Thin Gas Cell Research Articles

Influence of Interatomic Collisions on Intra-Doppler Absorption Resonances in Thin Gas Cells

Influence of Interatomic Collisions on Intra-Doppler Absorption Resonances in Thin Gas Cells

Ultra-High-Resolution Fluorescence Spectroscopy Upon Excitation of Atoms by Pulsed Radiation in Thin Gas Cells

Ultra-High-Resolution Fluorescence Spectroscopy Upon Excitation of Atoms by Pulsed Radiation in Thin Gas Cells

Sub-Doppler resonances caused by photoionization of atoms in thin gas cells

By analogy with the well-tested method of high-resolution spectroscopy in thin gas cells for the processes of optical pumping of atoms, this work shows the possibility of detecting narrow sub-Doppler optical resonances caused directly by straight photoionization of atoms (or molecules) in such cells. The structure of the established nontrivial resonances substantially depends on the probability of photoionization of atoms and dimensions of such a cell, the internal thickness of which is many times smaller than its diameter. Of particular interest is the broadening of the considered sub-Doppler resonances, which is determined directly by the photoionization cross section of atoms and the intensity of radiation causing ionization. Under certain conditions, such photoionization broadening can be measured experimentally with high accuracy, as a result of which it is possible to obtain new important information about ionization processes in atoms and molecules. Keywords: sub-Doppler resonances, photoionization of atoms, thin gas cell, transit-time relaxation of atoms.

Внутридоплеровские резонансы, обусловленные фотоионизацией атомов в тонких газовых ячейках

By analogy with the well-tested method of high-resolution spectroscopy in thin gas cells for the processes of optical pumping of atoms, this work shows the possibility of detecting narrow sub-Doppler optical resonances caused directly by straight photoionization of atoms (or molecules) in such cells. The structure of the established nontrivial resonances substantially depends on the probability of photoionization of atoms and dimensions of such a cell, the internal thickness of which is many times smaller than its diameter. Of particular interest is the broadening of the considered sub-Doppler resonances, which is determined directly by the photoionization cross section of atoms and the intensity of radiation causing ionization. Under certain conditions, such photoionization broadening can be measured experimentally with high accuracy, as a result of which it is possible to obtain new important information about ionization processes in atoms and molecules.

Sub-Doppler Fluorescence Spectroscopy in Thin Gas Cells Pumped with Broadband Radiation

Sub-Doppler Fluorescence Spectroscopy in Thin Gas Cells Pumped with Broadband Radiation

Effect of collisions on the shape of the coherent population trapping resonance detected by the Ramsey method

We report on a theoretical model of the effect of coherent population trapping in alkali atoms in an optically thin gas cell with a buffer gas at a nonzero temperature in a pulsed laser field. The shape of the Ramsey resonance is analysed for two different temperature ranges, i. e. for a ‘cold’ atomic ensemble and for an ensemble of ‘hot’ atoms in a cell with a buffer gas. The influence of the hyperfine structure of the excited level on the shift of the central Ramsey resonance is investigated.

Sub-Doppler Spectroscopy in Thin Gas Cells upon Excitation of Atoms by Broadband Pulsed Radiation

Sub-Doppler Spectroscopy in Thin Gas Cells upon Excitation of Atoms by Broadband Pulsed Radiation

Sub-Doppler Resonances Caused by Rabi Oscillations on Atomic Transitions in Thin Gas Cells

Excitation of atoms from the ground quantum level to a metastable state by traveling monochromatic radiation in a thin gas cell, the internal thickness of which is much smaller than its radius, is studied theoretically. Sub-Doppler resonances are detected and analyzed that arise in the population of a metastable atomic level due to light-induced Rabi oscillations on a given optical transition and specific features of transit-time relaxation of atoms in a considered cell with a rarified gas upon scanning the radiation frequency. Such nontrivial resonances can be recorded in fluorescence of atoms with the help of an additional probe light beam. A method is proposed for a direct measurement of the Rabi frequency from spectral intervals between detected sub-Doppler resonances. The results obtained can be applied in atomic spectroscopy for the precision determination of components of dipole moment matrix elements, as well as for the Einstein coefficients and oscillator strengths related to these components, for quantum transitions between the ground and metastable levels of atoms.

