THEORETICAL AND EXPERIMENTAL ASPECTS OF THE ROTATIONAL SPECTROSCOPY OF SMALL MOLECULES

Abstract

Theoretical and experimental aspects of high resolution spectroscopy are discussed. The theoretical work concentrates on the matrix elements of the rigid rotor and molecular hyperfine terms of the Hamiltonian of a rotating molecule. The experimental work consists of Stark cell and beam maser microwave spectroscopy on C1CN and HDO respectively. The theory of rotational spectroscopy is developed in a uniform manner based on the irreducible tensor method. It is shown how the effect of molecular symmetry may be incorporated into this method. The instrumentation required to observe the hyperfine splittings is discussed. This includes descriptions of the Stark cell and molecular beam maser microwave spectrometers. A new tunable C-band resonant cavity is described. The following molecular constants are reported for both common species of C1CN: rotational constant Bₒ, centrifugal distortion constant D(J), quadrupole coupling constant eqQ(k), and spin-rotation constant C(x). Values in MHz for C1CN are as follows: ³⁵C1CN; Bₒ = 5 907.820 ± 0.010, eqQ(C1) = -83.26 ± 0.06, eqQ(N) = -3.59 ± 0.08, C(C1) = -0.006 ± 0.006, C(N) = 0.013 ± 0.010; ³⁷C1CN; Bₒ = 5 847.246 ± 0.008, eqQ(C1) = -65.61 ± 0.06, eqQ(N) = -3.61 ± 0.08, C(C1) = -0.004 ± 0.006, C(N) = 0.010 ± 0.010. The following molecular constants for HDO are reported: quadrupole coupling tensor elements eqQ(D)(gg) and spin-rotation tensor elements C(x)(gg). The values in KHz are as follows: eqQ(D)(aa) = 276.45 ± .88, eqQ(D)(bb) = -110.97 ± 1.46, eqQ(D)(cc) = -165.77 ± 1.10, C(D)(aa) = 1.33 ± .20, C(D)(bb) = -4.38 ± .36, C(D)(cc) = -2.99 ± .24, C(H)(aa) = -58.42 ± .47, C(H)(bb) = -5.46 ± .83, C(H)(cc) = -23.28 ± .10. The values for the constants for C1CN agree with previous values but are more precise. Those for HDO do not agree with previous values but are thought to be more accurate since they are obtained for a larger data base.