Submillimeter Microwave Spectrum ofH2O18

The rotational spectrum of ${\mathrm{H}}_2$ ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ in the ground vibrational state has been investigated by means of high-resolution microwave spectroscopy. We report the measurement of ten new rotational transitions in the 1.0- to 0.4-mm wavelength region. Among these are several low-$J$ lines which are of significance in the atmospheric absorption of electromagnetic radiation in the microwave region. The frequencies of newly observed transitions are (in MHz); $5_{1,5}$←$4_{2,2}$, 322 465.17; $4_{1,4}$←$3_{2,1}$, 390 607.76; $4_{2,3}$←$3_{3,0}$, 489 054.26; $6_{2,4}$←$7_{1,7}$, 517181.96; $6_{4,3}$←$5_{5,0}$, 520 137.32; $5_{3,3}$←$4_{4,0}$, 537 337.57; $1_{1,0}$←$1_{0,1}$, 547 676.44; $6_{4,2}$←$5_{5,1}$, 554 859.87; $5_{3,2}$←$4_{4,1}$, 692 079.14; and $2_{1,1}$←$2_{0,2}$, 745 320.20. Assignments were based on a weighted analysis of combined microwave and infrared data.