Internal Conversion/Intersystem Crossing

PiJip)) is called the Lorentzian lineshape function. Its fwhm is equal to y, and is inversely proportional to the lifetime t = Ijy. It approaches zero as o) + oo, and maximizes at co == coq (Fig. 8.1). Physically, y itself will have several components in any real absorption line, arising from spontaneous emission (fluorescence or phosphorescence), nonradiative excited-state decay (intersystem crossing, internal conversion, photochemistry), collisional deactivation, etc. ... 

Fig. 5.6 A schematic representation of alternative pathways for formation and decay of excited states. Singlet states are labeled S and triplet states T superscripts 0, 1, 2,. .n,. .. denote the ground state and excited states of increasing energy. Radiative processes (absorption, fluorescence, phosphorescence) are indicated with solid arrows nonradiative processes (intersystem crossing, internal conversion, etc.), with wavy arrows. Internal conversion and intersystem crossing usually proceed via excited vibrational levels of the product state. Diagrams of this type were introduced by A. Jablonski in 1935 in a paper on the mechanism of phosphorescence [295]. The horizontal axis has no physical significance...

A rise in temperature increases the rate of vibrations and collisions, resulting in increased intersystem crossing, internal and external conversion. Consequently, the fluorescence intensity is inversely proportional to the temperature increase. Additionally, an increased temperature causes a red shift of the emission wavelength. 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

The natural processes of intersystem crossing and internal conversion will quickly (e.g. 50 ns) carry the molecule from this excited electronic surface to the ground electronic surface without a collision,... 

Energy level diagram for a molecule showing pathways for deactivation of an excited state vr Is vibrational relaxation Ic Is Internal conversion ec Is external conversion, and Isc Is Intersystem crossing. The lowest vibrational energy level for each electronic state Is Indicated by the thicker line. 

VD = vibrational deactivation IC = internal conversion F = fluorescence IX = intersystem crossing and P = phosphorescence. 

FIGURE 7.4 Modified Jablonski diagram showing transitions between excited states and the ground state. Radiative processes are shown by straight lines, radiationless processes by wavy lines. IC = internal conversion ISC = intersystem crossing, vc = vibrational cascade hvf = fluorescence hVp = phosphorescence. 

Thus if one starts with one pure isomer of a substance, this isomer can undergo first-order transitions to other forms, and in turn these other forms can undergo transitions among themselves, and eventually an equilibrium mixture of different isomers will be generated. The transitions between atomic and molecular excited states and their ground states are also mostly first-order processes. This holds both for radiative decays, such as fluorescence and phosphorescence, and for nonradiative processes, such as internal conversions and intersystem crossings. We shall look at an example of this later in Chapter 9. 

Figure 9.1. A Jablonski diagram. So and Si are singlet states, Ti is atriplet state. Abs, absorption F, fluorescence P, phosphorescence IC, internal conversion and ISC, intersystem crossing. Radiative transitions are represented by full lines and nonradiative transitions by dashed lines...

Figure 7. Potential energy diagram of CH2O. After excitation to specific rovibrational levels of Si, internal conversion leads to highly excited molecules in the ground electronic state So, whereas intersystem crossing populates the lowest triplet state Ti.

Section V.D described the competition of two pathways in the H2 + CO molecular channel. There are also multiple pathways to the radical channel producing H + HCO. In aU cases, highly vibrationally excited CH2O is prepared by laser excitation via the So transition. In the case of the radical channel discussed in this section, multiple pathways arise because of a competition between internal conversion (S o) and intersystem crossing ( i T ), followed by evolution on these electronic states to the ground-state H + HCO product channel. Both electronic states So and Ti correlate adiabatically with H + HCO products, as shown in Fig. 7. 

Thus we see that we have three processes which can compete for deactivation of the excited singlet fluorescence, internal conversion, and intersystem crossing. If we increase the rate of the latter by adding a heavy atom, this should result in a decrease or quenching of the fluorescence intensity ... 

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