Chemiluminescence infrared

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. 

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... 

The course of reactions was followed either by determining product concentrations by monitoring their infrared chemiluminescence or by... 

Also, potassium superoxide (KO2) decomposes DMD in acetone solution to release singlet oxygen, as has been detected by the characteristic infrared chemiluminescence . Furthermore, a catalytic amount of n-Bu4NI decomposes TFD into oxygen gas and triflu-oroacetone in high yield . Analogous to the Caroate decomposition by ketones, also the catalytic decomposition of peroxynitrite by ketones, e.g. methyl pyruvate, is rationahzed in terms of peroxynitrite oxidation by in-situ-generated dioxirane. ... 

Near-Infrared Chemiluminescence. The technique of chemiluminescence has been most successfully applied in the atmosphere to the measurement of NO and other oxides of nitrogen through various conversion procedures. Glaschick-Schimpf et al. (115) and Holstein et al. (116) reported results of laboratory kinetic studies in which the H02 concentration was determined through the chemiluminescence reaction system shown in equations 28 and 29. It involves the same molecular transitions in the near-infrared as discussed previously (for H02 emission from the 2A state). 

Highly energetic non-equilibrium rotational distributions are observed where they are not strictly required by the conservation laws, for example, in infrared chemiluminescence studies4 in which special efforts are made to reduce rotational relaxation. In the reaction... 

Infra-red chemiluminescence has been used to measure the vibrational state distributions of CO2 formed by reaction on Pt and Pd surfaces [45-50]. While the detection sensitivity of infrared chemiluminescence does not approach that of LIF or REMPI, it is an attractive way to probe molecules such as CO2 where there is substantial vibrational excitation and REMPI schemes are not available. In some cases Doppler measurement of the translational energy release can be achieved, giving direct information on the translational energy release of vibrationally excited C02 [45]. Recently infrared diode laser spectroscopy has been used to detect vibrationally excited CO2 from CO oxidation over... 

One flow system that has been designed to minimise the effects of relaxation and is commonly used in infrared chemiluminescence studies is designated the arrested relaxation system [77]. Figure 2 shows an... 

Fig. 2. An arrested flow apparatus used for measuring the infrared chemiluminescence from reaction products with a Fourier transform spectrometer. (Reproduced from ref. 81 by permission of the authors and the American Institute of Physics.)...

In addition to surprisal analysis of measured product energy distributions, surprisal synthesis has been applied [178] to the prediction of energy distributions either by induction from some more limited experimental data or by deduction from some dynamical calculation. In the inductive approach to surprisal synthesis, the available experimental data is used as a constraint to compute the surprisal parameter, X, by ensuring that the entropy is maximised. This surprisal parameter then determines a more detailed distribution. In a more modest way, this approach may be used to extend incomplete product energy distributions. For example, as mentioned before, infrared chemiluminescence measurements are incapable of determining the population of products in the vibrational ground state, v = 0, and this is often induced from the surprisal analysis of the other vibrational levels. 

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. 

For the reactions H, D + Br2 there has been less experimental confirmation about the product vibrational and rotational energy disposal. Fast-flow infrared chemiluminescence measurements [241] yield a product vibrational distribution that differs from earlier measurements [228]. A recent molecular beam study [235] for D + Br2, at a lower collisional energy (5.3 kJ mole-1) than the earlier beams studies ( 42 kJ mole-1), gives CFX> = 0.40, which agrees more closely with the 300 K chemiluminescence value, CFX> = 0.41 [228], than does the higher energy beams value [111] of (FT) = 0.31. 

An infrared chemiluminescence study [501] of the reaction 0(3P) + CH3 shows that it is direct with the formaldehyde product being highly vibrationally excited. A lower limit of (Fv) > 0.43 has been placed on this excitation. The high pressure reactions of 0(3P) atoms with various hydrocarbons show emission of vibrationally excited OH(d < 14) radicals which is attributed to the reaction [ 502]... 

A considerable wealth of dynamical information has been obtained from studies of halogen atom reactions. The majority of these studies concentrate either on the reactions of fluorine atoms which tend to be faster and more exoergic than the other halogen atom reactions or those reactions which involve the production of a hydrogen halide molecule which may then be studied using infrared chemiluminescence methods. From a chemical point of view, this makes a review of this nature incomplete, but it does reflect the scope of the experimental studies conducted so far. Because of experimental difficulties in reagent preparation, the reduced reactivity and the inability of infrared methods... 

Infrared chemiluminescence measurements [581] of the HF products from the reactions of fluorine atoms with a wide range of pure and halogenated olefinic and aromatic hydrocarbons (see list in Table 7)... 

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). 

The reaction of CH3 with CF3 has been studied [643] using infrared chemiluminescence detection of the HF product, which is produced with v < 4( 0.13). The vibrational population distribution is non-Boltzmann despite HF being formed by unimolecular decomposition from a chemically activated CH3CF3 intermediate. It is suggested that the HF acquires excess energy as it separates from the CH2CF2. 

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. 

Recently measurements on chemical lasers [282-285] and of infrared chemiluminescence [286] have provided information about the partitioning of energy in unimolecular elimination reactions, following production of... 

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... 

---