Intensity changes

Even if the intensity changes are only of the order of 1 %, they may nevertheless be reliably detected using... 

How do we find phase differences between diffracted spots from intensity changes following heavy-metal substitution We first use the intensity differences to deduce the positions of the heavy atoms in the crystal unit cell. Fourier summations of these intensity differences give maps of the vectors between the heavy atoms, the so-called Patterson maps (Figure 18.9). From these vector maps it is relatively easy to deduce the atomic arrangement of the heavy atoms, so long as there are not too many of them. From the positions of the heavy metals in the unit cell, one can calculate the amplitudes and phases of their contribution to the diffracted beams of the protein crystals containing heavy metals. 

T he measurement principle, based on the ratio of the intensities, is insensitive to fluctuations in the source intensity. Changes associated with wavelength dependence can cause error in the measurement. 

Systematic studies on disubstituted thiophenes are scarce, but some differences between the various series of disubstituted thiophenes have been detected. According to Imoto et al., the main effect of substituents in 2,5-disubstitution in thiophenes appears in the wavelength shift, whereas the main effect of substituents in the 2,4-disub-stituted thiophenes appears in the intensity changes of the bands. The intensities of the first absorption band of some 2,6-disubstituted thiophenes have been calculated from the spectroscopic momenta of the substituents. The UV absorption of thiophene was displaced... 

It is obvious that calculated values are systematically lower than the experimental data. Comparison of the experimental and calculated values of coefficient p shows that along with the changes in occupancy levels that appear at elevated temperatures, inter-particular interactions also make a significant contribution. Band intensity is generally defined as the derivative of the dipole moment with respect to the normal coordinate. It is, therefore, logical to assume that thermal extension and outer-sphere cation replacement have a similar influence on the potential of inter-ionic interactions, which, in turn, lead to the intensity changes. 

The ionic current intensity corresponding to the peak at 169 amu was analyzed under isothermal and polythermal conditions [383]. It was found that in a gaseous atmosphere, the intensity changes are in correlation with the CO content and in negative correlation with the C02 content. The presence of CO in vacuum systems equipped with heating elements is usually related to thermo-cycling and desorption of CO by nickel atoms [386]. Based on the above, the presence of NbF4+ ions in mass spectra is most probably related to the niobium reduction process, which can be represented as follows ... 

In order to avoid any source of inaccuracy that might arise from the fact that the absolute intensity line cannot be reproduced, on account of the nature of the instruments themselves, the intensity is always measured with respect to that of a standard sample. Let us suppose that I0/Is represents the ratio of the line height of the compound which is to be irradiated to that of the standard sample. After irradiation, the new ratio has become ///g. On eliminating Is then we get I/I0 which represents the intensity change on going from the irradiated to the nonirradiated compound. Suppose now that the concentration of the new chemical species or, in general terms, imperfections induced by irradiation be proportional to the amount of radiation absorbed in the sample. Then the relation which represents the impurity effect may immediately be written as follows ... 

The intensity changes observed, while consistent with the occurrence of Reaction 1, were small enough to have originated from discrimination effects. A new instrument is being constructed in our laboratory which... 

It is particularly difficult to study charge transfer reactions by the usual internal ionization method since the secondary ions produced will always coincide with ions produced in primary ionization processes. Indeed these primary ions frequently constitute the major fraction of the total ion current, and the small intensity changes originating from charge transfer reactions are difficult to detect. For example, Field and Franklin (5) were unable to detect any charge transfer between Xe + and CH4 by the internal ionization method although such reactions have been observed using other techniques (3, 9,22). 

In addition, intensity changes under increasing pressure have been observed. For example, the most intense Raman line at STP conditions is the flg component of v ( 220 cm ), but at about 2 GPa the intensity decreases in favor of the ag component of Vi ( 475 cm ) which on further compression gains more intensity (about a factor of 2 at 5 GPa) [120]. This behavior was explained by the anisotropy of the crystal s compressibihty [139] and differences in the components of the Raman tensor of the two modes [87] with respect to the crystal axes [109]. 

The intensity changes are believed [3,13-15,19] to arise from the geometry of the monolayer with respect to the metal. 

Figure 3 shows the UV-vis DRS spectra of the three groups of catalysts In all cases, a prominent Au plasmon peak around 525 run was observed. This peak was sharper for catalysts of both groups A and B, and broader for catalysts of group C. That is, catalysts of lower C.F. s had broader peaks. In addition, there were three peaks at 270, 230, and 200 nm. These bands were related to the hydroxyls on AljOj, since they were observed on pure AljO, also, and their intensities changed with the moisture content of the sample. 

Figure 5.11 shotvs the temporal profile of the intensity change in the SFG signal at the peak of the Vco mode (2055 cm ) at OmV induced by visible pump pulse irradiation. The solid line is the least-squares fit using a convolution of a Gaussian function for the laser profile (FWFJ M = 20 ps) and a single exponential function for the recovery profile. The SFG signal fell to a minimum within about 100 ps and recovered... 

Another method is to measure the disappearance rate of the excited parent molecules, that is, the intensity changes of the disk-like images at various delay times (therefore, at various photolysis laser positions) along the molecular beam. This is very useful when the dissociation rate is slow and the method described above cannot be applied. This measurement requires a small molecular beam velocity distribution and a large variable distance between the crossing points of the pump and probe laser beams with the molecular beam. The small velocity distribution can be obtained through adiabatic expansion, and the available distances between the pump and probe laser beams depend on the design of the chamber. For variable distances from 0 to 10 cm in our system and AV/V = 10% molecular beam velocity distribution, dissociation rates as slow as 3 x 103 s 1 under collisionless condition can be measured. 

It is relatively easy to decide which vibronic bands have a common origin. This is accomplished by observing the phosphorescence intensity change of each band upon microwave saturation at a frequency that corresponds to transitions between rz and tx. This is known as phosphorescence-microwave double resonance (PMDR) spectroscopy. These frequencies for 2,3-dichloroquinoxaline are given in Table 6.3. 

Figure 6.2. (I) Conventional phosphorescence spectrum of 2,3-dichloroquinoxa-line in durene at 1.6°K. (II) am-PMDR spectrum, obtained by amplitude modulation of microwave radiation that pumps the tv-t, (1.055 GHz) zf transition with the detection at the modulation frequency. Only bands whose intensities change upon microwave radiation (1.055 GHz) and thus originate from tv or rz appear in the am-PMDR spectrum. Transitions from r and rv appear with opposite sign (phase-shifted by 180°). (Hb, lie ) Polarization of the am-PMDR spectral transitions, relative to the crystal axes. The band at 0,0-490 cm-1 originates from both the r and t spin states its intensity does not change upon the 1.055-GHz saturation (no band in II) however, its polarization does rhanp. (bands in Hb and IIc ). (Reproduced with permission from M. A. El-Sayed.tt7W)...

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