Knowledge Bank

Chlorine nitrate plays an important role in upper atmospheric chemistry. It was first detected in the stratosphere by Rinsland $etal.a$ by observation of the $\nu 4$ band a-type Q-branch. There have been a number of laboratory infrared spectroscopic studies on the $\nu 4$ band since that $ime.b$ Among them is the work of Bell, $DuxburyandStuartc$ who used a tunable diode laser to study and model the a-type Q-branches of the two chlorine isotopic species $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">37</mrow>}Cl$ as well as that of the $\nu 4+\nu 9-\nu 9$ hot band. At room temperature, even at the high resolution of a diode laser, the bands are only partially resolved due to the overlapping of lines of the two main isotopic species and of lines from several strong hot bands. A beautifully resolved spectrum of the $\nu 4$ region over a limited range of J and $Ka$ was obtained later by $Xu,BlakeandSharped$ from diode laser spectra of a molecular jet at about 7 K. In this work, 29 spectra of the region 750 to $900cm-1$ at temperatures ranging from 190 to 297 K and air pressures ranging from zero to 156 hPa were recorded at DLR with resolutions of 0.00094 to $0.0083cm-1$. Using the ground-state constants of $M\backslash ""ulleretal.e$ a vib-rotational analysis of the $\nu 4$ and $\nu 3$ fundamental bands as well the hot band, $\nu 4+\nu 9-\nu 9$, of the most abundant $\mathrm{<mrow\; data-mjx-texclass="ORD">35</mrow>}Cl$ isotopomer was carried out using the spectrum recorded at 191 K with the highest resolution and a zero air pressure. The corresponding cold bands for the lesser abundant isotope were partially assigned. Upper state rotational Hamiltonian constants were determined allowing precise modeling of the contours of the $\nu 4$ fundamental Q-branches over the temperature and pressure ranges studied as shown by comparisons with the other spectra.