Line shapes, positions and intensities of water transitions near 1.28 μm

Abstract

We present measurements of approximately 70 isolated, self-broadened, water vapor lines which are assigned to the (1,0,1)–(0,0,0), (0,0,2)–(0,0,0), (1,2,0)–(0,0,0), and (2,0,0)–(0,0,0) vibrational bands and which occur in the transparency window region from 7710cm−1 to 7920cm−1. We acquired absorption spectra on room-temperature, water samples over the pressure range 150–800Pa with the frequency-stabilized cavity ring-down spectroscopy technique. In order to optimize measurement accuracy, we integrated mK-level temperature control and SI-traceable pressure measurements into our cavity ring-down spectroscopy measurements. This technique yielded relative uncertainties of 0.04% and 0.20% in sample density and measured line intensity, respectively. We also referenced our spectrum frequency axes to a Cs clock, which provided vacuum line positions with a combined standard uncertainty of 3MHz. Comparison of our measured intensities, positions and self-broadening parameters with literature values reveals that the present work substantially reduces uncertainty in these line parameters. For the (1,0,1)–(0,0,0) vibrational band, the measured line intensities agree at the 1%-level with published ab initio calculations. Our spectra exhibited signal-to-noise ratios up to ≈20,000:1 to enable stringent tests of theoretical line profiles through multispectrum least-squares data analysis. We show that the partially correlated, quadratic-speed-dependent Nelkin–Ghatak profile gives a quality of fit that is commensurate with the high spectrum signal-to-noise ratio, and unlike most other profiles considered here, reproduces the measured line shapes without systematic residuals over the entire pressure range. Our results confirm that mechanisms of (1) collisional narrowing, (2) speed-dependent effects and (3) partial correlation between velocity-changing and dephasing collisions, contribute to the self-broadened line shape of isolated water vapor transitions.