Metastable decay

Spengler, B. Kirsch, D. Kaufmann, R. Metastable decay of peptides and proteins in matrix-assisted laser-desorption mass spectrometry. Rapid Comm. Mass Spectrom. 1991, 5,198-202. 

Figure 2. (a) Reflection TOF mass spectrometer, (b) Depicts the electrostatic potentials. With a judicious selection of potential, the daughter ions arising from metastable decay arrive at the detector prior to the parent ions which have higher kinetic energy. MCP denotes a microchannel plate charged particle detector, (a) Taken with permission from ref. 22 (b) Taken with permission from ref. 19. 

Fragmentation Region metastable decay collisional induced dissociation photodissociation... 

Example The metastable decay of the o-nitrophenol molecular ion, m/z 139, by loss of NO to yield the [M-NO] ion, m/z 109, has been studied on a single-focusing magnetic sector instrument (Fig. 2.11. [45] The mass spectrum shows a... 

Katakuse, 1. Nakabushi, H. Ichihara, T. Sakurai, T. Matsuo, T. Matsuda, H. Metastable Decay of Cesium Iodide Cluster Ions. Int. J. Mass Spectrom. Ion Proc. 1984, 62, 17-23. 

Jones, J. J. et al, Ionic liquid matrix-induced metastable decay of peptides and oligonucleotides and stabilization of phospholipids in MALDl FTMS analyses. J. Am. Soc. Mass Spec., 16, 2000,2005. 

Photo-excited SO2 and SO2 clusters have been observed to undergo a number of excited state and ion-state processes. Ion-state studies have, for example, identified the energy threshold of the ion-state oxygen loss channel of the SO2 monomer and dimer [1], Additionally, studies investigating the metastable decay process of SO2 clusters and mixed S02-water clusters have identified the dissociation pathways and the nature of the charged core of these cationic clusters [2]. The dynamics of oxygen loss of SO2 and SO2 clusters following excitation to the C (2 A ) state, which couples to a repulsive state, have also been studied to determine the influence of the cluster environment on the dissociation process [3]. 

A theory of photochemical processes which relates macroscopic observables to molecular properties should have great appeal to the physical chemist, since in these cases, unlike the case of thermal reactions, the nature of the metastable decaying states can be unambiguously defined. Furthermore, since these states are routinely prepared in photochemical experimentation, they are worthy of extensive study. Perhaps a more complete understanding of the properties of isolated molecule metastable states can play the role of precursor to the understanding of the states involved in thermal reactions. 

Isomeric C Hn radical ions fragment not very differently by the different mass spectro-metric methods. The metastable decays are nearly identical, but the collisionally activated spectra of 14 isomeric hexenes, measured by Nishishita and McLafferty240, exhibit some quantitative differences. Bensimon, Rapin and Gaumann251 compared the metastable decay and the photoinduced fragmentation by infrared photons of long-lived parent ions of six hexene isomers and cyclohexane. If the linear isomers are practically identical, some notable differences are observed for branched isomers. Cyclohexane behaves similar to n-hexenes. The metastable fragmentation of H/D-labeled 4-Me-2-pentene, 2-Me-2-pentene... 

The term desorption is used in contrast to evaporation in cases in which a transition of a molecular or ion from the condensed into the gas phase is assumed to take place under non thermal equilibrium condition. The underlying idea is that at thermal equilibrium, temperatures for an evaporation would lead to a correspondingly high excitation of internal vibrational modes of excitation leading to fraigmentation of the molecule. As mentioned above, several characteristics of the ion spectra (2., 6.) cannot reasonably be fitted to an equilibrium temperature model. These properties seem to be the more pronounced, the higher the laser irradiance (i.e. usually the shorter the pulse) and are best documented for the LAMMA technique. Though metastable decay of ions is observed and will be discussed below, the decay rate for most of the ions is very small and decay... 

Figure 3. Positive ion spectra (up) of phthalic acid and of neutrals (down) from metastable decay of positive ions.

Figure 7. Positive- (top and negative ion (middle) spectra and of neutrals (bottom) from metastable decay of negative ions of benzopyrene.

A typical hard reflection time-of-flight mass spectrum of ammonia clusters is shown in Figure 6-8(a). When the reflecting voltage is reduced, only the daughter ions are reflected, as shown in Figure 6-8(b). To precisely measure the metastable decay fractions, the soft reflection mode is used for a number of reasons discussed elsewhere (Wei et al. 1990a,b). 

The sp chains are metastable and have the tendency to undergo cross-linking reactions to form sp phase [7]. In order to characterize the sp metastable decay we have monitored the sp chain stability through the evolution of the C peak intensity, either keeping the sample under UHV for several days (at a pressure of about 2 x 10 Torr) or exposing it to different atmospheres (H2, He, N2 and dry air) [40]. In the case of UHV conditions, we have observed a slow decrease of the intensity of both the two components of the C peak and small changes in the shape of the G and D bands. The temporal evolution of is well described by an exponential decay plus a constant ... 

The intensity of the C peak decreases readily following an exponential law and reaching an asymptotic value after a few hours from gas injection (Figure 2.7). Table 2.1 presents the results of the analysis of the evolution of fc, fitted with an exponential decay the time constants r and the percentage Rq of the nonreacted fraction of sp chains for the different gas exposures are shown, together with the parameters of the metastable decay in UHV as discussed above. 

We observe threshold effects at different temperatures both for the slow decay constants r2 and in the fraction of surviving carbynes after the thermal treatment Rq. This has a value of roughly 29% after the metastable decay at RT, it drops at 15% at lOO C remaining constant up to 150°C. We observe another drop to 8% at 200°C. This suggests the presence of two activated processes with energy barriers situated between 25 and 40meV. 

One possible mechanism for the stabilization of sp chains in the carbon matrix may be the formation of joints between the chain ends and sp island adatoms. Jarrold and co-workers have reported the observation of ball-and-chain dimers formed by fullerenes linked by sp chains obtained by laser desorption of fullerene films [44]. The same type of connections could be formed in our films, rich of fullerene-like fragments [35], thus allowing the stabilization of the carbynoid species. Different type of stabilization sites could be present in our films since we observe a metastable decay of the carbynoid population. 

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