Photoinitiated Hhyphen; and Dhyphen;atom reactions with N2O in the gas phase and in N2Ondash;HI and N2Ondash;DI complexes

摘要

Reactions of H atoms with N2O have two product channels yielding NH+NO and OH+N2. Both channels were observed via NHAthinsp;3Pgr;larr;Xthinsp;3sum; and OHAthinsp;2sum;larr;Xthinsp;2Pgr; laserhyphen;induced fluorescence spectra. Photoinitiated reactions with N2Ondash;HI complexes yield a much lower lsqb;NHrsqb;/lsqb;OHrsqb; ratio than under the corresponding bulk conditions at the same photolysis wavelength. For hot Dhyphen;atom reactions with N2O, this effect is somewhat more pronounced. These results can be interpreted in terms of entrance channel geometric specificity, namely, biasing hydrogen attack toward the oxygen. Another striking observation is that the OH and OD rotational level distributions (RLD) obtained under bulk conditions differ markedly from those obtained under complexed conditions, while the NH as well as the ND RLD are similar for the two environments. In addition, OH Doppler profiles change considerably in going from bulk to complexed conditions, while such an effect is not observed for NH. The changes observed with the OH RLD are most likely due to OHndash;halogen interactions and/or entrance channel specificity. Under bulk conditions, the Doppler shift measurements indicate a large amount of N2internal excitation (i.e., sim;25thinsp;000 cmminus;1) for the OH (v=0) levels monitored. This is consistent with a reaction mechanism involving an HNNOdeg;intermediate. The hot hydrogen atom first attaches to the terminal nitrogen of N2O and forms an excited HNNOdeg;intermediate having a relatively elongated Nndash;N bond compared with N2O. Then the H atom migrates from nitrogen to oxygen and exits to the N2+OH product channel, leaving N2vibrationally excited. A simple Franckndash;Condon model can reconcile quantitatively the large amount of N2vibrational excitation.