The subtle and varied structural modifications considered in the previous chapters represent only a narrow window of irradiance or fluence, in which material is heated or melted but (essentially) not vaporized. In the following, material effects pertaining to the open-ended interval of irradiances exceeding the evaporation threshold will be considered. The physics involved is quite different from that considered in Chap. 4, and so is our point of view: Rather than studying what are essentially relaxation phenomena occurring after irradiation, we shall again deal with true light-matter interactions and consider relaxation effects only occasionally. The material effects in this regime are also rather less subtle than those discussed so far: Instead of microscopic structural rearrangements we shall be concerned with the transport of macroscopic masses over macroscopic distances, with ablation and compression of material and with internal energies large compared to chemical activation energies.The main topic of this chapter, material evaporation, is the most traditional branch of laser processing. Related research activities started in the early sixties, soon after the first high-power ruby lasers became available. The early work was mainly motivated by the prospect of using laser beams as machining tools. Most of the phenomena governing evaporation by laser beams were understood by 1975, although, of course, important insights and refinements have been made in the meantime and continue to be made. Present research activities are mainly centered in two fields: improvements in the understanding and control of machining processes, and studies of the interaction mechanisms of ultra-intense radiation with matter.Speaking of applications, machining -in all its variations from cutting through welding to milling and drilling -was the first area in which laser processing crossed the threshold to widespread industrial use. More prospective areas of application are shock treatments of materials and material testing under extreme conditions. Somewhat elusive remains the production of ultra-dense plasmas capable of sustaining thermonuclear fusion for energy production. Dense and hot laser-produced plasmas promise other uses, however, e.g., as intense sources of X-ray radiation.This chapter is organized loosely following a sequence of increasing irradiance. We start again with a section discussing fundamentals. Section 5.2 then deals with "normal" material evaporation in which the laser beam just happens to furnish the energy, along with its main applications in 115 M. Allmen et al. (eds.), Laser-Beam Interactions with Materials © Springer-Verlag Berlin Heidelberg 1995 157

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To describe the laser drilling process, a theoretical model that includes expulsion of liquid material is developed. The model allows the calculation of drilling velocity and drilling efficiency as a function of the absorbed intensity. The same quantities were determined experimentally, using Nd-YAG-laser pulses of rectangular shape. Good agreement between measurement and calculation was found in the intensity region where efficient drilling is possible, i.e., where reflection losses and vapor absorption can be neglected. For most metals this region is between 1 and 100 MW/cm2.

Metastable crystalline and amorphous phases were obtained by pulsed-laser quenching of thin Au-Ti and Cr-Ti films. Subsequent furnace annealing was found to convert metastable crystalline modifications into amorphous ones for certain compositions in both systems. In Cr~"Ti (x -0.7), a further annealing step at a higher temperature leads back to the initial metastable crystalline phase, thus making possible repeated crystalline to amorphous to crystalline transitions by simple thermal cycling. This phenomenon is unrelated to fast diffusion, but can be explained on the basis of free-energy considerations.PACS numbers: 64.70. Kb, 61.55.Hg, 81.30.Bx, 82.40.Mw Irradiation of thin films on suitable substrates by short laser pulses is a proven method to achieve melt quenching at very large cooling rates. ' As a result, glass formation is observed in binary metallic systems over extended ranges of composition. Certain melts, however, resist glass formation even by laser quenching, but form metastable crystalline phases instead. Laser-quenched glassy films on Au-Ti and Cr-Ti have been described previously. 2 In the present Letter we describe laser-quenched crystalline phases of Au-Ti and Cr-Ti with the intriguing property of turning amorphous upon subsequent furnace annealing.The metallic films were electron-gun -deposited multilayers, 150 nm thick, on substrates of either sapphire or tungsten. By irradiation with 50-ns full width at half maximum ruby-laser pulses the films were melted, homogenized, and subsequently quenched. Numerically calculated cooling rates with these parameters are 1 x 109 K/s for films on sapphire and 3&& 109 K/s for films on tungsten substrates. ' After quenching, as well as after subsequent annealing in a highvacuum furnace (10 6 mbar or better), the films were investigated by x-ray diffractometry and electrical resistivity measurements.

Objective. Only limited data exists in terms of the incidence of intracranial bleeding (ICB) in patients with mild traumatic brain injury (MTBI). Methods. We retrospectively identified 3088 patients (mean age 41 range (7–99) years) presenting with isolated MTBI and GCS 14-15 at our Emergency Department who had undergone cranial CT (CCT) between 2002 and 2011. Indication for CCT was according to the “Canadian CT head rules.” Patients with ICB were either submitted for neurosurgical treatment or kept under surveillance for at least 24 hours. Pearson's correlation coefficient was used to correlate the incidence of ICB with age, gender, or intake of coumarins, platelet aggregation inhibitors, or heparins. Results. 149 patients (4.8%) had ICB on CCT. No patient with ICB died or deteriorated neurologically. The incidence of ICB increased with age and intake of anticoagulants without clinically relevant correlation (R = 0.11; P < 0.001; R = −0.06; P < 0.001). Conclusion. Our data show an incidence of 4.8% for ICB after MTBI. However, neurological deterioration after MTBI seems to be rare, and the need for neurosurgical intervention is only required in selected cases. The general need for CCT in patients after MTBI is therefore questionable, and clinical surveillance may be sufficient when CCT is not available.