Laser irradiation, transient

A 10 M solution of methyl vlologen (MV ) did not have slgnlflcant ground or excited state absorbance from 350 nm to 750 nm. A 10 M solution of methyl vlologen In Aldrich HSX had ground state spectra characteristic of humic substances before and after laser Irradiation. A transient absorption spectrum of the above solution measured under the standard experimental condltio described previously showed a spectrum similar to that of MV reported In the literature (31)., 2 Figure 7. These data show evidence of Interaction between MV and the humic substance via a photoprocess. 

Immediately after the laser light irradiation, the transient absorption band appeared at 780 nm with broad bands around 1500 and 840 nm. The transient absorption band decays according to two components. The decay rate constants of the fast- and slow-decaying components were 1.2 X 10 and 3.4 x 10 s, respectively. The fast- and slow-decaying components may be due to the doublet and quartet states, respectively. 

Irradiation of a benzene solution of DABA at room temperature with a nitrogen laser (Horn and Schuster, 1982) gives the transient absorption spectrum shown in Fig. 3. This spectrum was recorded 50 ns after irradiation of the diazo-compound and decays over a period of ca 250 ps by a path exhibiting complex kinetic behavior. This transient spectrum is essentially identical with the low temperature optical spectrum described above, and thus is similarly assigned to 3BA. 

The transient triplet carbene formed from irradiation of DABA in benzene can be observed to react with styrene. For this process laser spectroscopy reveals a bimolecular rate constant (ksty) equal to 1.2 x 107M-1s-1. The product of the reaction is the expected cyclopropane. This observation clearly supports the spectral assignment of the carbene made above. When deuteriated a-methylstyrene is substituted for styrene as a trap for the carbene, the cyclopropane that results is a 1 1 mixture of stereoisomers, (20) (Table 6). This finding indicates that BA is reacting in this sequence exclusively from its ground triplet state. 

Irradiation of DAX in pentane with a nitrogen laser creates a transient species whose spectrum is quite similar to that of the species formed by the low temperature irradiation. The half-life of this intermediate in very dry solvent is ca 50 ps. Its reaction under these conditions follows a complex kinetic path that gives ultimately dimeric azine and bi-xanthylidene (22). These findings help in the assignment of a structure to the transient. Neither XAH" or XAH+ are expected to give these products, but XA easily accounts for both. 

Irradiation of DMDAF in benzene solution at room temperature generates a transient intermediate that appears within the rise-time of the laser and decays by a pseudo first-order kinetic path with a half-life of 51 ns. The optical spectrum of the intermediate is essentially the same as that observed in the low temperature irradiation experiment. 

Experiments were conducted in our laboratory to evaluate many of the dynamical expectations for rapid laser heating of metals. One of the aims of this work was to identify those population distributions which were characteristic of thermally activated desorption processes as opposed to desorption processes which were driven by nontbennal energy sources. Visible and near-infrared laser pulses of nominally 10 ns duration were used to heat the substrate in a nonspecific fashion. Initial experiments were performed by Burgess etal. for the laser-induced desorption of NO from Pt(foil). Operating with a chamber base pressure 2 x 10 torr and with the sample at 200 K, initial irradiation of a freshly cleaned and dosed sample resulted in a short time transient (i.e. heightened desorption yield) followed by nearly steady state LID signals. The desorption yields slowly decreased with time due to depletion of the adsorbate layer at the rate of ca. 10 monolayer... 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

Irradiation of PFe02 in solutions by short, very intense, laser pulses produces transients such as 3 and 4. Absorbance changes following the production of 3 and 4 are ascribed to their decay and rate parameters can be estimated as shown in the Scheme. Note the units in (2.20) which are occasionally used for large rate constants. The mechanism shown is certainly a simplified one. 

Figure 11 Transient absorption spectra observed in photoinduced electron Uansfer from Cso (1-1 X 10 " M) to CI4Q (4.0 X 10 M) in the presence of Sc (0.11 M) after laser irradiation at X = 532 nm in deaerated PhCN at 298 K (black circles = 5 p.sec, black square = 25 xsec and black triangle = 100 (xsec). Inset Time profile of the absorption band at 740 nm and 980 nm due to Cso and Cso - (From Ref. 135.)...

A cooperative model for isomerization was proposed to explain the effect of intense laser irradiation. In this model, the formation of excited or transient states in close proximity can transiently provide enough free volume for isomerization to occur. This work with NOSH has been complemented by studies using 6-nitro-BIPS and NIPS [109,110]. This cooperative model is shown in Figure 12. 

The transient absorption formed from laser flash photolysis of 15 was not quenched in oxygen-saturated solutions, presumably because the photoenolization to form 16 takes place from a singlet excited state of 15. Furthermore, Schworer and Wirz did not observe any transient absorption that can be attributed to the formation of the triplet excited state of 15 from direct irradiation. In contrast. Hurley and Testa used energy transfer to estimate that the triplet excited state of 15 is formed in 67% yield,whereas Takezaki et al. have measured the yields for forming the triplet excited state of 15 to be slightly higher or above 80%. They estimated the lifetime of the lowest triplet excited state of 15 to be 350 ps in ethanol. 

When the reaction was conducted in O-deuteriated HFIP, (CF3)2CHOD, as a solvent, the deuterium was found exclusively at the 2-position of the 1,3-dimethoxybenzene 312. Laser flash photolysis at 248 nm irradiation carried out with 10 3 M solutions of substrates 311 with different silyl groups (SiMe3, SiMe2Ph, SiMePh2, SiMe2C6H4X-4 X = OMe, Me, F, I) show absorptions of the transient cations 313 with maxima at 380-390 nm and a weak band at 300 nm. The absorption spectra are similar to that of the parent 2,6-dimethoxybenzenium ion 314 (R = H) (kmax = 410 nm) but show a slight hyp-sochromic shift. 

However, the initial step of the electron transfer reaction strongly depends on the solvent polarity. By changing the solvent to less polar or nonpolar solvents like benzene or nonaromatic hydrocarbons the transient absorptions of 3C 0, G)0 and donor radical cation appear immediately after the laser pulse. The decay of all the absorptions is also completed at the same time. The fast appearance and the fast decay of the Go and donor radical cation absorption suggest that there is an interaction between fullerene and donor in less polar and nonpolar solvents before laser irradiation [120,125,133-139],... 

Typical configurations of laser flash photochemical apparatus for the detection of transients in a timescale longer than 1 ns are shown in Figure 6.8. The pump-probe configurations in Figure 6.8 are used for the front-face and side-on irradiations of the... 

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