Submillimeter Microwave Spectrum ofH2O16

Abstract

The absorption spectrum of water vapor, in addition to giving information about the structure and properties of the water molecule, is of practical importance because of the effects of water vapor on the propagation of electromagnetic radiation. However, precise measurements of only 6 ground-vibrational-state transitions of ${\mathrm{H}}_2^{16}$O have been reported in the microwave region. We report the measurement of nine new rotational transitions of ${\mathrm{H}}_2^{16}$O and the remeasurement to higher accuracy of the four previously known submillimeter lines. The frequencies of the newly observed transitions are (in Mc/sec) ${10}_{2,9}$←$9_{3,6}$, 321225.644; $7_{5,3}$←$6_{6,0}$, 437346.667; $6_{4,3}$←$5_{5,0}$, 439150.812; $7_{5,2}$←$6_{6,1}$, 443018.295; $6_{4,2}$←$5_{5,1}$, 470888.947; $5_{3,3}$←$4_{4,0}$, 474689.127; $6_{2,4}$←$7_{1,7}$, 488491.133; $5_{3,2}$←$4_{4,1}$, 620700.807; and $2_{1,1}$←$2_{0,2}$, 752033.227. These transitions have been measured with a submillimeter-wave spectrometer which employs a klystron-driven crystal harmonic generator and a 1.6°K InSb photoconducting detector. With this work, the lines contributing the major portion of the atmospheric absorption of water vapor in the region up to 800 Gc/sec have been precisely measured.