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Accueil > Séminaires > Archive des séminaires d’Utinam > 2017

Pasquale Amodio

A global approach for precision determination of molecular spectroscopic parameters

lundi 29 mai 2017, 14h

salle de conférences de l’observatoire

Pasquale AMODIO
Université de la Campanie "Luigi Vanvitelli" (Université de Naples II), Italie

Résumé :

In recent years there is a growing interest in an update of the International System of units (SI), it consists in a coherent system of unit measurements based on seven units, called fundamental units. Some of the basic units continue to be defined in terms of material substance or of a specific procedure of realization, as happens for the kilogram and the Kelvin. To overcome this problem, the General Conference on Weights and Measures proposed a new definition of the Kelvin and the Kilogram units. The changes proposed for the SI will be adopted only after a further refinement of the experimental results on the new definitional constants. In particular, the redefinition of the kelvin unit, will be built in terms of the Boltzmann constant, kB.

One of more interesting method to determine the kB value with high accuracy and precision is the Doppler Broadening Thermometry (DBT). Based upon precision laser spectroscopy in the linear regime of interaction, Doppler broadening thermometry (DBT) consists in retrieving the Doppler width from the highly-accurate observation of the absorption profile corresponding to a given atomic or molecular line in a gaseous sample at the thermodynamic equilibrium. This method, to determine the absolute temperature of a gaseous system to thermodynamic equilibrium, is very promising and has the advantage of being conceptually simple, applicable to any gas at any temperature, in whatever spectral region.

Even in the case of a well isolated spectral line, under the influence of binary collisions, the molecular line shape exhibit clear deviations from the time honored Voigt profile and can be quite complicated by joint occurrence of velocity-changing collision and speed dependent effects. To determine the kB value it is necessary to improve experimental techniques, on the one hand, and develop refined procedures of spectral analysis, on the other hand. In particular, when doing nonlinear least-square fits of experimental spectra by using quite sophisticated line-shape models that involve a relatively large number of free parameters, the statistical correlation becomes a relevant issue that limits the achievable precision in the retrieval of spectroscopic parameters.

To overcome this limitation we develop a global analysis procedure to simultaneously fit spectra from a single (and well isolated) line across a given range of pressure, sharing a number of free parameters, including the velocity-changing collision parameter, pressure broadening and shift parameter and Doppler Broadening. By calculating the covariance matrix, it comes out that the correlation coefficient can be significantly reduced as compare to individual fits.