Photon distribution curves

Fig. 15. Angle-integrated photoelectron energy distribution curves of uranium in the region of the giant 5 d -> 5 f resonance (90 eV < hv < 108 eV). The 5 f intensity at Ep is suppressed by more than a factor of 30 at the 5 ds/2 threshold (see the spectra for hv = 92 and 94 eV) and resonantly enhanced above threshold (see, e.g., the spectrum for hv = 99 e V). At an initial energy 2.3eV below Ep a new satellite structure is observed which is resonantly enhanced at the 5 d5/2 and 5 ds onsets. At threshold the satellite coincides with the Auger electron spectrum, which moves to apparently larger initial energies with increasing photon energy (from Ref. 67)...

Such distribution functions have been obtained for benzene, toluene, o-, m-, and p-xylene, mesitylene durene, aniline, methylaniline, di-methylaniline. Some of them are reproduced in Figures 9 and 10, where on the ordinate the relative numbers of electrons are plotted, and the corresponding kinetic energies (in e.v.) are indicated as abscissae. Each distribution curve corresponds to a definite photon energy noted at the curve. 

For benzene, naphthalene, aniline, and its iV-methylatcd derivatives the change in the shape of the kinetic spectra with photon energy is qualitatively similar. If the excess of energy hv — lv is small, there is only one maximum in the distribution curve and this is monotonously shifted to larger kinetic energies with photon energy increase. 

In the case of dimethylaniline, which possesses the lowest Ip (7.14 e.v.), there are at a photon energy of 11.0 e.v. four maxima on the distribution curve. For naphthalene vapor at a photon energy of 11.2 e.v., five maxima on the distribution curve are present. 

Thus on the distribution curve belonging to a definite photon energy there exists a minimum which appears simultaneously with the rise of the slow electron maximum, but remains stationary. For example, in the case of toluene [Fig. 9(6) ], such a minimum appears when the photon energy is 10.1 e.v. and remains stationary up to that of 11.0 e.v. 

Such a stationary minimum on the distribution curve has tentatively been ascribed by Kurbatov et al. to a pre-ionization process from a high excited molecular level, situated near the first-excited leyel of the ion. The neutral excited molecule M could switch, during its lifetime, to the state M+ + e (process 3 in Section III). This suggestion is, however, doubtful, since it implies that the excitation of the molecule to a higher state decreases the probability of the direct transitions to the ionized state. For benzene, o-, m-xylene, mesitylene, and durene, when the photons exceed Ip by 2.3-2.6 e.v., new sharp maxima of slow electrons appear. 

Two variables of a PES experiment are readily altered the input photon energy (hv) and the output photoelectron kinetic energy (KE). In a classical energy distribution curve (EDC) operation mode, one scans KE only and obtains information on the energy level manifold. While this is the only mode possible with fixed-energy VUV photon sources, SR permits two further combinations a constant final-state (CFS) mode where one scans hv and a constant initial-state (CIS) mode with both hv and KE scanned in such a way that their difference remains constant. CIS and CFS modes permit separate studies of the initial (ground electronic states, ionization probabilities) and final (photoelectron perturbed by the molecular ion) stages of photoionization events. 

Fig. 5. Energy distribution curves at a photon energy of 30 eV for three different conditions (a) after a-Si deposition (b) after H2 exposure at 10 2 Torr for 2 min (c) after 02 exposure at 5 X 10 5 Torr for 7 min at 110°C. The tops of valence tend of SiOx and a-Si H are also extrapolated (dashed lines). The top of SiO, VB shifts from 2.7 e V under the Fermi level to 2.45 eV (AE = 0.25 eV), while the top of a-Si H shifts from 0.6 to 0.35 eV, going from condition (b) to condition (c).

Fig. 2. Ozone effect on solar radiation (left) and dependence of ozone concentration on atmospheric altitute (right). In the left part the dotted curve represents the photon distribution of solar energy outside the atmosphere (based on the assumption of black body radiation at T = 5773 K). The full curve gives the photon distribution of solar radiation reaching earth surface (see Ref.8. The ozone effect is shown by shadowed area, the dicline above 800 nm is mainly due to absorption by water vapour. On the right side the full curve represents qualitatively a typical ozone profile, the real ozone distribution significantly depends on the local situation (geography), see Ref.15)...

This is shown in the lower half of Fig. 4 for one particular photon energy of hv B 7.9 eV. In the upper half of the figure we computed the distribution curve based on a simple joint density model with constant matrixelement assumption for the same photon energy. 

