Second Harmonic Detection

In most cases, you will use the first harmonic and the normal first-derivative of the absorption spectrum will be presented. If your spectrum has very good S/N and has some regions where you would like better resolution, a second-derivative presentation may help. However, second derivatives from second harmonic detection are very costly in terms of S/N ratio and so you really do have to have a strong signal  

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Reeder and Rieger6 used ESR spectra to identify complex ions and to estimate formation constants for aqueous oxovanadium(iv) complexes with lactic acid, thiolactic acid, glycolic acid, and thioglycolic acid. Through the use of second-harmonic detection, which produces second-derivative spectra, the resolution was good enough that several of the individual species could be separately... 

Using the trigonometric identity cos2x = (l/2)(l+cos(2x)), one sees the argument for second harmonic detection ... 

The signal-to-noise ratio is further improved if phase-sensitive second-harmonic detection is performed at a fixed reference frequency ct)ref is used— hence the need for a lock-in amplifier (Fig. 11.24). 

Reid, J., and Labrie, D. "Second-Harmonic Detection with Tunable Diode Lasers - Comparison of Experiment and Theory." Applied Physics B 26 (1981) 203-10. 

Once quantum chemistry has provided all the information required, that is, rotational and centrifugal distortion constants and, if the case, hyperfine parameters as well as line intensities, a graphical simulation of the rotational spectrum can be performed. The latter requires the knowledge of the experimental technique involved. For example, if the frequency modulation with second harmonic detection is performed, then the second derivative of the natural spectrum is obtained (as seen in Figure 6.2). The graphical representation of the computed spectrum can then be... 

Reid (1981) Second harmonic detection with tunable diode lasers - comparison of experiment and theory. Applied Physics B 26 203-210. 

To ensure interface-selective detection of the Raman-pumped vibrational coherence, one more incident electric field is required. A fourth-order optical response is thereby generated. The requirement is fulfilled by observing the second harmonic (SH) light generated at the interface, instead of the transmitted fundamental light. 

In addition to the fourth-order response field Tfourth, the probe light generates two SH fields of the same frequency 211, the pump-free SH field Eq(2 Q), and the pump-induced non-modulated SH field non(td> 211). The ground-state population is reduced by the pump irradiation and the SH field is thereby weakened. The latter term non(td, 211) is a virtual electric field to represent the weakened SH field. Time-resolved second harmonic generation (TRSHG) has been applied to observe E on (td, 211) with a picosecond time resolution [20-25]. The fourth-order field interferes with the two SH fields to be detected in a heterodyned form. 

Optical detection offers the most conventional technique to time-resolve the coherent phonons. It includes four-wave mixing [8], transient reflectivity [9,10] and transmission [7] measurements, as well as second harmonic generation (SHG) [15,32]. Coherent nuclear displacement Q induces a change in the optical properties (e.g., reflectivity R) of the crystal through the refractive index n and the susceptibility y,... 

By using a nonlinear optical process such as SHG, one can probe surface phonons and adsorbate-related vibrations exclusively [14,15,32,34]. Time-resolved SHG (TRSHG) detects the second harmonic (SH) of the probe beam as a function of time delay between pump and probe. The SH electric field is driven by the nonlinear polarization Pi 2w) at the surface, which... 

As stated before, the coefficients /, g, and h are linear combinations of the components of the tensors yeee, y eem, and ymee. For a system with Ccc, symmetry (i.e., chiral, isotropic surface symmetry) and -polarized second-harmonic light detected in transmission, these coefficients are given by (For the complete set of equations, see previous sections.)... 

Figure 9.23 s-Polarized second-harmonic signal detected in transmitted direction as function of the azimuthal rotation angle. Twofold pattern clearly indicates C2 symmetry of sample. 

To analyze the second-order susceptibility, the symmetry of the films was first analyzed by measuring the intensities of the second-harmonic light. The sample was irradiated with polarized light from a Nd YAG laser incident at 45°, and the second-harmonic light emanating from the sample was detected while the sample was rotated around its surface normal. No variation in the second-harmonic intensity was observed as the sample was rotated, indicating... 

Mizutani, G., Sonoda, Y, Sano, H., Sakamoto, M., Takahashi, T., and Ushioda, S. 2000. Detection of starch granules in a living plant by optical second harmonic microscopy. J. Luminesc. 87-89 824-26. 

Nonlinear optical spectroscopies such as second harmonic generation (SHG) and sum frequency generation (SFG) are finding increasing use in probing species at interfaces (e.g., Eisenthal, 1996). For example, SHG was used by Donaldson et al. (1995) to detect a surface-bound S02 species, and SFG has been applied to elucidate the structure of dimethyl sulfoxide at liquid interfaces (Allen et al., 1999). These techiques are... 

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