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.     

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.     

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.     

On the possibilities of sub-Doppler atomic spectroscopy in multilayer gas cells

Possibilities of high-resolution spectroscopy of atoms (or molecules) for optical pumping and probing with the use of a proposed cell with a series of plane-parallel thin gas layers between spatially separated regions of this cell are theoretically studied. It is shown that efficient velocity selection of optically pumped atoms is possible in view of their characteristic transit and collisional relaxation in such a cell, which leads to the formation of narrow sub-Doppler resonances in absorption of a probe monochromatic wave. The resolution of this spectroscopic method is analyzed in the cases of stationary and definite nonstationary optical pumping of atoms by broadband radiation for different geometrical parameters of these cells and pumping intensity. The proposed multilayer gas cell is a compact analog of a large number of parallel atomic (molecular) beams and can be the basis for new high-precision and compact optical frequency standards.     

Theoretical studies on the photoionization cross-sections of solid silver

An alternative expression for photoionization cross-section of atoms or molecules and a dielectric influence function （DIF） in a high-density system proposed recently are used to study the photoionization cross-sections of solid silver. It is suggested that a density turning point （DTP） of a photoionized system may be viewed as the critical point where the photoionization properties of atoms in a real system may have a notable change. The results show that the present theoretical photoionization cross-sections are in good agreement with the experimental results of a silver crystal both in structure and in magnitude.     

Possibility of the optical separation of the isomeric nuclei by laser radiation

The possibility of using laser radiation for fast optical separation of isomeric nuclei is considered. The proposed method of separation is based on selective two-step photoionization of atoms or photodissociation of molecules.     

An experimental setup for spectroscopy applications of optically generated VUV and XUV radiation

In this paper, we present an experimental setup to study photoionization of atoms and molecules using the high-order harmonic radiation generated by a highly non-linear process. We report on the characteristics of this table-top experimental apparatus showing also the results of the photoionization of krypton gas, by the use of the ninth order harmonic of fs Ti : S laser, observed both in the electron spectrum and in the mass spectrum.     

Hollow cathode discharge (HCD) dark space diagnostics with laser photoionization and galvanic detection

We report on a new technique for measuring the cathode dark space width and the variation of ground state atomic density within it by measuring the ionization current generated by laser photoionization of neutral ground state metallic atoms. The technique is supported by a theoretical model of charge displacement in the dark space based on the assumption of a Lorentzian gas and thus on Langevin equations. After verifying the applicability of the theoretical approach, measurements of dark space width with respect to pressure, current and nature of the buffer gas are presented for an uranium HCD. Results of variation in density of ground state neutral uranium, in the dark space, versus current and pressure in Xe are also given. These last results are of interest when using photoionization currents in the HCD dark space for laser spectroscopy, such as photoionization studies of elements like uranium.     

Interference effects on the photoionization cross sections between two neighbouring atoms: nitrogen as an example

Interference effects on the photoionization cross sections between two neighbouring atoms are considered based on the coherent scattering of the ionized electrons by the two nuclei when their separation is less than or comparable to the de Broglie wave length of the ionized electrons. As an example, the single atomic nitrogen ionization cross section and the total cross sections of two nitrogen atoms with coherently added photoionization amplitudes are calculated from the threshold to about 60~\AA (1~\AA=0.1~nm) of the photon energy. The photoionization cross sections of atomic nitrogen are obtained by using the close-coupling R-matrix method. In the calculation 19 states are included. The ionization energy of the atomic nitrogen and the photoionization cross sections agree well with the experimental results. Based on the R-matrix results of atomic nitrogen, the interference effects between two neighbouring nitrogen atoms are obtained. It is shown that the interference effects are considerable when electrons are ionized just above the threshold, even for the separations between the two atoms are larger than two times of the bond length of N₂ molecules. Therefore, in hot and dense samples, effects caused by the coherent interference between the neighbours are expected to be observable for the total photoionization cross sections.     

