Vibrational relaxation of NO(upsilon=1) by oxygen atoms

The rate constant k(O)(upsilon= 1) for NO(upsilon= 1) vibrational relaxatio n by O has been measured at room temperature using a laser photolysis-laser probe technique. Vibrationally excited NO and relaxer O atoms were formed using 355 nm laser photolysis of a dilute mixture of NO2 in argon bath gas. The time evolution of both the NO(upsilon= 1) and the O atoms was monitore d using laser-induced fluorescence (LIF). The required absolute O-atom dens ities were obtained through a comparison of O-atom LIF signals from the pho tolysis source and from a titrated cw microwave source. At early times the O atoms constitute the most important loss mechanism for the nascently prod uced NO(upsilon= 1). Possible effects from NO(upsilon= 1) vibrational ladde r-climbing and from thermal expansion have been shown to be minimal. The ra te constant k(O)(upsilon= 1) = (2.4 +/- 0.5) X 10(-11) cm(3) s(-1) determin ed herein is a factor of 2 to 3 lower than the generally accepted value of k(O)(upsilon= 1) used in thermospheric modeling. The present value for k(O) (upsilon= 1) is the same, within the error bars, as the k(O)(upsilon= 2,3) previously measured in this laboratory using an entirely different techniqu e, resonant infrared laser excitation of NO(upsilon= 0). This result sugges ts that the collisional relaxation rates are independent of upsilon. A rece nt quasiclassical trajectory calculation, in which both allowed NO-O surfac es have been explicitly considered, predicts a collisional relaxation rate which is in good agreement with the present result. The k(O)(upsilon= 1) va lue, along with previously measured rate constants for NO-O high-pressure r ecombination (k(rec)(infinity)) and isotope exchange (k(iso)), can serve as a proxy for the rate coefficient k(C) describing the formation of a long-l ived NO2* intermediate from O+NO collisions. The present value for k(O)(ups ilon= 1) is significantly lower, however, than a recent determination of k( rec)(infinity) and also the value of k(C) derived from k(iso). In the latte r case the comparison is not as straightforward. (C) 1999 American Institut e of Physics. [S0021-9606(99)00731-X].