The Quasi-equilibrium Theory QET

The quasi-equilibrium theory (QET) is the most widely used theoretical framework for the discussion of the fragmentation pattern of the parent ion in a uni-molecular process. Although other unimolecular theories (see Levine, 1966) have been subsequently proposed, the QET has traditionally been applied for 

Before we do this, though, we point out that for a simple diatomic molecule, assuming ideal conditions, one can in principle calculate the rate of the uni-molecular process. This is so because the lower excited states of the ion are (relatively) few and well separated. If the potential curves are then given, the value of the rate can be provided. For a polyatomic molecule, two great complications immediately arise (1) the number of lower excited states increases tremendously and (2) multidimensional potential energy surfaces make trajectory calculations intractable. 

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The quasi-equilibrium theory (QET) of mass spectra is a theoretical approach to describe the unimolecular decompositions of ions and hence their mass spectra. [12-14,14] QET has been developed as an adaptation of Rice-Ramsperger-Marcus-Kassel (RRKM) theory to fit the conditions of mass spectrometry and it represents a landmark in the theory of mass spectra. [11] In the mass spectrometer almost all processes occur under high vacuum conditions, i.e., in the highly diluted gas phase, and one has to become aware of the differences to chemical reactions in the condensed phase as they are usually carried out in the laboratory. [15,16] Consequently, bimolecular reactions are rare and the chemistry in a mass spectrometer is rather the chemistry of isolated ions in the gas phase. Isolated ions are not in thermal equilibrium with their surroundings as assumed by RRKM theory. Instead, to be isolated in the gas phase means for an ion that it may only internally redistribute energy and that it may only undergo unimolecular reactions such as isomerization or dissociation. This is why the theory of unimolecular reactions plays an important role in mass spectrometry. 

Two almost identical theories explaining the phenomena observed in the case of unimolec-ular reactions in the gas phase at high vacuum were proposed in 1952. One of them, the quasi-equilibrium theory (QET), was suggested by Rosenstock et al. [2] and applies to mass spectrometry. The other is named after the initials of its authors, RRKM, standing for Rice, Rampsberger, Kassel and Marcus [3], and deals with neutral molecules. 

It is well known that the mass spectra of some polyatomic molecules are interpreted fairly well by the quasi-equilibrium theory (QET) developed by Eyring and co-workers [80]. 

The problems associated with the classical RRK expression in Eq. (1.11) were eliminated by Marcus and Rice (1951) and by Rosenstock, Wallenstein, Wahrhaftig, and Eyring (1952). These quantum theories, which treat the vibrational (and rotational) degrees of freedom in detail, became known as the RRKM and the quasi-equilibrium theory (QET), respectively. The RRKM/QET expression, which will be discussed in detail in chapter 6, is given by... 

One of the earliest statistical theories of unimolecular decay of ionic species, developed explicitly to interpret mass spectra, is the quasi-equilibrium theory (QET). In the QET, the electronically excited molecular states that are accessed by the primary ionization event decay rapidly by internal conversion to the ground state of the ion, where... 

Quasi-Equilibrium theory (QET) Theory of mass spectra based on rapid internal conversion of excited electronic states produced by electron impact to the ground electronic state. Statistical redistribution of energy on the-ground state surface, followed by vibrational predissociation, determines the rate of fragmentation. 

Quasi-Equilibrium Theory (QET) Iorrization process in an El source is very fast, taking <10 s, which forms molecttlar ions in groimd and excited states. Ionization occurs by a vertical transitiorr, and therefore the interatomic distances remain... 

Theories of quasi-equilibrium (QET) [28] and RRKM [29] explain the monomolec-ular fragmentation of ions. R.A. MARCUS receives the Nobel Prize in 1992. 

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