Photodissociation of NH3 at 106–200 nm

Abstract

Cross sections and quantum yields of emissions from the photofragments of NH3 were measured in the 106–200 nm region using synchrotron radiation as a light source. The emission systems of NH2 (Ã 2A1→X̃ 2B1), NH(c 1Π→a 1Δ, b 1Σ+), and NH(b 1Σ+→X 3Σ−) were observed from the NH3 photodissociation. The dependences of emission intensities on the NH3 and/or He (buffer gas) pressures were studied. The NH(c→a,b) and NH(b→X) emissions were observed when the NH2(Ã2A1) species was quenched by 10 Torr of He. The NH(b→X) and NH(c→a) emissions start to appear at 182.3±1.5 and 127.9±0.9 nm, respectively. From these emission thresholds the threshold energy for photodissociation of NH3 into NH(X 3Σ−)+H2(X 1Σ+) is determined to be 4.17±0.06 eV, and the upper limit for the heat of formation of NH is 3.81±0.06 eV. For excitation wavelengths longer than 128 nm the NH(b 1Σ+) species is produced by a primary photodissociation process, and at shorter excitation wavelengths the NH(b 1Σ+) may be partly produced by a cascading process following the NH(c→b) transition. Contrary to previous theoretical interpretations, all the vibronic levels of the NH3 (B̃ and C̃) states produce the NH2(Ã→X̃) emission. The quantum yield for the C̃ state is lower than that of the B̃ state. The vibronic levels of the C̃ state are renumbered in accordance with their quantum yields.

Ammonia (J,K)=(1,1) to (4,4) and (6,6) inversion lines detected in the Seyfert 2 galaxy NGC 1068

“…It is lower than that of a few ×10 −7 for the central ∼500 pc of the Milky Way (Rodríguez-Fernández et al 2001), but higher than the extremely low value of 5×10 −10 determined for M 82 by Weiß et al (2001). M 82 is a special case, because it contains a starburst in a late stage of its evolution leaving NH 3 , which is a molecule particularly sensitive to UV radiation (e.g., Suto & Lee 1983), in only a few molecular cores that are still sufficiently shielded against the intense radiation field. With the three para-and two ortho-NH 3 lines observed, we can estimate the ortho-to-para abundance ratio to be R ∼ 0.9 (we do not give an error here because of the extrapolated (0,0) column density) as an upper limit because the (0,0) column density might be overestimated by extrapolating from the (3,3) column density, which could be affected by maser activity (e.g., Walmsley & Ungerechts 1983;Ott et al 2005).…”

Dense gas in nearby galaxies

“…This indicates that in contrast to a "late starburst" such as M 82, where the chemistry is affected by photodissociation, the chemistry in NGC 253 is dominated by slow shocks which are able to release molecules from grains without destroying them (see also García-Burillo et al 2001;Usero et al 2004Usero et al , 2006. Ammonia belongs to the same class of molecules, thought to be enriched in C-shock environments and which is easily destroyed by dissociating radiation due to its low dissociation energy (Suto & Lee 1983). Takano et al (2005a) presented VLA maps of the (1, 1), (2, 2), and (3, 3) transitions of NH 3 .…”

The spectroscopy of high Rydberg states of ammonia

“…The features observed in the NH 3 spectrum were too broad to allow determination of any upper state rotational constants or to verify the previously proposed 7 1 EЉ excited state symmetry. However, a band contour simulation of the less predissociation broadened D -X 0 0 0 transition in ND 3 , recorded under both beam and bulk conditions, shows conclusively that the D state in fact has 1 EЈ symmetry, consistent with it being derived from the electronic promotion 3deЉ 39,40 had insufficient resolution to permit any rotational analysis of these features.…”