Excitation of Ions

Non-pairwise hydrodynamical forces. We should finally take into account hydrodynamical interactions between two particles where, in some intermediate states, we would have a temporary excitation of ions. This type of effect would lead to a kind of effective hydrodynamical force and is indicated in Fig. 26. 

LITs capable of scanning, axial or radial excitation of ions, and precursor ion selection for MS/MS experiments [118,134-136] have lately been incorporated in commercial mass spectrometers (Fig. 4.39). The replacement of Q3 in a QqQ instrument with a scanning LIT, for example, enhances its sensitivity and offers new modes of operation (Applied Biosystems Q-Trap). Introduction of a scanning LIT [118,135] as MSI in front of an FT-ICR instrument (Thermo Electron LTQ-FT) shields the ultrahigh vacuum of the FT-ICR from collision gas and decomposition products in order to operate under optimum conditions. In addition, the LIT accumulates and eventually mass-selects ions for the next cycle while the ICR cell is still busy with the previous ion package. 

Ion cyclotron resonance o i 1 o o 1 Excitation of ions moving in circular orbits in a magnetic field Rates and equilibria for reactions of ions with neutral molecules in the gas phase (Section 27-8)... 

In addition to the processes just discussed that yield vibrationally and rotationally excited diatomic ions in the ground electronic state, vibrational and rotational excitations also accompany direct electronic excitation (see Section II.B.2.a) of diatomic ions as well as charge-transfer excitation of these species (see Section IV.A.l). Furthermore, direct vibrational excitation of ions and molecules can take place via charge transfer in symmetric ion molecule collisions, as the translational-to-internal-energy conversion is a sensitive function of energy defects and vibrational overlaps of the individual reactant systems.312-314... 

EIEIO Electron Impact Excitation of Ions from Organics uses... 

Mixed Mode Detection Excitation of ions with a broadband... 

Z-Mode Excitation Excitation of ions at their trapping... 

The monolayer also provides an environment of variable dielectric so that intermolecular association between photoactive molecules can readily occur. For example, molecular association of pyrene within a Langmuir-Blodgett film is clearly seen through time-resolved fluorescence measurements on the picosecond timescale [92], Attenuated total reflectance studies of dyes in cast films can similarly reveal their positions and photophysical interactions [93], Photochromism in a monolayer assembly has been attributed to excitation of ion-pair charge transfer complexes formed within the array [94]. 

R.K. JULIAN and R.G. COOKS develop broadband excitation of ions using the stored-waveform inverse Fourier transform (SWIFT) [67],... 

The dipolar or induced dipolar natime of molecules means that the impacting electron can cause rotational excitation but, because of conservation of momentum, very little of the kinetic energy of the electron can be imparted and little direct vibrational excitation can occur (Cottrell, 1965). Further, although ion-sources frequently operate at fairly high temperatures, the population of vibrationally excited states of molecules even at 500°K is very low and the source of the large vibrational excitation of ions must be sought elsewhere. For illustrative... 

The tandem-in-time instruments are mostly ion-trapping devices, including ion trap and FT-ICR. They operate in a time sequence in the scan function to yield MS/MS data, mostly product ion spectra. No additional mass analyzer is required. In the case of an ion trap, the scan function begins with the isolation of ions of interest with ejection of all other ions from the ion trap, followed by (a) translational excitation of ions by applying a supplementary RF voltage to the trap and (b) mass analysis of the product ions using resonant ejection. 

Cody, R. B. and Freiser, B. S. Electron impact excitation of ions from organics an alternative to collision induced dissociation. Anal. Chem. 51 547-551, 1979. 

BH Wang, FW McLafferty. Electron impact excitation of ions from larger organic molecules. Org Mass Spectrom 25 554—556, 1990. 

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. 

