^{1,a)}and Ian W. M. Smith

^{1,b)}

### Abstract

Rate coefficients are reported (a) for total removal of radicals from selected rotational levels (, 1, 6, 10, 15, and 20) and (b) for state-to-state rotational energy transfer between levels and other rotational levels in collisions with . CN radicals were generated by pulsed laser photolysis of NCNO at . A fraction of the radicals was then promoted to a selected rotational level in using a tunable infrared “pump” laser operating at , and the subsequent fate of this subset of radicals was monitored using pulsed laser-induced fluorescence (PLIF). Values of were determined by observing the decay of the PLIF signals as the delay between pump and probe laser pulses was systematically varied. In a second series of experiments, double resonance spectra were recorded at a short delay between the pump and probe laser pulses. Analysis of these spectra yielded state-to-state rate coefficients for rotational energy transfer,. The difference between the sum of these rate coefficients, , and the value of for the same level is attributed to the occurrence of chemical reaction, yielding values of the rotationally selected rate coefficients for reaction of CN from specified rotational levels. These rate coefficients decrease from for to for . The results are briefly discussed in the context of microcanonical transition state theory and the statistical adiabatic channel model.

The authors are grateful to the EPSRC (Engineering and Physical Sciences Research Council) for a grant in support of this research and Professor J. K. M. Sanders for providing laboratory facilities in the Cambridge Chemistry Department. The authors also thank Professor Chester M. Sadowski (York University, Canada) for experimental help in the early stages of this project and Professor Ian R. Sims (University of Rennes I, France) for the loan of a wavemeter. Finally, the authors thank a number of people for useful correspondence and sundry help: S. G. Arkless, Dr. P. W. Barnes, Dr. A. Goddard, and Dr. K. M. Hickson.

I. INTRODUCTION

II. EXPERIMENTAL METHOD

III. RESULTS

A. Kinetic experiments and rate coefficients for total removal from specified levels

B. Spectroscopic experiments and rate coefficients for state-to-state rotational energy transfer from specified levels

C. Relationship between the rate coefficients for reaction of , with and the thermal rate constant for the reaction

IV. DISCUSSION AND CONCLUSIONS

### Key Topics

- Reaction rate constants
- 24.0
- Collision induced chemical reactions
- 21.0
- Hydrogen reactions
- 20.0
- Chemical reaction cross sections
- 19.0
- Rotational energy transfer
- 18.0

## Figures

PLIF signals from CN[, , (●) ] radicals as a function of time delay between the pulses from the pump and probe lasers in a mixture of a trace of NCNO in of 3:1 mixture of and at . The continuous curve shows the simulation for achieved by the procedure outlined in the text [Eqs. (2a) and (2b)]. The inset shows PLIF signals from CN[, , (●) ] generated in of pure at .

PLIF signals from CN[, , (●) ] radicals as a function of time delay between the pulses from the pump and probe lasers in a mixture of a trace of NCNO in of 3:1 mixture of and at . The continuous curve shows the simulation for achieved by the procedure outlined in the text [Eqs. (2a) and (2b)]. The inset shows PLIF signals from CN[, , (●) ] generated in of pure at .

A plot of pseudo-first-order rate coefficients for the removal of CN(, , ) as a function of the concentration of included in at a total pressure of . Under these conditions rotational relaxation is very rapid and the gradient of the line corresponds to the second-order rate constant for reaction between and .

A plot of pseudo-first-order rate coefficients for the removal of CN(, , ) as a function of the concentration of included in at a total pressure of . Under these conditions rotational relaxation is very rapid and the gradient of the line corresponds to the second-order rate constant for reaction between and .

Plots of pseudo-first-order rate coefficients for the total removal of CN radicals from the rotational level in the state as a function of the total concentration of a 2:1 mixture of and . After correction for the effects of , the gradients yield the second-order rate coefficients for total removal (by rotational energy transfer and reaction) of CN from the specified rotational levels.

Plots of pseudo-first-order rate coefficients for the total removal of CN radicals from the rotational level in the state as a function of the total concentration of a 2:1 mixture of and . After correction for the effects of , the gradients yield the second-order rate coefficients for total removal (by rotational energy transfer and reaction) of CN from the specified rotational levels.

Comparison of the rate coefficients for total removal of CN radicals from specified rotational levels in collisions with (엯) and (●).

Comparison of the rate coefficients for total removal of CN radicals from specified rotational levels in collisions with (엯) and (●).

Double resonance spectrum obtained, following the excitation of CN to , , by scanning the wavelength of the probe laser with delay between the pump and probe laser pulses. The total pressure of the gas was . Insert (a) shows the time dependence of the PLIF signals from and and 2 following excitation of CN(, ) in the same gas mixture, with the vertical dashed line marking the time delay used to take double resonance spectrum. Insert (b) presents state-to-state rotational energy transfer rate coefficients derived by the procedure described in the text.

Double resonance spectrum obtained, following the excitation of CN to , , by scanning the wavelength of the probe laser with delay between the pump and probe laser pulses. The total pressure of the gas was . Insert (a) shows the time dependence of the PLIF signals from and and 2 following excitation of CN(, ) in the same gas mixture, with the vertical dashed line marking the time delay used to take double resonance spectrum. Insert (b) presents state-to-state rotational energy transfer rate coefficients derived by the procedure described in the text.

Comparison of the rate coefficients for total removal from selected rotational levels (엯) with the sums of the state-to-state rate coefficients for rotational energy transfer from the same set of levels (◻).

Comparison of the rate coefficients for total removal from selected rotational levels (엯) with the sums of the state-to-state rate coefficients for rotational energy transfer from the same set of levels (◻).

The main panel compares the rotationally state-selected rate coefficients for reaction between CN and (i.e., ) measured in the present work (●) and interpolated for other rotational levels (엯), assuming that the rotationally state-selected rate coefficients for reaction depend exponentially on , with the thermal rate constant (---) at and with (⋯) corresponding to the reaction rate based on the interpolated values. The insert shows (●) the contributions to the thermal rate constant from different CN rotational levels compared with a curve (——) showing the variation of the equilibrium rotational populations at .

The main panel compares the rotationally state-selected rate coefficients for reaction between CN and (i.e., ) measured in the present work (●) and interpolated for other rotational levels (엯), assuming that the rotationally state-selected rate coefficients for reaction depend exponentially on , with the thermal rate constant (---) at and with (⋯) corresponding to the reaction rate based on the interpolated values. The insert shows (●) the contributions to the thermal rate constant from different CN rotational levels compared with a curve (——) showing the variation of the equilibrium rotational populations at .

## Tables

State-to-state rate coefficients, , for the transfer of CN radicals from rotational levels to rotational levels in (, ) in collisions with at . Errors in the values of are given at the level of . Figures in brackets are values of estimated using the PGF analysis described in the text.

State-to-state rate coefficients, , for the transfer of CN radicals from rotational levels to rotational levels in (, ) in collisions with at . Errors in the values of are given at the level of . Figures in brackets are values of estimated using the PGF analysis described in the text.

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