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

Contributed Lectures, G4 109Spontaneous Emission between ortho- and para-Levels ofWater Ion, H 2 O +Keiichi Tanaka 1,2 , Kensuke Harada 2 , Sinkoh Nanbu 3 , and Takeshi Oka 41 National Chiao Tung University, Taiwan, ktanaka@nctu.edu.tw; 2 Kyushu University,Japan, ktanaka@chem.kyushu-univ.jp; 3 Sophia University, Japan, 4 University ofChicago, USANuclear spin conversion interaction of the water ion, H 2 O + , has been studied to derivethe spontaneous emission lifetime between the ortho- and para-levels. The H 2 O + ion isa radical with 2 B 1 electronic ground state and the off-diagonal electron spin-nuclear spininteraction term, T ab (S a ∆I b + S b ∆I a ), connects ortho and para levels, because ∆I = I 1 –I 2 has nonvanishing matrix elements between I = 0 and 1. The mixing by this term withT ab = 72 MHz, predicted by an ab initio theory in MRD-CI/Bk level 1 , is many orders ofmagnitude larger than that for closed shell molecules because of the large magneticinteraction due to the unpaired electron.With the molecular constants reported by Mürtz et al. by FIR-LMR 2 , we searched orthoand para coupling channels below 1000 cm -1 with accidental near degeneracy betweenortho and para levels. For example, hyperfine components of the 4 2,2 (othro) and 3 3,0(para) levels mix by 1.2 x 10 -3 due to the near degeneracy (∆E = 0.4174 cm -1 ), and givethe ortho-para conversion lifetime of about 0.03 – 10 year for these levels. Due to thelarge dipole moment of H 2 O + , µ b = 2.37 D 3 , the water ion generated in the higherrotational states by the reaction of OH + + H 2 H 2 O + + H + ∆ relax within a fewminutes, to the lowest rotational levels, 1 0,1 (para ) and 0 0,0 (ortho), respectively. Themost significant lower lying 1 0,1 (para) and 1 1,1 (ortho) levels, on the contrary, mix onlyby 8.8 x 10 -5 because of their large separation (∆E =16.267 cm -1 ) to give thespontaneous emission lifetime from 1 0,1 (para) to 0 0,0 (ortho) of 520-5200 year.These results qualitatively help to understand the observed high ortho to para ratio of4.8 ± 0.5 toward Sgr B2 4 , but they are too slow to compete with the reaction by collisionunless the number of density of H 2 in the region is very low (n ~ 1 cm -3 ) or the radiativetemperature is very high (T >100K).Tanaka K.Harada K.Nanbu S.Oka T.Fig 1: Spontaneous emission between 1 01 (para) and 0 00 (ortho).References[1] Mol. Phys. 80,1485 (1993). [2] J. Chem. Phys. 109, 9744 (1998). [3] J. Chem.Phys. 91, 2818 (1989). [4] A&A. 521, L11 (2010).