Kinetics and mechanism of the vibrational relaxation of NOX²Π(v= 1) in the temperature range 100–433 K

Abstract

The first vibrational level of NOX211 was excited with an intense light flash and relaxation times were measured from 100 to 433 K. At 210 K and below, the rate equation is). kl is the rate coefficient for self-relaxation in bimolecular collisions and varies inversely with temperature. The term k, [NO][M] corresponds to vibrational relaxation in ternary collisions.The negative temperature coefficient for relaxation in bimolecular collisions was interpreted qualitatively by postulating that the potential energy of the resonance for an electronic-vibrational transition is orientation dependent ; the lowest resonance potential may correspond to the most stable configuration of the (NO), dimer. The role of ternary collisions is probably to remove NOX'II(U = 1) as stabilized dimer. From the dissociation constants of the dimer, rates of bimolecular dissociation were computed assuming that k2 corresponds to the rate coefficient for dimerization. The magnitudes of the bimolecular rate coefficients were consistent with the energy transfer model, provided all degrees of freedom can contribute efficiently to bond rupture. Rate coefficients for vibrational exchange from NO to N, were measured at 298 and 433 K. Evidence for " vibrational equilibrium " was found in experiments with NO + CO mixtures at low temperature.It has been established with various experimental techniques that the probability of vibration-translation relaxation of NOX211(v = 1)' in collision with NOX211(v = 0), is abnormally high at temperatures 2 300 K 1-5 :At 300 K, the probability Plo of (i) per collision has been reported to be " 2 x which is approximately lo4 fold greater than predicted by standard theory.'. Nikitin has interpreted the abnormally fast relaxation rate by supposing that (i) involves an electronic transition in the intermediate (NO), collision complex. In the theory (linear complex), a resonance occurs between the ground state, " E l , with one molecule vibrationally excited, and the vibrationless '2; state, The resonance was predicted to have a potential energy of ~2 kcal/mol (relative to the separated molecules) in the "Z; state ; the relaxation rate would therefore be expected to exhibit a positive temperature coefficient as shown in fig. 1 of ref. (3), where an attempt was made to fit Nikitin's theory to the experimental results which existed at that time.We have measured the rate coefficient for relaxation of NOX211(v = 1) down to 100 K with the flash technique, and contrary to the prediction from Nikitin's theory as detailed by Wray," the temperature coefficient is n e g a t i ~e . ~ From the dependence of the relaxation rate on the pressures of nitric oxide and diluent gas M, it was suggested that relaxation of NOX211(u = 1) occurs both in bi-and termolecular collisions.8 The details of these experiments are reported here. NOX211(v = 1)+NOX211(v = 0)+2NOX211(v = 0).

Energy Transfer Processes in the Stratosphere

“…For M = H,O, CO,, N,, and CO there are only measurements at 300 "K (28). Of particular significance is the fact that, in contrast to the other diatomics N,, O,, and CO, the NO V + T process for M = NO is very fast at room temperature with k = 6 x 10-l4 cm3/molecule s. Also, Billingsley and Callear (35,36) have observed a very marked inverse temperature dependence around room temperature. However, recent work by Stephenson (38) using the well-established laser fluorescent technique casts doubt on some aspects of the Billingsley and Callear result.…”

“…For NO the V + T process has been studied over a wide temperature range for M = Ar and NO (26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38). For M = H,O, CO,, N,, and CO there are only measurements at 300 "K (28).…”

“…However, recent work by Stephenson (38) using the well-established laser fluorescent technique casts doubt on some aspects of the Billingsley and Callear result. The explanation of these observations is not clear at present and has been ascribed to NO dimer formation (26,36) or to spin-orbit coupling (39). In Table 2, a rate for M = N, is given based on some unpublished data of the author.…”

ChemInform Abstract: RESONANZ BEI STOSSINDUZIERTEN ENERGIEUEBERTRAGUNGSPROZESSEN

“…Die restlichen Schockwellendaten von B r e s c h e a r s und B i r d [4], W r a y [S] und Bradley und Lewis [6] stimmen gut iiberein. Auch die rnit Hilfe der anderen Methoden gemessenen Tieftemperaturwerte unterhalb von 500 K von B a u e r und S a h m [7], H o c h a n a d e l , G h o r m l e y und O g r e n [8] und Billingsley und C a l l e a r [3] Was die sehr hohen Temperaturen angeht (3000 bis 7000 K), so hat Wray [S] gezeigt, daD hier die SSH-Theorie Werte liefert, die in der richtigen GroBenordnung liegen. Zum gleichen Ergebnis kam auch Nikitin [11] bei einer theoretischen Behandlung dieses Problems.…”

“…Weiterhin hat sich gezeigt, daD speziell beim Stickstoffmonoxid, das bisher am genauesten untersucht worden ist, die neuesten MeDdaten bei tiefen Temperaturen mit der Theorie Nikitins nicht beschrieben werden konnen. Deshalb mu0 diese in wesentlichen Punkten verbessert werden, wie dies Billingsley und C a l l e a r [3] in qualitativer Weise versucht haben, bzw. nach neuen Gesichtspunkten gesucht werden.…”