Emission spectroscopy of dissociating molecules

1) The energies of the vibrational eigenstates in the electronic ground state, which are related to the frequencies of the emitted photons by E% T hu — Ef T Tuo.  

2) The intensities of the emission lines which reflect the dissociation dynamics in the excited electronic state and the vibrational dynamics in the ground electronic state. 

The example of CH3I and similar results for O3 (Imre, Kinsey, Field, and Katayama 1982 Imre, Kinsey, Sinha, and Krenos 1984) clearly demonstrate the possibility of accessing vibrational levels of polyatomic 

Like absorption spectra and final state distributions, emission or Raman spectra can be calculated either in the time-independent or the alternative time-dependent framework (Heller 1981a,b Williams and Imre 1988a). The two methods will be outlined in Section 14.1. At the present time a joint experimental and fully ab initio theoretical investigation has been performed only for H2O excited into the first continuum. Section 14.2 summarizes the main results which are characteristic for other molecules as well. Finally, we briefly discuss in Section 14.3 some recent 

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Imre, D., Kinsey, J. L., Sinha, A., and Krenos, J. (1984), Chemical Dynamics Studied by Emission Spectroscopy of Dissociating Molecules, J. Phys. Chem. 88, 3956. 

Imre, D., Kinsey, J.L., Sinha,. 4., and Krenos, J. (1984) Chemical dynamics studied by emission spectroscopy of dissociating molecules,. 7. Phys. Chem. 88, 3956-3964. Untcli, A., Weide, K., and Schinke, R. (1985) The direct photodissociation of ClNO(Si) An exact threc-dimeiisional wave packet analysis, J. Chem. Phys. 95. 6496-6507. 

Even if two vibrational states are degenerate they can yield completely different cross sections. The dissociation of excited vibrational states samples a considerably wider region of the upper-state PES than dissociation of the ground vibrational state. However, because the two quantum mechanical wavefunctions both have an oscillatory behavior, the interpretation of the various cross sections is not always obvious. The photodissociation of excited vibrational states is closely related to the emission spectroscopy of the dissociating molecule which is the topic of the following chapter. 

In cases like D2CO or NO2 comparison with experimental data on a state-specific level are ruled out entirely and one has to retreat to more averaged quantities like the average dissociation rate, (fc), and the distribution of rates, Q(k). If the dynamics is ergodic — the basic assumption of all statistical theories — one can derive a simple expression for Q k), which had been established in nuclear physics in order to describe the neutron emission rates of heavy nuclei [280]. These concepts have since developed into the field of random matrix theory (RMT) and statistical spectroscopy [281-283] and have also found applications in the dissociation of energized molecules [121,284-286]. 

In a dual-color cross-correlation fluorescence spectroscopy (DCCFS) experiment [46], a sample containing two fluorophores with different emissions in each molecule was irradiated with two lasers (or with one laser) to perform simultaneous excitation of the fluorophores. The DCCFS in combination with the confocal laser microscopy allows the separation of microscopic volume with two different fluorophores from volume with only one of them and, therefore, the monitoring of dissociation of the dual-labeled molecules or association of two single-labeled molecules. Optical setup as realized in an inverted microscope to perform simultaneous excitation of the fluorophores (Figure 11.14). 

In the case where the sample is introduced as a solution aerosol, the atomization source must evaporate the solvent, vaporize the resulting salt particles, and dissociate any analyte-containing molecules -tasks that are similarly required when wet aerosols are used in emission spectroscopy. Since collisions with high-temperature gases are the most efficient means of accomplishing the first two tasks, thermal sources such as flames are typically used (e.g., flame AA). 

In a flame atomizer, a solution of the sample is nebulized by a flow of gaseous oxidant, mixed with a gaseous fuel, and carried into a flame where atomization occurs. As shown in Figure 9-1, a complex set of interconnected processes then occur in the flame. The first is desolvation, in which the solvent evaporates to produce a finely divided solid molecular aerosol. The aerosol is then volatilized to form gaseous molecules. Dissociation of most of these molecules produces an atomic gas. Some of the atoms in the gas ionize to form cations and electrons. Other molecules and atoms are produced in the flame as a result of interactions of the fuel with the oxidant and with the various species in the sample. As indicated in Figure 9-1, a fraction of the molecules, atoms, and ions are also excited by the heat of the flame to yield atomic, ionic, and molecular emission spectra. With so many complex processes occurring, it is not surprising that atomization is the most critical step in flame spectroscopy and the one that limits the precision of such methods. Because of the critical nature of the atomization step, it is important to understand the characteristics of flames and the variables that affect these characteristics. 

Accurate potential functions for chemically stable diatomic molecules are well known thanks to extensive studies in the field of spectroscopy. The potential in the vicinity of the equilibrium position of the (usually deep) well has been determined for both ground and excited molecular states. Such studies have also yielded information on repulsive parts of potential curves, especially when light absorption or emission leads to dissociation, as in Chapter 7. Other important sources have been the measured temperature dependence of the deviation from ideal gas behavior and of u-ansport coefficients, and from lattice energies and compressibility of solids. 

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