https://research.cbc.osu.edu/wysocki.11/wp-content/uploads/2012/09/Wysocki-1987-Internal-energy-dist.pdf

Internal energy distributions of isolated ions after activation by various methods

It is now known that tunable dye laser light leads to resonance enhanced photoionization of molecules in a mass spectrometer via multiphoton absorption. In some cases this process also leads to ion fragmentation. Time delayed tandem laser pulses of different wavelengths are employed to study the mechanism of this multiphoton ionization and fragmentation process in benzene. The experimental results strongly indicate that excitation does not proceed up an autoionization ladder of the neutral molecule, rather the multiphoton process is shown to be due to the climbing of two independent ladders, one up to the ionization potential in the neutral molecule, the second in the new species of molecular parent ions produced. Hence multiphoton ionization could also form a new basis of ion spectroscopy.

Visible and UV multiphoton ionization and fragmentation of polyatomic molecules

Absolute fragmentation rates and average energy released in the gas phase dissociation of 2,4 and 1,5 hexadiyne have been measured by photoion–photoelectron coincidence. The decay rates over the metastable energy range, as measured by the asymmetric time‐of‐flight distributions of C4H4+ and C3H3+ fragments are identical, and equal to those measured by others for benzene. This indicates that over this energy range all three C6H6+ isomers rearrange to a common ion structure prior to dissociation. Calculated rates based on the statistical theory suggest that the C6H6+ ions fragment to all product channels by competitive pathways and that the existence of isolated states or noncommunicating isomeric structures is highly unlikely. Above this energy range, the branching ratios to the C3H3+ and C4H4+ fragments indicate that the 1,5 hexadiyne ion no longer isomerizes prior to dissociation.

The dissociation dynamics of internal energy selected C6H6+ a)

Publisher Summary This chapter discusses collision-induced dissociation. The determination of molecular structure by mass spectrometry requires that the molecule be ionized and undergo structurally informative fragmentations. Fragmentation may be induced by the ionization process itself or by some other means of excitation. Often the optimum methods of ion formation do not impart sufficient energy to cause appreciable fragmentation of the molecular ion. The internal energy of the ion in these circumstances may be increased by collisional activation (CA). Tandem mass spectrometry (MS/MS), in all its various configurations, has emerged as the most important technique for acquiring collisional-activated dissociation (CAD) [also termed collision-induced dissociation (CID)] mass spectra. The overall CAD process can be separated into two consecutive steps occurring on well-separated time scales. CAD mass spectra may be acquired on any of four distinct classes of mass spectrometer: magnet sector, quadrupole, time-of-flight (TOF), and Fourier transform (FT) mass spectrometers. There are also a number of hybrid configurations employing a combination of sector and quadrupole analyzers.

Collision-induced dissociation.

Decay rate measurements of internal energy selected ions were performed with a novel technique employing highly selective two‐photon laser ionization in a reflectron time‐of‐flight mass spectrometer. Resonantly enhanced two‐photon ionization leads to vibrational state selected C6H+6 cations. These are further excited with a second laser pulse of variable frequency beyond the dissociation threshold for the four decay channels of lowest energy. The excitation results in a metastable decay of C6H+6 with typical decay rate constants of some 106 s−1. Decay rate constants for two C‐loss channels and for the first time also for the two H‐loss channels are measured in the energy range from 5.1 to 5.5 eV. It is shown that H‐loss and C‐loss dissociation occurs from a common electronic state and is competing. From the total decay rate constants and the measured branching ratios individual decay rate constants for all decay channels are obtained and found to be in good agreement with RRKM calculations. This confirms the statistical character of the C6H+6 ion dissociation.

Multiphoton ionization in a reflectron time-of-flight mass spectrometer: Individual rates of competing dissociation channels in energy-selected benzene cations

Unimolecular Ion Decompositions: Dependence of Rate Constants on Energy from Charge Exchange Experiments

Abstract Ions of the benzonitrile, benzene and thiophene molecules were produced in well-defined states of excitation by charge exchange with Xe+, Kr+, Ar+, CO+, and N2+. Their subsequent unimolecular decomposition was followed, as a function of time, by allowing the dissociations to take place within a strong homogeneous draw-out field and measuring the kinetic energy of the product ions. The decompositions were found to be purely exponential within experimental error, and the corresponding decay rate constants k (for 2 · 105 sec-1 ≦ k ≦ 5 · 108 sec-1) proved to be monotonically increasing functions of the excitation energy. These are the first unambiguous measurements of this function k(E) for any molecule. In the case of benzene, the reactions C6H6+ → C6H5+ + H and C6H8+ → C4H4+ + C2H2 were found to be definitely not in competition with one another. Furthermore the dependence of the excitation energy on the impact energy was measured. A variation of the impact energy between 10 and 200 eVc.m. only changes the internal energy E by about 0.1 eV. This value appears to be in qualitative agreement with calculations on near-resonant charge transfer by Gurnee and Magee.

B. Andlauer

Papers

Unimolecular Ion Decompositions: Dependence of Rate Constants on Energy from Charge Exchange Experiments

Dissociation Lifetimes of Molecular Ions Produced by Charge Exchange