Vibrational overtone absorption excitation

Figure 3. Energy schemata of transitions involving vibrational states (a excitation of 1st vibrational state - mid-IR absorption b excitation of overtone vibrations - near-IR absorptions c elastic scattering - Rayleigh lines d Raman scattering - Stokes lines e Raman scattering - Anti-Stokes lines f fluorescence).

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, 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. 

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... 

Two experiments have dealt with the effect of higher levels of vibrational excitation on a bimolecular reaction. Overtone absorption from v—0 to o = 6 can be induced in HCl using visible light from a dye laser. Due to the small cross-section associated with overtone transitions, the high laser intensities obtainable intracavity are needed. For reaction (17), the activation energy... 

In recent experiments lasers have been used to excite unimolecular reactants. In overtone excitation, states in which an MH or MD bond contains n quanta of vibrational energy are prepared by direct single-photon absorption. Here, M is a massive atom in contrast to H or D. For states with large n, the energy in the bond may exceed the molecule s unimolecular threshold. Overtone excitation of CH and OH bonds has been used to decompose molecules (Crim, 1984, 1990). One limitation of this technique is the very weak oscillator strengths of overtone absorptions. 

Fig. 6.22. Optoacoustic overtone absorption spectrum of acetylene around y = 15 600 cm corresponding to the excitation of a local mode by five quantum vibrations [6.56]...

What distinguishes vibrational overtone initiated chemistry from that driven by electronic excitation is that the chemistry takes place exclusively in the ground electronic state. In general, following absorption of a photon, chemistry is in competition with energy dissipation in the lower atmosphere this is often driven... 

The examples discussed above illustrate the utility of vibrational overtone excitation by red sunlight in atmospheric photochemistry. The low absorption cross-section of vibrational overtones limits the importance of such light-initiated chemistry. However, when reactive electronic states are high in energy (as is the case with most alcohols and acids) or when UV radiation is suppressed at high solar zenith angles, vibrational overtone initiated photochemistry has been used to explain discrepancies between measurements and model results. 

A property of organic samples that can also affect the results of FT-Raman measurements is the reabsorption of the Raman light by the sample itself. This arises from the overlap between the Raman emission and the direct absorption by vibrational overtones and combinations that occur between 1.064 tm (the excitation) and 1.786 tm (the end of the Raman spectrum at the 3800-cm shift from the laser). These absorptions will affect relative intensities in the Raman spectrum, especially in large samples where the laser is penetrating deep below the surface. The effects of self-absorption will be especially... 

For the ideal harmonic oscillator only the fundamental vibrations are allowed and there would be no NIR spectrum. An important consequence of the an-harmonic nature of molecular vibrations is that transitions between more than one energy levels are allowed. These transitions give rise to overtone absorption bands. The near-IR bands result from transitions between the ground state and second or third excited vibrational states. The near-IR region of the spectrum thus contains mainly overtones and combination bands of fundamental mid-IR absorption bands cfr. Fig. 1.5). The intensity of the overtones depends on the anharmonicity of the vibration. Near-IR intensities are some 10 to 100 times lower than the corresponding fundamentals in mid-IR to compensate this, samples are 0.1 to 1 mm thick, which is a large virtue in comparison to mid-IR. There is no special theory of near-IR spectroscopy. 

Fig. 2 Jablonski energy level diagram illustrating possible transitions, where solid lines represent absorption processes and dotted lines represent scattering processes. Key A, IR absorption B, near-IR absorption of an overtone C, Rayleigh scattering D, Stokes Raman transition and E, anti-Stokes Raman transition. S0 is the singlet ground state, S, the lowest singlet excited state, and v represents vibrational energy levels within each electronic state.

An additional point that should be considered is that in the harmonic oscillator approximation, the selection mle for transitions between vibrational states is Ay = 1, where v is the vibrational quantum number and Ay > 1, that is, overtone transitions, which involve a larger vibrational quantum number change, are forbidden in this approximation. However, in real molecules, this rule is slightly relaxed due to the effect of anharmonicity of the oscillator wavefunction (mechanical anharmonicity) and/or the nonlinearity of the dipole moment function (electrical anharmonicity) [55], affording excitation of vibrational states with Ay > 1. However, the absorption cross sections for overtone transitions are considerably smaller than for Ay = 1 transitions and sharply decrease with increasing change in Av. 

Experimentally one can investigate resonances by various spectroscopic schemes, as indicated in Fig. 1 by direct overtone pumping [11] from the ground vibrational state, by vibrationally mediated photodissociation [12] using an excited vibrational level as an intermediate, or by stimulated emission pumping (SEP) [13-15] from an excited electronic state. In all cases it is possible to scan over a resonance and thereby determine its position j4s aHd its width hkU). A schematic illustration of an absorption or emission spectrum is depicted on the left-hand side of Fig. 1 all of the more or less sharp structures at energies above threshold are resonances. Figure 2 shows an overview SEP spectrum measured for DCO [16]. It consists of... 

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