It seems likely that the 3400-Mc/sec transition between the lambda doublets of the lowest rotational state of CH in interstellar space will be soon observed. 1 ' 2 The absorption line intensity for such a transition depends upon the concentration of CH molecules, the state temperature, the transition frequency, and the square of the CH electric dipole moment. 3 We report here an experimental determination of the CH electric dipole moment.The electric dipole moment of CH was determined from simultaneous observation of Stark splittings in the J = i, 2 n i/2 states of CH and OH. The J = i, 2 n i/2 level splits into two components whose separation is, except for small corrections due to the finite lambda doubling, proportional to the applied electric field and the electric dipole moment. The transitions studied here were the P 12 (l) line of CH at 3890.22 A, arising from the transition C 2 S + -X 2 n i/2 , and the R 2 (l) line of OH at 3084.05 A, arising from the transition A 2 Z + *X 2 n i/2 . For these lines, second-order Stark effects in either the upper or lower states shift both first-order Stark components but do not affect the measured separation. 3 Electronically excited molecules were produced in a direct-current discharge tube containing methyl alcohol vapor and helium gas at a total pressure of 1 mm Hg. The discharge tube was constructed to provide electric fields up to 50 000 V/cm in the cathode dark space. 4The CH and OH emission lines were simultaneously observed at a dispersion of 4 cm""Vmm on a stigmatic grating spectrograph. Measured Stark splittings are listed in Table I. Figure 1 is a plot of the observed Stark splittings of CH corrected for lambda doubling against observed Stark splittings for OH similarly corrected. The corrections are small, approximately 2%. From a least-squares determination of the slope of the line through the origin we obtain the ratio of the electric dipole moments

The ultraviolet spectrum of the OH molecule has been obtained in electric fields up to 64 000 volts per cm. Stark line splittings, broadenings, and field-induced parity-forbidden transitions have been observed. The electric dipole moment of OH in its electronic and vibrational ground state (X2Π) has been determined to be (1.727 ± 0.02) Debyes. The dipole moment in the first excited vibrational state has been found to be about 4% lower.

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