Photoionization efficiency measurements

The photoionization efficiency is defined as the number of ions, produced by one incident photon on unit path length at unit gas pressure. The curves in Figures 3-6 represent spectral efficiency curves. On them the ratio of the measured photocurrent to the number of incident monochromatic photons is plotted as function of the photon energy (in e.v.). At the low pressure used in the mass spectrometer (< 10 3 torr) the efficiency curves are proportional to the photoionization cross-section curves.3... 

Reese and Rosenstock359 measured the photoionization efficiency curve between 1360 and 600 A. They suggested vibrational progressions in the 1130-900 A region. 

Photoionization studies of rare gas dimers have been carried out . Photoionization efficiency curves for the vdW molecules (Ar. .. Ar, Kr. .. Kr, Xe. .. Xe) were obtained by means of molecular beam techniques. The dissociation energies of the ions were calculated from the measured first ionization potentials (Table 6) and the dissociation energies of the parent vdW molecules. As the dissociation energy of vdW molecules is neghgible compared the dissociation energy of the rresponding cations, the latter energies (available from other sources) can be used to estimate first ionization potentials of vdW systems which have not yet been studied (values in parentheses in Table 6). 

Photoemission from excited single states produced by photoionization of anthracene crystals occurs after two step laser excitation Biphotonic excitation of phenanthrene under 208 nm irradiation is a complex process involving both ionization andT-T annihilation. Change transfer exciton band structures have been characterized with samples of crystalline tetracene . Measurement of the photoionization efficiency in trans-stilbene crystals as a function of excitation energy shows that ionization occurs after rapid vibronic relaxation o. 

Ionization of the SiCl radical may produce the ground state of SiCl. Marijnissen and ter Meulen [96Mar] have measured the adiabatic ionization potentials IP of the two isotopomers Si Cl and Si Cl using (1+1) two-color photoionization efficiency spectroscopy (PIE). They started from the 2 =1/2 and Q = 3/2 spin-orbit components of the X II ground state of SiCl, with the following results ... 

A major source of energetic information is the photoionization efficiency (PIE) curve in which the relative numbers of photoions produced per number of photons transmitted is measured as a function of photon wavelength (energy). A typical PIE curve is shown in Figure 1. 

As illustrated in Fig. 7.15, the electromagnetic radiation measured in an XRF experiment is the result of one or more valence electrons filling the vacancy created by an initial photoionization where a core electron was ejected upon absorption of x-ray photons. The quantity of radiation from a certain level will be dependent on the relative efficiency of the radiationless and radiative deactivation processes, with this relative efficiency being denoted at the fluorescent yield. The fluorescent yield is defined as the number of x-ray photons emitted within a given series divided by the number of vacancies formed in the associated level within the same time period. 

Figure 1. Versions of photoionization spectroscopy wherein not only the dependence of the multiphoton ionization efficiency on the laser wavelength is subject to measurement, but also the mass spectrum of photons and energy spectrum of photoelectrons (a) energy-level diagram (b) collision of a neutral particle with laser photons.

Photoionization always produces two species available for analysis the ion and the electron. By measuring both photoelectrons and photoions in coincidence, the kinetic electron may be assigned to its correlated parent ion partner, which may be identified by mass spectrometry. The extension of the photoelectron-photoion-coincidence (PEPICO) technique to the femtosecond time-resolved domain was shown to be very important for studies of dynamics in clusters [131, 132]. In these experiments, a simple yet efficient permanent magnet design magnetic bottle electron spectrometer was used for photoelectron... 

Mahan (56) found a value of 1.2 0.1 by mass spectrometric analysis for CO with NO photoionization being used to monitor lamp intensity. Warneck (59) found a value of 1.0 with mass spectrometric analysis for CO and lamp intensity measurements by O2 actinometry with an assumed value of 2.0 for the quantum efficiency of O3 formation. Recently Slanger et al. (70) have measured the rate of CO production in the CO2 photolysis at different wavelengths and expressed the results relative to the rate at 147 nm. Their results are 0.76 0.11 at 121.5 nm,... 

From Eq. (9) it is clear that correct determination of QY depends on accurate knowledge of the overall detection efficiency/, which is generally about 0.5. It can be determined in two ways. One is to compare the measured ratio C2+/C+ with the calculated ratio for an atomic rare gas such as Xe at a wavelength where the true relative production ratio A2+/N+ is known from photoionization mass spectrometric measurement of the ion yields. This method has been used in deriving the Xe data of Fig. 15. As expected, the quantum yield in the atomic case is unity (100%) within experimental error this must be generally true because deactivation of superexcited states by light emission is very rare. Thus the second and quicker method is simply to measure the apparent QY for an atomic gas and determine / accordingly. 

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