Tuning rates

Remarkable tuning of reaction rates has been achieved for the isomerization of several dye molecules in supercritical fluid solvents using both small pressure changes and small additions of cosolvents. Rates of the thermal cis-trans relaxation were measured spectroscopically following irradiation for three dyes in supercritical carbon dioxide and ethane, pure and with several polar and protic cosolvents. These results demonstrate the versatility of supercritical fluid solvents, both to examine reaction mechanisms and as a means to tune rates (DiUow et al., 1998). 

Tsang, W.T, N.A. Olsson and R.A. Logan High-Speed Direct Single-Frequency Modulation with Large Tuning Rate and Frequency Excursion in Cleaved-Coupled-Cavity Semiconductor Lasers. Applied Physics Letters 42(8), 650-652 (April 15. 1983). 

Figure 6. Infrared COl stretching frequency and Stark tuning rate as a function of particle size, determined from SNFTIR spectra of CO species resulting from the adsorption of methanol from 0.5 M CH3OH in 0.1 M HCIO4 solution. ...

On Pd, the tuning rates for adsorbed CN are 12cm Vv and 20cm Vv for the linear CN and bridge-bonded CN respectively. The difference cannot be explained on the basis of a pure Stark effect but on the differences in backdonation between linear and bridged adsorbed CN [124]. The tuning rates and the band eenter frequencies on different metals are summarized in Table 3. 

Table 3. Compilation of the frequency range of the band center frequency and tuning rate for adsorbed CN on different metals.

Potential-dependent frequencies in spectra of adsorbates in electrochemical interfaces are commonly observed. Thus so-called Stark tuning rates, Qv/QE, of 30cm have been reported for adsorbed CO on platinum [65, 111] (Fig.59) and adsorbed CN on silver [109, 111, 162]. Even higher values were found for sulfate species adsorbed on polycrystalline platinum (100 cm V [36, 38]) or on single crystal Pt(lll) [141, 143] (120cm V ). In some cases, as for adsorbed tetra-cyanoethylene [163] and anthracene [164], vibrational features which are forbidden by the surface selection rule become active under the influence of the electric field at the interface. 

To a good approximation the Stark tuning rate is given by... 

Applying the model to adsorbed CO, Lambert has evaluated the coefficients a from experimental data. Thus 020 was taken from the unperturbed frequency (vq), 03 0 from gas-phase values of a Morse potential, o, j from the IR cross-section of adsorbed CO and 021 from the first overtone observed by EELS. Equation (18) has been applied successfully to CO/Ni(lll) and CO/Pt, both in the gas phase and at electrodes, reproducing the observed Stark tuning rate with a good accuracy. 

It is very interesting that the Stark tuning rate is coverage-dependent for adsorbed CO, in electrochemical media [55-57] as well as in vacuum [172]. It is found that at lower degrees of coverage the Stark tuning rate increases significantly. This depen-... 

Alabugin, I. V., Manoharan, M., Kovalenko, S. V. Tuning Rate of the Bergman Cyclization of Benzannelated Enediynes with Ortho... 

The main effect of charging a cluster is to alter its HOMO and LUMO levels that correctly reflect the fact that the electrode potentials change the Fermi level of the electrode to control the electrochemistry of the system. The observed large tuning rate of the Pt-H frequency should be... 

Hzf and Hfield are respectively the zero-field and field-dependent parts of H and Mfii is the tuning rate for the energy of basis function iM) in the field F. The Hamiltonian matrix for this A J = 0 two-level problem is... 

Rather than measuring the widths and locations of anticrossings, another kind of useful information is obtained by direct measurement of the tuning rate of each eigenstate in the applied field,... 

The important point to notice is that, since the sign of the mixing coefficient C21 is determined by the signs of Ef( — E and H12, the field tuning rate, gj,... 

Under UHV, the proportionality factor between the vibrational frequencies and the local electric field is termed the Stark-Tuning Rate (STR), 6ve, and has units cm . (V.cm" ) . Note that this factor is also the one which is detamined by quantum chemical calculations. In solution, the accessible quantity is the Stark-Tuning Slope (STS), dvf, expressed in cm. V . We shall extract the STR by multiplying this latter quantity by the thickness k of the double layer [34] ... 

Table 1 Experimental Stark-Tuning Slopes (in cnr ,V ) for CO adsorbed on various metallic surfaces dipped into various solvents. The theoretical values are those obtained in this work, using the presently calculated Stark-Tuning Rate along with a thickness d (in A) for the EDL calculated via the Gouy-Chapmann model.

Stark-Tuning Rates and Stark-Tuning Slopes for CO on Pd(lOO)... 

Tables Computed Stark Tuning Rates (in cm. (V.cm y O for different Pd CO clusters, for both the Pd-C and C-0 stretching modes.

Fig. 6a. The heterodyne signal is mixed with a local oscillator set to the heterodyne center frequency. The mixer output is then amplified in order to supply a feedback current that is proportional to the phase shift ofthe heterodyne signal. Note that because the tuning rate of the external cavity laser was 25 MHz/mA, the peak-to-peak variation in the feedback current over several seconds was less than 5 yA. Figure 6b shows the heterodyne signal under phase-locked conditions. The observed signal indicates that the phase-locked heterodyne linewidth is less than the 15 Hz resolution halfwidth of the spectrum analyzer.

LiNb03 by different pump wavelengths. Angle tuning has the advantage of faster tuning rates than in the case of temperature tuning. 

Because the vibrations that underlie IR absorption spectra must affect the electric dipole of a molecule, we would expect the frequencies of these modes to be sensitive to local electric fields, and this is indeed the case. Shifts in vibration frequencies caused by external electric fields can be measured in essentially the same manner as electronic Stark shifts, by recording oscillations of the IR transmission in the presence of oscillating fields. The Stark tuning rate is defined as S = dv/dE , where v is the wavenumber of the mode and is the projection of the field (E) on the normal coordinate [88, 89]. To a first approximation, is given by —u Aft + E Aa)/hc, where ti is a unit vector parallel to the normal coordinate, Afi is the difference between the molecule s dipole moments in the excited and groxmd states, and Aa is the difference between the polarizability tensors in the two states (Sect. 4.13, Box 4.15 and Box 12.1). However, anharmonicity and geometrical distortions caused by the field also can contribute to vibrational Stark effects [90, 91]. 

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