Comment pubs.acs.org/JPCA
Comment on “An ab Initio Study of the E 3Πg State of the Iodine Molecule” Trevor Ridley EaStCHEM School of Chemistry, Joseph Black Building, The King’s Buildings, Edinburgh, EH9 3JJ, U.K.
J. Phys. Chem. A 2012, 116 (9), 2366−2370. DOI: 10.1021/jp3000202 J. Phys. Chem. A 2013, 117. DOI: 10.1021/jp309853g
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Both of these factors suggest that the comparison reported by Kalemos et al.3 of the ab initio PEC for the E state with that derived from experimental measurements7 is not valid and the experimental observations themselves appear to confirm this. First, there is no evidence for the predicted avoided crossing ∼52 000 cm−1.7 However, four E state vibrational levels in the range v′ = 190−208 do interact with v′ = 7−10 of the X[2Π1/2]c;6s 0g+ state, the lowest energy Ω = 0g+ Rydberg state, ∼55 000 cm−1. A second interaction region, ∼63 500 cm−1, involving coupling of v′ = 500−600 of the E state with v′ = 1−3 of the “singlet” X[2Π3/2]c;5d 0g+ state has been observed.8 We note that in the original paper this was wrongly attributed to coupling of the X[2Π3/2]c;7s 1g and β 1g states. It is possible that the calculated PEC can be seen as a reasonable representation of the X[2Π3/2]c;6s 2g/D′ 2g coupled states and a comparison could be made with experimental data for this state. The X[2Π3/2]c;6s 2g Rydberg state has not been observed experimentally but is believed to have a minimum that is close in energy to its 1g partner, i.e., ∼48 400 cm−1. In addition, the dominant configuration of the D′ 2g IP state is believed to be 3Πg (1432). Turning points of a PEC for the D′ 2g IP state up to ∼60 000 cm−1 have been proposed on the basis of a combination of an RKR analysis of v′ = 0−30 and a simulation of the coupling with an X[2Π3/2]c;5d 2g Rydberg state.8,9 Unfortunately, there are no experimental data available for the region of the proposed avoided crossing ∼52 000 cm−1. Nevertheless, it is proposed that it is much more realistic to compare the calculated PEC with experimental data for the D′ 2g IP state rather than with that for the E 0g+ state. The factors discussed above may also impact on a subsequent ab initio calculation of the E 3Πg state of He−I2.10
umerous experimental studies have shown that the spectroscopy of the valence, ion-pair (IP) and Rydberg states of I2 can best be understood by describing their symmetries using Hund’s case c coupling. In a previous paper,1 we presented a full description of how Hund’s case c coupling is applied to the Rydberg states of I2. As an illustration, a Rydberg state might be described as X[2Π3/2]c;6p 1u where X[2Π3/2]c refers to the lower of the two spin−orbit (SO) components of the ground state of I2+, 6p refers to the nl values of the occupied Rydberg orbital, and 1u is Ω, the component of total angular momentum along the molecular axis. The total spin is not always well-defined in this basis. States are either linear combinations of equal weights of singlet and triplet spin, sometimes abbreviated to “singlet”, states or pure triplet states. This description of the Rydberg states was used to interpret which states are excited most strongly by oneand two-photon absorption from the ground state. Similarly, the intensity of IP → valence state emission is determined by the Ω values of the states where, in general, ΔΩ = 0 transitions are more than 2 orders of magnitude more intense than ΔΩ = 1 transitions.2 Kalemos et al.3 have reported ab initio calculations on a mixed Rydberg/IP state, labeled E 3Πg, in which they used Hund’s case a to describe the electronic states. In their calculations the authors included eleven 3Πg states but none of any other spin multiplicity. In this Comment we propose that there are serious oversimplifications in their approach to the problem specifically with the treatment of SO coupling and that the comparisons with experimental data that they presented are not the most appropriate. The Authors’ calculated E 3Πg state potential energy curve (PEC) has minima at 2.59 and 3.59 Å that are Rydberg and IP in nature, respectively, with Te values of ∼48 300 and 41 300 cm−1. Only two Rydberg states, X[2Π3/2]c;6s 2g and 1g (3Π2g and 3Π1g in Hund’s case a), have minima ∼48 300 cm−1.4 The E state in the region of its IP minimum has Ω = 0g+ and hence cannot homogeneously couple to either of these two Rydberg states; only the D′ and β, with Ω = 2g and 1g, respectively, can do so. The X[2Π1/2]c;6s 0g+ Rydberg state inherits the Hund’s case a 3Π0g+ state and lies ∼5000 cm−1 higher in energy because of the SO splitting of the core states.5 In addition, in the calculations the IP state is considered as a pure 3Πg state with electronic configuration (1432) (see ref 3 for shorthand notation), based on a very early experimental study.6 However, this configuration is now commonly believed to be only a minor component of the E state with the major component being 3Σg− (2242).5 © XXXX American Chemical Society
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
REFERENCES
(1) Ridley, T.; Beattie, D. A.; Cockett, M. C. R.; Lawley, K. P.; Donovan, R. J. Phys. Chem. Chem. Phys. 2002, 4, 1398. (2) Lawley, K. P.; Jewsbury, P. J.; Ridley, T.; Langridge-Smith, P. R. R.; Donovan, R. J. Mol. Phys. 1992, 75, 811. (3) Kalemos, A.; Valdés, Á .; Prosmiti, R. J. Phys. Chem. A 2012, 116, 2366. (4) Lehmann, K. K.; Smolarek, J.; Goodman, L. J. Chem. Phys. 1978, 69, 1569.
Received: September 3, 2012
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(5) Miller, J. C. J. Phys. Chem. 1987, 91, 2589. (6) Rousseau, D. L. J. Mol. Spectrosc. 1975, 58, 481. (7) Wilson, P. J.; Ridley, T.; Lawley, K. P.; Donovan, R. J. Chem. Phys. 1994, 182, 325. (8) Lawley, K. P.; Ridley, T.; Min, Z.; Wilson, P. J.; Al-Kahali, M. S. N.; Donovan, R. J. Chem. Phys. 1995, 197, 37. (9) Tellinghuisen, J.; Fei, S.; Zheng, X.; Heaven, M. C. Chem. Phys. Lett. 1991, 176, 373. (10) Kalemos, A.; Valdés, Á .; Prosmiti, R. J. Chem. Phys. 2012, 137, 034303.
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