Vibrational Stark Effects

In EMIRS and SNIFTIRS measurements the "inactive" s-polarlsed radiation is prevented from reaching the detector and the relative intensities of the vibrational bands observed in the spectra from the remaining p-polarised radiation are used to deduce the orientation of adsorbed molecules. It should be pointed out, however, that vibrational coupling to adsorbate/adsorbent charge transfer (11) and also w electrochemically activated Stark effect (7,12,13) can lead to apparent violations of the surface selection rule which can invalidate simple deductions of orientation. 

The ab initio SCF cluster wavefunction has been used to investigate the bonding of CO and CN- on Cu,0 (5,4,1), (5 surface layer, 4 second layer and 1 bottom layer atoms), and to calculate their field dependent vibrational frequency shifts in fields up to 5.2 x 107 V/cm(46). A schematic view of the Cu10 (5,4,l)CO cluster is shown in Figure 8. In order to assess the significance of Lambert s proposal, that the linear Stark effect is the dominant factor in the field dependent frequency shift, the effect of the field was calculated by three methods. One is by a fully variational approach (i.e., the adsorbate is allowed to relax under the influence of the applied field) in which the Hamiltonian for the cluster in a uniform electric field, F, is given by... 

H The Stark effect is the result of altering the charge of the metal surface, which can strengthen or weaken the adsorption of the surface species. The effect can be quantified by the change in vibrational peak against the surface potential (i.e., Stark tuning slopes). 

On the basis of these formulae one can convert measurements of area, which equals the integral in the latter formula, under spectral lines into values of coefficients in a selected radial function for electric dipolar moment for a polar diatomic molecular species. Just such an exercise resulted in the formula for that radial function [129] of HCl in formula 82, combining in this case other data for expectation values (0,7 p(v) 0,7) from measurements of the Stark effect as mentioned above. For applications involving these vibration-rotational matrix elements in emission spectra, the Einstein coefficients for spontaneous emission conform to this relation. 

Fundamental studies of CO binding to platinum surfaces have shown that the vibrational stark effect and binding energetics are related to the nature of surface coordination of CO as terminally bound or bridging [9, 10]. The spectroelectro-chemical behavior of polynuclear platinum... 

Rotational constants obtained for both the ground and the three first excited vibrational states allowed one to derive the equilibrium molecular structures of GeF2 (re = 1.7321 A, 6>e = 97.1480211) and GeCl2 (re = 2.169452 A, <9e = 99.8825°285). From measurements of the Stark effect the dipole moment of GeF2 has been determined to be 2.61 Debye283. The harmonic and anharmonic force constants up to the third order have been obtained for both molecules and reported too283,285. 

The first term of (3.289) represents a translational Stark effect. A molecule with a permanent dipole moment experiences a moving magnetic field as an electric field and hence shows an interaction the term could equally well be interpreted as a Zeeman effect. The second term represents the nuclear rotation and vibration Zeeman interactions we shall deal with this more fully below. The fourth term gives the interaction of the field with the orbital motion of the electrons and its small polarisation correction. The other terms are probably not important but are retained to preserve the gauge invariance of the Hamiltonian. For an ionic species (q 0) we have the additional translational term... 

Meerts and Dymanus [142, 153] extended their studies of the OH and SH radicals by examining the Stark effect and determining the electric dipole moments, but an even more extensive study of the Stark effect for OH and OD in several different vibrational levels was described by Peterson, Fraser and Klemperer [154], The effect of an applied electric field on the hyperfine components of the A-doublets for the. 7 = 3 /2 level of the 2n3/2 state is illustrated in figure 8.47. Measurements were made of the MF = 2, A MF = 0 transition in a calibrated electric field of approximately 700 V cnr1 and the Stark shift from the zero-field line position measured. The observations were made on resonances from 0 = 0, I and 2 for OH, and v = 0 and 1 for OD. 