Внутридопплеровские резонансы, обусловленные осцилляциями Раби на атомных переходах в тонких газовых ячейках

Theoretical research is carried out of excitation of atoms from the ground quantum level to a metastable state by the running monochromatic radiation in thin gas cell, whose inner thickness is much less than its radius. As a result, sub-Doppler resonances has been discovered and analyzed, which appear in the population of the metastable atomic level at scan of the radiation frequency because of light induced Rabi oscillation on the given optical transition and also transit relaxation of atoms in the considered cell with the rarefied gas. Such nontrivial resonances may be detected in atomic fluorescence by additional probe light beam. The method for direct measurement of the Rabi frequency on spectral intervals between established sub-Doppler resonances is proposed. Obtained results may be applied in atomic spectroscopy for precise determination of components of matrix elements of dipole moments and related Einstein coefficients and oscillator strengths of quantum transitions between ground and metastable atomic levels.

On the Sub-Doppler Spectroscopy of Optically Excited Atoms in a Thin Gas Cell

This work continues a theoretical investigation of the capabilities of the well-known method based on using a monochromatic probe light beam in combination with optical pumping of atoms (molecules) of a rarefied-gas medium by a broadband radiation in a thin cell the diameter of which is much larger than its internal thickness. In contrast to calculations carried out in the previous publications on this method of spectroscopy, here, we consider the case of arbitrary values of pump intensity and thickness of a cylindrical gas cell. Thus, all the possible mechanisms and specificities of velocity selection of atoms in optically excited levels caused by transit-time relaxation of such atoms in gas cells of this kind are analyzed. Within the framework of this approach, sub-Doppler absorption resonances of the probe light beam corresponding to quantum transitions from the upper level excited by optical pumping are investigated. The obtained results can be used in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization to established narrow spectral resonances.

On the capabilities of sub-Doppler photoionization spectroscopy in thin gas cells

A high-sensitivity photoionization method of registration of narrow sub-Doppler resonances in the spectral distribution of a flow of metastable atoms (or molecules) excited from the ground quantum term by a monochromatic laser beam propagating at normal incidence through an ultrathin gas cell (with a micrometer-scale or even nanoscale gas layer thickness) is proposed. Based on density matrix equations for atomic particles, various mechanisms of broadening of the considered resonances, such as time-of-flight, field, and Doppler broadening, are analyzed. The requirements for laser beam parameters and gas cell dimensions that allow obtaining the narrowest resonances are established. The proposed method can be used in ultrahigh resolution spectroscopy of atoms and molecules, as well as high-precision optical frequency standards.

Influence of transverse configuration of running laser beam on sub-Doppler structure of its absorption spectrum in thin gas cell

The theoretical investigation is conducted of the strong dependence of sub-Doppler resonances of absorption on the transverse intensity profile of a monochromatic laser beam propagating through a thin cell with rarefied gas medium. The resonances under study correspond to centers of quantum transitions and are caused by the specificity of the optical pumping of atoms during their flight between the walls of a thin cell whose internal thickness is many times smaller than its diameter. The detection of narrow high-contrast sub-Doppler absorption resonances, which are prospective for efficient laser-frequency stabilization, can be facilitated by the proper selection of the radial distribution of the light beam intensity in the cell’s transverse cross section.

Limiting resolution of linear absorption spectroscopy in thin gas cells

The most narrow sub-Doppler frequency resonances in the linear absorption of monochromatic radiation that propagates in the normal direction through a cell containing a layer of rarefied gas medium with a thickness smaller than or on the order of the wavelength of this radiation are theoretically studied. The calculation is performed using as an example a three-dimensional gas cell shaped like a rectangular parallelepiped. It is shown that the width and amplitude of considered sub-Doppler resonances (in the vicinity of centers of rather weak quantum transitions) significantly depend on the transit relaxation of atomic particles, which is determined by their transit times through the irradiated region of the cell both in longitudinal and in transverse directions. The restrictions of the approximation of the planar one-dimensional cell that was previously used in such calculations are determined. Possible applications of linear absorption resonances in ultrathin (nanometer) gas cells as references for optical frequency standards are discussed.

Photoprocesses with the sub-Doppler spectral selectivity excited by the monochromatic optical pulse in a thin gas cell

The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.

Sub-Doppler spectroscopy of atoms excited in the regime of Rabi oscillations in a thin gas cell

We carried out theoretical investigation about velocity-selective atomic excitation on long–lived (metastable) levels of an atomic vapour in a thin cell by a monochromatic laser beam, running in the normal direction. The regime of coherent Rabi oscillations is considered on the light-induced transition from a sublevel of the ground quantum term to a metastable atomic level. On the basis of density matrix equations for the two-level system, we analysed the atomic population density of the metastable level, when the sample is irradiated by resonant monochromatic laser beam with an annular cross-section versus atomic velocities and versus the detuning, the amplitude, and the geometry of the laser beam. It is shown that, in the centre of the annular region, it can be obtained a population distribution on the metastable level as a function of the laser detuning, characterized by a sharp narrow resonance profile, whose width is reduced with respect to the thermal Doppler width roughly by the ratio between the diameter of the irradiated region and the inner thickness of the cell. We suggest high-sensitive schemes, in order to detect these sub-Doppler resonances, by probing the population of the metastable state with a second laser beam, resonant with a transition leaving from the metastable level. The case of 1S 0 → 3P 1 spin-forbidden transition of Ca is discussed in more detail

