Collision-induced excitation

Collision-induced dissociation mass spectrum of tire proton-bound dimer of isopropanol [(CH2)2CHOH]2H. The mJz 121 ions were first isolated in the trap, followed by resonant excitation of their trajectories to produce CID. Fragment ions include water loss mJz 103), loss of isopropanol mJz 61) and loss of 42 anui mJz 79). (b) Ion-molecule reactions in an ion trap. In this example the mJz 103 ion was first isolated and then resonantly excited in the trap. Endothennic reaction with water inside the trap produces the proton-bound cluster at mJz 121, while CID produces the fragment with mJz 61. 

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... 

CoIIisional activation. An ion/neutral process wherein excitation of a (fast) projectile ion is brought about by the same mechanism as in collision-induced dissociation. (The ion may decompose subsequently). 

The 16 ns natural lifetime of excited Na is much shorter than the 140 /rs mean time between collisions, thus the fine broadening due to collision-induced... 

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... 

Finally, we would like to point out that in the off-resonance region, the response time of the nonlinearity is limited only by the optical pulse width r, as long as (Ea -Tiaj)/h >>2ir(x ). (8) This is no longer true when collisions (or phonons in solids) are present. For optical frequencies close enough to the absorption edge, the collision induced transitions to the excited state will cause the x s response time to be limited by the relaxation time of the excited states. (8)... 

In order to record excitation spectra, the radical ions must first be thermalized to the electronic ground state, which happens automatically if they are created in condensed phase (e.g. in noble-gas matrices, see below). In the gas-phase experiments where ionization is effected by collision with excited argon atoms (Penning ionization), the unexcited argon atoms serve as a heat bath which may even be cooled to 77 K if desired. After thermalization, excitation spectra may be obtained by laser-induced fluorescence. 

The validity of Johnston s interpretation of the experimental facts in terms of the simple unimolecular dissociation (1) has been questioned by Lindars and Hinshelwood120 and by Reuben and Linnett121. These workers maintain that isothermal plots of k versus p are not smooth curves, but consist of a number of straight lines linked by markedly curved portions. To explain such behaviour they incorporate into their mechanism a collision-induced crossover of vibrationally excited N20 (XS) to repulsive 3II and 3E states. While we incline towards the simpler view held by Johnston105 and others106-116, we feel that this feature of the decomposition kinetics merits further investigation. 

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. 

Fig. 8. Collision-induced satellite spectrum of Naj around X = 5290 A, due to fluorescence originating from rotational levels populated by collision from the V = 6, / = 43 level in the state, which was excited by the X = 4880 A argon laser line. (From ref.

The Intensity ratio between these satellite lines" and the main fluorescence line yields the absolute cross sections for the different collision-induced rotational transitions, provided the He pressure and the lifetime of the excited Na state are known. Both parameters have been measured, giving cross sections between = 1.5 A for A/ = 1 transitions and 6 = 0.3 A for A/ = 5... 

In spite of the fact that in alkali vapors, which contain about 1 % diatomic alkali-molecules at a total vapor-pressure of 10 torr, the atoms cannot absorb laser lines (because there is no proper resonance transition), atomic fluorescence lines have been observed 04) upon irradiating the vapor cell with laser light. The atomic excited states can be produced either by collision-induced dissociation of excited molecules or by photodissociation from excited molecular states by a second photon. The latter process is not improbable, because of the large light intensities in the exciting laser beam. These questions will hopefully be solved by the investigations currently being performed in our laboratory. 

Detailed information about collision-induced, vibrational, or rotational excitation and deactivation has been collected from much work on molecular lasers, expecially the COj laser 1. If the intensity of a laser line is sufficiently high, its gain in an amplifying medium saturates and is then determined by the number of molecules excited per second into the upper laser level and by the depopulation rate of the lower level. Measuring the saturation behaviour under different pressures and discharge currents allows the excitation and deactivating processes to be studied Observation of the... 

Collision induced dissociation the dissociation of ions after collisional excitation... 

The deviation from first-order kinetics for the uridylic acids is also reflected in quantum yields which vary with concentration and which therefore vary during the course of a particular photolysis.7 These deviations from first-order kinetics have been discussed in terms of a collision-induced transition of an excited-singlet pyrimidine to a long-lived state.7 We shall say more about the probability of long-lived states later. 

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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