Metastable dissociation

In other related studies,186 we determined the structures of mixed water-methanol clusters the intensity distributions of (H20)n(CH30H)JBH+ showed magic numbers at n + m = 21, 0 < m 8 due to the enhanced stabilities of the dodecahedral cage structures in the mixed clusters. Studies of the metastable dissociation of (H20)B(CH30H)mH+ (n + m < 40) provided evidence that the dissociation channels... 

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. 

More recently, Audier and McMahon have shown that the unimolecular dissociation spectrum of transient ions can be directly obtained from a simple manipulation of a series of FTICR spectra. The data arising from this approach very closely resemble those obtained from metastable dissociations in conventional sector spectrometers (MIKES), and it has been consequently dubbed metastable ion cyclotron resonance (MICR) spectrometry. Very briefly, the method functions as follows ... 

The excess energy within the cluster ion owing to the above exothermic reaction and the relaxation around the newly formed ion, as well as possible further multiphoton excitations, contributes to heating of the cluster ions and concomitant evaporative dissociation. As the cluster ions cool evaporatively, the dissociation extends to longer times, and the present study is directed to an investigation of the metastable dissociation processes of cluster ions (NH3) H+ in the field-free region (where the time window is about one to a few tens of microseconds). In the case of the ammonia cluster ion system, the dissociation process can be expressed as... 

Metastable Dissociations Dynamics Kinetic Energy Release... 

The RETOF technique has applications far beyond mass analysis and determination of metastable dissociation fraction. In particular, it provides a very valuable approach to determining kinetic energy release (KER) during evaporative dissociation. As shown in this section, these data also find application in determining absolute values of cluster bond energies for systems without a barrier to... 

The procedure is to see how fragments are eliminated from model olefins and to compare the results with the metastable dissociation from an olefin fragment of an alkane ion. In this section we will limit ourselves mainly to the C7H14 ions formed either from different isomeric C7H14 or from alkanes with nine or more carbon atoms. A particularly... 

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. 

The wide distribution of rotational states in tetrazine 16a is an indication that vdW bending modes are always excited in the metastable (dissociative) levels produced by IVR, whatever rotational states of the 6a level were initially excited. We consider this as another indication for the mixed character of the van der Waals vibrational levels. 

Example The curved-field reflectron of a coaxial ReTOF instrument occupies a large portion (D) of the total flight path s = li + D) and uses 86 lens elements whose voltages are set by 85 precision potentiometers located between them [47], The distance f between ion source and reflector entrance provides sufficient field-free flight path for metastable dissociations. 

Example GB and PA of cyclohexanecarboxamide were determined by both experimental methods. The kinetic method based on both metastable dissociation and CID yielded GB = 862 7 kJ mol" and PA = 896 5 kJ mor, while the... 

The decomposition of Ar(N2)2 studied with a reversed double-focusing mass spectrometer, proceeds via Ar(N2)2 - ArN + N2 and Ar(N2)2 Ar + (N2) by both collision-induced and metastable dissociation [33]. 

This metastable dissociation of C5H5+ ions will produce a peak at apparent mass 23.4 Da. Clearly these phenomena can (and do) assist in the interpretation... 

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