Photoionization single ionization

All the previous reviews [3-9] have been concerned mainly with what is called direct double photoionization in atoms. This is a process in which a single photon ejects two electrons simultaneously, without the intervention of any real intermediate state of either the neutral molecule or the singly ionized cation. It can be written... 

As a result of the photoionization, a singly ionized atom is formed, which can also be produced by electron impact. The core hole (eg, in the K shell) can be filled by an electron from a higher shell (eg, the Li shell) and the energy of this de-excitation process can be released by emission of an X-ray photon (X-ray fluorescence, XRF) or can be transferred to another electron (eg, in the L2 shell), which is then emitted with a well-defined kinetic energy (Auger process). This... 

The photoionization process with which we shall be concerned in both UPS and XPS is that in Equation (8.4) in which only the singly charged is produced. The selection mle for such a process is trivial - all ionizations are allowed. 

Photoionization ti me-of-fli ght mass spectrometry is almost exclusively the method used in chemical reaction studies. The mass spectrometers, detectors and electronics are almost identical. A major distinction is the choice of ionizing frequency and intensity. For many stable molecules multi photon ionization allowed for almost unit detection efficiency with controllable fragmentation(20). For cluster systems this has been more difficult because high laser intensities generally cause extensive dissociation of neutrals and ions(21). This has forced the use of single photon ionization. This works very well for low i oni zati on potential metals ( < 7.87 eV) if the intensity is kept fairly low. In fact for most systems the ionizing laser must be attenuated. A few very small... 

Notes AA, acetic acid AAc, ammoniuin acetate ACN, acetonitrile AH, airnnonium hydroxide FA, formic acid MeOH, methanol TFA, trifluoroacetic acid THF, Tetrahydrofuran APCI, atmospheric pressure chemical ionization APPI, atmospheric pressure photoionization ESI, electrospray ionization IS, ion spray IT, ion trap Q, single quadrupole QqQ, triple quadrupole TOF, time of flight. 

A further possibility is that the signals arise from hydrated electrons or base radical ions produced by monophotonic ionization of the polymers. However, the quantum yield for photoionization of adenosine is reported to be approximately the same as that of poly(A) and poly(dA) [25], It is unlikely that photoionization of the polymers can account for the signals seen here since there is no detectable signal contribution from the photoionization of single bases [4], The most compelling argument that our pump-probe experiments monitor excited-state absorption by singlet states is the fact that ps and ns decay components have been observed in previous time-resolved emission experiments on adenine multimers [23,26-28]. 

C,HJ (Benzene) sf6- >0 Unspecified Photoionization -mass spectrometry Single-source electron-impact ionization-mass spectrometry CeH (QHj —)C,2H SF6 (XFVSFJXFj- X-Se.Te,U SF6-(TeF6 SF6,F)TeF5- The ion SF - produced by thermal electron capture of SF in a vibrational degree of excitation equal to electron affinity of the neutral 116a... 

Additional details on some of these methods are described in other sections of this review. Attempts have also been made to determine excited-state populations in single-source mass-spectrometric experiments from an analysis of ionization efficiency curves.38ad There are several difficulties in applying such methods. For instance, it is now known from photoionization studies that ionization processes may be dominated by autoionization. Therefore, the onset of a new excited state is not necessarily characterized by an increased slope in the electron-impact ionization-efficiency curve, which is proportional to the probability of producing that state, as had been assumed earlier. Another problem arises because of the different radiative lifetimes that are characteristic of various excited ionic states (see Section I.A.4). 

There are two alternative methods for determining internal-state distributions in single-source photoionization experiments, depending on the prevailing type of ionization process. 

---