Infrared from vibrationally excited

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)... 

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]. 

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. 

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... 

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... 

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... 

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. 

Fig.4 proves that this idea works. It shows in the upper trace the photoacoustic signal at the microphone as a function of the infrared laser wavelength. The lines originate from vibrational excitation of... 

Keil and co-workers (Dhamiasena et al [16]) have combined the crossed-beam teclmique with a state-selective detection teclmique to measure the angular distribution of HF products, in specific vibration-rotation states, from the F + Fl2 reaction. Individual states are detected by vibrational excitation with an infrared laser and detection of the deposited energy with a bolometer [30]. 

Color from Vibrations and Rotations. Vibrational excitation states occur in H2O molecules in water. The three fundamental frequencies occur in the infrared at more than 2500 nm, but combinations and overtones of these extend with very weak intensities just into the red end of the visible and cause the blue color of water and of ice when viewed in bulk (any green component present derives from algae, etc). This phenomenon is normally seen only in H2O, where the lightest atom H and very strong hydrogen bonding combine to move the fundamental vibrations closer to the visible than in any other material. 

Perhaps the first evidence for the breakdown of the Born-Oppenheimer approximation for adsorbates at metal surfaces arose from the study of infrared reflection-absorption line-widths of adsorbates on metals, a topic that has been reviewed by Hoffmann.17 In the simplest case, one considers the mechanism of vibrational relaxation operative for a diatomic molecule that has absorbed an infrared photon exciting it to its first vibrationally-excited state. Although the interpretation of spectral line-broadening experiments is always fraught with problems associated with distinguishing... 

Therefore, the various stretching and bending vibrations of a bond usually take place at particular quantized frequencies. Thus, in a situation where upon the infrared light having the same frequency is incident on the molecule, energy is absorbed, and the net effect could be observed by an increase in the amplitude of that vibration. In another situation, whereby the molecule reverts from the excited state to the ground state, the absorbed energy is released in the form of heat. 

Very large rate constants have been found for near resonant energy transfer between infrared active vibrations in CO2 Such near-resonant transitions and their dependence on temperature have also been studied for collisions between vibrationally excited CO2 and other polyatomic molecules as CH4, C2H4, SF et al. The deactivation cross-sections range from 0.28 for CH3F to 4.3 for SFs at room temperature, and decrease with increasing temperature. 

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. 

The laser action originates from electronically excited I atoms. This type of laser is termed a photodissociation laser. Since there are no vibrational and rotational modes in the I atom, the efficiency of I production may be 100%. These systems emit in the infrared region. 

Vibrations drive chemical reactions. This often-heard statement sounds trivial if understood as a description of chemical changes occurring via vibrational excitation of chemical bonds. A more difficult question to answer is, which and how many vibrations are reaction-relevant and which amount of energy (number of vibrational quanta) is needed in which mode. The barriers of chemical reactions are typically between 1 and 3 eV in comparison with the energy regime of infrared quanta ranging from. 03 to. 3 eV. This relation shows that one generally has to assume multiphoton excitation in the relevant infrared modes. 

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.

---