Excited ions vibrational excitation

A look at Table IV will show the general simplicity of the spectra. This is due to the fact that ionization occurs by electron tunneling, which does not excite the ions vibrationally, as is the case with impact ionization. Thus the combination of field ionization and mass spectrometry has considerable analytical potentialities, when it is considered that acetone, for example, gives rise to 19 peaks of comparable intensity by impact ionization and shows no peaks over 0.1% except the parent in field ionization. 

The enhancement of collisional dissociation cross sections by reactant-ion vibrational excitation is also observed for interactions at relatively low translational energies, particularly in the energy region near threshold. This was demonstrated in studies of dissociation of ( = 0-5) in which... 

Figure 15. Kinetic energy release (KER) curves for (a) 0+ ions (upper curve and axis), and (b) photoelectrons (lower axis) obtained from the two images shown in Figure 2. Total kinetic energy release is plotted, thus the kinetic energy of each O atom is one-half of the KER, The height of the 0+ curve is multiplied by a factor of 2 with respect to that of the e curve. Note the axis direction for photoelectrons is reversed and displaced to line up with the 0+ curve. Also indicated are positions for the ground-state ion vibrational levels and for excited atoms formed with 0(3P) partners, except for the 0 (3ssS) peak at 0.30eV in the 0+ image, which has an 0(lD) partner.

The temperature-dependent Raman spectra are depicted in Fig. 4-27a, b. Figure 4-27a shows the spectra of H2O-I (the water molecules in the inner coordination sphere) from 133-223 K. Figure 4-27b shows the spectra of H2O-II (the water molecules in the outer sphere). The spectra above 223 K are not shown because of the overlap with fluorescence that is observed with the 514.5 nm excitation. Plots of the variations of band frequency with temperature are illustrated in Fig. 4-28a, b for H2O-I and H2O-II. Two discontinuities are observed at 195 5K and 140 5K, indicative of three distinct phases occurring in the temperature range studied, as indicated in Fig. 4-28a. The higher-frequency OH stretch region, as shown in Fig. 4-28b does not show any discontinuities for H2O-I. A plot of full width at half maximum intensity (FWHM) vs. T for H2O-I shows a discontinuity at 140 K (Fig. 4-28c, d). Additional support for these phase transitions was found from the temperature dependences of the UO vibrational mode, lattice vibrations and the NO3 ion vibrations (translations and rotations). 

FIGURE 2.1 Ion vibration in external potential well correlated with electronic excitation [39] or spin excitation [48] as described by entangled state with mixing angle 0. Application in quantum information processing [49]. 

The measurements of T and by LIE for diatomic ions (particularly N2 ) drifting in He have revealed no cooling due to inelastic effects, but Tef in those studies (<600 K) was far too low to populate the first excited vibrational state of N2. It is tempting to ascribe the small increase of inelastic effect from Cl or NO to N02 and NOs (Figure 2.19c) to the inelastic energy loss via excitation of ion vibrations growing as they get softer and their number increases, but that would be an overinterpretation of scarce data. 

Photoeiectrons can be emitted from three different energ> levels in the nitrogen molecule, labelled A, B and C. The other peaks appearing in the spectrum are caused by vibrational excitation of the nitrogen molecule ion that is formed. This reduces the kinetic energy of the photoeiectrons because it takes additional energy to make the ion vibrate, and therefore these peaks all appear to the left of the principal peaks. 

Unstable species such as O, FI and N atoms, molecular radicals and vibrationally excited diatomics can be injected by passmg the appropriate gas tluough a microwave discharge. In a SIFT, the chemistry is usually straightforward since there is only one reactant ion and one neutral present in the flow tube. 

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

In (a), an ion and a gas atom approach each other with a total kinetic energy of KE, + KEj. After collision (b), the atom and ion follow new trajectories. If the sum of KE, + KEj is equal to KE3 + KE4, the collision is elastic. In an inelastic collision (b), the sums of kinetic energies are not equal, and the difference appears as an excess of internal energy in the ion and gas molecule. If the collision gas is atomic, there can be no rotational and no vibrational energy in the atom, but there is a possibility of electronic excitation. Since most collision gases are helium or argon, almost all of the excess of internal energy appears in the ion. 

If the applied potential is positive, a positive ion (M +) is produced, and, if negative, a negative ion (M ) is formed. Since there is no vibrational excitation, no fragment ions are produced. 

For molecules and ions having more than one atom, the extra energy can make the component bonds rotate and vibrate faster (rovibrational energy). Isolated atoms, having no bonds, cannot be excited in this way. 

As excited atoms, molecules, or ions come to equilibrium with their surroundings at normal temperatures and pressures, the extra energy is dissipated to the surroundings. This dissipation causes the particles to slow as translational energy is lost, to rotate and vibrate more slowly as rovibrational energy is lost, and to emit light or x-rays as electronic energy is lost. 

Quasiequilibrium statistical theory was applied to the negative ion mass spectra of diphenylisoxazoles. Electron capture by the isoxazole leads to molecular ions having excited vibrations of the ring and of bonds attached to it. The dissociation rate constants were also calculated (77MI41615, 75MI416U). 

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