General discussion of lasers

GENERAL DISCUSSION ON LASER CONTROL OF CHEMICAL REACTIONS... 

These practical issues of particle shape and dispersion are not intended to cast aspersions on the laser diffraction technique rather, these factors have been discussed to bring awareness around the analytical results that are obtained when these factors are present. Laser diffraction has proven itself to be a reliable, robust technique for particle size analysis. When the assumption of nonaggregated spherical particles is violated, there are clear manifestations in the calculated particle size distribution. When analyzing drug substances that are used in low-dose solid oral formulations, the impact of these manifestations can be particularly impactful as there is often a limited number of API lots to be used for method development. Therefore, the analyst must be aware of these issues prior to the commencement of method development to avoid these pitfalls. In addition to the information contained in ISO 13320, Snorek et al. have written a summary around the general practices of laser diffraction measurements in the pharmaceutical industry.19... 

Atoms in strong laser fields 9.24 General discussion of strong field effects... 

These considerations lead to a general discussion of exciplex lasers. This is a well-established laser scheme for dye lasers where charge-taransfer complexes of excited molecules form the upper lasing state. Examples are the dyes 4-methyl-umbelliferone or N-methylacridin 92h... 

Laser photons are the most important ingredients in any laser chemistry experiment. Hence, it is essential in a textbook on laser chemistry to incorporate a description of the principles of lasers and laser radiation. On the other hand, a complete discussion of laser theory and an exhaustive list of specific lasers, including their construction, operation and description of characteristics, are well beyond the scope of this short introductory part - a wealth of general laser textbooks and books on specific laser types have been written on the subject. Rather, we restrict our outline of laser sources to a summary of the principles behind laser action and to a discussion of the parameters, with which a user will very likely be confronted with in laser chemistry problems. If the reader wishes to delve deeper into the basics of laser physics, he/she is referred to general (e.g. SUfvast, 2004) or specialist texts see the Further Reading list for Part 1. 

In this chapter we will discuss the general principles of lasers and study the most important tunable lasers that are of primary spectroscopic interest. Since many tunable lasers are optically pumped by fixed-frequency lasers we will also describe the most useful types of such lasers. For a more thorough account of laser physics we refer the reader to standard textbooks [8.1-10]. 

In this chapter we will discuss the general principles of lasers. Since we mainly consider spectroscopic aspects in this book, we will focus on tunable lasers for laser spectroscopy in the frequency (wavelength) domain and short-pulse lasers for spectroscopy in the time domain. Short-pulse lasers are also required for the generation of ultra-intense laser pulses, the use of which has opened up a new field of spectroscopy ultra-intense laser/matter interaction. In addition to the many types of spectroscopically interesting lasers, we will also cover a number of the fixed-frequency lasers that are used to pump them. For more detailed accounts of the field of laser physics, frequently also referred to as quantum electronics, we refer the reader to standard textboolcs [8.1—8.13]. 

Several reviews of the chemical laser literature have appeared, but until quite recently no discussion was available which documented the engineering development of specific laser devices. Fortunately the Handbook of Chemical Lasers has recently appeared which reviews the detailed historical development of the chemical laser field and provides specific discussion of virtually every type of chemical laser that had been developed by the end of 1974. This book also provides general discussions of the optical, kinetic, and gas dynamic aspects of chemical laser operation. The present review is restricted primarily to discussions of the use of the chemical laser in the laboratory and of current research directed toward the discovery of new chemical laser systems. No attempt is made here to provide a complete bibliography of the chemical laser literature since 1974 the Handbook of Chemical Lasers should be consulted for references prior to 1974. 

This review is organized as follows Section 3.2 presents a brief discussion of the HF(DF) and CO chemical lasers which are at present the most highly developed systems for practical applications Section 3.3 outlines recent uses of small chemical lasers in laboratory research and Section 3.4 is devoted to a general discussion of problems in the search for new chemical lasers, with particular emphasis on electronic transition lasers capable of operation at visible wavelengths. 

The development of lasers has continued in the past few years and 1 have included discussions of two more in this edition. These are the alexandrite and titanium-sapphire lasers. Both are solid state and, unusually, tunable over quite wide wavelength ranges. The titanium-sapphire laser is probably the most promising for general use because of its wider range of tunability and the fact that it can be operated in a CW or pulsed mode. 

The simplified theory is adequate to obtain qualitative agreement with experiment [1,16]. Comparisons between the simplified and more advanced versions of the theory show excellent agreement for the dominant (electronic) contribution to the time-dependent dipole moment, except during the initial excitation, where the k states are coupled by the laser field [17]. The contributions to the dipole from the heavy holes and light holes are not included in the simplified approach. This causes no difficulty in the ADQW because the holes are trapped and do not make a major contribution to the dynamics [1]. This assumption may not be valid in the more general case of superlattices, as discussed below. 

In Section II, the basic equations of OCT are developed using the methods of variational calculus. Methods for solving the resulting equations are discussed in Section III. Section IV is devoted to a discussion of the Electric Nuclear Bom-Oppenhermer (ENBO) approximation [41, 42]. This approximation provides a practical way of including polarization effects in coherent control calculations of molecular dynamics. In general, such effects are important as high electric fields often occur in the laser pulses used experimentally or predicted theoretically for such processes. The limits of validity of the ENBO approximation are also discussed in this section. 

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