The role of orbiting resonances in the vibrational relaxation of

1. The role of orbiting resonances in the vibrational relaxation of I2(B,v’=21) by collisions with He at very low energies: A theoretical and experimental study A. García-Vela1, Iván Cabanillas-Vidosa2, J.C. Ferrero2, and G.A. Pino2 1Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas, C/ Serrano 123, 28006 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 I2(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 I2(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 I2(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 I2(B,v’=21)-He complex [6]. Calculated total and inelastic partial cross sections for the I2(B,v’=21) + He collisions vs energy Total (elastic plus inelastic) cross sections are obtained for different initial rotational states j’ of I2(B,v’=21,j’) (left figure). In the middle figure partial inelastic cross sections for the channel I2(B,v’=21,j’) + He  I2(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 I2(B,v’=21)-He complex. The positions of three of the theoretical peaks coincide very nicely with those of the experimental peaks (Fig. 1). Fig. 1. Experimental cross sections [1]. Averaged inelastic partial cross sections corresponding to the vibrational relaxation channels I2(B,v’=21) + He  I2(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. 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 I2(B,v’=21)-He orbiting resonances. 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. Conclusions: The cross sections calculated for the low energy collisions of I2(B,v’=21) with He exhibit a pronounced structure of peaks originated by orbiting resonances of the I2(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 I2(B,v’=21)-He complex, and the role played by these resonances in enhancing the I2 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, 144303 (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-2011-29596-C02-01, the Consolider program, MCINN, Spain, Grant No. CSD2007-00013, 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.