C.M. Sadowski

Absolute rate constant for the reaction of CH with O2

Abstract CH(X3II) has been produced in the multiple-photon dissociation of methylamine by a TEA CO2 laser. Laser-induced fluorescence has been used to measure relative CH concentrations as a function of time and added gas pressures. Chemiluminescence which accompanies the reaction is due to the production of OH(A 2Σ+) in the reaction. The rate constant obtained from the fluorescence experiments is (3.3 ± 0.4) × 10−11 cm3 molecule−1 s−1 at 298 K, in agreement with that deduced from the chemiluminescence signal.

Direct measurement of thermal rate constants for state‐to‐state rotational energy transfer in collisions of CN(X 2Σ+, v=2, N) with He

Stimulated emission pumping state preparation and laser induced fluorescence state detection in the CN(B 2Σ+–X 2Σ+) violet system have been employed to study relaxation of single rotational states of CN(X 2Σ+, v=2) in collisions with He at 295 K. Approximately 2/3 of the value of the total removal rate constant for CN(X 2Σ+, v=2, N=2, 11, or 14) corresponds to changes in the rotational quantum number of ‖ΔN‖≤3 and a strong propensity is evident which favors even changes in N for ‖ΔN‖≤4. The measured rate constants are consistent with detailed balance, can be described with statistical power‐gap and exponential energy‐gap fitting functions and, where compared, are in accord with an IOS‐based scaling function. No significant difference is found between a sum of state‐to‐state rate constants and separately measured total removal rate constants for N=2, 11, and 14. The total removal rate constants decrease monotonically between N=0 and N=41 to about 1/3 of their value at N=0.

Photodissociation of BrCN and ICN in the a continuum: Vibrational and rotational distributions of CN(X 2Σ+)

Abstract We report results of experiments on photodissociation of ICN and BrCN in the long-wavelength (A) continuum in which primary rotational and vibrational distributions of CN in its electronic ground state are determined. For the CN fragment from ICN, our observations at 222 and 249 nm, reported here, together with our earlier results at longer wavelengths, show that the fraction of energy which goes into rotation (in the ground-state channel) is very nearly independent of exciting wavelength in the range 222–351 nm. This holds for each of CN vibrational levels 0, 1, and 2, where the fractions are 15, 25, and 33% respectively. The vibrational distributions decrease monotonously with υ, but show little change as a function of exciting wavelength. CN distributions are also presented for the photodissociation of BrCN at 193, 222, 249, and 308 nm. Results for both parent molecules are briefly discussed in relation to other recent experimental and theoretical work.

Absolute rate constant for the CH + O reaction

Abstract CH(X 2 Π) was produced by multiple infrared photon dissociation of CH 3 OH vapour in the presence of excess atomic oxygen. Time-resolved measurements of relative CH concentrations were made at 298 K with a tunable dye laser. From the dependence of CH decay rate on atom concentration, an absolute rate constant (9.4 ± 2)×10 −11 cm 3 molecule −1 s −1 was deduced for the CH + O reaction.

State-to-state and total rotational energy transfer rate constants for CN(B 2Σ+,v=0,N)+H2, CN(X 2Σ+,v=2,N)+H2, D2, and CN(X 2Σ+,v=3,N)+NO

State-to-state and total rotational energy transfer (RET) rate constants were measured for collisions of CN(B 2Σ+,v=0,Ni=4,7,8,11) with H2, CN(X 2Σ+,v=2,Ni=4,11) with H2 and D2, and CN(X 2Σ+,v=3,Ni=4) with NO at room temperature and under single, or near-single, collision conditions. Rate constants were also measured for electronic quenching of CN(B 2Σ+,v=0,Ni=4,7,8,and 11) by H2. In general, state-to-state RET rate constants showed very small or no even–odd alternations as the final rotational state varied. Total rate constants for CN(X 2Σ+,v=2,N)/H2, D2 were found to decrease with increasing rotational quantum number, N. By contrast, total rate constants for CN(B 2Σ+,v=0,N)/H2 were found to be relatively independent of N. Exponential energy gap and angular momentum fitting functions were found to represent measured state-to-state RET rate constants very well and were substantially equally effective in this regard.

State-to-state and total rotational energy transfer rate constants for CN(B 2Σ+,v=0,N)+He and C2N2

Total rotational energy transfer (RET) rate constants, state-to-state RET rate constants, and radiative lifetimes have been measured for CN(B 2Σ+, v=0, Ni=0, 4, 6, 7, 8, 11 and 13). All experiments were conducted under single collision conditions in a static cell filled with previously prepared C2N2/He mixtures at various total pressures up to a maximum of 800 mTorr. The above initial states were prepared by pumping CN(X) with a pulsed dye laser tuned to appropriate frequencies in the P-branch of the CN-violet (0,0) band following the production of CN(X) by the excimer laser photolysis of C2N2 at 157 nm. Total RET rate constants and radiative lifetimes were determined from decay constants obtained with a transient digitizer over the above range of cell pressures. State-to-state RET rate constants were obtained from time-averaged fluorescence spectra in the R branch of the (0,0) band of the CN violet system at various cell pressures within the above range. A weak propensity for odd jumps in N was discernab...

Single rotational state preparation and relaxation in CN(X2Σ+, ν=2). Stimulated emission pumping and laser-induced fluorescence detection

Abstract Stimulated emission pumping (SEP) in the CN(B 2 Σ + −X 2 Σ + ) violet system has been employed to prepare substantial populations of CN(X 2 Σ + , ν=2) molecules in single rotational states. These populations are detected by laser-induced fluorescence (LIF) and the combined SEP-LIF procedure is used to observe the population decay of the prepared states in collisions with helium atoms.

Photofragmentation of CH3CN at 193 nm: CN fragment energy distribution

Abstract Following dissociation of acetonitrile at 193 nm, where the absorption is very weak, CN(X; ν=0) is found with about 16% of the available energy in rotational motion. Distribution over the lowest three vibration levels is according to the ratios 10: 4: 1 with ν=0 most populated; about 10% of the available energy is in vibration. No CN(A 2Π) was observed. We compare the results with distributions for other states of CN from CH3CN and with distributions from CN(X) from other cyanides.

ICN photodissociation at 193 nm; CN(B2Σ) rotational distributions and fluorescence polarization

The rotational distribution of CN(B2Σ,v=0) dissociated from ICN at 193 nm is well represented by a thermal rotational distribution for T=250 K, giving an average energy of 175 cm−1. The energy of the highest rotational state detected (N=28) suggests that some or all of the molecules dissociated have appreciable internal energy. We discuss recent values of the available energy. Polarization of the R(8) line of the CN(B) fluorescence agrees with other experiments in this absorption feature, and gives the same upper-state symmetry observed at shorter wavelengths. CN(A2Π) was also detected promptly after the fragmentation.

Sodium atom emission from nitrogen afterglows: excitation process and doppler temperature

Abstract We have investigated the dependence of Doppler temperature of the D 1 sodium emission (λ 5896 A) from a nitrogen afterglow, on the afterglow conditions. We found that afterglows having low N-atom concentrations give emission lines for which T (λ 5896 A) = 900 ± 200°K. By contrast, in an afterglow emitting the N 2 bands brightly, the temperature of the λ 5896 A line was 1300°K to 1500°K while in a similar environment (but with much more sodium present) the temperature of the Na λ 6154 A line was around 900°K.