Infrared emission, from vibrationally excited

Perona et al. (1 ) have studied the infrared emission from vibrationally excited HCl and DCl formed in the reaction of H and 1 atoms on S C1.,. From their observation that the highest HCl vibrational level occupied was v = 7, they obtained an upper limit... 

John Polanyi and his collaborators have used the infrared emission from vibrationally excited HCl to give information about the potential energy surface for the reaction... 

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)... 

Millikan20 has described an ingenious fluorescence technique for measuring relaxation rates of the CO(t> = 1) molecule. In a flow tube at 5-20cm.sec-1, CO is excited to (o = 1) at the inlet with infrared emission from the CO fundamental (2143 cm-1), a suitably intense source being a CH4(rich)/02 flame. There are two competing processes by which the vibrational excitation can decay... 

The reaction of 0(3P) + CS2 has been especially well studied by a variety of techniques, although the energy disposal is by no means completely described. Of the three possible reaction channels (giving CS + SO, CO + S2 and OCS + S), the first is the major route [470] and has been the most extensively studied. CO emission has been observed from the second channel [451, 466], but no infrared emission could be seen from vibrationally excited OCS produced in the third channel [466]. These last two channels constitute only 15—30% of the total reactions of O + CS2 [470]. 

So far, we have been concerned mainly with emission of radiation from electronically excited states. Emission may also arise from vibrational transitions in various reaction systems. The species HO2 has long been postulated as an important chain carrier in combustion reactions, although emission from electronically excited HO2 has yet to be demonstrated unequivocally. However, Tagirov has observed radiation in flames at a frequency of 1305 cm which he ascribes to transitions from vibrationally excited HO2. Investigations of vibrational quenching processes are of great interest, and if the vibrationally excited species emit infrared radiation, then emission spectrometry may be the most satisfactory way of following the reaction. Davidson et describe a shock-tube study of the relaxation of... 

The second approach to the study of reactive scattering involves the use of some spectroscopic method for the detection of the products in specified internal quantum states. Molecular spectroscopy is well suited to the determination of the relative populations in individual states since the quantum numbers of the upper and lower states of a molecular line in an assigned transition are known. Moreover, the intensities may be directly related to concentrations of specific internal states. The original implementation of this approach for the study of reactive scattering involved observation of spontaneous infrared emission from the radiative decay of vibrationally excited products [4, 5]. This approach is still being employed, however now usually with detection of the emission with Fourier transform [6], rather than grating-tuned spectrometers. In some cases, emission from electronically excited products can be observed for highly exothermic reactions. 

D24.5 Infrared chemiluminescence. Chemical reactions may yield products in excited states. The emission of radiation as the molecules decay to lower energy states is called chemiluminescence. If the emission is from vibrationally excited states, then it is infrared chemiluminescence. The vibrationally excited product molecule in the example of Figure 24.13 in the text is CO. By studying the intensities of the infrared emission spectrum, the populations of the vibrational states in the product CO may be determined and this information allows us to determine the relative rates of formation of CO in these excited states. 

F+Ha HF- +. H AH =-139.9 kj is also exothermic and can produce energy rich HF molecules. The heat of chemical reaction is distributed in various vibrational-rotational modes to give vibrationally excited HF or HC1 in large numbers. Emission from these hot molecules can be observed in the infrared region at h 3.7 (j-m. The reaction system in which partial liberation of the heat of reaction can generate excited atoms or molecules is capable of laser action (Section 3.2.1). They are known as chemical lasers. The laser is chemically pumped, without any external source of radiation. The active molecule is born in the excited state. Laser action in these systems was first observed by Pimental and Kasper in 1965. They had termed such a system as photoexplosion laser. 

Figure 3. The overall temporal profile of the infrared emission as seen by the detector at four positions of optical path difference 5, in the vicinity of the position of zero optical difference <5 = 0. The data shown are for emission from highly vibrationally excited C02, taken with a temporal resolution of 3 ns and with one shot of the C02 laser per mirror position, and illustrate how both the intensities and the time profiles of the emission, arising from the production and decay of many vibrational levels, change as a function of 5. Reproduced with permission from Ref. 37.

ORM assumes that the atmosphere is in local thermodynamic equilibrium this means that the temperature of the Boltzmann distribution is equal to the kinetic temperature and that the source function in Eq. (4) is equal to the Planck function at the local kinetic temperature. This LTE model is expected to be valid at the lower altitudes where kinetic collisions are frequent. In the stratosphere and mesosphere excitation mechanisms such as photochemical processes and solar pumping, combined with the lower collision relaxation rates make possible that many of the vibrational levels of atmospheric constituents responsible for infrared emissions have excitation temperatures which differ from the local kinetic temperature. It has been found [18] that many C02 bands are strongly affected by non-LTE. However, since the handling of Non-LTE would severely increase the retrieval computing time, it was decided to select only microwindows that are in thermodynamic equilibrium to avoid Non-LTE calculations in the forward model. 

Sonobe and Rosenfeld (48,49) have measured the 4.7 pm infrared emission of CO. The extent of the CO vibrational excitation can be estimated using a cold gas filter containing CO and a 4.7 pm filter. If CO is vibrationally excited there is a smaller amount of attenuation of 4.7 pm fluorescence by the cold gas filter. When ketene is photolyzed at 193 nm, they estimate from their data that the rotational and vibrational temperatures are about 6700 and 3700 K, respectively. A high rotational temperature suggests that the C-C-0 angle is bent in the excited state. The CO vibrational excitation becomes less for longer excitation wavelengths. 

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