Dependence on the Beam Intensity

18 In hase and out-of-phase SRGs inscribed on the same potymer film due to different light intensities. From reference 39. 

Below a certain threshold intensity, no in-phase grating could be formed even if the out-of-phase grating was completely saturated and the writing continued. This threshold intensity at 488 nm was about 24 W/cm for PD03 and 22 W/cm for HPAA-N02. These systems also showed interesting optica erasure characteristics, which are discussed in Section 14.4.10. 

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One of the main characteristics of the laser emission is the huge amount of energy that is concentrated within a narrow beam and can be delivered on a tiny area. In order to take full profit of the high power density available, it is also necessary to use photosensitive systems which obey the reciprocity law, i.e. where the energy required for the reaction is not dependent on the light intensity, which means that the quantum yield remains constant. This condition appears to be almost fullfilled in the present case since the fluence, expressed in J cm-2, was found to increase by only a factor of 4 when the light-intensity was increased by over 4 orders of magnitude (Table I). 

Samples of 1 (200 mg) were sealed in evacuated Pyrex ampoules (inner diameter 4 mm) and immersed in a 500-mL Pyrex beaker filled with ice and water in such a way that no ice blocked the laser beam. The beam of an excimer laser (Lambda Physics, EMC 201 XeCl 17 ns pulses 50 Hz repetition rate 3 h X = 308 nm) was positioned vertically using two dielectric mirrors and focused to the desired intensity by a quartz-lens with a focal length of 20 cm. For low intensity irradiations, the ampoules were placed in front of a mercury arc at a distance of 5 cm. The product ratio depended on the light intensity. The compounds 1, 2, 3 and 4 were separated by gas chromatography or HPLC on RP18 and spectroscopically characterized after 93-97% conversion to 3 and 4. 

The generic experiment in hyper-Rayleigh scattering4 consists of measuring the intensity of the second harmonic l(2a>) that is scattered by a fundamental laser beam impinging on a solution of nonlinear optical chromophores in a suitable solvent. This intensity should be quadratically dependent on the fundamental intensity l(o>). 

The foregoing can be applied to the case of two-photon (TP) excitation. However, there are difficulties with obtaining the general expressions for the order parameters in elementary functions in this case. For this reason, for the moment, we do not go beyond the low-intensity approximation in consideration of the TP-induced x anisotropy. The TP absorption depends on the peak intensity of the pumping beam, and so the development of the distribution function Nj Q) gives... 

The processes taking place in laser Py-MS are not well characterized because more than one effect may happen when the sample is irradiated with the laser beam—laser induced desorption (LID), melting, pyrolysis, ionization, etc. These processes depend on the laser intensity and energy (wavelength) and on the substrate and sample composition. Also, the vacuum in the MS system may play a role regarding the result of irradiation by diminishing any secondary reactions of the pyrolysate. 

The basic principle of TLM is illustrated in Fig. 1 [1]. In a TLM, two laser beams are utilized an excitation laser beam and a probe laser beam. The wavelength of the former is tuned to the light absorption band of the sample and that of the latter is adjusted to avoid the absorption band. The excitation laser is tightly focused into the sample. The diameter of the beam waist is usually around 1 xm. The sample absorbs the excitation beam, and heat is released to the solvent around the molecule, increasing temperature of the solution. The light absorption is linearly dependent on the light intensity distribution which is a Gaussian distribution, and the resultant temperature distribution is sim-... 

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