Il

FIGURE 3.9 Jablonski diagram illustrating photophysical radiative and nonradiative transitions. S0 = ground singlet state, S, = first excited singlet state, T, = first triplet state, A = absorption of light, F = fluorescence, P = phosphorescence, IC = internal conversion, ISC = intersystem crossing. Radiative transitions are shown by solid lines, and nonradiative transitions by wavy lines. Photochemical processes are not indicated.

mid and Laudenslager, 1982; Crosley, 1989). Chemilu-minescence is similar to fluorescence except that the excited state is generated in a chemical reaction.

Phosphorescence is defined as the emission of light due to a transition between states of different spin multiplicities. Because this is theoretically not an allowed transition for an ideal unperturbed molecule, phosphorescence lifetimes tend to be relatively long, typically lO^-lO"2 s.

Intersystem crossing (ISC) is the intramolecular crossing from one state to another of different multiplicity without the emission of radiation. In Fig. 3.9 (ISC), shows the transfer from the first excited singlet state S, to the first excited triplet state T,. Since the process is horizontal, the total energy remains the same and the molecule initially is produced in upper vibrational and rotational levels of T,, from which it is deactivated as shown by the vertical wavy line. Similarly, (ISC)2 shows the intersystem crossing from T, to upper vibrational and rotational states of the ground state S„, from which vibrational deactivation to v" = 0 then occurs.

Internal conversion (IC) is the intramolecular crossing of an excited molecule from one state to another of the same multiplicity without the emission of radiation. As seen in Fig. 3.9, the horizontal wavy line (IC), represents internal conversion from the lowest excited singlet state S, to high vibrational levels of the ground state S0; this is generally followed by vibrational deactivation to v" = 0.

Intramolecular photophysical processes available to an excited molecule shown in Fig. 3.9 predominate at low pressures where collisions with other molecules are relatively infrequent. However, at 1 atm pressure, or in the liquid state, the excited molecule can undergo many collisions with ground-state molecules; this can lead to collisional deactivation of the excited species by several paths. For example, an electronically excited molecule, A*, in the S, >() state could undergo a series of collisions, be vibrationally (and rotationally) deactivated, and fall into the S,' =() state. The energy lost by A* is carried off as translational energy of the ground-state collision partner, B. From here, A (S, =0) can undergo photophysical or photochemical processes. Alternatively, energy transfer from A* to the collision partner can occur in which the excitation energy appears as excess vibrational, rotational, and/or electronic energy of molecule B.

Collisional deactivation and energy transfer play important roles in tropospheric chemistry. For example, electronically excited S02 in the B, state can be deactivated by 02 (as well as by N2 and H20) to the ground ('A,) state, with part of this process occurring via triplet-triplet energy transfer to generate singlet electronically excited states of 02:

Note that in the transfer of electronic energy between an excited atom or molecule and a second atom or molecule, the Wigner spin conservation rule generally applies. This states that the overall spin angular momentum of the system should not change during the energy transfer (see Herzberg for details). Because 02 has the unusual property of having a ground triplet state, energy transfer from triplet collision partners can produce the reactive singlet states of molecular oxygen. Indeed this is the mode of action in some photody-namic therapies in medicine.

Similarly, collisional deactivation is an important factor in trying to detect and measure various gaseous species in the troposphere using the technique of induced fluorescence. For example, as discussed in Chapter 11, induced fluorescence is one of the techniques applied to determine the concentration of OH free radicals in the troposphere. The OH is excited to the A2X+ state, from which it fluoresces as it returns to the ground state. However, collisional deactivation of excited OH by 02 and N2 is significant at f atm pressure; this reduces the emitted light relative to interfering signals. Expansion of the air sample to lower pressures reduces this quenching and increases the overall sensitivity (see Chapter 11).

b. Photochemical Processes

In contrast to the photo physical processes just described, photochemical processes produce new chemical species. Such processes can be characterized by the type of chemistry induced by light absorption: photodissociation, intramolecular rearrangements, photoisomer-ization, photodimerization, hydrogen atom abstraction, and photosensitized reactions.

Of these, photodissociation is by far the most pervasive and important in atmospheric chemistry. For example, the photodissociation of N02 into ground-state oxygen atoms,

followed by the reaction of OC P) with Oz, is the sole known source of anthropogenically produced 03 in the troposphere.

The reader will encounter numerous other examples of photodissociation throughout this text, so it will not be treated further here. However, as will become obvious in examining the chemistry of both the troposphere and stratosphere in later chapters, it is photochemistry that indeed drives the chemistry of the atmosphere.

c. Quantum Yields

The relative efficiencies of the various photophysical and photochemical primary processes are described in terms of quantum yields, 4The primary quantum yield, for the ith process, either photophysical or photochemical, is given by Eq. (I):

Number of excited molecules proceeding by process i

Total number of photons absorbed

For example, the nitrate radical, which plays an important role in nighttime chemistry (see Chapter 6), absorbs light in the red region of the visible (600-700 nm). The electronically excited state formed on light absorption can dissociate into either N02 + O or into NO + 02, or it can fluoresce:

The primary quantum yields for each process are defined as follows:

Number of N02 or Of P) formed in the primary process

Number of photons absorbed by N03

Number of NO or 02 formed in the primary process

Number of photons absorbed by NO-,

Number of photons emitted by N03

Number of photons absorbed by N03

04c. is also known as the fluorescence quantum yield,

By definition, the sum of the primary quantum yields for all photochemical and photophysical processes taken together must add up to unity, i.e.,

S(0f + + 0dcact + ...& + <£„ + ...) = 1.0, where <£p, and <£dcai;1 are the primary quantum yields for the photophysical processes of fluorescence, phosphorescence, and collisional deactivation, respectively, and (f>.d, (f>b, and so on are the primary quantum yields for the various possible photochemically reactive decomposition paths of the excited molecule.

For example, in the case of N03, 04a is, within experimental error, 1.0 up to 585 nm and then decreases to zero at 635 nm. As path (4a) falls off above 585 nm, path (4b), <^4b increases to a peak of 0.36 at approximately 595 nm and then also decreases at longer wavelengths (Orlando et al., 1993; Davis et al., 1993; Johnston et al., 1996). As the quantum yields for both (4a) and (4b) decline, fluorescence, (4c) is observed (Nelson et al., 1983; Ishiwata et al., 1983), increasing toward unity at ~ 640 nm. In short, at different wavelengths the contribution of the various processes, (4a), (4b), and (4c), varies, but always consistent with their sum being unity.

While the aim of photochemical studies is generally to measure primary quantum yields, this is not always experimentally feasible. For example, NO reacts rapidly with N03 to form N02. Thus determination of (f>4a or <f)4h by measuring the NO and N02 formed can be complicated by this secondary reaction of NO with N03, and the measured yields of NO and N02 may not reflect the efficiency of the primary photochemical processes.

In some cases, then, the overall quantum yield, rather than the primary quantum yield, is reported. The overall quantum yield for a particular product A, usually denoted by <f>A, is defined as the number of molecules of the product A formed per photon ab sorbed. Because of the potential contribution of secondary chemistry to the formation of stable products, the overall quantum yield of a particular product may exceed unity. Indeed, in chain reactions, overall quantum yields for some products may be of the order of 10h or more.

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