Photoexcitation cross section

It is often the goal of photoconductivity studies to measure the photoexcitation cross sections, i.e., the crvni and avpi. Although it is hard to determine the magnitudes of these cross sections, because of the other terms in Eq. (44) that are unknown, it is often easy to determine their energy dependences. The reason is that none of the other terms are usually very energy dependent. In 1965, Lucovsky published an oft-quoted equation for [Pg.101]

Fig. 5. Photoconductivity, a, and photoexcited-carrier-concentration (An and Ap) spectral data for three GaAs-Cr samples, A, B, and C. Also shown is the Lucovsky photoionization cross section [Eq. (45)] for excitation from a level at 0.52 eV. The theory is fitted to the data at 0.53 eV. [From Look (1977a).]...

Regardless of the choice of the sample thickness, the total amount of sample particles in the x-ray probe beam under optimized conditions is directly proportional to the x-ray spot size and inversely proportional to the x-ray absorption cross section, whose photoinduced (small) changes we want to measure [12]. Typical x-ray foci at synchrotrons are in the 0.1 - 0.3 mm range. For the examples treated below, this means that we have between 1014 and 1016 molecules in the probed volume. In order to achieve a reasonable photoinduced signal we should excite as many solute molecules as possible. Neglecting the optical absorption cross sections for photoexcitation for the moment, this requires on the order of 1015 laser photons per pulse, or ca. 0.25 mJ of pulse energy (e.g., at 800 nm). In other words, one should aim to... 

Similar arguments apply to charge exchange and photoexcitation, and the basic result is the same the cross section for the production of Rydberg atoms is the continuation below the limit of the ionization cross section, leading to an n 3 dependence of the excitation cross section. 

In Chapter 3 we considered briefly the photoexcitation of Rydberg atoms, paying particular attention to the continuity of cross sections at the ionization limit. In this chapter we consider optical excitation in more detail. While the general behavior is similar in H and the alkali atoms, there are striking differences in the optical absorption cross sections and in the radiative decay rates. These differences can be traced to the variation in the radial matrix elements produced by nonzero quantum defects. The radiative properties of H are well known, and the radiative properties of alkali atoms can be calculated using quantum defect theory. 

Fig. 8.13 Power spectra of measured Na photoexcitation cross sections from the 3p states vs the classical action S in atomic units (a) o polarization and (c) ic polarization and calculated power spectra of H (b) from the 2p m = 1 state with o polarization and (d) from the 2p m = 1 state with jt polarization. All are for fixed scaled energy W = W rE = —2.5...

Fig. 19.6 A schematic view of an apparatus for measuring photoexcitation cross sections and photoelectron energy and angular distributions. The atom beam comes out of the page, and Di and D2 are the electron and ion detector, respectively (from ref. 25).

Absorption of X-rays of the energies corresponding to binding energies of electrons in atoms by electronic shells as a rule leads to photoexcitation of an atom and to the photoeffect. The microscopic quantity describing absorption of X-rays is the so-called effective absorption cross-section om, characterizing the absorption of X-rays of frequency a> by a single atom, and is defined as... 

The results summarized here illustrate the important role surface states play in C>2 evolution from photoexcited TiC>2 and provide an example of a quantitative determination of the density and electron capture cross section of these states. 

The cross-section for photoexcitation from an initial state V ,- to a final state 0/ is /81/... 

Details of the photoexcitation and dissociation of seeded supersonic molecular beams of I2 by 514.5 nm radiation from a CW Ar laser have been provided by Bernstein and co-workers.A number of measurements were made, including LIF, I2 beam attenuation, and I2 TOF distributions as functions of carrier-gas pressure and nozzle temperature. A value for the direct photodissociation cross-section for I2 was determined to be (2.4 0.5) x 10 cm from measurements of the laser-induced beam loss. Use of additional spectroscopic information enabled calculation of the fraction of molecules excited on the basis of a simple model, and comparisons of the degree of excitation for different beam conditions and beam/laser geometries were made. 

Gas-phase ions may be fragmented by photoexcitation (PID), particularly by IR photons tuned to the vibrational frequency of covalent bonds. The cross section of an ion for photon absorption is low compared with its collisional cross section, so PID is most commonly associated with the use of intense light sources (lasers) and FTICR where the period the ion is exposed to the photons can be lengthened to increase the... 

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