Infrared chemiluminescence experiments

The first mfonnation on the HE vibrational distribution was obtained in two landmark studies by Pimentel [39] and Polanyi [24] in 1969 both studies showed extensive vibrational excitation of the HE product. Pimental found that tire F + H2 reaction could pump an infrared chemical laser, i.e. the vibrational distribution was inverted, with the HF(u = 2) population higher than that for the HF(u = 1) level. A more complete picture was obtained by Polanyi by measuring and spectrally analysing tlie spontaneous emission from vibrationally excited HE produced by the reaction. This infrared chemiluminescence experiment yielded relative populations of 0.29, 1 and 0.47 for the HF(u =1,2 and 3)... 

Spectroscopic detemiination of the HE rotational distribution is another story. In both the chemical laser and infrared chemiluminescence experiments, rotational relaxation due to collisions is faster or at least comparable to the time scale of the measurements, so that accurate detemiination of the nascent rotational distribution was not feasible. However, Nesbitt [40, 41] has recently carried out direct infrared absorption experiments on the HE product under single-collision conditions, thereby obtaining a fiill vibration-rotation distribution for the nascent products. 

In the reaction F + C6D6, it appears that the distribution of energy is random when attention is focused on the C6DSF vibrational distribution measured in infrared chemiluminescence experiments [583], but is non-random for the product recoil distribution measured in a molecular-beams experiment [588]. This could be rationalised if certain modes in the complex do not take part in the randomisation or if a few specific modes are coupled to the reaction coordinate at the transition state (the exit channel barrier). 

Infrared chemiluminescence experiments are now being used to investigate how the product energy distributions change if excess energy of a particular kind is supplied to the reactants. The reactions Cl + HI [335], F + HC1... 

Chemical-laser emission spectra up to 1967 have been compiled by Patel 6 ). There is some inconsistency between HF laser spectra obtained in different laboratories and with different experimental set-ups. This is probably due in part to the absorption of several HF lines by the atmosphere inside or outside the laser cavity. However, there is additional inconsistency between the emission spectra of infrared chemiluminescence experiments and HF chemical lasers. While spontaneous luminescence predicts a peak in the v=2 - 1 transitions around /=614>, the laser emission usually... 

Energy distributions of alkyl radical dissociation products, as measured by crossed molecular beam and infrared chemiluminescence experiments, show deviations from the predictions of statistical theories such as the RRKM or phase space theory. In these experiments an atom X is added to a C=C double bond of an olefin RY to form... 

With the development of the first chemical laser sources it became possible to use the lasers themselves to obtain the specific rate constant data formerly available only from the infrared chemiluminescence experi-ments. " Some very useful techniques have been developed by Pimentel and co-workers for the determination of the partitioning of the energy release of chemical reaction among the vibrational levels of diatomic reaction products/ These techniques are based on the relationship between gain or absorption and the ratio of vibrational populations on a given vibration-rotation transition. These methods were originally developed for the study of reactions of interest for chemical lasers however, they have been generalized to include many other reactions which do not provide useful population inversions. 

Another approach to the determination of surface kinetics in these systems has been to combine molecular beams in the 10 2-10 1 mbar pressure range with the use of the infrared chemiluminescence of the C02 formed during steady-state NO + CO reactions. This methodology has been used to follow the kinetics of the reaction over Pd(110) and Pd(l 11) surfaces [49], The activity of the NO + CO reaction on Pd(l 10) was determined to be much higher than on Pd(lll), as expected based on the UHV work discussed in previous sections but in contrast with result from experiments under higher pressures. On the basis of the experimental data on the dependence of the reaction rate on CO and NO pressures, the coverages of NO, CO, N, and O were calculated under various flux conditions. Note that this approach relied on the detection of the evolution of gas-phase... 

If there is no difference in fractional energy disposal with isotope, then a discrepancy arises between the molecular beam measurements of (Ft) and the values of (FT> derived from the chemiluminescence experiments. For D + C12, the beams result is = 0.40 which is much lower than the chemiluminescence estimate, = 0.60, obtained under comparable initial conditions [196], No satisfactory explanation for this difference has yet been advanced. The good agreement between the H, D + Cl2 infrared chemiluminescence results measured by different groups suggests that a remeasurement of the product translational energy distribution might be required to resolve this discrepancy. 

Polanyi and his co-workers have observed infrared chemiluminescence from vibrationally excited molecules formed in simple chemical reactions. In some cases [101-103], excitation was thought to occur in recombination reactions. The highest vibrational level observed in these experiments was always considerably below the dissociation energy of the excited molecule, but no firm conclusion can be drawn from this fact because there is little doubt that the observed distribution was considerably relaxed from the first stabilizing collisions. 

Next, we briefly consider some reactions where measurements of the energy partitioning have been made by both beam [373, 374] and infrared chemiluminescence [228, 265, 376] experiments, namely the family... 

The experiments discussed at the end of the previous section provided information about the translational excitation of the products of unimolecular fragmentation of energized species formed in association reactions. The distributions of vibrational energy in the products of some reactions of this, and related, types have been determined by chemical laser measurements and by observations of infrared chemiluminescence. Some of these studies were referred to in Section III.C, other reactions have been studied more recently [388-392], In all of these investigations, the product which has been observed is HF or HC1 formed in what is frequently termed a snap-out reaction. These processes require that, almost simultaneously, two bonds break, the HX bond forms, and the order of a bond in the other product is increased. The reverse reaction, a four-centre (bimolecular) one, has a high activation barrier, so in the snap-out process a considerable proportion of the total energy is released after the system passes through the activated state. Thus reaction (120)... 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

The H- F2 reaction was the first one. beyond H- H2 and Its Isotopes, for which it was meaningful to compare large scale DW calculations with experiment (Clary and Connor [21]). This reaction has the advantage that a reasonable LEPS potential surface Is available (Jonathan et al. [49]) and the product distributions are insensitive to variations in j and Etr for the thermal energy range. This last property allows a fixed energy calculation for H+F2(v=0. j=0) to be meaningfully compared with thermal infrared chemiluminescence data (Polanyi and Sloan [69]. Brandt and Polanyi [11]). 

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