Vibrational overtone absorption

Keywords Cloud chemistry Dust Heterogeneous chemistry Ice photochemistry Organic aerosols Urban grime Vibrational overtone absorption... 

Like aU photolysis reactions, those initiated by vibrational overtone absorption are analyzed as first-order kinetic processes with a photochemical rate, /, which depends upon the absorption coefficient a(A) of the absorbing compound, the... 

Room and low-temperature electronic absorption spectra for the (110) face of a Os2piv4Cl2 crystal are shown in Figs. 3 and 4. No absorption attributable to electronic transitions could be found in the 1300-2000 nm region. Strong vibrational overtone absorption prevents extension of the measurements beyond... 

The functional groups almost exclusively involved in NIRS are those involving the hydrogen atom C-H, N-H, O-H (see Figure 5.1). These groups are the overtones and combinations of their fundamental frequencies in the mid-infrared and produce absorption bands of useful intensity in the NIR. Because the absorptivi-ties of vibrational overtone and combination bands are so much weaker, in NIRS the spectra of condensed phase, physically thick samples, can be measured without sample dilution or the need to resort to difficult short-path length sampling techniques. Thus conventional sample preparation is redundant, and fortunately so, because most PAT applications require direct measurement of the sample " either in situ, or after extraction of the sample from the process in a fast loop or bypass. 

However, in polyatomic molecules, transitions to excited states involving two vibrational modes at once (combination bands) are also weakly allowed, and are also affected by the anharmonicity of the potential. The role of combination bands in the NIR can be significant. As has been noted, the only functional groups likely to contribute to the NIR spectrum directly as overtone absorptions are those containing C-H, N-H, O-H or similar functionalities. However, in combination with these hydride bond overtone vibrations, contributions from other, lower frequency fundamental bands such as C=0 and C=C can be involved as overtone-combination bands. The effect may not be dramatic in the rather broad and overcrowded NIR absorption spectrum, but it can still be evident and useful in quantitative analysis. 

Donaldson et al. (1997) have proposed that absorption in the visible due to OH vibrational overtones could be important in the lower stratosphere at large solar zenith angles. Transfer of energy from the absorbing mode to the HO—NOz bond may then cause dissociation, as observed, for example, in HOC1 (e.g.,... 

Donaldson, D. J., G. J. Frost, K. H. Rosenlof, A. F. Tuck, and V. Vaida, Atmospheric Radical Production by Excitation of Vibrational Overtones via Absorption of Visible Light, Geophys. Res. Lett., 24, 2651-2654 (1997). 

A rule of thumb for hydride stretches [56, 57] is that the intensities of the vibrational overtone and combination transitions decrease, approximately, as IQ-Ay jjjg drop-off in intensity for the first few quanta of excitation may be even steeper, by another factor of 10. This implies that, in a specific spectral interval, the strongest vibrational transitions from the vibrationless ground state level correspond to the transition with the smallest Av and the greatest anharmonicity. However, as shown later, even these small absorption cross sections of vibrational overtone transitions can be sufficient for overtone preexcitation. 

Anharmonicity leads to deviations of two kinds. At higher quantum numbers, AE becomes smaller, and the selection rule is not rigorously followed as a result, transitions of A 2 or 3 are observed. Such transformations are responsible for the appearance of overtone lines at frequencies approximately two or three times that of the fundamental line the intensity of overtone absorption is frequently low, and the peaks may not be observed. Vibrational spectra are further comphcated by the fact that two different vibrations in a molecule can interact to give absorption peaks with frequencies that are approximately the sums or differences of their fundamental frequencies. Again, the intensities of combination and difference peaks are generally low. 

The infrared absorption of 1-butene that occurs at 1830 cm 1 (Figure 10-1) falls in the region where stretching vibrations of alkene bonds usually are not observed. However, this band actually arises from an overtone (harmonic) of the =CH2 out-of-plane bending at 915 cm 1. Such overtone absorptions come at exactly twice the frequency of the fundamental frequency, and whenever an absorption like this is observed that does not seem to fit with the normal fundamental vibrations, the possibility of its being an overtone should be checked. 

However, what unite all applications of NIRS for PAC are the unique features of the NIR spectrum. The NIR is in effect the chemical spectroscopy of the hydrogen atom in its various molecular manifestations. The frequency range of the NIR from about 4000 cm-1 up to 12 500 cm-1 (800-2500 nm) covers mainly overtones and combinations of the lower-energy fundamental molecular vibrations that include at least one X—H bond vibration. These are characteristically significantly weaker in absorption cross-section, compared with the fundamental vibrational bands from which they originate. They are faint echoes of these mid-IR absorptions. Thus, for example, NIR absorption bands formed as combinations of mid-IR fundamental frequencies (for example v + u2), typically have intensities ten times weaker than the weaker of the two original mid-IR bands. For NIR overtone absorptions (for example 2v, 2v2) the decrease in intensity can be 20-100 times that of the original band. 

D20 has an absorption band centered around 1.9 /(m that corresponds to the first vibrational overtone. The D20 transmits 75—80% of the T-jump pulse in a cell with a 50 pm path length. The energy from the fraction that is absorbed produces the T-jump. The specific heat capacity of D20 is 4.22 J K g 1. Assuming 20% absorption and a sample volume of... 

Figure 9.40 Cavity ring-down absorption spectrum of HCN showing the 1q3q vibrational overtone/combination band. (Reproduced, with permission, from Romanini, D. and Lehmann, K. K., J. Chem. Phys., 99, 6287, 1993)...

We elected to study coherent up-pumping dynamics in solution-phase metal-hexacarbonyl systems because of their strong vibrational infrared absorption cross sections, relatively simple ground-state spectra, and small (ca. 15 cm ) anharmonic overtone shifts. It was felt that these systems are ideal candidates to demonstrate that population control could be achieved for polyatomic species in solution because the excited state population... 

Three main types of absorption occur, namely fundamental, overtone, and combination. Fundamental bands are the primary absorption bands for each mode of vibration, overtone bands occur at... 

The vapor spectra of the M(thd)3 compounds with M = Pr, Nd, Sm, Eu, Dy, Ho, Er, and Tm are shown in Figures 5 and 6. The arrows indicate absorption owing to vibrational overtone and combination bands of the organic chelate moiety. The remaining absorption bands arise from f f transitions of the rare-earth constituents. The energies and molar absorptivities of the f f absorption maxima are shown in Table VII. 

Absorption bands that are attributed to overtone and combination vibrations are also observed in the IR spectrum of polyatomic molecules. Overtone vibrations occur at frequencies of approximately integral multiples of the fundamental frequencies. Combination vibrations appear at frequencies that are the sum or the difference of the frequencies of two or more frmdamental vibrations. Overtone and combination bands are much less intense than fundamental bands. 

---