Infrared emission spectroscopy of NH: Comparison of a cryogenic echelle spectrograph with a Fourier transform spectrometer

The high-resolution emission spectrum of NH has been observed in the near infrared using a Fourier transform spectrometer (FTS) and a cryogenic echelle spectrograph (called Phoenix) at the National Solar Observatory at Kitt Peak. By using a large format InSb array detector, the newly constructed Phoenix is calculated to offer a large increase in sensitivity over a Fourier transform spectrometer for measurements near 5 μm (2000 cm⁻¹). In order to test the performance of Phoenix, we recorded vibration–rotation emission spectra of the free-radical NH. The infrared bands of NH were produced in a microwave discharge of a mixture of $NH3$ and He. The rotational structure of five bands, 1–0, 2–1, 3–2, 4–3, and 5–4 in the 2200–3500 cm⁻¹ region has also been measured using two FTS spectra. An analysis of these bands combined with the previous electronic, vibration–rotation, and pure rotation measurements provides improved molecular constants for the ground electronic state. In particular, we have extended the range of measured J values so that the new constants are suitable for predicting line positions in high-temperature sources such as stellar atmospheres and flames. A comparison of the Phoenix spectra with the FTS spectra confirms the higher sensitivity of the Phoenix spectrometer. The relative advantages and disadvantages of instruments like Phoenix are discussed. Although designed for astronomical work, cryogenic echelle spectrographs have applications in the ultrasensitive detection of molecules in chemical physics.