Volk M, Kholodenko Y, Lu HSM, Gooding EA, DeGrado WF, Hochstrasser RM. Peptide conformational dynamics and vibrational Stark effects following photoinitiated disulfide cleavage. J Phys Chem B 1997 101 8607-8616. 

From accurate measurements of the Stark effect, when electrostatic fields are applied, information about the electron distribution is also obtained. Further information is obtained from nuclear quadrupole coupling effects and Zeeman effects <1974PMH(6)53>. Microwave studies also provide important information regarding molecular force fields, particularly with reference to low-frequency vibrational modes in cyclic structures <1974PMH(6)53>. 

Because of the short lifetime of ions in gaseous atmospheres, even at low pressure, gas-phase IR measurements are limited to adsorption of neutral molecules. Electrochemical applications of the IR method offer the interesting possibility of providing data on the adsorption properties of charged particles (Secs. 8 and 9). In the electrochemical environment the applied potential allows ionic adsorbates to be studied under energetically controllable conditions. Otherwise the electrochemical double layer offers exceptional conditions to investigate the Stark effect on vibrational transitions by setting tunable electric fields of the order of 10 V cm at the interface. This phenomenon will be discussed in Sec. 10. 

Changes in rotational and vibrational transitions of molecules under the influence of strong electric fields are known as the Stark effect. The effect of strong electric fields was found first on atomic spectra and extended by Condon to vibrational transitions in molecules in 1932 [161]. 

In the next sections we discuss the available data on the electrochemical Stark effect on the vibrational spectrum of adsorbed carbon monoxide and adsorbed sulfate ions at platinum. 

In contrast to the Stark effect on the vibrational frequency, the changes of the transition probability with the electric field have been investigated less. For electrochemical systems an expression for the integrated absorption coefficient, B, was proposed by Korzeniewski et al. [164] as follows ... 

It must be stressed that the polarizability gradient da/dQk also appears in the equation for Raman intensities [175], as indicated also by Lambert [176]. Thus, in view of Eq. (25), we can extend the consequences of the static electric field to vibrations which are forbidden by the surface selection rule the high electric field in the double layer can induce a dipole moment component in the direction of the field on permanent dipoles which are parallel to the surface. Thus the effect of orientation due to the electric field is just a manifestation of the Stark effect. 

The electrochemical double layer offers the exceptional possibility of investigating the Stark effect at very high electric fields. Some important progress has been made in the theoretical treatment of the problem. Experimental data of potential effects upon the frequency and/or the intensity of vibrational modes must discriminate between the pure electric field effect and the secondary effect of potential on the coverage and, consequently, on the lateral interactions. 

Lambert DK, Tobin RG (1990) CO on Pt(335) Vibrational Stark effect, mode coupling, and local field effects on a stepped surface. Surf Sci 232 149... 

Integrated i.r. intensity data have been used to calculate bond moments and their derivatives [453] the calculated dipole moment was -0.93 D (3.10 x 10" ° C m), compared with an experimental value (Stark effect upon the microwave spectrum) of -0.95 D (3.17 x 10-30 c m) [1215]. The original vibrational intensity data for COFj [981] has now been corrected [1363], and the CNDO/2 calculations [1827] of the dipole moment derivatives have been revised [289,292,1363,1417] many of the bond moment derivatives (the squares of which are approximately proportional to band intensities) were shown to be transferable [1675]. Bond dipole moments have also been calculated by ab initio methods (with a STO 5-31G basis set) [1941]. In addition, the dipole moment derivatives have been calculated under the MINDO/3 formalisms [1586], but little reliance can be placed on the results obtained. 

Lambert, D.K. (1996) Vibrational Stark effect of adsorbates at electrochemical interfaces. Electrochimica Acta, 41, 623-630. 

Oklejas, V., Sjostrom, C., and Harris, J.M. (2003) Surface-enhanced Raman scattering based vibrational stark effects as a spatial probe of interfacial electric fields in the diffuse double layer. Journal of Physical Chemistry E, 107, 7788-7794. 

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