Abstract book - Prof. Per Jensen, Ph.D. - Bergische Universität ...

Poster session, J22 185Rovibrational spectroscopy of bending modes of DMSO: “WhenTHz/FIR sources reveal an unusual rotational behaviour”Arnaud Cuisset 1 , Olivier Pirali 2 , Dmitrii A. Sadovskii 11 Laboratoire de Physico-Chimie de l’Atmosphère, Dunkerque, France,arnaud.cuisset@univ-littoral.fr; dima@purple.univ-littoral.fr;2 Synchrotron SOLEIL, Saint-Aubin, France, olivier.pirali@synchrotron-soleil.frCuisset A.Pirali O.Sadovskii D.A.In addition to its importance for industrial and environmental studies, the monitoring ofDiMethylSulfOxyde (DMSO, (CH 3 ) 2 SO) concentrations is of considerable interest forcivil protection. From a spectroscopic point of view, DMSO was long known for beinga near symmetric top and was used to discuss K-doubling and Watson’s S-reduction 1 .The existing high resolution gas phase spectroscopic data only cover the pure rotationaltransitions in the ground state. Due to a weak vapor pressure, conventional far infra-red(FIR) sources are ineffective for high resolution spectroscopic studies, and theinformation on the rotational structure of the bending vibrational states of DMSOremained inaccessible up until most recently 2 . The exceptional properties of the FIRsynchrotron radiation sources 3 , specifically those of the AILES beamline of SOLEILthat became operational at the end of 2008, made this study possible.The ν 23 and ν 11 perpendicular and parallel bands associated with the asymmetric andsymmetric bending modes of DMSO were recorded at room temperature with aresolution of 0.0015 cm −1 using the IFS 125 FTIR spectrometer connected to amultipass cell with a 150 m optical path. The detailed analysis of the simplest ν 11parallel band yielded accurate rotational and centrifugal distortion constants for both theground and excited states. More than 2700 b-type and c-type transitions have beenassigned within the experimental accuracy. A long and difficult work of analysis hasbeen undertaken to assign the very congested ν 23 bending vibration of DMSO. The keyto the ν 23 band was in the series of fairly regular P Q and R Q “branchettes” with ∆K c = ±1situated off the unresolved main Q branch. After finding and combining the reciprocalP Q Kc (J ) and R Q Kc-2 (J ) lines so that their frequency differences reproduce to at least 10 −3the lower state splittings known very accurately from 4 , a computer aidedassignment procedure based on a systematic search of all combination frequenciesbecame possible. Subsequently to the assignment of ν 23 more than 7600 a-type and c-type rovibrational lines, we discovered a sequence of four-fold degenerate clusters ofrotational levels at high J value (J>40). This unusual system of localized statescorresponds to classical rotations about a pair of “tilted” axes which become stationaryat high J after the principal axes A looses stability and bifurcates for J=27. Recentexperiments, using a frequency multiplication chain, aim to observe the formation ofdegenerate four-fold clusters probing pure rotational transitions in the ν 23 and ν 11excited states.cm −1References[1]. V. Typke, J. Mol. Struct. 384, 35, (1996).[2]. A. Cuisset, L. Nanobashvili, I. Smirnova, R. Bocquet, F. Hindle, G. Mouret, O.Pirali, P. Roy, and D. A. Sadovskii, Chem. Phys. Lett. 492, 30 (2010).[3]. A. R. W. McKellar, J. Molec. Spectrosc. 262, 1 (2010).[4]. L. Margulès, R. A. Motiyenko, E. A. Alekseev, and J. Demaison, J. Molec.Spectrosc. 260, 23 (2010)