Triplet state excitation cross section

Note Differential elastic and excitation transfer cross sections have been measured for He(2 S) + Nc and for He(23S) + Ne for energies between 25 and 370 meV (1). Some of the data are shown in Fig. 52. It was possible to measure the differential excitation cross sections for the triplet system, too. A semiclassical two-state calculation was performed for the pumping transition of the red line of the HeNe-laser Hc(2 S)+ Nc— Hc + Ne(5S, lPt), which is the dominant transition for not too high energies (2). A satisfactory fit is obtained to the elastic and inelastic differential cross sections simultaneously, as well as to the known rate constant for excitation transfer. The Hc(215)+ Ne potential curve shows some mild structure, much less pronounced than those shown in Fig. 36. The excitation transfer for the triplet system goes almost certainly over two separate curve crossings. This explains easily the 80 meV threshold for this exothermic process as well as its small cross section, which is only 10% of that of the triplet system. 

If we substitute amplitude (4.32) into formula for the differential excitation cross section (4.9) and integrate it, making the substitution (1/27r) dil = q dqlk0kn, we will get a factor (hk0)6 in the total cross section. Thus, while the direct scattering cross section decreases as 1IE with decrease of the energy, the exchange cross section behaves as 1/E3, meaning that the triplet states can be excited only by the slow electrons. 

The excitations of lower triplet states of benzene in these two approximations have been calculated in Ref. 133. A similar study of excitation cross sections for triplet states of a nitrogen molecule has been done in Ref. 134. 

From the point of view of establishing a CR model for H2, it is necessary to have v — v resolved cross-sections for all transitions between the electronic states with N <3 and for the high-A states (P, R, S, V-singlets and p, r, s, m-triplets) from the group of N = 4 states. For the transitions to (and between) the states with higher N, the excitation cross-sections can be obtained by scaling those for N < 3(N < 4) (within the same series 1,3Aa of states) according to the N 3 rule. 

Optical limiting by materials exhibiting RSA has been widely studied. - These materials, for example, C60 molecules, possess a molecular energy level scheme as depicted in Figure 12.27, in which the excited state absorption cross sections G] (of the singlet state) and/or CTj of the triplet excited state are greater than the ground state cross section ct. ... 

The subject of delayed fluorescence was discussed in Section 5.2a. It was seen that there are two common types of delayed fluorescence, that arising from thermally activated return from the triplet state to the lowest excited singlet (E-type delayed fluorescence) and that arising from collision of two excited triplet molecules resulting in a singlet excited molecule and a ground state molecule (P-type delayed fluorescence). The P-type delayed fluorescence can be used as a convenient tool for the determination of intersystem crossing efficiencies[Pg.125]

When Hammond and co-workers(59) found that the intersystem crossing quantum yield for aromatic ketones was unity (see Chapter 3) it was a short but very important step to realize that these compounds should be ideal triplet sensitizers. Thus one can excite the triplet state of molecules that otherwise would be formed inefficiently, if at all, by intersystem crossing. This idea resulted in a number of papers in the early 1960 s from the Hammond group on this topic. It is not possible in this short section to survey this area, but a few of the early studies are indicated by the following reactions ... 

Barnes et al. (1998) have measured the yield of OH from HOC1 photolysis and find, in addition to the strong absorption shown in Fig. 4.39, a weak absorption feature at 380 nm due to excitation to the lowest triplet state. Although the absorption cross section of this weak absorption is only 4 X 10 21 cm2 molecule-1, its contribution lowers the calculated stratospheric lifetime of HOC1 by 10-20%. 

The study of the short-lived excited states is limited by the low concentrations in which they are created on excitation with normal light sources. The use of high intensity sources such as flash lamps with suitable flashing rates (Section 10.3) and laser sources (Section 10.2) have helped in the study of the triplet states of molecules. The triplet-triplet absorption, triplet lifetimes, intersystem crossing rates from triplet to ground singlet State, etc. can be effectively measured because of high concentrations of the... 

For the reactions described so far in this section, the ketone substrates have lowest excited states that are (n.ii ) in character aliphatic ketones may react by way of the singlet or the triplet state, and aryl ketones normally through the triplet because intersystem crossing is very efficient. The efficiency of photochemical hydrogen abstraction from compounds such as alcohols or ethers is very much lower if the ketone has a lowest (Ji,n triplet state, as does I - or 2-acetylnaphthalene (CmH-COMe). However, all aryl ketones, regardless of whether their lowest triplet state is fn,Jt l or (Jt.Ji ), react photochemically with amines to give photoreduction or photoaddition products. A different mechanism operates (4.38), that begins... 

One may well ask why the isomerization of alkenes discussed in the preceding section requires a sensitizer. Why cannot the same result be achieved by direct irradiation One reason is that a tt — tt singlet excited state (5,) produced by direct irradiation of an alkene or arene crosses over to the triplet state (Ij) inefficiently (compared to n —> it excitation of ketones). Also, the Si state leads to other reactions beside isomerization which, in the case of 1,2-diphenyl-ethene and other conjugated hydrocarbons, produce cyclic products. For example, cw-l,2-diphenylethene irradiated in the presence of oxygen gives phenanthrene by the sequence of Equation 28-8. The primary photoreaction is cyclization to a dihydrophenanthrene intermediate, 6, which, in the presence of oxygen, is converted to phenanthrene ... 

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