* Corresponding author. Tel.: 00-91-55367; fax: 00-91-08-554-55244. Journal of Quantitative Spectroscopy & Radiative Transfer 67 (2000) 85}90 Note Ionization potential of astrophysically important molecules R.R. Reddy*, Y. Nazeer Ahammed, K. Rama Gopal, P. Abdul Azeem, T.V.R. Rao Department of Physics, Sri Krishnadevaraya University, Anantapur 515 003, India Received 25 January 1999 Abstract Some formulae relating electronegativity di!erences, ionization potentials and dissociation energies for astrophysically important molecules are proposed. The relations are as follows: I  "I M  #0.143e (when I M  (I  ), I  "I M  #0.54(D  !D M  ) (when I M  (I  ), I  "I M  !e (when I M  'I  ), I  "I M  !1.85(D  !D M  ) (when I M  'I  ), where I M  "(I  I  ) and D M  "(D  D  ). I  and I  are the ionization potentials and D  and D  the dissociation energies of homonuclear molecules of the corresponding atoms of the heteronuclear molecule. D  represent the dissociation energy of the heteronuclear molecule and  the Pauling elec- tronegativity di!erence. The estimated ionization potentials are in good agreement with the values cited in the literature.  2000 Elsevier Science Ltd. All rights reserved. 1. Introduction In recent years the discovery of the presence of various types of molecules, from simple to relatively complex forms, in various astronomical objects has excited many scientists. It was realized that the chemical processes involved in the constitution of stars and galaxies. Chemical composition studies of processed material, as found in the envelopes of highly evolved stars and in planetary nebulae, provide checks on our concept of stellar evolution and element building. The 0022-4073/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 0 7 3 ( 9 9 ) 0 0 1 8 6 - 7