High-resolution absorption spectroscopy of room-temperature and jet-cooled ammonia between 59,000 and 93,000 cm−1

We present new high-resolution photoabsorption spectra of ammonia spanning the region between 59,000 cm⁻¹ and 93,000 cm⁻¹ that were recorded by using the Fourier Transform Spectrometer at the Synchrotron SOLEIL. This region extends from just above the Franck-Condon envelope for the $\tilde{A}$ ¹$A_2″$ ← $\tilde{X}$ ¹$A_1\prime$ transition to well above the NH₃⁺ ${\tilde{X}}^{+}$ ²$A_2″$ ionization threshold. The spectra were recorded at a measured resolution of 0.23 cm⁻¹ in both a room-temperature cell (293 K) and in a slit-jet supersonic expansion (∼70 K). The absolute photoabsorption cross section with an uncertainty of ± 5% is also reported for the room-temperature spectrum. The present resolution is a factor of 10 – 100 times higher than in other recently reported broad band spectra of ammonia, and many of the observed bands show partially resolved rotational structure. We have attempted to assign this structure for a number of these bands. The oscillator strengths extracted from the data are in good agreement with previous measurements but, in the case of structured bands, the present higher resolution measurements return higher peak absorption cross sections, that increase further when the sample is cooled. The present higher resolution spectra suggest that a number of previous vibronic band assignments that were based on quantum defect considerations may require some revision. Finally, we discuss the substantial differences between the photoabsorption and photoionization data just above the first ionization threshold.