Of more fundamental importance is the different saturation coverage adsorption behaviour of Cl2 on 111 7x7 and 111 2x1 silicon surfaces. This difference is clearly shown by the angle-integrated energy distribution curves at different photon energies for both s- and p-polarizations [Fig. 28(a) and (b)]. Much more pronounced polarization effects occur for... 

Figure 8.4 Photoinduced electron energy distribution curve for an EUV resist with 15% photoacid generator concentration, showing both the low-energy region and an inset of the blowup of the valence band region for a photon energy of 92 eV. (Courtesy of T. Madey.)...

An example of a measurement obtained by a delay-line MCP and two TCSPC cards is shown in Fig. 5.98. One TCSPC card measures the delay of the photon pulses between the outputs of the delay line, i.e. the position of the photon in the fluorescence spectrum. The second card measures the times of the photons in the decay curve. It receives a position-proportional routing signal from the first card and thus builds up the photon distribution over time and wavelength, see Fig. 3.14, page 42. 

The resulting signal shape is a frequent souree of eonfusion. Sometimes the step at time A is even mistaken for the rise of a fluoreseenee signal. Of course, the reeorded curve is absolutely correct. The photons left of the eutoff point, A, are not lost. They were recorded shortly before the previous reference pulse and appear where they should be, i.e. in the late part of the period, B, at the right end of the photon distribution. The curve can be eentred in the reeorded time interval by adjusting the signal delay in the detector or reference channel. 

Fig. 4.26. Electron distribution curve in a-Ge at various photon energies in a-Ge (after Spicer and Donovan (1970b)).

In a photoemission experiment monochromatic photons at uv or X-ray frequency are incident upon a clear surface of the solid. The number of emitted electrons in a specific energy range is counted. With incident photon energy hv the number of photoelectrons in the energy interval (E, E -I- AE) is denoted by AT(E, hv)AE, and the quantity N(E, hv), named energy distribution curve (EDC),... 

Fig. 3.50. The energy distribution curves for Yb at various photon energies (Broden et al., 1973).

When fluorescence spectra having different shapes and X. distributions are to be compared quantitatively it is necessary to correct for the wavelength-dependent sensitivity of the EmW MT combination. If the spectra of a solute under different sample conditions have identical shape and only vary in intensity, then no instrumental correction is necessary. PMTs do not have constant photon sensitivity across the useful UV-VIS spectral range. A typical photon sensitivity curve is shown in Figure 10 for a Hamamatsu R955 PMT, a PMT that is used in many instruments. It would be desirable is to have a flat wavelength response. 

Fig. 3. Construction of an electron energy distribution curve (EDC) from a density of states. The top panel depicts a parabolic density of states with structure centered around E-,. For simpUcity, we show photoexcitation of three initial state levels by a photon energy hv with no account taken of dipole matrix element effects. p is the Fermi energy, Vo is the inner potential,

Fig. 5. The origin of energy distribution curves (EDCs), of constant initial state spectra (CISs), and constant final state spectra (CFSs). For the EDC, the photon energy is fixed and the electron energy is scanned. For the CIS, the photon and the electron energies are scanned synchronously. For the CFS, the electron energy is fixed while the photon energy is scanned. Matrix element and escape effects will distort all these spectra.

Fig. 19.12 Energy distribution curves for UBe and UPt at AO-eV photon energy. The long scans are at 20 K and 0.3-eV resolution. The short scans are at 0.13-eK resolution at the temperatures indicated ttfter Arko et ed. (73)].

The energy distribution curves of the photoelectron emission from cleaved single crystals at 300 K for photon energies hv = 6.5 to 9.7 eV (see Fig. 115) reveal peaks attributed to4f levels at 1.6 eV below Ep with a peak location and peak width independent of exciting photon energy, and to p states ca. 3 eV below Ep. The peak at 5.5 eV below Ep, observable only for hv>9 eV, cannot be explained it may result from scattered electrons [1,3]. Earlier studies at hv = 6.5 eV on single crystals [2] and at hv = 6.5 to 10.2 eV on (ordered) polycrystalline films [6] showed that the 4f levels lie above the p valence band but that emission from 4f is very weak. Studies with 40 eV synchrotron radiation photons reveal the intense 4f peak at 1.8 eV below Ep and a broad p band peak around 3 eV below Ep. But there also is an unidentified broad peak between 8 and 11 eV (not observable for 61 eV photons) and a weak broad peak at 13 eV below Ep (not studied for 61 eV), which was tentatively attributed to the outermost s band of selenium, Sato etal. [7]. 

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