved in SD spectra, and the frequency spacings (30–50 MHz) between the atomic transitions of a hyperfine structure and the relative probabilities of the atomic transitions are correctly displayed. CDN is shown to be applied in practice. Additional narrow velocity selective optical pumping (VSOP) resonances appear in the SD spectra of the resonance fluorescence of the Rb vapors. A developed theoretical model well describes the experimental results. [ABSTRACT FROM AUTHOR]