Molecular ions, lifetime

As in standard kinetics experiments, an amount of substance formed or consumed in a finite time interval, At, is what is actually measured. If the sampling interval At (i.e. observation window) is between times ti and t2 (i.e. between molecular ion lifetimes and t2), the number (per time) of fragment ions formed in this interval is... 

The figure shows two maxima. The first maximum at a molecular ion lifetime of 10 s corre-... 

The first deals with the elimination of ethylene from the molecular ion of cyclohexene, the second and third with the dynamics of stereochemically controlled dissociation reactions and the fourth with time-resolved MS-MS to identify the structures of product ions generated at different molecular ion lifetimes. 

CgHe over all molecular ion lifetimes studied, while the corresponding endo isomer [2a] eliminates both CgHe and CgHg, but not C5HI7, see Figures 4A and 4B, respectively. 

Figure 6 Collision-induced dissociation MS-MS spectra of (M-methyl) ions, generated upon FI from the molecular ion of pent-3-en-2-ol at molecular ion lifetimes of (A) 10- ° s and (B) 10- ° ° s. Reproduced with permission from Zappey HW, Ingemann S and Nib-bering NMM (1991) Study of the rate of methyl loss and the structure of the product ion as a function of the lifetime of the molecular ion of penten-3-en-2-ol. Organic Mass Spectrometry 26 241-246.

Valproic acid was quite volatile and vaporized as soon as it was admitted to the source of the mass spectrometer. Only a very weak ion was detectable in the molecular ion region at m/e 145. This would correspond to (M3-H)+, but exact mass measurement was not possible because of the peak s small size and the short lifetime of the sample in the mass spectrometer. 

As the IE of a molecule is governed by the atom of lowest IE within that neutral (Chap. 2.2.2), the EA of a molecule is basically determined by the atom of highest electronegativity. This is why the presence of halogens, in particular F and Cl, and nitro groups make analytes become attractive candidates for EC (Table 7.3). [78] If EC occurs with a neutral of negative EA, the electron-molecule complex will have a short lifetime autodetachment), but in case of positive EA a negative molecular ion can persist. 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

Depending on the kind of the intermediate molecular ion, all resonance processes can be divided into two groups.116 The first group is the so-called shape resonances, where the electron is trapped in a potential well formed in the ground electron state of the molecule by centrifugal or polarization forces. The lifetime of such states is between 10 15 and 10 s. 

A major thrust of recent work on FTMS of large biomolecules has dealt with questions concerning the lifetime of molecular ions formed by high-energy particle bombardment. Chait and Field have reported that a large fraction of the molecular ions of chlorophyll A formed by 252Cf fission fragment ionization decomposes with lifetimes of less than a few... 

The detection efficiency of C6H5X (X = F, Cl, Br and I) was investigated with the laser multiphoton ionization method152. The laser-induced ion yield depends mainly on the cross sections of the transitions available to the molecule ground state and on the lifetime of the intermediate electronic state that is initially excited. If a species has a radiative lifetime that is very short compared to the pulse duration, it may relax after excitation and will not be ionized. Molecular ions will therefore be obtained when laser pulses that are at least as short as the excited-state lifetimes are employed. The S excited states of halobenzenes are estimated to have subnanosecond lifetimes, with the exception of fluorobenzene for which a lifetime of the order of 9-10 ns has been calculated at 2ex = 266 nm. Picosecond laser pulses are therefore found effective in producing ionization of halobenzenes with short lifetimes, whereas nanosecond pulses are not152. 

Mass spectrometry is typically a flow experiment in that molecular ions are being continuously formed and are continuously reacting and it is ion currents or count rates which are measured. The molecular ions may be formed according to some sequence of pulses, but the essential situation remains the same. Generally, the molecular ions are formed at some more or less well-defined position in space and the lifetimes, t, of ions correlate with transport of the ions away from their position of formation through the analysers and towards the detector. 

Ion lifetimes from nanoseconds down to microseconds have been determined in experiments involving ionization within the confines of a molecular beam and decomposition within a strong electric field [381, 667, 672]. Using electron impact ionization, rates of decomposition have been determined as a function of time over the range 3 ns to 1 p s for ions formed from butane, heptane, perdeuteromethane, benzene, carbon dioxide and benzonitrile. 

The lifetimes of ions studied by ion cyclotron resonance (ICR) are commonly of the order of milliseconds [254], In terms of eqn. (9), the limits tx and t2 of the observation window are zero and of the order of milliseconds, respectively. Reactive ions of longer lifetimes are not distinguished from those with the more usually encountered lifetimes (< /is). Using ICR, it has been found that decomposition of the molecular ion of 1, 5-hexadiyne (c.f. Sect. 5.7) to lose H occurs predominantly at times greater than microseconds [344], An ICR mass spectrometer constituting the second half of a tandem mass spectrometer has been used to study decomposition of propane ions up to times of milliseconds [775], The observation window in this case extended from tx — /is to... 

The decay curves (lifetime distributions) in the microsecond time frame for the formation of (C3HS)+ from the molecular ions of but-l-ene, cis- and frcms-but-2-ene, methylcyclopropane and 2-methylpropene have been determined but the curves could not be described by single rate coefficients. The interpretation put forward was, in effect, that there were two (or more) reacting configurations [20]. Breakdown curves have been reported for butadiyne [210]. 

Detailed studies show that the rate coefficients for 3-body association increase with decreasing temperature (Smith and Adams, 1978) probably due to the longer lifetime of the excited complex (AB ). Arnold (1979) suggest i that radiative association of Hj to molecular ions may proceed with high rate constants at the low temperatures of dense molecular clouds, (iii) Laboratory observation of a large number of chemical reactions which lead to the formation of complex molecules. Often though, it is not easy to assess the precise relevance of these new data to the interstellar conditions. 

As meteoric material is evaporated, a number of high energy gas-phase collision processes occur. These processes, examples of which are listed in Fig. 1, involve both collisions between neutrals and neutrals and ions. Metal atoms, Me, are abundant in all meteoric bodies, and play an important role for several reasons (i) Metal atoms and ions frequently have low lying excited states with high oscillator strengths and are, therefore, easily identified and traceable (ii) Metal atoms have low ionization potentials and can be ionized fairly efficiently in high velocity neutral collisions (iii) Atomic metal ions have very long lifetimes with respect to ion-molecule reactions and electron-ion recombination compared with molecular ions, which rapidly dissociatively recombine with electrons. 

---