Excited-state reactions

Figure 9.6. Stability of a lifetime-based fiberoptic oxygen sensor over a period of 100 h of continuous operation. Lifetime techniques are insensitive to the process of photobleaching only in the absence of excited state reactions. Excited state reactions of the sensor-carrier system cause drifts in the observed lifetime with photobleaching. They are avoided by limiting the concentration of the sensor in the carrier. (From Ref. 21 with permission.)...

In the course of an electrode reaction excited states can arise, which are deactivated via radiation of light quanta. These states may be divided into two types (a) excited electrons and holes in the semiconductor and (b) excited products in the solution near the electrode surface. 

Miscellaneous Reactions.- Excited state alkyl nitrites undergo competing nitrogen-oxygen bond homolysis and exchange by labelled... 

Besides the hydrogen abstraction reaction, excited states of simple carbonyl compounds can undergo several other types of reactions in the solid state rearrangement (26-29), [2 + 2] cycloaddition (5,30,31), a-cleavage (24), and elimination of CO (32). Here,only examples done by us will be mentioned. 

The excess electrons sooner or later react with the positive ions. If the parent ion has not fragmented or reacted in an ion-molecule reaction, excited states of the parent molecule will be formed. In the gas phase fragmentation of the parent ion is important, in the condensed phase less so, because of collisional deactivation and the cage effect. 

The most abundant literature is that bearing on solar eneigy conversion, mainly centered on the use of Ru(2,2 -bipyridine)3 and its analogues. The excited state of the parent compound was found some years ago to be a powerful reducing agent [212], allowing the following spontaneous reactions to be written ... 

Heterogeneous photochemical reactions fall in the general category of photochemistry—often specific adsorbate excited states are involved (see, e.g.. Ref. 318.) Photodissociation processes may lead to reactive radical or other species electronic excited states may be produced that have their own chemistry so that there is specificity of reaction. The term photocatalysis has been used but can be stigmatized as an oxymoron light cannot be a catalyst—it is not recovered unchanged. 

Second-order effects include experiments designed to clock chemical reactions, pioneered by Zewail and coworkers [25]. The experiments are shown schematically in figure Al.6.10. An initial 100-150 fs pulse moves population from the bound ground state to the dissociative first excited state in ICN. A second pulse, time delayed from the first then moves population from the first excited state to the second excited state, which is also dissociative. By noting the frequency of light absorbed from tlie second pulse, Zewail can estimate the distance between the two excited-state surfaces and thus infer the motion of the initially prepared wavepacket on the first excited state (figure Al.6.10 ). 

Quack M 1982 Reaction dynamics and statistical mechanics of the preparation of highly excited states by intense infrared radiation Adv. Chem. Rhys. 50 395-473... 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Wynne K, Haran G, Reid G D, Moser 0 0, Dutton P L and Hochstrasser R M 1996 Femtosecond infrared spectroscopy of low-lying excited states in reaction centers of Rhodobacter sphaeroides J. Rhys. Chem. 100 5140-8... 

Boxer S G, Goldstein R A, Lockhart D J, Middendorf T R and Takiff L 1989 Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers J. Rhys. Chem. 93 8280-94... 

Stanley R J, King B and Boxer S G 1996 Excited state energy transfer pathways in photosynthetic reaction centers. 1. Structural symmetry effected. Phys. Chem. 100 12 052-9... 

Keil and co-workers (Dhamiasena et al [16]) have combined the crossed-beam teclmique with a state-selective detection teclmique to measure the angular distribution of HF products, in specific vibration-rotation states, from the F + Fl2 reaction. Individual states are detected by vibrational excitation with an infrared laser and detection of the deposited energy with a bolometer [30]. 

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

Altliough an MOT functions as a versatile and robust reaction cell for studying cold collisions, light frequencies must tune close to atomic transitions and an appreciable steady-state fraction of tire atoms remain excited. Excited-state trap-loss collisions and photon-induced repulsion limit achievable densities. 

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