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\title{ELECTRON SPIN-TORSION AND SPIN-ROTATION EFFECTS IN THE\\OPEN-SHELL CH$_3$CO RADICAL$^1$}
\author{\FirstName L.\ H.\ \LastName COUDERT, \FirstName O.\ \LastName PIRALI, \FirstName R.\ \LastName CHAHBAZIAN, \FirstName M.-A.\ \LastName MARTIN-DRUMEL}
\address{Institut des Sciences Mol\'eculaires d'Orsay, Universit\'e Paris-Saclay, Orsay, France} 
\author{\FirstName L.\ \LastName ZOU, \FirstName R.\ A.\ \LastName MOTIYENKO, \FirstName L.\ \LastName MARGUL\`ES} 
\address{UMR 8523 - PhLAM - Physique des Lasers Atomes et Mol\'ecules, University of Lille, CNRS, F-59000 Lille, France} 
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\fancyhead[C]{{\footnotesize\textit{HRMS, 28th Colloquium, Dijon, France, Aug.\ 28 - Sep.\ 1, 2023}}}
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\footnotetext[1]{Supported by the Programme National ``Physique et Chimie du Milieu Interstellaire"
(PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES}\refstepcounter{footnote}%
Although open-shell molecules displaying a fine interaction and
non-rigid molecules displaying a large amplitude motion have
been the subject of numerous spectroscopic investigations,
fewer results are available about molecules displaying {\em
both} a fine interaction and a large amplitude motion. A non-exhaustive
list of such species include the open shell CH$_2$,
CH$_2$OH, and Na$_3$ species displaying\footnote{Ohashi, Tsuura,
Hougen, Ernst, and Rakowsky, {\em J.\ Mol.\ Spec.}~{\bf
184} (1997) 22; Coudert, {\em J.\ Chem.\ Phys.}~{\bf 153}
(2020) 144115; and Coudert, Chitarra, Spaniol, Loison,
Martin-Drumel, and Pirali, {\em J.\ Chem.\ Phys.}~{\bf 156}
(2022) 244301} respectively a large amplitude bending, torsion,
and pseudorotation.  The acetyl radical
CH$_3$CO is a benchmark molecule for this subclass of molecules
as it presents a low barrier internal rotation of its methyl
group and a fine interaction. In the first spectroscopic
investigation of the radical,\footnote{\label{hirota}Hirota, Mizoguchi, Ohsima,
Katoh, Sumiyoshi, and Endo, {\em Mol.\ Phys.}~{\bf 105} (2007)
455} a few low $J,$ $K_a=0$ transitions were measured in its microwave
spectrum and a new theoretical model was developed to model
its spectrum.

In this poster, new measurements carried out in the
submillimeter wave spectrum of CH$_3$CO
will be presented. More than 300 transitions have
been assigned up to $N=22$ and $K_a=5$ in the ground torsional state
for both $A$ and $E$ torsional species
and for both $F_1$ and $F_2$ electron spin-rotation components.
Based on a newly developed theoretical approach, a line
frequency analysis has been carried out and line frequencies
were reproduced with a unitless standard deviation of 1.2.

The analysis shows that two effects, ignored in the
pioneering approach of Hirota {\em et al.,}\addtocounter{footnote}{-1}\footnotemark\
are quite important. These are the electron spin-torsion coupling,
described by a term in $p_{\alpha} S_z$, where $p_{\alpha}$
is the momentum conjugated to the torsional angle $\alpha$,
and a dependence of the electron spin-rotation coupling tensor
on this torsional angle. In the poster the new
measurements will be presented and the  spectroscopic
parameters will be reported. These include
the usual torsion-rotation parameters
$A$, $B$, $C$, $D_{ab}$, $\rho$, and $V_3$, and the
new fine interaction-torsion parameters.

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