The role of orbiting resonances in the vibrational relaxation of I 2 (B,v’=21) by collisions with He at very low energies: A theoretical and experimental study A. García-Vela 1, Iván Cabanillas-Vidosa 2, J.C. Ferrero 2, and G.A. Pino 2 1 Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, C/ Serrano 123, Madrid, Spain 2 Centro Láser de Ciencias Moleculares, INFIQC, Departamento de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, 500 Córdoba, Argentina Introduction: In the last years there has been a controversy about whether the origin of the unexpectedly large cross sections found experimentally for the I 2 (B,v’) vibrational relaxation induced by collisions with He at very low collision energies was related to the presence of orbiting resonances [1-3]. More recently, measured cross sections for I 2 (B,v’=21) vibrational relaxation upon low temperature collisions with He exhibited for the first time clear peaks at given collision energies (Fig. 1) that were attributed to orbiting resonances of the I 2 (B,v’=21)-He vdW complex formed in the low energy collisions [4,5]. Further recent wave packet simulations (assuming zero total angular momentum) confirmed that the peaks in the experimental cross sections are the signature of orbiting resonances of the I 2 (B,v’=21)-He complex [6]. Conclusions: The cross sections calculated for the low energy collisions of I 2 (B,v’=21) with He exhibit a pronounced structure of peaks originated by orbiting resonances of the I 2 (B,v’=21)-He van der Waals complex formed upon the collisions. This structure of peaks is similar to that found in the experimental cross sections. Actually, out of the five peaks found in the measured cross sections, the first four peaks (at 0.82, 1.17, 1.67, and 2.7 cm -1 ) have nearly coincident positions with those of four of the theoretical peaks. This result confirms that the peaks of the experimental rate constants and cross sections are originated by orbiting resonances of the I 2 (B,v’=21)-He complex, and the role played by these resonances in enhancing the I 2 vibrational relaxation. References [1] J. Tusa, M. Sulkes, and S.A. Rice, J. Chem. Phys. 70, 3136 (1979). [2] C. Cerjan and S.A. Rice, J. Chem. Phys. 78, 4952 (1983). [3] W.R. Gentry, J. Chem. Phys. 81, 5737 (1984). [4] I. Cabanillas-Vidosa, G.A. Pino, C.A. Rinaldi, and J.C. Ferrero, Chem. Phys. Lett. 429, 27 (2006). [5] I. Cabanillas-Vidosa, C.A. Rinaldi, G.A. Pino, and J.C. Ferrero, J. Chem. Phys. 129, (2008). [6] A. García-Vela, I. Cabanillas-Vidosa, J.C. Ferrero, and G.A. Pino, Phys. Chem. Chem. Phys. 14, 5570 (2012). Acknowledgements: A. G.-V. was funded by CICyT, Ministerio de Ciencia e innovación (MCINN), Spain, Grant No. FIS C02-01, the Consolider program, MCINN, Spain, Grant No. CSD , COST Action program, Grant No. CM1002, and the Centro de Supercomputación de Galicia (CESGA).The experimental work was supported by CONICET, FonCyT, SeCyT, and MinCyT Córdoba. Fig. 1. Experimental cross sections [1]. Averaged inelastic partial cross section corresponding to the Δv’=0 channel, obtained by weighting each j’ contribution to the cross section with an equiprobable distribution assigning a weight 1/10 to each j’ contribution. The positions of the peaks are essentially the same as those of the peaks of the cross sections averaged with the Maxwell- Boltzmann distribution, indicating that the peak positions found theoretically are independent on the weighting distribution used to average the cross section, and that these peaks actually reflect the positions of the I 2 (B,v’=21)-He orbiting resonances. Calculated total and inelastic partial cross sections for the I 2 (B,v’=21) + He collisions vs energy Total (elastic plus inelastic) cross sections are obtained for different initial rotational states j’ of I 2 (B,v’=21,j’) (left figure). In the middle figure partial inelastic cross sections for the channel I 2 (B,v’=21,j’) + He  I 2 (B,v’’=21,j’’) + He (due to tunneling) obtained for several initial j’ states are shown. Averaged total and inelastic partial cross sections are obtained by summing the j’=0-9 contributions weighted with a Maxwell-Boltzmann distribution corresponding to a temperature T=0.5 K (right figures). The averaged cross sections present a series of peaks which correspond to the positions of the orbiting resonances of the I 2 (B,v’=21)-He complex. The positions of three of the theoretical peaks coincide very nicely with those of the experimental peaks (Fig. 1). Experimental rate constants and cross sections for the Δv’=0 and Δv’<0 vibrational relaxation channels [6]. In these new experimental cross sections a new peak (albeit weak) at around 2.7 cm -1 is found. It is noted that this peak position agrees very well with a theoretical peak found at 2.71 cm-1 for the Δv’=0 channel. Another point of agreement between experiment and theory is that the structure of peaks is more pronounced for the Δv’=0 channel than for the Δv’<0 vibrational relaxation channels. Averaged inelastic partial cross sections corresponding to the vibrational relaxation channels I 2 (B,v’=21) + He  I 2 (B,v’’=v’-1, v’-2, v’-3) + He (the Δv’=-1, -2, and -3 channels) using the Maxwell-Boltzmann distribution. The cross sections exhibit two peaks at 0.11 and 0.39 cm -1, and a broader bump around 2 cm -1. The vibrational relaxation process being faster than the tunneling process associated with the Δv’=0 channel would produce broader peaks that would give rise to the less resolved structure of these cross sections. The absence of J>0 contributions in the calculation could also be responsible of the less resolved structure of peaks.