Metastable dissociation experiments

An important aspect of studying metastable dissociation of these clusters is that the measurements enable a determination of the surface composition of mixed systems. This is important in designing experiments to study the heterogeneous chemistry of aqueous systems. For example, the loss channel of H20 is found to be open to all (H20)n(CH30H)mH+ except (H20)(CH3OH)mH+ for which the water loss is relatively small. For the water-rich composition mixed clusters, the results show that water molecules have a tendency to build a cage structure in the cluster size region m + n = 21, with 0 < m < 8. 

Figure 2 Excitation scheme for the two laser pump-pump experiment. State-selected benzene cations are produced in a resonantly enhanced two-photon ionization process. A second laser pulse of variable photon energy excites the ions to a well defined energy level above the dissociation threshold and metastable dissociation takes place (taken from ref. /16/).

Neutral products of dissociation of ions can be analyzed using collision-induced dissociative ionization (CIDI) and neutral fragment reionization (NfR) methods. In the first method, unlike NR MS, no collision gas is introduced in the first collision cell. All precursor and product ions are deflected before reaching the second cell. Neutral products of metastable dissociation are colli-sionally analyzed in the second cell and detected. In NfR experiments, similar to NR MS, a collision gas is introduced to the first collision cell to increase the degree of fragmentation. The requirement to this gas is that it should not cause neutralization of the ions. Helium is used for this purpose. 

Tandem quadrupole and magnetic-sector mass spectrometers as well as FT-ICR and ion trap instruments have been employed in MS/MS experiments involving precursor/product/neutral relationships. Fragmentation can be the result of a metastable decomposition or collision-induced dissociation (CID). The purpose of this type of instrumentation is to identify, qualitatively or quantitatively, specific compounds contained in complex mixtures. This method provides high sensitivity and high specificity. The instrumentation commonly applied in GC/MS is discussed under the MS/MS Instrumentation heading, which appears earlier in this chapter. 

Fig. 1.1. TOF data for I2 as a function of time delay in a pump-probe experiment showing a variety of vibrational signals in the I+ signal. The horizontal band at 5,145 ns corresponds to metastable t] 1. The pair of bands at 5,020 and 5,270 ns corresponds to the I+ + I+ dissociation channel, while the signal in between these bands corresponds to I+ + I. Pump and probe are both 25 fs, 800 nm pulses...

The relatively long timescales of the ionization, isolation, thermalization, reaction, and detection sequences associated with low-pressure FTICR experiments are generally thought to preclude the use of this technique as a means of examining the unimolecular dissociation of conventional metastable ions occurring on the microsecond to millisecond timescale. Nonetheless, as just demonstrated (Section IIIC), intermediates with this order of magnitude of lifetime are routinely formed in the bimolecular reactions of gaseous ions with neutral molecules at low pressures in the FTICR cell, as in Equation (13). 

In principle, the neutral desorbed products of dissociation can be detected and mass analyzed, if ionized prior to their introduction into the mass spectrometer. However, such experiments are difficult due to low ejfective ionization efficiencies for desorbed neutrals. Nevertheless, a number of systems have been studied in the groups of Wurm et al. [45], Kimmel et al. [46,47], and Harries et al. [48], for example. In our laboratory, studies of neutral particle desorption have been concentrated on self-assembled monolayer targets at room temperature [27,28]. Under certain circumstances, neutrals desorbed in electronically excited metastable states of sufficient energy can be detected by their de-excitation at the surface of a large-area microchannel plate/detector assembly [49]. Separation of the BSD signal of metastables from UV luminescence can be effected by time of flight analysis [49] however, when the photon signal is small relative to the metastable yield, such discrimination is unnecessary and only the total yield of neutral particles (NP) needs to be measured. 

Until now, most studies of dissociation dynamics of metastable cluster ions have been made using a double-focusing mass spectrometry method (Lifshitz et al. 1990 Lifshitz and Louage 1989, 1990 Stace 1986). As discussed herein, the novel technique of reflectron time-of-flight mass spectrometry is a valuable alternative approach to more standard methods. With carefully designed experiments, it is possible to derive both kinetic energy releases and decay fractions for... 

Recent experiments on the coverage dependence of the sticking of 02 to Ag(l 1 0) revealed a remarkable drop shown in Fig. 20 [172-179]. This drop has been explained by electrostatic effects [179]. Butler et al. have attributed the drop to the build-up of added rows at the surface [180], If the picture of adsorption into a metastable intermediate is valid, it implies that the intermediate should be stabilized at a step edge. Subsequently the molecule can dissociate at the site and form an Ag-O pair, that is inserted into an added row. This involves mass transport at the surface, which has been observed in... 

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