Original Line Shape Research Articles

A systematic study of the hydration and drying process of silica xerogels using Cu(II) EPR spectroscopy

The influence of the hydration and drying process on the line shape and signal intensity of the electron paramagnetic resonance spectra recorded from Cu(II) ions present in silica xerogels calcined at various temperatures was investigated. The experimental Cu(II) electron paramagnetic resonance spectra were found to consist of a superimposition of three individual subspectra (Γ1, Γ2 and Φ), which reflect different local environments in which the Cu(II) ions were located. The results demonstrate that: (i) Within experimental error, the spin Hamiltonian parameters of each individual subspectrum remain, in the course of the experiments, identical. (ii) The hydration process changed the relative contribution from the individual subspectra (Γ1, Γ2 and Φsignificantly, and increased the overall electron paramagnetic resonance signal intensity by a factor of more than ten, as compared with the non-hydrated silica xerogels. (iii) On re-drying the hydrated silica xerogel samples, the original line shape and original signal intensity values were restored. Thus, measurement of the relative contributions of the individual subspectra can be used as a sensitive method with which to monitor the hydration/drying process in silica xerogels. As a caveat, we conclude that the influence of the hydration/drying process should be taken into account in the interpretation of Cu(II) electron paramagnetic resonance spectra of calcined silica xerogel samples, which provides the real novelty of the present report. Relative contribution of subspectra (Γ1, Γ2 and Φ) to Cu(II) EPR spectra of silica xerogels calcined at the temperatures quoted and then hydrated for 15 min and for 3 days.

Absorption-line-shape recovery beyond the detection-bandwidth limit: Application to the precision spectroscopic measurement of the Boltzmann constant

A theoretical model of the influence of detection bandwidth properties on observed line shapes in laser absorption spectroscopy is described. The model predicts artificial frequency shifts, extra broadenings and line asymmetries which must be taken into account in order to obtain accurate central frequencies and other spectroscopic parameters. This reveals sources of systematic effects most probably underestimated so far potentially affecting spectroscopic measurements. This may impact many fields of research, from atmospheric and interstellar physics to precision spectroscopic measurements devoted to metrological applications, tests of quantum electrodynamics or other fundamental laws of nature. Our theoretical model is validated by linear absorption experiments performed on H2O and NH3 molecular lines recorded by precision laser spectroscopy in two distinct spectral regions, near- and mid-infrared. Possible means of recovering original line shape parameters or experimental conditions under which the detection bandwidth has a negligible impact, given a targeted accuracy, are proposed. Particular emphasis is put on the detection bandwidth adjustments required to use such high-quality molecular spectra for a spectroscopic determination of the Boltzmann constant at the 1 ppm level of accuracy.

SURPRISES: when ab initio meets statistics in extended systems

The surface photoelectron and inner shell electron spectroscopy (SURPRISES) program suite performs ab initio calculations of photoionization and non-radiative decay spectra in nanoclusters and solid state systems by using a space-energy similarity procedure to reproduce the band-like part of the spectra. This approach provides an extension of Fano resonant multichannel scattering theory dealing with the complexity arising from condensed matter calculations at a computational cost comparable to that of molecules. The bottleneck of electron spectroscopy ab initio calculations in condensed matter is the size of the Hilbert space where the wavefunctions are expanded and the increase in number of final decay states in comparison to that of atoms and molecules. In particular, the diagonalization of the interchannel interaction to take into account the correlation between the double ion and the escaping electron is impracticable when hole delocalization on valence bands and electronic excitations are included in the model. To overcome this problem SURPRISES uses a 'space-energy similarity' approach, which allows the spreading of the Auger probability over the bands without tuning semi-empirical parameters. Furthermore, a completely new feature in the landscape of ab initio resonant decay processes calculations is represented by including energy loss through a statistical approach. Using the calculated lineshape as electron source, a Monte Carlo routine simulates the effect of inelastic losses on the original lineshape. In this process, the computed spectrum can be directly compared with acquired experimental spectra, thus avoiding background subtraction, a procedure not free from uncertainty. The program can exploit the symmetry of the system under investigation to reduce the calculation scaling and may compute photoemission and Auger decay angular distribution patterns including energy loss for the electrons emitted in resonance-affected photoionization processes. In this paper, we present general methods, computational techniques and a number of numerical tests applied to the calculation of Si K–LL and O K–LL Auger spectra from different SiO2 nanoclusters.

