Regular Article

Excitation of infrared-inactive vibrational modes in a polyatomic molecule through hot band transitions

Department of Physics, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan

^a e-mail: yamakawa@iis.u-tokyo.ac.jp

Received: 27 July 2017
Received in final form: 7 January 2019
Published online: 7 March 2019

Abstract

The excitation of an infrared-inactive mode (ν_m) in a polyatomic molecule through the electric dipole transition is discussed taking into account the high-order derivatives of the dipole moment with respect to the normal coordinates. It is revealed that this excitation requires one of the specific degenerate modes (ν_t) to be thermally excited in advance of infrared absorption. The third-order dipole-derivatives responsible for the hot band transition of “ν_m + ν_t − ν_t” are identified. The author also deals with examples of hot band transitions in benzene and recommends reassignment of weak absorption features detected in previous infrared spectra of gaseous polyatomic molecules.