The far ultra-violet continuum of nitric oxide was assigned to the dimer by obtaining the following experimental evidence : (a) the intensity of the spectrum is proportional to; (b) the possibility that the carrier may be some other known oxide of nitrogen was eliminated ; (c) from the temperature dependence the enthalpy change accompanying dissociation was 2.24 (f 0.1) kcal/mol (100-140 K), in good agreement with 2.4 (f0.2) kcal/mol deduced from the dimer infra-red intensities ; ( d ) approximate upper and lower limits to the dissociation constant, and the temperature dependence, were consistent with Guggenheim's method of analyzing the second virial coefficients by means of the principle of corresponding states.An absolute scale of dissociation constants was fixed from the excess of the reduced second virial coefficient at its normal boiling point, and the temperature dependence found spectroscopically. The dissociation energy of the dimer is 1.6 (fO.l) kcal/mol. Evidence for substantial population of excited states of the dimer at 300 K was discussed. The dimer continuum exhibits a maximum at 2050& and the oscillator strength of the system was recorded as 0.26.
A new absorption spectrum in the region 4600-5600 A has been discovered in the flash photolysis of silyl iodide, SiH,I. A rotational analysis, together with the observation of a deuterium isotope shift, has shown that the spectrum is due to the HSiI radical.The lower electronic state of HSiI is a 'A' state with a bond angle of -103", and the upper state is 'A" with angle -116". Axis-switching effects, due to the increase in bond angle, cause the appearance of A K = 0, +2 subbands, in addition to the ordinary A K = + 1 subbands.--
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).
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