期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2021
卷号：118
期号：52
DOI：10.1073/pnas.2113315118
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
It has long been observed that the coma of a comet is often green while its tail is not. While the explanation for this must be that the molecules responsible for the green emission, C
2, are photodissociated, the mechanism was, until now, unknown. We have observed the photodissociation of C
2 in the laboratory for the first time and, in doing so, have determined its bond dissociation energy with unprecedented precision. Invoking the observed mechanism, the calculated lifetime of cometary C
2 is found to be consistent with astronomical observations.
The dicarbon molecule (C
2) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C
2. From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ
·
mol
−
1
is determined, with an uncertainty comparable to its more experimentally accessible N
2 and O
2 counterparts. The value is within 0.03 kJ
·
mol
−1 of high-level quantum theory. This work shows that, to break the quadruple bond of C
2 using sunlight, the molecule must absorb two photons and undergo two “forbidden” transitions.
