Lasers power dependence

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. 

As discussed at length by Shimizu et al. [7], the on time distributions show temperature and laser power dependencies—for example, exponential cutoffs of power-law behavior. Although no direct observations of cutoffs in the off time distribution have been reported, ensemble measurements by Chung and Bawendi [3] demonstrate that such a cutoff should also exist, but at times of the order of tens of minutes to hours. Our analysis here, employing power-law decaying distributions, is of course applicable in time windows where power-law statistics holds. 

From the laser power dependence, the reaction scheme was written as... 

Pressure and laser power dependence of the product 637 distributions in the IRMPD of CCljCHCl. At moderate pressures elimination of HCl found to be the major decomposition channel but at low pressures results indicate that C-Cl bond fission becomes primary decomposition step... 

A schematic illustration of the laser tsunami and laser power dependence of shockwave generation, plasma emission, and cavitation bubbling is given in Figure 28.2. It is noticeable that the threshold of the shockwave is lowest and that of the bubbling is highest [35]. Of course this tendency is rather qualitative, as direct... 

At high n, Ciocca et al. found the laser-power dependence for Na formation to be quadratic, suggesting the reaction mechanism (5) above. If the product Na ion is in its ground state (excited Na can be dismissed on energetic grounds), then reaction 5 is 5.5 eV exothermic. This led Ciocca et al. to suggest three possible explanations for this rather remarkable observation. 

The diffusion cloud method can thus be seen to be a potentially useful technique for studying the reactions of laser-excited polyatomic molecules. Since the reactant sodium is monitored, the same technique can be used for a large number of molecular reactants. By measuring the laser power dependence of the reaction rate information can be obtained on both the vibrational energy requirements and the steady-state value of the reaction rates. 

Figure 2. Top Photoinduced absorption spectra of polyCPDA. T = 80 (full line), 120 (dotted line), and 180 K (dashed line), laser power P = 200 mW and chopper frequency f = 13 Hz. Bottom a) chopper frequency dependence of the 1.1 eV signal at 80 K, P = 300 niW, other conditions as above b) laser power dependence of the 1.1 eV peak for f = 13 Hz (o) and 850 Hz ( ). Other parameters as above.

Guenther, S., Koestler, M., Schulz, O., and Spengler, B. (2010) Laser spot size and laser power dependence of ion formation in high resolution MALDl imaging. Int.J. Mass Spectrom., 294, 7-15. 

Guenther, S. Koestler, M. Schulz, O. Spengler, B. Laser Spot Size and Laser Power Dependence of Ion Formation in High Resolution MALDl Imaging. Int. J. Mass Spectrom. 2010,294,7-15. 

Figure 5. Laser power dependence of absorption-time profiles of triplet excited at 750 nm. Inset Analysis of the triplet-triplet annihilation process according to the relation —d[lnA o]/4t = - -...

From the laser power dependence of the absorptiontime profiles of the Qq fine particles, it was confirmed that the fast-decaying part appears when the excitation laser power becomes high only the slow-decaying part appears with lower laser power excitation. This finding indicates that the triplet-triplet annihilation process in the fine particles becomes apparent only when the density of the triplet excited states becomes sufficiently high by the higher laser power. 

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