Slow light in a thin gas cell by means of a traveling monochromatic wave

In this paper we demonstrate that the interaction of a traveling monochromatic light wave with atoms of a dilute gas in a cell whose length is much less than the diameter of the light beam can slow down the velocity of light. This slowing down is achieved by the specific transit relaxation of particles between walls of the cell (along the wave vector). For a two-level atomic system the group index is in the order of 103. The results show that such a slow down could be achieved by a single traveling light wave in a gas medium.

New sub-Doppler absorption resonances in a thin gas cell produced by means of a running monochromatic wave

A theoretical investigation is carried out through the interaction of the plane running monochromatic light wave, having an arbitrary intensity, with atoms (molecules) of a rarefied gas in the plane cell (at the normal incidence of the wave). Cases of closed and open resonance transitions from the non-degenerate ground (or metastable) quantum level are considered. Possible sub-Doppler resonances are analysed in the wave absorption, caused by the transient establishment of the optical coherence on the transition, Rabi oscillations between its levels, and optical pumping during the free flights of particles between the walls of the cell. Results of the previous works on given problems are generalized, which were obtained at definite restrictions on the wave intensity and cell length. Moreover, non-trivial sub-Doppler spectral structures, resulting as a consequence of the dependence of the absorption saturation on the transit relaxation of particles, have been established and investigated. Such structures may consist of a number of peaks and dips caused by Rabi oscillations between the transition levels. The results obtained can be used in sub-Doppler spectroscopy and for the stabilization of laser frequencies in thin gas cells.

Coherent excitation in electron capture collisions

We have calculated the coherent excitation cross sections induced in electron capture collisions with fast impinging ions, within the eikonal approximation taken in its prior form. The cross sections and components ϱ 0 k ( l 1, l 2) of the irreducible tensor associated with the l 1, l 2 density matrix elements, are given in a closed analytical form, which yields directly the total intensity and the polarization. This result generalizes the one obtained by F.T. Chan and J. Eichler (Phys. Rev. A20 (1979) 1811) for electron capture into arbitrary nl shells. The sub-shell cross section values agree with those calculated within the Born approximation. The polarization — about 20% — is higher than that obtained in close coupling methods (i.e. H.G. Morrison and U. Opik, J. Phys. B11 (1978) 473). It is also higher than that measured in electron capture experiments with thin gas-cells.

Ar40(p,d)Ar39reaction atEp=35MeV

The $^{40}\mathrm{Ar}(p,d)^{39}\mathrm{Ar}$ reaction has been studied at a bombarding energy of 35 MeV. A thin planar gas cell target allowed an energy resolution of 30 keV to be achieved. Excitation energies of levels in $^{39}\mathrm{Ar}$ up to 5.6 MeV were obtained, angular distributions being taken from 6\ifmmode^\circ\else\textdegree\fi{} to 40\ifmmode^\circ\else\textdegree\fi{} (laboratory). Distorted-wave Born approximation fits were made to the observed angular distributions of the stronger states and spectroscopic factors extracted. The results are compared with previous experiments and to shell-model calculations. The data support the contention that much of the antianalog state strength is contained in the 3.379 MeV state.NUCLEAR REACTIONS $^{40}\mathrm{Ar}(p,d)$, ${E}_{p}=35$ MeV, $\ensuremath{\Delta}E=30$ keV, measured $\ensuremath{\sigma}({E}_{d},\ensuremath{\theta})$; DWBA analysis, deduced $l$ values and spectroscopic factors. Planar gas cell target.

Auger Spectra of Carbon and Argon Following Ionization by Equal-VelocityαParticles and Deuterons

The spectra of carbon $K$- and argon $L$-shell Auger electrons resulting from ionization produced by 0.15-0.50-MeV/amu $\ensuremath{\alpha}$ particles and deuterons incident on a thin gas cell have been measured. Absolute ionization cross sections were obtained from the Auger-electron yields and, in addition, the yields obtained with equal-velocity projectiles were used to test the projectile $z$ dependence of the ionization process. Intense satellite structure was observed in the Auger spectra and these features were attributed to Auger transitions from multiple vacancy states formed during the collision.