Absorption spectra diatomic electronic

To apply this concept for a simple diatomic molecule, let s consider the example given in Figure 3. At room temperature, according to the Boltzman distribution, most of the molecules are in the lowest vibrational level (v) of the ground state (i.e., v = 0). The absorption spectrum presented in Figure 3b exhibits, in addition to the pure electronic transition (the so-called 0-0... 

The reflection principle, outlined in Sections 6.1 and 6.2, explains the energy dependence of absorption spectrum as a mapping of the initial coordinate distribution in the electronic ground state onto the energy axis. Rotational state distributions of diatomic photofragments in direct dissociation can be explained in an analogous manner. 

Vibrational Relaxation. Stochastic processes, including vibrational relaxation in condensed media, have been considered from a theoretical standpoint in an extensive review,502 and a further review has considered measurement of such processes also.503 Models have been presented for vibrational relaxation in diatomic liquids 504 and in condensed media,505 using a master-equation approach. An extensive development of quantum ergodic theory for relaxation processes has been published,506 and quantum resonance effects in electronic to vibrational energy transfer have been considered.507 A paper has also considered the coupling between vibrational relaxation and molecular electronic transitions.508 A theory has also been outlined for the time-resolved electronic absorption spectrum of a molecule undergoing collisional vibrational relaxation.509... 

We note that the induced absorption bands mentioned are associated with single or double electronic transitions. Such induced bands are not limited to oxygen similar bands of a few other systems (benzene, for example) were known for some time. However, most of the common diatomic molecules have electronic states that are many eV above the ground state. As a consequence, for those molecules, electronic induced transitions commonly occur in the ultraviolet region of the spectrum where interference from allowed electronic spectra may be strong. Electronic induction is not nearly as common as rovibro-translational induced absorption. 

In diatomic spectra, one distinguishes between individual bands each corresponding to a definite pair of quantum numbers v, v", and band systems, each composed of an ensemble of bands associated with a particular electronic transition. In polyatomic spectra, often (a), the individual bands of an electronic transition are so numerous and strongly overlapping that it is difficult or impossible to distinguish them individually, or (b), the electronic transition gives rise only to continuous absorption in both these situations the entire spectrum of an electronic transition is commonly called a band. IT IS RECOMMENDED (REC. 39) that the word band be reserved for definite individual bands, and that electronic transition or transition be used for the entire spectrum, whether discrete, pseudo-continuous, or strictly continuous, associated with an electronic transition or band system if the spectrum consists of discrete bands. ... 

There are several recent experimental studies on the CeO diatomic molecule. Schall et al. (1986) have studied CeO using the sub-doppler Zeeman spectroscopy. Again, the ligand-field model is so successful in explaining the observed spectra due to the ionic nature of the diatomic lanthanide oxide. Linton et al. (1979, 1981, I983a,b) as well as Linton and Dulick (1981) have studied the electronic spectrum of CeO using absorption, emission as well as laser spectroscopic method. There are many 0-0 bands for... 

Arsenic, as the element with the shortest primary absorption wavelength in AAS, produces several AsO molecular bands around 250 nm if the conventional air/acetylene flame is used. An overview of the structures produced by the diatomic molecule is shown in Figure 7.15. Flere the electronic transition takes place between the X and the B state, while the vibrational excitation is changed in some cases. Apart from the two AsO bands with Av= +1 around 243.5 nm and Av= -1 around 256.3 nm, two systems without a change of the vibrational excitation exist. The latter can be divided into the so-called subsystems I and II, showing pronounced band heads around 250.4 nm and 257.0 nm, respectively. In Figure 7.16 the enlarged spectrum of the electronic transition to the B state with Av = 0 (subsystems I) is depicted. 

In Chapter 7 several electron excitation spectra of diatomic molecules that have been recorded in a flame were shown. Some of these spectra are only observed in the presence of specific mafrices or acids, such as those of the PO and CS molecules, whereas others, particularly ihe spectrum of OH, is present in any spechoscopic flame, even when no sample is introduced. The concentrations of radicals such as O, OH, CN and H in a flame are determined by the reactions between the natural components of the flame. The influence of any sample constituents on these components is negligible, since every reaction that leads to a reduction or an increase in the concentration of such a species is immediately counteracted by infinitesimal shifts in the equilibria of the main components. This means that these molecular absorption structures in a flame should remain constant, and be eliminated in the calibration process. However, this is not exactly the case, as will be shown in the following examples. 

Gas phase electronic spectra of polyatomic molecules are more complicated than the spectra of diatomics. The number of vibrational modes and the possibility of combination bands usually lead to numerous vibrational bands, and these may be overlapping. Also, the rotational fine structure tends to be more complicated, as we might expect from the differences between diatomic and polyatomic infrar (IR) spectra. Conventional absorption spectra can prove to be a difficult means of measuring and assigning transitions, and so numerous experimental methods have been devised to select molecules in specific initial states and to probe the absorption or the emission spectrum with narrow frequency range lasers. 

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