CARS polarization

CARS spectroscopy utilizes three incident fields including a pump field (coi), a Stokes field (CO2 C02nonlinear polarization at cOcars = 2c0i — CO2. When coi — CO2 coincides with one of the molecular-vibration frequencies of a given sample, the anti-Stokes Raman signal is resonantly generated [22, 23]. We induce the CARS polarization by the tip-enhanced field at the metallic tip end of the nanometric scale. 

In our tip-enhanced near-field CARS microscopy, two mode-locked pulsed lasers (pulse duration 5ps, spectral width 4cm ) were used for excitation of CARS polarization [21]. The sample was a DNA network nanostructure of poly(dA-dT)-poly(dA-dT) [24]. The frequency difference of the two excitation lasers (cOi — CO2) was set at 1337 cm, corresponding to the ring stretching mode of diazole. After the on-resonant imaging, CO2 was changed such that the frequency difference corresponded to none of the Raman-active vibration of the sample ( off-resonant ). The CARS images at the on- and off- resonant frequencies are illustrated in Figure 2.8a and b, respectively. 

A remedy obviously should be available using polarization tricks. In conventional Raman spectroscopy, the isotropic and anisotropic components are deduced from linear combinations of the polarized and depolarized spectra, while a nonresonant part is not clearly recognized (41). In frequency-domain CARS it is known how to suppress the nonresonant contribution and solely measure resonant scattering (isotropic plus anisotropic part) (42). In time-domain CARS, polarization interference can do an even better job with three magic cases (derived in Refs. 35,39). These authors derived explicit expressions for the coupling factors F in Equations (2)-(4) ... 

Clip acts in phase (the same Fourier component) with the first action of cii to produce a polarization that is anti-Stokes shifted from oi (see fV (E) and IFj (F) of figure B 1.3.2(b)). For the case of CSRS the third field action has frequency CO2 and acts in phase with the earlier action of CO2 (W (C) and IFj (D) of figure Bl.3.2 (b). Unlike the Class I spectroscopies, no fields in CARS or CSRS (or any homodyne detected Class II spectroscopies) are in quadrature at the polarization level. Since homodyne detected CRS is governed by the modulus square of hs lineshape is not a synmretric lineshape like those in the Class I... 

A RIKES experunent is essentially identical to that of CW CARS, except the probe laser need not be tunable. The probe beam is linearly polarized at 0° (—>), while the polarization of the tunable pump beam is controlled by a linear polarizer and a quarter waveplate. The pump and probe beams, whose frequency difference must match the Raman frequency, are overlapped in the sample (just as in CARS). The strong pump beam propagating tlirough a nonlinear medium induces an anisotropic change in the refractive mdices seen by tlie weaker probe wave, which alters the polarization of a probe beam [96]. The signal field is polarized orthogonally to the probe laser and any altered polarization may be detected as an increase in intensity transmitted tlirough a crossed polarizer. When the pump beam is Imearly polarized at 45° y), contributions... 

Plenary 10. Hiro-o Hamaguchi, e-mail address lilrama ,chem.s.u-tokvo.ac.ip (time and polarization resolved multiplex 2D-CARS). Two-dimensional (tune and frequency) CARS using broadband dye source and streak camera timing. Studies dynamic behaviour of excited (pumped) electronic states. Follows energy flow within excited molecules. Polarization control of phase of signal (NR background suppression). 

Toleutaev B N, Tahara T and Hamaguchi H 1994 Broadband (1000 cm multiplex CARS spectroscopy application to polarization sensitive and time-resolved measurements Appl. Phys. 59 369-75... 

The reverse contrast, i.e. bright characters on dark, can be achieved by orientating one polarizer parallel to tlie mbbing direction and tlie otlier one periiendicular to it, so tliat tlie device is dark in tlie off state. Backlit versions of tliis device are used in car dashboard displays [111]. 

There is a pronounced tendency for boron to become bonded to the less substituted car bon of the double bond Thus the hydrogen atoms of diborane add to C 2 of 1 decene and boron to C 1 This is believed to be mainly a steric effect but the regioselectivity of addition does correspond to Markovmkov s rule m the sense that hydrogen is the neg atively polarized atom m a B—H bond and boron the positively polarized one... 

H3P—CH2 where it can be seen that the electron distribution is highly polarized m the direction that makes carbon electron rich The carbon has much of the character of a car banion and can act as a nucleophile toward C=0... 

An example of the contribution of polar interactions between an acrylic PSA and a substrate is shown in Fig. 6. By copolymerizing iso-octylacrylate and acrylic acid, using a monomer ratio of, respectively, 95/5 and 90/10, two otherwise identical PSAs were made. The PSAs were laminated to both sides of a foam core to make an attachment tape as used in the automotive industry for the application of body side moldings to a car. One side of the foam tape was laminated against an aluminum foil backing. The other side was laminated against an automotive paint-coated panel to make the final test sample. The test sample was allowed to... 

Recall that the car bon atom of car bon dioxide bear s a partial positive charge because of the electron-attracting power of its attached oxygens. When hydroxide ion (the Lewis base) bonds to this positively polar ized car bon, a pair- of electrons in the car bon-oxygen double bond leaves carbon to become an unshared parr of oxygen. 

Neither bromine nor ethylene is a polar molecule, but both are polarizable, and an induced-dipole/induced-dipole force causes them to be mutually attracted to each other. This induced-dipole/induced-dipole attraction sets the stage for Br2 to act as an electrophile. Electrons flow from the tt system of ethylene to Bi, causing the weak bromine-bromine bond to break. By analogy to the customary mechanisms for electrophilic addition, we might represent this as the formation of a car bocation in a birnolecular- elementary step. 

Turning now to electrophilic aromatic substitution in (trifluoromethyl)benzene, we consider the electronic properties of a trilluorornethyl group. Because of their high electronegativity the three fluorine atoms polarize the electron distribution in their- a bonds to car bon, so that car bon bear s a partial positive charge. 

With electrons flowing from ethylene to zirconium, the Zr—CH3 bond weakens, the carbons of ethylene become positively polarized, and the methyl group migrates from zirconium to one of the carbons of ethylene. Cleavage of the Zr—CH3 bond is accompanied by formation of a a bond between zirconium and one of the car bons of ethylene in Step 3. The product of this step is a chain-extended form of the active catalyst, ready to accept another ethylene ligand and repeat the chain extending steps. 

Section 17.2 The carbonyl carbon is 5/r -hybridized, and it and the atoms attached to it are coplanarc Aldehydes and ketones are polar- molecules. Nucleophiles attack C=0 at car bon (positively polarized) and electrophiles, especially protons, attack oxygen (negatively polarized). 

You have already had considerable experience with caibanionic compounds and their- applications in synthetic organic chemistry. The first was acetylide ion in Chapter 9, followed in Chapter 14 by organometallic compounds—Grignaid reagents, for exanple—that act as sources of negatively polarized car bon. In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic, and that this property can be used to advantage as a method for carbon-carbon bond formation. 

This suggests that either the P-CAR +, which is more polar than the parent p-CAR, can efficiently reorientate so as to interact with the vitamin C in the aqueous phase, or that the ascorbic... 

---