Laser power effect

C. Viets and W. Hill, Laser Power Effects in SERS Spectroscopy at Thin Metal Films, J. Phys. Chem. B 105, 6330 (2001)... 

The fluorescence signal is linearly proportional to the fraction/of molecules excited. The absorption rate and the stimulated emission rate 1 2 are proportional to the laser power. In the limit of low laser power,/is proportional to the laser power, while this is no longer true at high powers 1 2 <42 j). Care must thus be taken in a laser fluorescence experiment to be sure that one is operating in the linear regime, or that proper account of saturation effects is taken, since transitions with different strengdis reach saturation at different laser powers. 

From the write and read process sketched so far, some requirements for MO media can be derived (/) a high perpendicular, uniaxial magnetic anisotropy K in order to enable readout with the polar Kerr effect (2) a magnetoopticady active layer with a sufficient figure of merit R 0- where R is the reflectivity and the Kerr angle (T) a Curie temperature between 400 and 600 K, the lower limit to enable stable domains at room temperature and the upper limit because of the limited laser power for writing. 

Raman spectroscopy of graphite can be an experimental challenge, because the material is a strong blackbody absorber. Generally, low (1—10-mW) laser power is used to minimise heating, which causes the band positions to change. In addition, the expansion of the graphite causes the material to go out of the focus of the optical system, an effect which can be more pronounced in microprobe work. 

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

Figure 5. Dependence of photon Figure 6. The Zeeman sub-levels of the ground state return per Watt of laser power on second excited state, and the effects of optical...

B) Calculated Ed (solid circles left axis) is seen to fluctuate significantly in time-lapse experiments. After 30 min a large intensity fluctuation in acceptor excitation was simulated by manually diminishing laser power with 60%. The open circles depict the correction factor y, calculated according to Eq. (7.6) from cells expressing acceptors only. Calculating Ed with the online-updated y-factor (solid squares) abolished the effects of excitation fluctuations. 

Collection of multiple data sets for each time span, with frequent alternation of the polarization, is an essential feature of our protocol. This provides some protection against the effects of drifts in laser power, photomultiplier quantum yield, and absolute calibration of the TAC, photochemical decomposition of the dye, and any other long-term processes that may alter the measured fluorescence response curves. Separate analysis of each data set is necessary to provide an indication of the uncertainty in run-to-run reproducibility and to detect and delete the rare spurious data set. 

The mby fluorescence method allows us to perform pressure measurements in a short time scale (1-10 s), providing a real-time access for pressure control comparing to the time scale of many solid-state chemical processes. As a matter of fact, real-time pressure measurements are necessary when studying kinetic processes [117], but it is also important to minimize the laser power used for measuring the mby fluorescence in order to avoid undesired photochemical effects on the sample, whenever these are possible. In the case of IR absorption studies, which are commonly used for kinetic purposes, the advantage of using the mby fluorescence method, once photochemical effects are prevented, with respect to the employment of vibrational gauges is that no additional absorption bands are introduced in the IR spectmm. 

S2 - Sq fluorescence in condensed media has so far been found in several types of molecules. However, metalloporphyrins are contrasted with these compounds by another arresting feature such that the S2 fluorescence can be observed even upon photoexcitation to the state. Stelmakh and Tsvirko have first noticed the anomalous S2 - Sq fluorescence in metalloporphyrins (15,16). Figure 1(a) shows the fluorescence spectra of ZnTPP in EPA taken by the 540 nm excitation of a nitrogen pumped dye laser. The fluorescence band at around 430 nm observed by visible excitation is safely assigned to the S2 state fluorescence. The laser power dependence of the fluorescence intensity is quadratic at low power density of excitation (<5 x 10 photons cm"2 pulse ) but shows typical saturation effect with increasing the laser intensity. It should be emphasized here that the S2 fluorescence of ZnTPP can be observed without focusing of the laser beeim. 

The dependence of and S2 fluorescence intensities on the sample concentration was investigated between 10 - 10 M, and a sample concentration of less than 10 M was chosen to minimize the effect of concentration quenching. Two-photon absorption measurements were carried out by the excitation by relatively low power density (<10 photons cm" pulse ) in order to avoid the saturation effect on the laser power. Under these experimental conditions, the following rate equations for the concentration of S2 molecule is derived ... 

Analysis of binding experiments required a careful comparison of (i) the MYKO 63 bands, either in the presence or absence of DNA bands and (ii) the DNA Raman bands, either in the presence or absence of MYKO 63 bands. This comparison was achieved by computer-subtracting variable amounts of one spectrum from another. Previously, the various spectra were normalized to the same relative Raman intensity, with the 934 cm band (CIO symmetric stretch) as an internal standard. The intensity of the CIO. scattering measures the combined effect of such experimental factors as counting time, optical alignment and laser power. 

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