Collision-induced excitation energy

COLLISION-INDUCED INTRAMOLECULAR ENERGY TRANSFER IN ELECTRONICALLY EXCITED POLYATOMIC MOLECULES... 

The title of this chapter seems to promise a general discussion of the nature of collision-induced intramolecular energy transfer in electronically excited polyatomic molecules. If interpreted as just stated, the title promises more than can be delivered at this time. It is only recently that advances in experimental technique have permitted the study of the pathways of intramolecular energy redistribution following collision, and the few results now available were neither anticipated nor can they yet be fully accounted for by the available theories of collision-induced energy transfer. This chapter describes a preliminary synthesis of the limited experimental and theoretical information in hand and discusses some of its implications. It will be seen that more questions are raised than are answered. 

Another common feature of relaxation in benzene and R2 aniline is that a very frequent first step in the collision-induced vibrational energy exchange is endoergic up-pumping of the excited molecule, even when exoergic channels are available. The ubiquity and overall importance of this first endoergic step must be explained by any plausible mechanism for the processes observed. 

The efficiency of collision-induced intramolecular energy transfer on an excited electronic state energy surface is very much higher than on the ground-state energy surface. 

Unlike the case of collision-induced vibrational energy transfer, collision induced rotational energy transfer seems to be free of strong restrictions on the changes in the rotational quantum numbers. When account is taken of the spectral widths of the excitation sources used, the nature of the rotation-vibration structure in the fluorescence and absorption spectra, and the possibility of resonant ener f transfer in the collision, it is concluded that the studies of Bj aniline are the weakest, those of B2 benzene better, and those of glyoxal the best available. With this hierarchy of quality of information kept in mind, the following weaker conclusions can also be obtained from the studies cited. 

The experimental data surveyed in Section III have several striking features. Although the data base is too small to permit definitive conclusions to be drawn, it seems likely that the extremely large cross-sections and the existence of propensity rules will be characteristic of all collision-induced intramolecular energy transfer in electronically excited molecules. 

The survey of theory and data presented in this chapter shows that we are barely on the threshold of an imderstanding of collision-induced intramolecular energy transfer in electronically excited molecules. So few experimental data are available that there are likely many qualitatively new phenomena as yet undiscovered. And, at least at present, the theory of collisions has not provided predictions or qualitative concepts which can be used to guide experimental studies, nor has it been developed to the point that it can fully account for the observations in hand. Clearly, much remains to be done in this subject area. 

Collision Induced Intramolecular Energy Transfer in Electronically Excited Polyatomic Molecules... 

S.A. Rice, Collision-induced intramolecular energy transfer in electronically excited polyatomic molecules. Adv. Chem. Phys. 47, 237 (1981)... 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

In most cases, ion activation in the reaction region or fragmentation zone is applied to increase the internal energy of the ions transmitted from the ion source. The most common means of ion activation in tandem mass spectrometry is collision-induced dissociation. CID uses gas-phase collisions between the ion and neutral target gas (such as helium, nitrogen or argon) to cause internal excitation of the ion and subsequent dissociation... 

Scans based on resonant ejection may either be carried out in a forward, i.e., from low to high mass, or a reverse manner. This allows for the selective storage of ions of a certain m/z value by elimination of ions below and above that m/z value from the trap. Thus, it can serve for precursor ion selection in tandem MS experminents. [156,158] Axial excitation can also be used to cause collision-induced dissociation (CID) of the ions as a result of numerous low-energy collisions with the helium buffer gas that is present in the trap in order to dampen the ion motion. [150,156] A substantial increase of the mass range is realized by reduction of both the RF frequency of the modulation voltage and the physical size of theQIT. [154,159,160]... 

CgH (n = 6, 7, 8). A novel collision-induced isomerization of CgH7 (10a), which has a sttained allenic bond, to (lOyS) has been reported to occur upon SIFT injection of (10a) at elevated kinetic energies (KE) and collision with helium. In contrast, radical anions (9) and (11) undergo electron detachment upon collisional excitation with helium. Bimolecular reactions of the ions with NO, NO2, SO2, COS, CS2, and O2 have been examined. The remarkable formation of CN on reaction of (11) with NO has been attributed to cycloaddition of NO to the triple bond followed by eliminative rearrangement. 

Collision-induced emission. Emission spectra of ordinary atoms and molecules correspond to downward transitions, from an initial energy level higher than the final one, whereas absorption involves the inverse transition. Both exist in supermolecules as well and have recently been seen in shocktube studies. Emission spectra are generally much richer than absorption spectra and may include hot bands, which involve transitions between excited vibrational (or electronic) states [116]. 

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