Ionization action spectra

Figure 2.6 CHjNHj vibrational spectra (a) H action spectrum obtained via 243.135 nm dissociation of vibrationally excited molecules and (b) ionization-loss stimulated Raman spectroscopy exhibiting the depletion of parent molecule ionization signal due to vibrational preexcitation. Reproduced with permission from Ref. [86]. Copyright (2009) AlP Publishing LLC.

The action spectra of these products can be obtained by monitoring the wavelength dependence of the Ba" " and BaF signals originating from the depletion of the parent complex. The action spectrum for the BaF reaction channel is displayed in the top trace of Figure 24.13 it was obtained by ionization with photons of wavelength 266 nm. A resemblance between the BaF and the Ba- -FCHa photodepletion signals is clearly manifested. 

Figure 9.15 Comparison among absorption spectrum of a PPPV film (curve labeled a, dashed bands indicate inhomogeneously broadened So <-5, 0 <-0, and 1 <-0 transitions) photocurrent action spectrum (/p) and fraction of photons absorbed [1 - exp(-oaf)] (right scale). The low-energy peak of Ip is due to defect ionization. The photocurrent was measured on a 6-pm-thick sandwich sample at a field of 10 V cm- and 295 K. (From Gailberger, M. and Bassler, H., Phys. Rev., B44, 8643, 1991. With permission.)...

To explain this observation, another difference between the two measurements needs to be considered With the helium droplets, the spectra represent direct IR absorption, while in REMPI experiments the IR spectrum is obtained indirectly as it is detected by an action spectrum via the Sj electronically excited state. If the lifetime of that excited state is short with respect to the laser pulse, two-photon ionization may not take place and the technique becomes blind to such a case. With the typical 10-ns laser pulses used in these experiments, the method may fail to detect species with electronic excited states in the picosecond or faster timescale. This explanation is borne out completely by quantum computational modeling. 

The stirring is contd for 5 hours, the pptd solid filtered off, and recrystd from w to give 8.4g of K dinitromethane, yield 23.3%, expln temp 208° (Ref 8). More recently it is conveniently prepd on a lab and comml scale by the interaction of NMe and Na nitrite with a Ag salt, most commonly the nitrate. This re- action, developed by Shechter and Kaplan of the Purdue Research Foundation, is called the Shechter-Kaplan reaction (Ref 14). The IR spectrum is given in Ref 13. The ionization constant in w at 25° is 2.26 0.01 (Ref 20). 

Self-Luminescence. The action of UV light or ionizing radiation on pure alkali-metal halide crystals causes intense luminescence particularly at low temperature. The emission spectrum is characteristic for each individual compound. This fluorescence is comparable with the recombination luminescence which occurs upon capture of an electron by a VK center (defect electron). 

Specific examples illustrate that similar principles affect the absorption spectra. For example, as we have pointed out above, the neutral form of the C-2 benzyl ester is red in MeOH and orange in methylene chloride. Thus it has the spectrum of the ionized form in the polar, protic solvent and of the nonionized form in the nonpolar solvent methylene chloride [248]. The tributyl ammonium salt of the C-2 octyl ester is soluble in solvents ranging from ethanol-water to toluene. Its spectrum in an essentially nonionizing solvent such as toluene is that of the ionized xanthene [249], The spectrum of the pyrillium salt in ethanol is concentration dependent. In dilute solution the compound is totally ionized and is red, whereas in concentrated solution the compound is not fully ionized and the orange form predominates, as predicted by the law of mass action. 

In order to study the photochemical action of solar radiation on tropospheric, stratospheric, and mesospheric constituents, the solar spectrum must be divided in various ranges.1 The radiation at wavelengths less than 100 nm, which is absorbed by nitrogen and oxygen in the thermosphere above 100 km, leads essentially to ionization processes and is, therefore, not considered there. Only X-rays of wavelengths less than 1 nm can penetrate into the atmosphere below 100 km, and lead indirectly to the dissociation of molecular constituents. Nevertheless, their principal role is the photoionization in the D region of the ionosphere below 100 km where the solar line Lyman-a at 121.6 nm ionizes the nitric oxide molecule, NO. 

No corresponding detailed theory of the action of slow electrons exists. Their fraction in the degradation spectrum is nevertheless so substantial that their contribution to the radiolysis must be considered as very important. The reason is that they are formed abundantly as secondary electrons from optical ionizations and contribute considerably... 

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