Atomic Reservoirs Research Articles

Structure and energetics of boron nitride fullerenes: The role of stoichiometry

We apply first-principles calculations to study the structure and the relative stability of several stoichiometric and nonstoichiometric boron nitride fullerenes of sizes ranging from 28 to 128 atoms. Among the structures considered, we observe an overall tendency for the formation of octahedrally faceted shapes as the atomic number increases, with more spherical shapes being prevalent only for small nonstoichiometric cages. From these observations, we expect that experimentally observed BN fullerenes of diameters of $\ensuremath{\sim}12 \AA{}$ or larger to be polyhedrally faceted regardless of their stoichiometry. We also address the relative stability of the fullerenes in both nitrogen-rich and boron-rich environments. For the smaller molecules, we find that the stoichiometric ${\mathrm{B}}_{16}{\mathrm{N}}_{16}$ is more stable than the nonstoichiometric structures with similar dimensions, in both environments. For the larger sizes, the nonstoichiometric ${\mathrm{B}}_{32}{\mathrm{N}}_{36}$ and ${\mathrm{B}}_{60}{\mathrm{N}}_{64}$ are found to be more stable than their stoichiometric counterparts in a nitrogen-rich environment, while the stoichiometric structures are the most stable under boron-rich conditions. These conclusions remain largely unchanged, when we consider the gas-phase limit of atomic reservoirs.

Role of lasers in analytical atomicspectroscopy: where, when and why

Nowadays, a laser source that can be tuned to a particular atomic or molecular transition is a standard tool in many laboratories and traditionally, this source has been identified with the dye laser. Yet, many laser systems tend to be too complex and expensive to acquire, so that their deployment in routine applications has always been, and still remains, rather limited. In addition, the field of analytical spectrometry seems to be more need-driven than technique-driven. Laser and associated methods, in particular fluorescence and ionisation with excimer-pumped dye lasers, and absorption with diode lasers, are the only methods capable of performing sophisticated diagnostics in most commonly used atomic and molecular reservoirs such as flames, plasmas and graphite furnaces. Indeed, lasers have been put to work in these atomisers to derive highly resolved (spatial and temporal) maps of essential physical parameters such as temperature, electron density, atomisation and ionisation efficiency, and to study complex excitation–ionisation mechanisms, line profiles and matrix interferences. From the stricter analytical point of view, despite the fact that the use of lasers has led to the ultimate performance in terms of detection limits (single atom and molecule), other techniques, such as ICP-MS are superior in terms of versatility, due to their multielement capabilities. It is therefore natural to ask the question of who would need lasers in analytical atomic spectrometry. In this context, there are some areas in which the use of laser techniques will still be the best choice, either because it is the ‘absolute’ limit of detection that matters, as for example in laser ablation and surface studies (microanalysis) or because they can provide the ultimate best in terms of selectivity and sensitivity for typical elements and applications (e.g., the determination of radionuclides), or finally because they can provide data much faster than other conventional methods (e.g., real time monitoring of selected metals and pollutants in the atmosphere). Several of these applications will be presented here, in particular, those that better respond to the three questions expressed in the title, i.e., where, when and why lasers would be the only choice. Some examples will be taken from on-going activities on the use of laser photofragmentation, fluorescence and plasma emission in the characterisation of atmospheric aerosols. The development of new, all solid state laser sources, continuously tuneable from the low UV to the IR is growing at an ever increasing rate due to the continuous search and discovery of new lasing materials and to new pumping and frequency conversion techniques. Diode-pumped, solid state lasers offer the most potential for increased commercial acceptance, new vibronic lasers extend their operating wavelength range and optical parametric oscillators are becoming serious candidates for replacing standard pulsed excimer-pumped dye lasers. Last but not least, semiconductor diode lasers are used as alternative sources to HCLs in atomic absorption measurements. A ‘universal’ laser spectrometer, based on these developments and capable of responding to all essential analytical and spectroscopic requirements and applications, is foreseen on the horizon. With such a system, all remaining hurdles that have limited the use of lasers to special situations where ultrahigh sensitivity is critical and cost and complexity are secondary criteria, should be surmounted. Finally, brief mention will be made of how lasers have been used to make an atomic system transparent at selected frequencies (electromagnetically induced transparency), making laser action possible without population inversion. This outcome can be exploited to push practical laser operation into the vacuum UV and X-ray regions.

Detection limits of metal species extended by OGE

Scientists at the National Bureau of Standards (NBS) recently reported on the opto‐galvanic effect (OGE) which can be monitored electrically to eliminate problems associated with laser scatter in analytical spectrometry and spectroscopic applications. These studies of interference effects have also expanded the limits of detection of some metal species.Peter K. Schenck, John C. Travis, Kermit C. Smyth, and David S. King of NBS described their experiments with discharges involving both continuous wave and pulsed dye lasers to irradiate hollow cathode lamps usually used for spectroscopic frequency calibration and atomic absorption analysis. Since the lamps are convenient atomic reservoirs, changes induced in the conductivity of their discharges may readily be observed.

The Fokker-Planck equations for the continuum mode laser with spatially inhomogenous dissipation and excitation

The multivariate nonlinear Fokker-Planck equation is derived from first principles in the case that noth the losses and the inversion in the laser are spatially inhomogeneous. After introducing the second quantized multimode hamiltonian for such a system the secular master equations for the radiation density operator is calculated. The atomic operators are eliminated by tracing over the atomic reservoirs. The master equations is transformed into a multimode Fokker-Planck equation by means of the coherent state formalism. The linear gain, saturation and diffusion matrices are presented explicitly in the quasi-plane-wave approximations.

Stationary momentum space solution of the Fokker-Planck equation for a simple model of a laser oscillator exhibiting spatial dispersion

Starting from a multimode hamiltonian for a system of radiating oscillators coupled with atomic reservoirs, the secular master equation for the radiation-density operator is calculated in the interaction picture after elimination of the atomic variables. Using the differential operator representation for coherent states this equation is transcribed into a multimode Fokker-Planck equation. The stationary solution in momentum space is given for the threshold region. Fourier transformation to configuration space results in a quasi-free energy formula for a laser oscillator exhibiting spatial dispersion.

Quantum Theory of an Optical Maser. II. Spectral Profile

In Paper I of this series, we derived equations of motion for a quantum-laser field interacting with atomic reservoirs. In the usual region of sustained oscillation, the off-diagonal elements of the radiation density matrix ${\ensuremath{\rho}}_{n,n+k}(t)$ were found to have an expontential decay associated with phase diffusion. In I we found the spectrum of the laser radiation by calculating the single time-ensemble average electric field implied by ${\ensuremath{\rho}}_{n,n+1}(t)$. This electric field was then treated as a classical variable whose Fourier analysis gave the spectrum. In the present paper, we establish the validity of this procedure by analyzing a simple model for a spectrometer. It is also shown that the same spectrum can be obtained from a two-time correlation function derived from the equations of motion.