Collision-induced absorption dissociation

Christie et al. have also studied the photodissociation of ICl at 4300 A and from 30 to 60 °C. Iodine molecule concentrations were measured as a function of time using absorption at 5700 A. Christie et al. suggest that since the upper electronic state is bound at the energy of this wavelength, the I2 will not dissociate. However, under the conditions of nearly all their experiments, the total pressure is large enough to cause collision-induced dissociation of the excited I2... 

The absorption band of Oj. .. Oj was assigned to collision-induced absorp-tion . Moreover, discrete features were found whkh certainly correspond to bound state O2. .. O2 molecules. The dissociation energies of both the ground and electronically excited states were obtained from analysis of the dimer vibrational levels. The upper electronic state was identified as a combined state (v = 0) -h (v = 1) the ground state is (v = 0). All the observed dimer features are rfiiftod to higher frequencies (17278-17321 cm" ) with respect to the isolated double molecule transition (17248 cm ). 

Fig. 8.21 Collision-induced dissociation of high vibrational levels populated either by multiple IR-photon absorption (a) or by stimulated emission pumping (b)...

In addition, new tandem mass spectrometry technologies were also among the important innovations. Apart from traditional collision-induced dissociation (CID) [89-91], a variety of activation methods (used to add energy to mass-selected ions) based on inelastic collisions and photon absorption have been widely utilized. They include IR multiphoton excitation [92,93], UV laser excitation [94—97], surface-induced dissociation (SID) [98-100], black body radiation (101, 102], thermal dissociation [103], and others. As the fragmentation of peptide/protein ions is a central topic in proteomics, there is strong interest in such novel ion dissociation methods as electron capture dissociation (ECD) [104, 105] and electron transfer dissociation [22]. These new methods can provide structural information that complements that obtained by traditional collisional activation. Also, very recently, ambient ion dissociation methods such as atmospheric pressure thermal dissociation [106] and low temperature plasma assisted ion dissociation [107] have been reported. 

The recent experiments on IR lasing of molecules have revealed a new type of molecular photodissociation caused by absorption of a large number of quanta in a time shorter than the interval between collisions (the so-called laser-induced collisionless dissociation). 

Because of the low collision rate in the high vacuum environment of a Fourier transform mass spectrometer (FTMS), vibrationally excited molecular ions cool predominantly by IR fluorescence. For typical IR transition dipole moments and frequencies in the mid-IR, spontaneous emission is expected to occur at a rate in the range of 1-100 s To energize an ion efficiently using IR multiple-photon excitation (MPE), the rate of photon absorption - the product of absorption cross section and photon flux - should exceed the emission rate. From such a back-of-an-envelope estimate, one finds that radiation sources producing several Watts/cm are required to induce efficient dissociation [141], Note that the demands on laser power may further increase because of the limited residence time of the ions in the laser field, collisional deactivation in traps at higher pressures, limited spectral overlap between molecular absorption and laser emission profiles, etc. 

In an FT-ICR instrument, fragmentation may be achieved by colliding ions with neutrals (CID) but various other strategies are available, such as collision with surfaces (surface-induced dissociation) or bombardment with ultraviolet or infrared radiation from a laser (ultraviolet photodissociation and multiphoton infrared photodissociation). Dissociation may also be achieved by the absorption of black-body radiation produced by a heated vacuum chamber walls (blackbody infrared dissociation). An advantage of these radiation-induced fragmentation methods is that gas pulses are no longer required. Sustained off-resonance irradiation is the preferred, radiation-based method for FT-ICR MS because it is the simplest to implement and tune. Very low energy and multiple excitation collisional activation techniques are also available. 

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