According to these simulations, an 800 pm-diameter laser beam traveling through the laser holes along the y-axis will intersect ca 36% of the ion cloud. Put another way, an ion will spend about 36% of its time within the path of an 800 pm-diameter laser beam. This interpretation and, indeed, our construction of an ion cloud of 500,000 points from 50 ion trajectories, assumes that collisions provide sufficient randomization to negate the effects of starting conditions. These results indicate the photo-excitation of ions can be quite efficient. This situation contrasts with the fluorescence collection efficiency. As discussed in Section 9.7.3, fluorescence is radiated in all directions so only a small portion of the ions (ca 0.25% in our set-up) that are excited optically will emit fluorescence in the direction of the fluorescence collection hole. 

The difficulty in producing stimulated emission in the ultraviolet regions is well known, and arises from the basic relation that the probability for spontaneous emission. A, varies as v B, where B is the probability for stimulated emission. Thus, losses in excited state population due to spontaneous emission increase rapidly at short wavelengths, and put severe demands on pumping sources in order to achieve inverted population. Various techniques are being explored to overcome these difficulties including recombination processes and excitation of ions and in principle, the free-electron laser could operate at these short wavelengths. Much effort in this direction is... 

The cylindrical device is easier to construct than a 3D ion trap having hyperbolic electrodes, and variation of the ratio rc 2zc (radius of cylinder to height of CIT) is facile. Research into QTs has focused upon small CITs, described as mini-CITs, for application as tandem mass spectrometers and for the development of arrays of mini-CITs. In order to achieve in a mini-CIT a given value of z(= qx from eqn [2]), an elevated drive frequency is required to compensate for the reduced value of r. In this manner, a potential well of sufficient magnitude is facilitated that permits excitation of ions confined within the mini-CIT. 

This relatively straightforward resonant excitation process is complicated somewhat in a stretched ion trap (see above). Since ion axial secular frequencies increase as an ion approaches an end cap electrode, it is necessary to adopt one of several strategies to maintain resonance and thus ion excitation. There are three such methods of resonant excitation of ions of a selected miz value isolated within an ion trap, namely, single frequency irradiation (SFI), secular frequency modulation (SFM) and multifrequency irradiation (MFI). 

Resonant excitation of ions in a 3D IT mass analyzer can be conducted by applying a low amplitude supplementary RF potential to one or both end-cap electrodes. The amplitude of the supplementary RF potential is typically a few volts, and its frequency is mass-dependent, typically 10 -1000 kHz. It correlates with the axial oscillation frequency of ions. The axial oscillation frequency is + where n represents the order of the stable... 

Kriegel et al 1987). Vibrational excitation of molecular ions is more efficient in collisions with buffer gas atoms or molecules heavier tlian helium (e.g. N2, Ar, etc.). This is demonstrated below. However, the occurrence of vibrational excitation is unfortvmate from the viewpoint of astrochemistry, since vibrational excitation of ions in drift tubes can obscure the influence of kinetic excitation on the rate coefficients of molecular ion-neutral reactions. Such information is needed in some astrophysical situations such as in interstellar MHD shocks. In these situations, the mean free times between collisions of the ions with the ambient gas are usually longer than the radiative lifetimes of the vibrational states of the ions and therefore the ions will generally be vibrationally relaxed. Thus data are required on the variation of the rate coefficients with Ej. for the reactions of ions in their ground vibrational state. So, drift tube data on molecular ion reactions must be applied with caution to astrophysical situations except that is for data obtained at low e/N where vibrational excitation of the molecular ions is minimal. 

An inverted version of the messenger tagging technique for detecting ion absorption uses the fact that electronic and/or vibrational excitation of ions hinders formation of weakly-bound clusters. This effect, explored years ago in relation to laser isotope separation [82], has recently been demonstrated for spectroscopy of N2" ions, cooled to 10.6 K in a 22-pole trap by collisions with He and termed laser-induced inhibition of cluster growth (LIICG) [83]. An electronic spectrum is generated by monitoring the reduction of the steady-state concentration of ion-He complexes as a function of the excitation laser wavenumber. 

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