Standard laser spectroscopy

Infrared diode laser spectroscopy has been used for the measurement of hydrocarbon and CO concentrations in exhaust (13, 14, 15). The adsorption path length, and thus the absorption cell volume, required for hydrocarbon measurement is rather large, limiting the time-resolution of the measurement. The absorption path length required for CO measurement, however, is relatively short and approximately equal to the diameter of a standard exhaust pipe. This allows CO to be measured with high time-resolution by an infrared laser beam passed through an... 

Figure 18-19 Laser spectroscopy of water vapor showing individual rotational transitions of H2I60, H2170, and H2,8O.The upper trace is from a standard water sample and the lower trace is from an unknown. Relative peak areas In the two spectra provide isotope ratios to an accuracy of 0.1 %. [E. R. Th. Kerstel, R. van Trlgt,...

There has been a long standing question about the nature of the defect equilibria in the fluorites (13-22) Measurements of the conductivity and diffusion in fluorites is commonly interpreted with simple mass action relationships but the site selective laser spectroscopy (as well as other techniques) has shown that the situation is more complex and that simple mass action relationships don t even describe the observed equilibria qualitatively (23-28). The principal reasons for the failure of standard relationships is either other equilibria (in particular, the f scavenging equilibria) compete for F, that F is an unusual ion that is known for forming associates with itself that could change the defect equilibria, or there are abnormally large nonideality effects. These effects were studied in all of the alkaline earth fluorides and we found the same anomalies were always observed. 

The main significance of the work so far carried out at Oxford is that it demonstrates the feasibility of making measurements of the IS - 2S transition with a frequency doubled cw system. This opens the way to considerable improvements in precision, but the interpretation of the measurements will depend on what other information is available - the transition frequency is sensitive to QED effects, the proton size, and the values of fundamental constants. Cynics might point out that after well over a decade of work on the IS - 2S transition, the results have produced no real surprises. A more optimistic view is that it is only over the last year or two that it has become possible to study the transition with the standard techniques of high resolution cw laser spectroscopy, and the next decade will bring dramatic developments. We shall see. 

The jet-cooled species can be studied using standard laser techniques (Figure 13-4a) like laser-induced fluorescence (LIF or fluorescence excitation spectroscopy) or R2PI, as well as using their sophisticated double-resonance variants, like UV/UV or IR/UV double resonance spectroscopy (Figure 13-4b and 4c), to investigate molecules or clusters existing under various isomeric forms... 

Both microwave and optical frequency standards have benefited greatly from the development of the laser and the methods of laser spectroscopy in atomic physics. In particular, the ability to determine both the internal and external (that is, motion) atomic states with laser light - by laser cooling for example - has opened up the prospect of frequency standards with relative uncertainties below lO, for example, the Cs atomic fountain clock. The best atomic theories in some cases at starting to match in accuracy that of measurement, providing thereby refined values of the fundamental, so-called atomic constants. Even quite practical measurements (such as used in GPS navigation and primary standards of length) have advanced in recent years. 

Aman J, Hammersberg M, Pendrill L R, Talvitie H, Zarka A and Chartier J-M 1996 Laser spectroscopy of molecular iodine with a tuneable semiconductor laser around 633 nm , Proc. Symposium on Frequency Standards and Metrology, Woods Hole, MA (USA), (World Scientific Publishing, ed. J. C. Bergquist), 437 - 40... 

The second scheme to be treated is based on a frequency modulation of the monochromatic incident wave. It was not designed specifically for laser spectroscopy, but was taken from microwave spectroscopy where it is a standard method. The laser frequency co] is modulated at the modulation frequency 2, which changes coi periodically from cul — Acol/ to cul + Acul/2. When the laser is tuned through the absorption spectrum, the difference APr = Py(col Al/2) is detected with a lock-in amplifier (phase-sensitive detector) tuned to the modulation frequency (Fig. 1.4). If the modulation sweep Acol is sufficiently small, the first term of the Taylor expansion... 

We would like to mention one further practical application of standard Raman spectroscopy, namely the method of Raman lidar, which is now routinely used to monitor the upper atmosphere for composition (e.g. the presence of water vapour), chemical processes (e.g. the generation or depletion of ozone (O3)), and the determination of temperature profiles at high altitudes. Although absorption and fluorescence lidar systems are also widely used, Raman lidar has the distinct advantage that it is a simultaneous multispecies measurement technique, and that only a single fixed-wavelength laser is required. 

Without going into any further detail, we will mention one other type of interferometer that is encountered in molecular spectroscopy experiments, namely the scanning Michelson interferometer (for detailed descriptions refer to standard textbooks on optics or laser spectroscopy). These are used in so-called Fourier transform spectrometers for high-resolution molecular spectroscopy in the IR such instruments are commonly known as FTIR spectrometers. While rather popular in analytical molecular spectroscopy of IR wavelengths, namely to record, identify and quantify molecular vibrations, they are less suitable in laser chemistry experiments because of the rather long acquisition times required to record... 

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]. 

Fig. 6. A typical experimental apparatus used in sub-Doppler FLN laser spectroscopy of a molecular beam of Cs. In the above experiment photomultiplier PMl monitors the total fluorescence emitted at the interaction region while PM2 records the narrowed fluorescence after dispersal through analyzers. In this case the monochromator acts as a narrow band filter for the Fabry-Perot signals while the Th lamp serves as a frequency standard. After Demtroder (1981).

The first method is based on a frequency modulation of the monochromatic incident wave. It has not been designed specifically for laser spectroscopy but was taken from microwave spectroscopy where it is a standard method. The laser frequency is modulated at a modulation frequency f, which sweeps periodically from to When the laser is tuned through the absorp-... 

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