Electronic to Vibrational Energy Transfer from Cl* (3 ^2P1/2) to N2O(?1): 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 ^2P1/2 , 882 cm-1) to N2O(?1) (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(?1) fluorescence at 7.8 ?m were analyzed to obtain rate coefficients for Cl* quenching and N2O(?1) 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(?1). 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(?3) 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 cm^3 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(?1) 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 cm^3molecule-1s-1. The rate coefficient for N2O(?1) relaxation by N2O was determined to be (1.0 ± 0.2) x 10-12 cm^3molecule-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 cm^3molecule-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 cm^3molecule-1s-1, and an N2O(?1) relaxation rate coefficient of kVN2O = (2.3 ± 0.4) x 10-12 cm^3molecule-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 "d 5 x 10-14 cm^3molecule-1s-1. The rate coefficient for the relaxation of N2O(?1) by argon was found to be k V Ar = (2.9 ± 0.8) x 10-13 cm^3molecule-1s-1. Argon kinetic studies indicate that the rise portion of the N2O(?1) 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(?1) fluorescence with addition of argon suggests that a simple Cl*?N2O(?1)?N2O mechanism may not apply.
School:Wright State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:electronic to vibrational energy transfer failure of a simple kinetic mechanism
Date of Publication:01/01/2005