Collision Transfer of Vibrational Energy from Highly Excited Polyatomics. Transition Probabilities and Cross Sections for Inefficient Bath Gases

Abstract

Collisional de‐excitation of vibrationally excited (∼45 kcal mole⁻¹) 2,4‐dimethylpentyl‐2* and $\text{2,4‐}\mathrm{dimethylpentyl}{\text{‐2‐d}}_1^{*}$ alkyl radicals by bath molecules H₂ and D₂, respectively, was studied in both the ``high'' and ``low'' pressure regions. The behavior with the stronger collider CF₄ was also investigated at ``high'' pressures. Both symmetry and unimportance of the endothermic (5ts) isomerization of this radical promoted greater accuracy of experimentation. Comparison of experimental rate behavior with model stochastic computations yields values of $\mathrm{〈\; \Delta \; E〉}$ , the average amount of energy transferred per collision, of 1.5, 2.2, and 4.6 kcal mole⁻¹ for H₂, D₂, and CF₄, respectively. Independently of any uncertainty in collision cross sections, the data establish that the preferred forms of the distribution of the transition probabilities p_ij are exponential (or related type) for weak colliders, and stepladder (or Gaussian‐like) for CF₄. The data support earlier conclusions that gas kinetic (viscosity)‐derived collision cross sections are appropriate for this type of energy transfer process; that inefficient bath gases are not such because of a preponderance of elastic collisions.