原子分子真空紫外光吸收和光电离绝对截面的精确测量

介绍了一种精确测量原子分子真空紫外光吸收和光电离绝对截面的实验方法。该方法的主要特点是使用圆筒形多级电极，有效地收集光电离产生的电子和离子，并利用高通量的同步辐射作光源，精确地测量分子（原子）的绝对光吸收、光电离和光离解截面，以及光电离量子产率和光学振子强度分布等。利用该方法获得了Ｘｅ原子和Ｎ２分子的一些实验结果。     

A laser photoionization method for the study of the CdS evaporation kinetics

The evaporation rates and saturation vapor pressures of cadmium sulfide and pure metallic cadmium are measured in the temperature ranges 585–950 K and 350–535 K, respectively, using laser resonance multistep photoionization. It is shown that CdS thermally dissociates into Cd atoms and S₂ molecules. The study is carried out in the framework of a scheme proposed earlier for photoionization of cadmium atoms. The measured minimum vapor pressure of pure cadmium is 10^?11 Torr.     

Highly efficient state-selective submicrosecond photoionization detection of single atoms

We experimentally demonstrate a detection scheme suitable for state analysis of single optically trapped atoms in less than 1 μs with an overall detection efficiency η exceeding 98%. The method is based on hyperfine-state-selective photoionization and subsequent registration of the correlated photoion-electron pairs by coincidence counting via two opposing channel electron multipliers. The scheme enables the calibration of absolute detection efficiencies and might be a key ingredient for future quantum information applications or precision spectroscopy of ultracold atoms.     

Contribution to the theory of non-self-maintaining volume discharges in molecular and noble gases

Results are presented of investigations of non-self-maintaining volume (NV) gas discharges. The structure is investigated and the current-voltage characteristics are plotted of NV discharges in molecular gas. The characteristic times of the transient processes in the cathode layer of a nonstationary electron-beam-controlled (EBC) discharge are obtained. It is proposed to use an NV discharge as the nonlinear element of a relaxation-oscillation laser. Results are presented of computer experiments with such a laser. The influence of the hydrodynamic motion of the mixture on the characteristics of the cathode layer of an open-cycle cw EBC laser is investigated. It is shown that the ultraviolet emission of excimer molecules alters radically the structure of the cathode layer of an EBC discharge in a noble gas, viz., the intense photoeffect on the cathode leads to the appearance of a negative cathode potential drop. Self-maintaining photoionization and EBC discharges in a noble gas are considered; in these discharges the plasma density is maintained by impurity ionization or by electron-excitation of the atoms by emission from excimer molecules.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 142, pp. 46–94, 1983.     

Cooling and trapping of atoms and molecules by a resonant laser field

A method is suggested for simultaneous cooling and trapping of atoms and molecules in a low-pressure gas under forces caused by recoil during spontaneous or induced transitions of the particles in the resonance field of a three-dimensional standing light wave. It is shown that at light field intensities ～0.01–0.1 W/cm² it is possible to cool atoms and molecules to the resonance field photon momentum and to hold the particles in the light field volume during a long period of time. The proposed approach opens the way for high-resolution Doppler-free spectroscopy of a small number of atoms and molecules.     

Synchrotron radiation VUV double photoionization of some small molecules

The VUV double photoionizations of small molecules (NO,CO,CO2 ,CS2 ,OSC and NH3 ) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO2+ ,CO2+ ,CO2+2 ,CS2+2 ,OSC2+ and NH2+3 ) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then,the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.     

Sub-doppler absorption resonances induced in a gas cell by transverse optical pumping

New sub-Doppler resonances at central frequencies of atomic (molecular) transitions that appear in the spectrum of absorption of the probe optical radiation under the influence of optical pumping propagating in the orthogonal direction through a relatively narrow area of a cylindrical cell containing dilute gas medium are discovered and analyzed. These resonances are induced by specific optical pumping of atoms as they fly freely from the inner cell surface through the pumped region toward the probe optical beam. The obtained mathematical relations are used to investigate the dependence of the discussed resonances on the intensity and spatial distribution of the localized optical pumping. The proposed method could allow reducing the Doppler broadening of the detected spectral lines by the factor equal to the ratio of the effective width of the narrow pumped region to the cell radius. The obtained results may find application in high-resolution spectroscopy of atoms (molecules), as well as for laser-frequency stabilization by using the discovered sub- Doppler resonances.     

Photoionization of atoms in parallel electric and magnetic fields: Cross-section calculations using the Lanczos algorithm and the complex coordinate method in a discrete variable representation

A combination of the complex-coordinate method and the Lanczos recursion scheme is implemented in the discrete variable representation (DVR) to obtain total photoionization cross-sections using an iterative procedure. Applications to photoionization of hydrogen atoms in electric fields and sodium atoms in electric and parallel electric and magnetic fields are presented and discussed. Received 15 May 2000 and Received in final form 4 October 2000