Threshold fluence

Maximum energy Transmitted energy Threshold energy Focusing distance Maximum fluence Threshold fluence Intensity Spatial position Center wavelength Ripple period Density of electrons Number of pulses per spot Lateral extension parameter Spatial coordinate Reflectivity... 

It should be noted that the diameter of a laser-produced cavity may differ significantly from the Gaussian beam diameter 2w0. Figure 8 illustrates this phenomenon. The distance 2w0 is indicated on the x-axis (projection at F0le2 to the x-axis). For ablation, a material- and laser-dependent fluence threshold Fth has to be surpassed. On the sample, one can realize a laser-induced hole with a diameter smaller than 2w0 (projection at Fth to the x-axis) for example. By adjusting F0 very close to Fth (with F0>Ft l), the production of tiny structures should be possible. 

Fig. 34 The removal rate versus pulse fluence for 266-nm laser ablation of PTFE. The fluence threshold can be observed and a single laser pulse can ablate more than a molecular layer (a few A). Reproduced with permission from J Phys Chem 2000, B26, 6212-6217. Copyright 1998 Am Chem Soc...

Srinivasan R, Braren B, Casey KG, Yeh M. (1990) The significance of a fluence threshold for ultraviolet laser ablation and etching of polymers./ylpp/TAj/s 67 1604. 

JR = intensity (fluence) threshold for the onset of ablative photodecom-... 

The etch depth is independent of the atmosphere in which the experiment is performed. Fluence thresholds for etching and etch depth plots for a variety of polymers have been reported [137, 304, 475, 578, 611, 712, 1298, 2029, 2031, 2032, 2048]. 

MALDI ionization has a high-order (6 or higher has been reported) empirical dependence on laser fluence. This leads to a perceived fluence threshold, above which ions are easily observed in a given instrument. The model predicts a rapid increase in signal, an apparent threshold, at a fluence where the vaporization of the sample becomes rapid. The threshold phenomenon is therefore not associated with ionization itself. Ions are formed at low fluence, but are not released if vaporization is poor. Because the of this, positive and negative ions should have the same threshold, as found experimentally. ... 

The apparent fluence threshold is wavelength-dependent and is inversely correlated with the absorption coefficient of the matrix. This is reproduced by the model and is again a vaporization effect. A higher absorption coefficient heats the upper sample layers more efficiently, so they vaporize at lower fluence. 

So far powerful lasers with picosecond to nanosecond pulse duration have usually been used for the ablation of material from a solid sample. The very first results from application of the lasers with femtosecond pulse duration were published only quite recently. The ablation thresholds vary within a pretty wide interval of laser fluences of 0.1-10 J cm , depending on the type of a sample, the wavelength of the laser, and the pulse duration. Different advanced laser systems have been tested for LA ... 

The depth profiling of the Cu-Ag-Si samples was performed with the laser beam focused on to the sample surface to the spot of approximately 30-40 pm. To obtain the best depth resolution the laser fluence was maintained near the 1 J cm level, close to the threshold of the LIBS detection scheme. The intensity profiles of Cu and Ag emission lines are shown in Fig. 4.43. The individual layers of Cu and Ag were defi-... 

OL behavior is assessed simply by monitoring the transmission of a (usually solution) sample as a function of the incoming laser fluence measured in joules per square centimeter (rather than intensity in watts per square centimeter).22,23 Limiting thresholds Fth, defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, are then commonly quoted. Since excited-state absorption processes generally determine the OL properties of molecules, the excited-state structure and dynamics are often studied in detail. The laser pulse width is an important consideration in the study of OL effects. Compounds (1-5)58-62 are representative non-metal-containing compounds with especially large NLO and/or OL... 

Fig. 12. Light threshold of photophorogenesis in dependence of residual (3-carotene synthesis. Mycelia, 50h after inoculation, were irradiated at various fluence rates for 1 min. The threshold was determined by extrapolation. From Gal-land and Russo (1979,inpress)...

The molecular ions produced in the MALDI process have relatively high initial velocities, which can cause reduction in mass resolving power and transmission, primarily for TOF analyzers with axial ion extraction (see Section 2.2.1). Hence, the MALDI-MS mass resolving power depends strongly on laser fluence and is highest when the laser fluence is close to the threshold level. 

The fluorescence spectra measured just upon ablation are given in Figure 2A as a function of laser fluence. The contribution below 370 nm was suppressed, as a Hoya L37 filter was used in order to cut off the laser pulse. Fluorescence spectra of this polymer film consist of sandwich (max. 420 nm, lifetime 35 ns) and partial overlap (max. 370 nm, lifetime 16 ns) excimers (20). The latter excimer is produced from the initially excited monomer state, while the sandwich excimer from the partial overlap excimer and the monomer excited states. Since these processes compete with efficient interactions between identical and different excimers (Si - Si annihilation) (12), the sandwich excimer is quenched to a greater extent compared to the partial overlap one under a high excitation. Actually the fluence-dependent spectral change around the threshold can be interpreted in terms of Si - Si annihilation. 

Fluorescence intensity increases with the laser fluence, while its change was quite smooth even around the threshold. If a new process leading to ablation was involved in addition to the Si - Si annihilation, the relative fluorescence intensity of two excimers would not change furthermore above the threshold. Although the details are beyond our current knowledge, we conclude that the Si -Si annihilation is the origin of laser ablation in thiB fluence range. 

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