Knowledge Bank

The $\alpha$-bands of ammonia which lie in the region 4000-8300 \AA have long been known in emission from oxyammonia flames and discharges through streaming ammonia. The bands were recently observed in absorption during the flash photolysis of ammonia and hydrazine. Isotopic shifts were observed when $N14$ was replaced by $N15$ and when H was replaced by D. The bands were assigned to $NH2$ for a variety of reasons and this assignment has now been conclusively confirmed by a rotational and vibrational analysis of the bands. The bands are found to be due to a Type C transition between a ground state in which $NH2$ is bent to an excited state in which it is linear. A progression of sixteen bands of the bending frequency in the upper state has been identified, the quantum number $v2\prime$ taking the probable values 5, 6, 7,... 20. The vibrational intervals are found to increase from $\sim 650cm-1$ to $\sim 1000cm-1$ with increasing vibrational quantum number. A vibrational perturbation has been observed in the region $v\prime =13-16$. No bands involving excitation of the N-H stretching vibrations in the upper state have yet been identified. Ground state combination differences are found to repeat in alternate bands, but not in successive bands of the progression. This provides strong confirmation that the upper state is linear since, for a progression of the bending vibration of a linear molecule, the vibrational symmetries alternate between $\Sigma ,\Delta ,\Gamma \dots$ for the even members and $\Pi ,\Phi ,\dots$ for the odd members of the progression. The expected $3:1$ alternation in the intensities of successive rotational lines of various branches has been observed and provides conclusive proof of the presence of a two-fold axis of symmetry. The sign of the intensity alternation is consistent with the assignment of the ground state to an electronic $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$ state as predicted by Mulliken. Rotational constants for the ground state have been determined by the application of Mecke’s Sum Rules are $Aooo\prime \prime =23.62cm-1$ $Booo\prime \prime =12.94cm-1$ $Cooo\prime \prime =8.36cm-1$. From these constants, the following bond lengths and angles may be deduced: {A}^{\prime\prime}{{ooo}}({NH})=1.02{5} \AA {H}-{N}^\backslash-{H}=103^{\circ} Detailed rotational constants for the upper state have not yet been obtained due to large displacements of sub-bands with different values of l. Very roughly these displacements are of the form $gl2$ where $g\sim 30-40$ $cm-1$. The upper state is probably an electronic $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\pi$ state and the displacement of sub-bands with different l values is probably due to interaction between the electronic orbital angular momentum $\Lambda h2\pi$ (where $\Lambda =1$) and the vibrational angular momentum $lh2\pi$ associated with the bending vibration. A small spin splitting has also been observed.