The equilibrium dipole moment of the water molecule has been determined from Stark effect measurements on two H2O, one D2O, and six HDO rotational transitions. The variation of the dipole moment projection operator with rotational state is taken into account and expressions are given for this operator evaluated in the ground vibrational states of the three isotopes. The value obtained for the equilibrium dipole moment is |0μx| = 1.8473 ± 0.0010 D. The effective dipole moments in the principal axis energy representation are |μb (HOH)| = 1.8546 ± 0.0006 D, |μb (DOD)| = 1.8558 ± 0.0021 D and |μb (DOH)| = 1.7318 ± 0.0009 D, |μa (DOH)| = 0.6567 ± 0.0004 D.
Thi s pa pe r fir st di sc usses th e tec hniqu e of mak ing S ta rk meas u re me nt s a t millim ete r wave' le ng th s. Th e de tail s of correcting for res idu a l ove rl a p betwee n th e lin es, th e effec ts of modu la ti on , a nd of th e fi e ld inh o moge ne it y a re di sc usse d. Finall y th e meas ure d fre qu e nc ies a nd th e e mpiri ca l S ta rk coe ffi cie nt s for one H,O, a nd one D,O, an d fiv e HDO lin es be tw ee n 85 and 250 GHz a re give n .Th e fi na l a nalys is of the data to giv e values of th e dipole mome nt wi ll be give n in a noth e r pap er.Key word s: Millimeter wav es; S ta rk e ffec t; wat e r.
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