Kinetics of excited-state Cr(a5S2, a5DJ, a5GJ) depletion by simple hydrocarbons

Abstract

The depletion kinetics of excited states of Cr(a⁵S₂, a⁵D_J, a⁵G_J) upon interaction with simple hydrocarbons (CH₄, C₂H₆, C₃H₈, cyclopropane, C₂H₂, C₂H₄, and propene) are studied in a discharged flow reactor at He pressures of 0.7 Torr. On interaction with alkanes, Cr(a⁵S₂, a⁵D_J) show no depletion but an increase in their populations. The higher excited state, Cr(a⁵G_J), shows depletion upon interaction with alkanes. Good correlation was observed between the depletion rate constants for Cr(G_J) and the formation rate constants for Cr(a⁵S₂, a⁵D_J), and it is suggested that collisional relaxation from Cr(a⁵G_J) to Cr(a⁵S₂, a⁵D_J) is important for the interaction with alkanes. On the other hand, all excited states show depletion upon the interaction with unsaturated hydrocarbons. Among the three states, Cr(a⁵G_J) shows the most efficient depletion. Rate constants were almost the same as the gas kinetics ones, which suggest no or very small energy barriers for the interaction of this state with alkenes and acetylene. More efficient depletion of Cr(a⁵G_J) by alkenes and acetylene compared with alkanes is consistent with the presence of stable π-complexes on interaction potential surfaces of the former systems.