The Stark and Zeeman effects in methyl silane

Abstract

The Stark and Zeeman effects in methyl silane in the ground vibronic state have been studied in detail using the molecular-beam electric-resonance method. For a symmetric rotor without internal rotation, the rotational dependence of the effective dipole moment for matrix elements diagonal in J has been shown by Watson, Takami, and Oka to have the form μ Q = μ 0 + μ J J( J + 1) + μ K K 2. It is shown here that, to this order, a complete characterization of the Stark effect requires only one more parameter, namely, the effective anisotropy ( α | - α ⊥) eff in the polarizability. From Stark measurements alone, the true anisotropy cannot be separated from the additional dipole distortion constant shown by Aliev and Mikhaylov to enter dipole matrix elements off-diagonal in J. By studying nine different transitions ( J, K, m J ) → (J, K, m J ± 1) in CH 3 28SiH 3, values were obtained for the four Stark parameters: μ 0 = 0.7345600(33) D, μ J = 8.83(35) μD, μ K = −32.82(37) μD, and ( α | - α ⊥) eff = 1.99(16) × 10 −24 cm 3. These errors reflect only the internal consistency in the data; the absolute error in μ 0 is 32 μD. The modification of the Stark effect by internal rotation is discussed; it is shown that the only significant effect here is to modify the interpretation of μ 0. The change in μ 0 upon isotopic substitution of 30Si for 28Si was determined: μ 0( 30Si) - μ 0( 28Si) = 67.0(2.0) μD . A study of molecular magnetic effects in CH 3 28SiH 3 has yielded the two molecular g factors, g ⊥ = −0.036391(21) nm and g | = −0.10667(13) nm, as well as the anisotropy in the susceptibility (χ | - χ ⊥) = −44.9(2.3) × 10 −30 J T 2 . The molecular quadrupole moment has been calculated.