## 1 Introduction  
Alkanes and other simple molecules based on carbon-hydrogen bonds are the building blocks of complex organic molecules and materials. Functionalization of C-H bonds in these kinds of materials are of great significance for environmental sustainability and economic development [1; 2; 3; 4]. Besides, ultrathin organic films have been studied extensively due to their potential applications in materials science. For instance, they could be used as organic microelectronic and optoelectronic devices [5; 6; 7]. To carry out the applications, the technique of cross-linking between carbon atoms in different organic molecules is crucial [8; 9; 10; 11]. Take [PERSON] et al.'s work as an example, it shows the evidence for cross-links formed between the long-chain molecules on dotriacontane (C\({}_{32}\)H\({}_{66}\)) thin film [12]. The cross-linking of organic materials can be realized through various methods. The most common method is through using chemical additives or catalysts, the so-called wet procedure [13; 14; 15; 16; 17]. However, this procedure may be harmful to microelectronic devices because the chemical additives or catalysts may dramatically change the dielectric constant and other electronic properties, and hence the electronic transport of the devices. To avoid these disadvantages, a more physical method, a so-called dry procedure, was proposed to realize the cross-linking, which is based on collision-induced hydrogen dissociation by using electron beam or other charged particle beam bombardments [18; 19; 20; 21; 22; 23]. This method, however, still has some drawbacks, such as the defects induced by the electrons damaging the original structure. The high energy cost is also a challenge for the charged particle accelerator or control instrument. These factors restrict the application range of this method. Therefore, a suitable method to realize the cross-linking of organic materials should have following characters: (1) avoiding introducing extra dopants or elements; (2) selectively dissociating the C-H bonds, without impact on C-C or other bonds; (3) low system complexity and energy-cost, suitable for large-scale industrial applications. The research of [PERSON] et al. showed that a low-energy proton beam could be used to bombard the organic thin film and realize the cross-linking of organic molecules by selectively cleaving C-H bonds while keep other bonds unaffected [24; 25; 26]. As the proton is a positive charged particle, its application is restricted owing to the electronic insulating behavior of most organic materials. For these materials, a surface charging problem, i.e., the accumulated charge on the surface, will cause the irreversible change in the properties of the organic materials. [PERSON] et al. proposed that H\({}_{2}\) molecules could be used as the incident particles to make the cross-linking [27]. The employment of neutral particle beams provides a new perspective for the fabrication of ultrathin organic films. However, systematic investigations of the role of H or H\({}_{2}\) played in this procedure are limited so far.  
In this article, we investigate the structural and electronic properties of hydrocarbon chains with hydrogen collision [28]. The results are compared with the cases of the incidence of H\({}_{2}\) molecules. To the best of our knowledge, no such research has been reported from ab-initio calculations up to now. In this research, we chose the alkane-type hydrocarbon chain as the organic material because alkanes are a common polymer precursor and widely used as a functional organic material by appending appropriate functional groups [29; 30]. Furthermore, the structure of an alkane is quite simple (C\({}_{\rm n}\)H\({}_{2n+2}\)). Here, we first study the structural and electronic properties of infinitely long hydrocarbon chains with hydrogen dissociations. The cross-linking of two adjacent hydrocarbon chains was then explored and discussed. The case of C\({}_{20}\)H\({}_{42}\), a finitely long molecule, was further investigated to see what happens.