Formation of CO, CH 4 , H 2 CO and CH 3 CHO through the H 2 CCO + H surface reaction under interstellar conditions.

The reaction of ketene (H 2 CCO) with hydrogen atoms has been studied under interstellar conditions through two different experimental methods, occurring on the surface and in the bulk of H 2 CCO ice. We show that ketene interaction with H-atoms at 10 K leads mainly to four reaction products, carbon monoxide (CO), methane (CH 4 ), formaldehyde (H 2 CO) and acetaldehyde (CH 3 CHO). A part of these results shows a chemical link between a simple organic molecule such as H 2 CCO and a complex one such as CH 3 CHO, through H-addition reactions taking place in dense molecular clouds. The H-addition processes are very often proposed by astrophysical models as mechanisms for the formation of complex organic molecules based on the abundance of species already detected in the interstellar medium. However, the present study shows that the hydrogenation of ketene under non-energetic conditions may also lead efficiently to fragmentation processes and the formation of small species such as CO, CH 4 and H 2 CO, without supplying external energy such as UV photons or high energy particles. Such fragmentation pathways should be included in the astrophysical modeling of H 2 CCO + H in the molecular clouds of the interstellar medium. To support these results, theoretical calculations have explicitly showed that, under our experimental conditions, H-atom interactions with the CC bond of ketene lead mainly to CH 3 CHO, CH 4 and CO. By investigating the formation and reactivity of the reaction intermediate H 3 C-CO radical, our calculations demonstrate that the H 3 C-CO + H reaction evolves through two barrierless pathways to form either CH 3 CHO or CH 4 and CO fragments.