A simple scanning semiconductor diode laser source and its application in wavelength modulation spectroscopy around 825 nm

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The diode laser controller is stable within +/−8 μA and +/−10 mK, respectively. The noise spectrum of the current controller is studied by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating temperature to probe (2, 1, 1)←(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier help to obtain a S/ N ratio ranging from ∼100 to ∼7 using a small sample path length of 1.5 m. Experimentally obtained derivative spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from HITRAN database.

Line-shape study of water vapour by tunable diode laser spectrometer in the 822?832�nm wavelength region

A near-infrared tunable diode laser absorption spectrometer is set up to measure the air-induced broadening coefficients and the line-strength parameters of water-vapour overtone transitions within the (2,1,1)←(0,0,0) band in the 822–832 nm wavelength region. A Hitachi HL8311 E double hetero-junction structure diode laser is used as a probe. The diode laser controller is home-built and stable within ±10 μA and ±10 mK, respectively. The laser-head mount has a simple design and provides easy access whenever changing of the laser head is required. The diode laser emission wavelength is thermally tuned between 50 °C and 12 °C. Thermal tuning of the diode laser emission wavelength is used to reveal the mode structure of the diode laser and to probe the overtone-band transitions of water vapour within its operating wavelength range. Current tuning of the diode laser is used at a fixed laser temperature to study the transitions one at a time. A balanced detector is used to improve the S/N ratio of the spectrum. A phase sensitive detection technique is followed to obtain the first-derivative spectra of the overtone transitions. The first-derivative spectra have been recorded at different air pressures inside the sample cell while the water-vapour pressure is kept fixed. The first-derivative spectrum is numerically integrated to obtain the original line shape. The original line shape is fitted with a Voigt profile by using a nonlinear least-squares fit program to extract the air-broadening coefficient and the line-strength parameter. The data obtained in our work is compared with the results of the HITRAN database.

Highly accurate smoothing filter

An iterative filter for the smoothing of degraded line shapes with Gaussian noise, and a signal to noise ratio defined in a wide interval 20–∞ is presented. The smoothed line shape shows errors below 2% in its height and width parameters with regard to the true values used in the simulation. The filter reduces the Gaussian noise to less than a half. In the tests carried out with a PC computer, the new filter proved that the original line shape for a linewidth of 5–30 is better recovered than the Savitsky–Golay filter. The application of the procedure only involves the use of the linewidth parameter of the degraded peak which simplifies the smoothing process. The filter follows two basic operations: first, an oversmoothing is applied to the degraded line and second, an iterative enhancement on the oversmoothing spectrum is made. The resulting spectrum of these two operations is compared with the degraded line by a least squares fitting method. When the filter is used in a simplified version, it is useful not only to determine very overlapped lines in a degraded spectrum, but also to calculate the background in an experimental spectrum.

Influence of multiple elastic and inelastic scattering on photoelectron line shape.

The influence of multiple elastic and inelastic scattering on photoelectron spectra from semi-infinite random solids has been investigated. A theoretical expression describing the energy and angular yield has been derived which adequately accounts for the anisotropic source emission of photoelectrons. Comparison of theory with results from an efficient Monte Carlo code yields very satisfactory agreement. The resulting energy and angular distribution for four subshell transitions in gold are presented. A pronounced dependence of the particle transport on the line shape is seen in the results for different geometries. As an application of the presented theory, a procedure to correct the observable experimental spectrum in the quasielastic regime for multiple elastic and inelastic collisions is developed and tested. In all studied cases the proposed approach yields background corrected spectra agreeing within 3% with the original line shape, while methods not accounting for the anisotropic source emission generally display discrepancies of up to 30% in the background intensity. The main advantage of the proposed approach lies in its generality. It not only accurately describes the influence of particle transport on the energy and angular distribution of Auger electrons and photoelectrons, but can also be applied in a straightforward manner to many other spectroscopic techniques using electrons, ions, or other probing particles.