Raman spectroscopy and inelastic collisions of cold molecules

Conferencia invitada; AMOC 2015, Anharmonicity in médium-sized molecules and cluster, CSIC, Madrid (Spain), 26-30 April 2015; http://tct1.iem.csic.es/AMOC2015.htmRaman spectroscopy of supersonic gas jets can be employed to record the spectra of medium-sized molecules at temperatures well below 100 K, and to study rotationally-inelastic collisions of a number of small molecules [1-5]. Number densities and rotational populations can be directly measured along the jet axis from the rotational and/or vibrational Raman spectra. These measurements can be linked to calculations of state-to-state rate coefficients (sts-rates) by means of the kinetic Master Equation, which accounts for the time evolution of the rotational populations. After an introduction to the experimental technique, we will show some recent results on jets involving O2, H2O(gas), and C2H6 molecules diluted in He. Oxygen molecules are cooled down to ~2 K, showing the relaxation among the fine structure triad of the ground rotational state. The rotational populations of H2O deviate from the Boltzmann distribution, preventing to define a proper rotational >temperature>. In C2H6, the simplification caused by the rotational cooling allows to analyze the complex spectrum of the CH stretching region, subject to a strong Fermi resonance [6]. Our measurements will be compared with calculated sts-rates for O2:He [7], and H2O:He collisions with three different PES¿s [8-10]. The collisional broadening of H2O rotational lines of astrophysical interest will also be discussed.Peer Reviewe