Fourier transform linewidths measurements in NH3 in the vibrational ground state

Abstract

We have measured linewidths of several pure rotational transitions in the vibrational ground state in the far infrared (80–200 cm−1 ). The experiments were conducted by using a Fourier transform spectrometer and gases (NH3 diluted by H2 or He) contained in a 15 cm absorption cell at room temperature. The aim of our study is to discuss the vibrational effect by comparing our measurements with those done in the infrared, 3 and 6 μm regions (ν3 and ν4 bands) and to find evidence for vibrational dependence. The measured pressure broadening cross sections were typically found to be 26 Å2 for the NH3 –H2 collisions and 16 Å2 for the NH3 –He collisions, by comparing these values with those obtained for the ν4 mode, 35 Å2 for the H2 perturber and 17 Å2 for the He perturber, we see a vibrational effect for the NH3 –H2 collisions but not significant for the NH3 –He ones. Our measurements are in good agreement with the theoretical calculations done by Billing by using the semiclassical coupled states method.