Residual absorption, lasers

Plotted curves illustrating this relation, Fig. 5, resemble very much the curves of Fig. 3. Consequently, one cannot infer from a measured intensity or energy saturation curve reliable values of molecular data without additional information, as for instance an independent measurement of ksr Another possibility is a measurement of the temporal characteristics of the bleaching as demonstrated in an experiment by Hercher et al. 14>. These authors bleached a thoroughly degassed solution of metal-free phthalocyanine in 1-chloronaphthalene by a ruby laser pulse (694.3 nm) of about 59 nsec pulse width. At the same time they measured the absorption at 632.8 nm using a He-Ne-laser, and the result of this measurement is shown in Fig. 6. It clearly demonstrates that the sample was almost completely bleached even before the laser pulse reached its maximum intensity, and that almost all of the molecules were stored in the triplet state because the transmission did not decrease with the fall of the laser intensity for at least 100 nsec. A small residual absorption indicates triplet-triplet absorption. 

There are many dyes which fulfill the above requirements, few of which have been used until now. The first dyes used with a ruby laser were phthalocyanines the free base as well as chloro-aluminum- and vanadium-phthalocyanine in 1-chloronaphthalene or nitrobenzene 22>. While these dyes show little residual absorption 14> and good photochemical stability, the solubility is relatively low and the solvents are not well suited to operation in a laser resonator. 

From a practical viewpoint, an increase of the absorption of CdTe near the RT band gap ( 1.5eV) has been correlated with 10.06 pm laser illumination [73]. It has been attributed to the temperature-induced shift of the band gap to lower energies generated by residual absorption of the crystal at 10.06 pm. The band-gap increase of silicon between RT and LHeT is 50 meV, and recent measurements at ultra-high resolution of the shift with temperature of the strongest B acceptor bound exciton line of qmi 28Si between 4.8 and 1.3 K show a band-gap increase of 1 GHz or 4 peV in this temperature domain [17]. 

An example of this process is the two-photon ionization of 2-aminopurine (2AP) - a nucleic acid base analog that can site-specifically inserted into any DNA sequence [27-31]. We have shown that photoexcitation of 2AP, which has a broad absorption maximum at 305 nm, by 308-nm intensive nanosecond XeCl excimer laser pulses, induced efficient and selective photoionization of 2AP residues. Absorption of the first photon results in the formation of the 2AP singlet excited state ( 2AP) and absorption of the second photon causes photoionization according to ... 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Excitation of an aqueous solution of poly(A/St/Phen) with a 355-nm, 22-ps laser pulse in the presence of MV2+ generated a transient absorption band peaking at about 600 nm due to MV + [120]. As shown in Fig. 16, the buildup of the 600-nm band completes immediately after the pulse excitation, indicating that the photoinduced ET from the singlet-excited Phen residue ( Phen ) to MV2 + occurs on a time scale comparable to or shorter than the duration of the laser pulse (ca. 22 ps) [120], Figure 16 also shows that a fast decay of the absorbance at 600 nm owing to the back ET from MV + to the Phen cation radical (Phen+ )... 

Fig. 16. Time-resolved transient absorption spectra for poly(A/St/Phen) (29) in aqueous solution in the presence of 5 mM MV2 + [Phen] (residue) = 0.66 mM. Delay times after the laser pulse are indicated [102]...

The above discussion of the Q-switching process immediately gives some requirements for a good Q-switching dye. First, this dye must exhibit a saturable absorption at the laser wavelength. Second, the residual losses by excited state absorption should be as low as possible. Third, the photochemical stability (and, of course, also the chemical stability in the dark) should be as high as possible. Fourth, the... 

Parallel sets of polymerization experiments were carried out using the monomer mixture described above with an Eosin-triethanolamine initiator system irradiated (514 nm) for short periods of time. The resulting prepolymer (or partly polymerized) solution was then exposed to UV radiation, as above, and the extent of crosslinking was measured. Since the polymerization at 514 nm was not carried out to completion, bleaching of the dye was incomplete at the concentration of the initiator used. Film formation was observed, but only after 3 minutes of irradiation. The lower levels of crosslinking can be attributed to partial masking of the oxime absorption by the residual initiator. Like experiments with a He-Cd laser (325 nm) produced gel along the laser path. 

Figure 18. Decay curves of the I 700 absorption at 695 nm, 645 nm, and 410 nm after excitation of Pr ( 24kDa) with a 15ns-laser flash Uctc = 650 nm) at 275 K. The experimental values have been fitted with the sum of two exponential decays and a constant (cf. Eq. (2)). On top of each decay curve a weighted residuals plot indicates the deviations of these computer-fitted parameters from the measured decay. (Curves from Figure 4 in Ruzsicska et al. [113].)...

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