We present Stark widths and shifts of the Hα line measured in a flash tube plasma in the electron density range N e = (1-4)×10 18 cm −3 and the temperature range T = 1-2 eV. The plasma parameters were measured independently of the width and shift of the Hα line. We found that the Hα widths and shifts, measured at two different plasma sources, producing 'warm dense matter' of similar parameters-the underwater discharge and the flash tube-are consistent with each other. Our experimental results are in a good agreement with Oks' theory of Stark widths and shifts. As a conventional reference, we also compared our experimental results with Griem's theory of Stark widths and shifts. For Griem's shifts we found discrepancies of up to 76%. Griem's widths lie mostly within the experimental error bars; however, their rms deviation from the most probable experimental widths is up to 44% greater than for Oks' widths. Therefore our experiment provides further proof of some new warm-dense-matter effects incorporated in Oks' theory, the most important of which is the acceleration of the perturbing electrons by the ion field.

Hydrogen H(alpha) shift is measured in a plasma characterized by a noticeable coupling due to a high electron density (N(e)>/=2x10(18) cm(-3)) and a relatively low temperature (T=8900 K). The plasma is produced by a single laser breakdown in an underwater medium, creating a stable and radially homogeneous plasma. The results cannot be explained by the known contributions to the shift, which predict shifts by a factor of 2 greater than the experimental shifts. A contribution called dipole ionic-electronic shift (DIES) is introduced. The data presented in this work constitute the experimental discovery of this phenomenon. Indeed, the total theoretical shifts obtained taking into account the DIES are in excellent agreement with our experimental values.

Stark-broadened emission profiles of the Balmer series Hbeta lines are measured subsequent to nanosecond laser-induced optical breakdown in gaseous hydrogen. Electron number densities are found from time-resolved spectra from Hbeta emissions to be in the range 10(15)-10(18) cm(-3). These results are compared with Halpha measurements for which number densities as high as 10(19) cm(-3) are determined from Stark widths and Stark shifts. Good agreement is reported for number densities inferred from Halpha and Hbeta emissions, down to an electron number density 3 x 10(16) cm(-3), by accurate treatment of ion dynamics in the theory.

Intra-Stark Spectroscopy (ISS) is the spectroscopy within the quasistatic Stark profile of a spectral line. In the ISS some local depressions ('dips') occur at certain locations of the quasistatic Stark profile of a spectral line. This phenomenon arises when radiating atoms/ions are subjected simultaneously to a quasistatic field F and to a quasimonochromatic electric field E(t) at the characteristic frequency. The present paper advances the study of the relativistic laserplasma interaction from our previous paper (Oks et al, Optics Express 25 (2017) 1958). First, by improving the experimental conditions and the diagnostics, it provides a systematic spectroscopic study of the simultaneous production of the Langmuir waves and of the ion acoustic turbulence at the surface of the relativistic critical density. It demonstrates a reliable reproducibility of the Langmuir-wave-induced dips at the same locations in the experimental profiles of Si XIV Lybeta line, as well as of the deduced parameters (fields) of the Langmuir waves and ion acoustic turbulence in different laser shots. Second, this study employs for the first time the most rigorous condition of the dynamic resonance, on which the ISS phenomenon is based, compared to all previous studies in all kinds of plasmas in a wide range of electron densities. It shows how different interplays between the Langmuir wave field and the field of the ion acoustic turbulence lead to distinct spectral line profiles, including the disappearance of the Langmuir-wave-induced dips.

We consider an isolated two-electron Rydberg atom/ion, where the average distance of the inner electron from the nucleus is much smaller than the average distance of the outer Rydberg electron from the nucleus. We calculate analytically the shape of any spectral line emitted by the inner electron under the rotating electric field of the outer electron-by using the O 4 symmetry of hydrogenic atoms/ions. By applying the obtained analytical expressions to the example of the Ly-alpha line, we arrive, in particular, to a peculiar result. Namely, as the spectral line splits into several components, the most intense components exhibit a quadratic Stark effect with respect to the electric field of the outer electron (though it is linear with respect to the dipole moment of the inner electron), while one would intuitively expect a linear dependence on the electric field of the outer electron since the subsystem Z + e is hydrogenic. We provide a physical explanation of this peculiar result.