Electronic to Vibrational Energy Transfer from Cl* (3 ²P_1/2) to N₂O(ν₁): Failure of a Simple Kinetic Mechanism

Experimental kinetic studies were carried out examining the electronic to vibrational (E-V) energy transfer from spin-orbit excited Cl*(3 ²P_1/2 , 882 cm^-1) to N₂O(ν₁) (symmetric stretch 1285 cm^-1). All studies were carried out in the gas phase at room temperature (298 ± 2 K). Cl* was generated by pulsed laser photolysis of ICl at 532 nm in the presence of various mixtures of N₂O, ICl,Ar, and SO₂. Time-resolved IR signals of N₂O(ν₁) fluorescence at 7.8 μm were analyzed to obtain rate coefficients for Cl* quenching and N₂O(ν₁) relaxation. E-V branching ratios, measured in ICl/N₂O mixtures show that al l Cl*/N₂O quenching collisions result in the excitation of the N₂O(ν₁). Additional studies were carried out,observing the competitive Cl* quenching between N₂O and SO₂. An indirect measurement of the rate coefficient for N₂O quenching of Cl* was obtained from time-resolved IR signals of SO₂(ν₃) fluorescence at 7.3 om with varying N₂O concentrations. Kinetic studies carried out in ICl/N₂O gas mixtures found the total Cl* quenching rate coefficient for N₂O to be k*_N2O = (9.8 ± 1.6) x 10^-12 cm³ molecule^-1s^-1. An E-V branching ratio, k*_E-V/k*_N2O = 1.09 ± 0.28, was determined in these experiments. This value indicates that all of the quenching collisions excite N₂O(ν₁) via the E-V pathway, and the absolute rate coefficient for E-V quenching may be determined as k * EV = (9.8 ± 3.0) x 10^-12 cm³molecule^-1s^-1. The rate coefficient for N₂O(ν₁) relaxation by N₂O was determined to be (1.0 ± 0.2) x 10^-12 cm³molecule^-1s^-1in ICl/N₂O mixtures. The N₂O competitive quenching studies in ICl/N₂O/SO₂ gas mixtures yielded a Cl* quenching rate coefficient for N₂O of k*_N2O = (1.59 x 0.13) x 10^-11 cm³molecule^-1s^-1

It was discovered in this project that the kinetic results in the presence of argon are quite different than in argon's absence. N₂O kinetic studies carried out in a ~5 Torr argon bath yield a quenching rate coefficient for N₂O of k*_N2O = (7. 3 ± 1.7) x 10^-12 cm³molecule^-1s^-1, and an N₂O(ν₁) relaxation rate coefficient of k^V_N2O = (2.3 ± 0.4) x 10^-12 cm³molecule^-1s^-1. This relaxation rate coefficient agrees with previous studies carried out by J.C. Batson (M.S. Thesis, Wright State University, 2003), but is in poor agreement with literature values. Batson's rate coefficient assignments are reversed because she was unaware of the unusual and unexplained argon effects discovered here.

Argon kinetic studies yielded an upper limit of the Cl* quenching rate coefficient of k*_Ar ≤ 5 x 10^-14 cm³molecule^-1s^-1. The rate coefficient for the relaxation of N₂O(ν₁) by argon was found to be k^V_Ar = (2.9 ± 0.8) x 10^-13 cm³molecule^-1s^-1. Argon kinetic studies indicate that the rise portion of the N₂O(ν₁) fluorescence signal is due to vibrational relaxation. This assignment is in direct conflict with competitive quenching studies carried out in this project with CD4 and CCl4. The unexplained behavior of N₂O(ν₁) fluorescence with addition of argon suggests that a simple Cl*→N₂O(ν₁)→N₂O mechanism may not apply.