Electric fields lasers

To discuss Raman activity, let us consider the nature of the polarizability (a) introduced in Section 1.4. When a molecule is placed in an electric field (laser beam), it suffers distortion since the positively charged nuclei are attracted toward the negative pole, and electrons toward the positive pole (Fig. 1-14). This charge separation produces an induced dipole moment (P) given by... 

Here, E is the strength of the applied electric field (laser beam), a the polarizability and / and y the first and second hyper-polarizabilities, respectively. In the case of conventional Raman spectroscopy with CW lasers (E, 104 V cm-1), the contributions of the / and y terms to P are insignificant since a fi y. Their contributions become significant, however, when the sample is irradiated with extremely strong laser pulses ( 109 V cm-1) created by Q-switched ruby or Nd-YAG lasers (10-100 MW peak power). These giant pulses lead to novel spectroscopic phenomena such as the hyper-Raman effect, stimulated Raman effect, inverse Raman effect, coherent anti-Stokes Raman scattering (CARS), and photoacoustic Raman spectroscopy (PARS). Figure 3-40 shows transition schemes involved in each type of nonlinear Raman spectroscopy. (See Refs. 104-110.)... 

The high-field output of laser devices allows for a wide variety of nonlinear interactions [17] between tire radiation field and tire matter. Many of tire initial relationships can be derived using engineering principles by simply expanding tire media polarizability in a Taylor series in powers of tire electric field ... 

Figure 9.24 shows part of the laser Stark spectrum of the bent triatomic molecule FNO obtained with a CO infrared laser operating at 1837.430 cm All the transitions shown are Stark components of the rotational line of the Ig vibrational transition, where Vj is the N-F stretching vibration. The rotational symbolism is that for a symmetric rotor (to which FNO approximates) for which q implies that AA = 0, P implies that A/ = — 1 and the numbers indicate that K" = 7 and J" = 8 (see Section 6.2.4.2). In an electric field each J level is split into (J + 1) components (see Section 5.2.3), each specified by its value of Mj. The selection mle when the radiation is polarized perpendicular to the field (as here) is AMj = 1. Eight of the resulting Stark components are shown. 

External stimulus flow meters are generally electrical in nature. These devices derive their signal from the interaction of the fluid motion with some external stimulus such as a magnetic field, laser energy, an ultrasonic beam, or a radioactive tracer. 

An interesting development of the PHB technique leads to four-dimensional data storage. By variation of an electric field appHed to the sample the spectral profile of the absorption holes can specifically be altered. This adds two more dimensions to the geometrically two-dimensional matrix frequency of laser light and electrical field strength (174). 

The existence of isotope shifts and of tunable lasers with narrow Hnewidth leads to the possibHity of separating isotopes with laser radiation (113,114). This can be of importance, because isotopicaHy selected materials are used for many purposes in research, medicine, and industry. In order to separate isotopes, one needs a molecule that contains the desired element and has an isotope shift in its absorption spectmm, plus a laser that can be tuned to the absorption of one of the isotopic constituents. Several means for separating isotopes are avaHable. The selected species may be ionized by absorption of several photons and removed by appHcation of an electric field, or photodissociated and removed by chemical means. 

A schematic representation of a PR apparatus is shown in Figure 2. In PR a pump beam (laser or other light source) chopped at frequency 2 creates photo-injected electron-hole pairs that modulate the built-in electric field of the semiconductor. The photon energy of the pump beam must be larger than the lowest energy gap of the material. A typical pump beam for measurements at or below room temperature is a 5-mW He-Ne laser. (At elevated temperatures a more powerful pump must be employed.)... 

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