Stark effect transition moments

In the case of strong excitation, in addition to the effects considered in Section 5.4, under the action of the dynamic Stark effect, transition from alignment to orientation may also take place [36, 243], which means that polarization moments of odd rank may emerge. This manifests itself in the second order of the expansion. Thus, we have... 

From a study of the microwave spectrum of 2-methylselenophene, the second-order Stark effect in the ground state was determined.11 The technique used was double radiofrequency-microwave resonance. For the identification by the double resonance method transitions of chiefly the A-state were chosen. From these observations the components of the dipole moment of 2-methylselenophene and the total dipole moment were determined. 

Aliphatic ketones show broad, low-intensity absorption maxima in the vicinity of 280 nm which are a result of n - n transitions. By use of the Stark effect, Freeman and Klemperer estimated that the dipole moment in the n, n singlet state is reduced to 1.48 D from its ground state value of 2.34 D [253]. 

Sheridan and co-workers144 took the microwave spectra of oxadiazole after its synthesis in 1962.39 From an analysis of Stark effects for a number of transitions, they concluded that the dipole moment should be 1.2 0.3 D (Table II). Davies and Mackrodt139 calculated the dipole moment to be 1.34 D, within the experimental error of Sheridan s value, by the CNDO/2 method of Pople and Segal. Other calculations133,136,138 indicate that the dipole moment of 1,2,4-oxadiazole should be notably less than that of the 1,2,5- and 1,3,4-isomers. Direct measurements of dipole moments by Milone145 had portended this much earlier. Besides the ones given in Table II, Milone had also found the same range of dipole moments for 3-methyl-5-phenyl, 5-methyl-3-phenyl, and other derivatives of the three sets of isomeric oxadiazoles. 

When a) l/n3, the field required for ionization is E = 1/9n4, and as a> approaches l/n3 it falls to E=0.04n. These observations can be explained qualitatively in the following way. At low n, so that a> 1/n3, the microwave field induces transitions between the Stark states of the same n and m by means of the second order Stark effect. With only a first order Stark shift a state always has the same dipole moment and wavefunction, as indicated by the constant slope dW/d of the energy level curve. Thus when the field reverses, — — , the Rydberg electron s orbit does not change. With a second order Stark shift as well, the slope dW/d is not the same at E and —E, and as a result the dipole moment and wavefunction are not the same. If the field is reversed suddenly a single Stark state in the field E is projected onto several Stark states of the same n and m when E — - E. Since all the Stark states of the same n make transitions among themselves they ionize once the field is adequate to ionize one of them, the red one, at E = 1/9n4 for m n. 

Pochan, Baldwin and Flygare have analyzed the microwave spectra of cyclopropanone and the isotopic isomers 13Ci, 13C2, and 2,2-dideutero-cyclopropanone.63) The rotational transitions were determined by studying the Stark effect (the shifts and splittings of lines produced by an electric field). The type of transition observed for cyclopropanone was consistent with C v symmetry and the sum of the moments of inertia (/a + /b — Ic) suggested that all four protons are out-of-plane. These data eliminate such structural alternatives as the dipolar oxyallyl tautomer 82 and allene oxide 83. The electric dipole moment (fi ) was calculated to be 2.67 0.10 D, which corresponds to an average of those of acetone (2.93 D) 65> and formaldehyde (2.34D).6 )... 

The principle behind this investigation is electrochromism or Stark-effect spectroscopy. The electronic transition energy of the adsorbed chromophore is perturbed by the electric field at the electric double layer. This is due to interactions of the molecular dipole moment, in the ground and excited states, with the interfacial electric field induced by the applied potential. The change in transition frequency Av, is related to the change in the interfacial electric field, AE, according to the following ... 

The dipole moments can be obtained from the Stark effect of the rotational transitions. The mean value was p = 5.612 0.01 D, an enhancement of 0.80 D over the vector sum of the monomer values. 

Diagonalisation of the Stark matrices enables us to plot the Stark energies, given values of B and /M), and the results are shown in figure 8.27 for the first three rotational levels, J = 0, 1 and 2. The parameter X is defined by A.2 = iJ E jB. In figure 8.28 we show plots of the effective electric moment of the molecule in the different J, M states listed in figure 8.27. With the aid of both diagrams, we are able to understand the principles of electric state selection, and the electric resonance transitions. 

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. 

We must now say more about the nature of the resonance transitions, and also describe additional measurements of the Stark effect which enable the electric dipole moment of the molecule to be determined. In both SO and NF the transitions detected are actually electric-dipole allowed, so perhaps the spectrum ought not to be described as a magnetic resonance spectrum. 

The molecules C4H480Se, C4a-D2H280Se, and C4D480Se were measured for the Stark effect of their l01-20i transitions, to give electric dipole moments of 0.368 + 0.005, 0.392 + 0.005, and... 

The ANI permanent dipole moment has not yet been well established. Earlier dielectric constant measurements (at a temperature of 459 K in the gas phase) provided a value of = 1.53 D62 (in units of Debye, ID = 3.33564 x 10 3° Cm). An earlier study using the Stark effect determination from the rotational spectroscopy reported a smaller value, /r = 1.15 0.02 D for C6H5NH2 and /r = 1.13 0.02 D for C6H5ND238. However, an independent MW study evaluated the p., component at 1.07 D and did not detect any h-and c-type transitions in the MW spectra37. More recent MW studies43 using also Stark effects in gas-phase electronic spectra pointed out that the values determined from MW spectra are actually the a-axis projection of the dipole moment. Therefore, the in-plane component of the dipole moment can be established as p.,(,Y0) = 1.13 D. 

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