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\fancyhead[C]{{\footnotesize \textit{HRMS, 28th Colloquium, Dijon, France, Aug. 28 - Sept. 1, 2023}}}
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\title{SATURATED ABSORPTION SPECTROSCOPY AND RESONANCE SHAPE OF HD AROUND 1.4 $\upmu$m}

\author{\underline{\FirstName PATRICK \LastName DUPR\'E}}
\address{Laboratoire Interdisciplinaire Carnot de Bourgogne, 9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France}

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The observations of ``non-standard'' molecular resonance shapes
under Saturated Absorption spectroscopy (like Lamb dip) have generated
an abundant literature. Molecular spectroscopy for metrology applications,
as well as for probing the Physics beyond the Standard Model or for
benchmarking the quantum chemistry codes are the core of recent and
highly accurate experiments on simple systems. However, the available
interpretations of the observed resonance shapes of weak transitions
remain largely controverted. The interaction potential between species
plays probably a key role, but it remains extremely difficult to investigate,
and only simplified collisional models can be tested experimentally.
Nevertheless, the low pressure regime emphasizes the relative role
of the spectral ``broadening'' due to the finite interaction-time
versus the collisional relaxations.

A tentative to bring another brick to the wall will be presented.
More specifically, recent progresses in the simulation of the line
profile to evaluate the relative contributions of the following effects
will be addressed:
\begin{itemize}
\item The crossover resonances arising when 2 close energy levels are coupled
by counter propagating electromagnetic fields through a third level
($V$- or $\Lambda$-configuration), this regards typically with molecular
systems exhibiting fine and/or hyperfine structures, even when weak
dipole moments are involved.
\item The collisional regime under Gaussian (at the opposite of purely monochromatic)
electromagnetic fields, such conditions are currently obtained at
low pressure when sensitive absorption techniques are employed.
\item Collisional quadratic speed dependence (broadening and frequency shift)
can generate asymmetry profiles (among other options).
\end{itemize}

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