McCall Benjamin J
Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801, USA.
Philos Trans A Math Phys Eng Sci. 2006 Nov 15;364(1848):2953-62; discussion 2962-3. doi: 10.1098/rsta.2006.1876.
H3+ plays a key role in interstellar chemistry as the initiator of ion-molecule chemistry. The amount of H3+ observed in dense interstellar clouds is consistent with expectations, but the large abundance of H3+ seen in diffuse clouds is not easily explained by simple chemical models. A crucial parameter in predicting the abundance of H3+ in diffuse clouds is the rate constant for dissociative recombination (DR) with electrons. The value of this constant has been very controversial, because different experimental techniques have yielded very different results, perhaps owing to varying degrees of rotational and vibrational excitation of the H3+ ions. If the value of this rate constant under interstellar conditions were much lower than usually assumed, the large H3+ abundance could be easily explained. In an attempt to pin down this crucial rate constant, we have performed DR measurements at the CRYRING ion storage ring in Stockholm, using a supersonic expansion ion source to produce rotationally cold H3+ ions. These measurements suggest that the DR rate constant in diffuse clouds is not much lower than usually assumed and that the abundant H3+ must be due to either a low electron fraction or a high ionization rate.
H₃⁺作为离子 - 分子化学的引发剂，在星际化学中起着关键作用。在致密星际云中观测到的H₃⁺数量与预期相符，但在弥漫云中看到的大量H₃⁺却难以用简单的化学模型解释。预测弥漫云中H₃⁺丰度的一个关键参数是与电子的解离复合（DR）速率常数。这个常数的值一直存在很大争议，因为不同的实验技术得出了非常不同的结果，这可能是由于H₃⁺离子的转动和振动激发程度不同所致。如果星际条件下这个速率常数的值比通常假设的低得多，那么大量的H₃⁺丰度就很容易解释了。为了确定这个关键的速率常数，我们在斯德哥尔摩的CRYRING离子储存环上进行了DR测量，使用超声速膨胀离子源产生转动冷的H₃⁺离子。这些测量表明，弥漫云中的DR速率常数并不比通常假设的低很多，而且大量的H₃⁺必定是由于低电子分数或高电离率所致。
