Enhancement of the bimolecular reaction of hydrogen iodide by vibrational excitation

The bimolecular reaction of HI in CO₂, which was excited vibrationally by irradiation of a continuous-wave CO₂ laser light, was investigated in the temperature range of 721–980 K. An enhancement of the reaction rate by a factor of about 2.5 was observed in the 1:1 HI? CO₂ mixture in comparison with the rate in pure HI when both sample gases were irradiated by a CO₂ laser (50 W) at 1 torr. However, in the HI-SF₆ mixtures the decomposition rate of HI was not accelerated by irradiation of the CO₂ laser. Thus the enhancement is attributed to vibrational excitation of HI through collisional energy transfers from laser-excited CO₂ (00°1). At lower total pressures or at lower partial pressures of HI in HI-CO₂ mixtures the enhancement was more significant because of inefficient vibrational deactivation of excited HI. A model calculation gave the result in agreement with the experimental one if the effective activation energy is assumed as E_a? = E_a - αE_vib, where E_a is the activation energy for the thermal reaction, E_vib is the vibrational energy of two colliding HI molecules, and α is estimated to be about 0.7. This means that a part of the vibrational energy of reacting HI is employed to reduce the activation energy for the translational or rotational degree of freedom.