Effects of Magnetic and Electric Fields on Perturbations

Molecular rotational levels can be split into M-components and shifted in static electric and magnetic fields. Stark and Zeeman effects have been very useful in revealing otherwise unobservable perturbations and in diagnosing the electronic symmetry of previously unknown perturbers. Long before the phenomena of perturbations and the Zeeman effect were understood, there were several studies of the effect of a magnetic field on perturbed lines (Fortrat, 1913 Bachem, 1920). 

Between 1929 and 1935, several perturbations in the CO A1 and N2 B2E+ states were studied in magnetic fields up to 36 kG (Crawford, 1929, 1934 Watson, 1932 Parker, 1933 Schmid and Gero, 1935). Lines were observed to split, to broaden symmetrically and asymmetrically, to gain or lose intensity each line was a special case. Spectral resolution and sensitivity were seldom adequate to resolve individual M-components. Although many qualitative features were satisfactorily explained, several perturbing states were conclusively but incorrectly assigned, and no quantitative theory of the effect of an external field on perturbed line positions, shapes, and intensities emerged. 

Radford (1961, 1962) and Radford and Broida (1962) presented a complete theory of the Zeeman effect for diatomic molecules that included perturbation effects. This led to a series of detailed investigations of the CN B2E+ (v — 0) A2II (v = 10) perturbation in which many of the techniques of modern high-resolution molecular spectroscopy and analysis were first demonstrated anticrossing spectroscopy (Radford and Broida, 1962, 1963), microwave optical double resonance (Evenson, et at, 1964), excited-state hyperfine structure with perturbations (Radford, 1964), effect of perturbations on radiative lifetimes and on inter-electronic-state collisional energy transfer (Radford and Broida, 1963). A similarly complete treatment of the effect of a magnetic field on the CO a,3E+ A1 perturbation complex is reported by Sykora and Vidal (1998). The AS = 0 selection rule for the Zeeman Hamiltonian leads to important differences between the CN B2E+ A2II and CO a/3E+ A1 perturbation plus Zeeman examples, primarily in the absence in the latter case of interference effects between the Zeeman and intramolecular perturbation terms. 

Moehlmann, et al., (1972) observed both magnetic and electric field induced perturbations in HCP and discussed the theory of such perturbations. 

A crucial feature of the CN system is that, when CN molecules are formed in a flame of 1 Torr active nitrogen plus CH2CI2, the A2II state is populated 

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