Before we began on the subject of astrochemistry in item, allow us look at the atomic energy degree of an atom. In this subdivision, the illustration of the H atom will be used because it is the simplest theoretical account of an atom and that 80 % of seeable affair in the Universe is made up of H.
The most common perceptual experience of a H atom is that of a proton at the Centre while as negatron orbits around it. However, based on quantum mechanics, the negatron can non be pictured this manner. It is pictured as a moving ridge map in which the chance of happening an negatron is found by work outing the moving ridge map. Based on this theoretical account, an negatron can be right following to the karyon or at the other terminal of the existence. Besides, unlike classical natural philosophies that the negatron has a scope of energy in which a negatron can travel up or down, quantum mechanics suggests that negatron has discrete energy degrees every bit long as proton-electron separation is finite.
Figure 1.1 Spectra and energy degree of an H atom
Obtained from hypertext transfer protocol: //hyperphysics.phy-astr.gsu.edu/hbase/hyde.htmlAs shown in Figure 1.1, Hydrogen atom has a figure of energy degrees. Electron can travel up or down the energy degrees by absorbing a photon or breathing a photon severally. However, since the difference in energy between the energy degrees are distinct, merely photon of the specific energy can do the negatron to travel up or down the energy degrees. Based on the Planck ‘s equation as shown in figure 1.1, the specific wavelength of the photon emitted or absorbed can be calculated. This phenomenon would be used in designation of elements in the ISM ( interstellar medium ) .
1.2 Energies in molecules
Molecules, like atoms besides posses energy and the entire sum of energy can be calculated by summing the electronic energy, vibrational energy and rotational energy. Like an atom, there are a set of distinct energy degrees for each type of energy. The electronic energy in molecules is similar to the electronic energy in an atom where passage between these degrees can go on if radiation is absorbed or emitted. However, the forces of attractive force between the atoms differ in the different energy degrees as negatron distribution will be altered. In fact, molecules may even interrupt as it moves through the different electronic provinces.
Vibrational energy refers to the energy at which the molecule is vibrating. If the vibrational energy is high, so the molecule can vibrate more and frailty versa. However a molecule will non halt vibrating even when it is at its lowest vibrational province.
Figure 1.2 Vibrational energy degrees
Obtained from: hypertext transfer protocol: //www.medicinescomplete.com/mc/clarke/current/login.htm? uri=http: //www.medicinescomplete.com/mc/clarke/2009/CLK9024F002_1.htm
Rotational energy refers to the energy degree at which a molecule is revolving approximately. Like vibrational energy, rotational energy besides have distinct energy sets where a molecule can pass through among these sets when radiation is absorped or emitted.
1.2 Cooling and Heating
The existence was created by an event known as the Big Bang. After a period of clip, gravitation causes affair to travel towards each other and prostration to organize protogalaxies. As gravitation causes the protogalaxies to fall in more and more, most of this gravitative energy is converted into heat energy. this causes the temperature of the gas in the protogalaxy to increase. Based on Gay-Lussac ‘s Law, as temperature additions, force per unit area besides increases. The force per unit area created would hold retarded the prostration due to gravitation and galaxies will non hold formed. Therefore, a chilling mechanism would hold existed.
The first manner was that two H atoms collided with each other with adequate energy to ionise one of the H atoms. Thus energy was used in ionizing and the motion of the merchandises. When the negatron and the ion recombined once more, the energy was radiated off. Overall, this causes the energy from the gas to be converted to radiation which could non be utilized by any other atom. This consequences in a net chilling.
This type of chilling merely operated at temperatures above 1000 Kelvin. Below which, another mechanism was used. This clip, the H2 molecules are the chief coolants. The H2 would clash with other H atoms or H2 molecules and in the procedure, addition adequate energy to raise the molecule to a higher vibrational or rotational degree. After which, the molecule would drop back to land province and emit energy in the signifier of radiation which can non be utilised by other affair. Hence, a net chilling consequences.
If this is the instance, so temperatures would maintain on dropping in the existence which is non the instance. Heating mechanisms besides exists to maintain temperature at a changeless degree. The beginnings of warming are viz. UV radiation and cosmic beams. They heat up gases by ionising a atom. The negatron will now transport the energy from the UV radiation or cosmic beam, and will reassign the energy to impersonal gases through hits. This will ensue in a net warming.
1.3 Common reaction mechanisms
Figure 1.3 Common reaction mechanisms ( Fraser,2002 ) The reactions shown in figure 1.3 can be classified into 2 types of reactions, impersonal reactions and ion-molecule reactions. Impersonal reactions are non really effectual because the atoms of molecules need to clash with sufficient energy to get the better of their comparatively high activation energy in order for reaction to happen. Besides, since they are impersonal, they must come within influence of each others ‘ negatron cloud in order for any interactions to happen. Even so, they might merely resile off if their symmetricalnesss or energies are non right. On the other manus, for ion-molecule reactions, it is much more effectual. This is because they can happen at a larger distance. The positive ion will polarise the molecule and pull the negatrons towards it. This force of attractive force is big plenty for a bond to organize between the positive ion and one of the atoms form the molecule. If the charge denseness of the positive ion is excessively high, the molecule might be ionised alternatively.
2. Interstellar chemical science
2.1 The physical conditions
The interstellar medium ( ISM ) consist of 4 different medium, viz. , the diffuse interstellar medium, elephantine molecular clouds, circumstellar medium and photon-dominated medium. The diffuse interstellar medium largely consists of empty infinite and the atomic denseness is between 1 to 102 atoms per cm3. Therefore, chemical science in this medium is minimum. In the elephantine molecular clouds, atomic denseness is about 106 cm-3 and a temperature of 10K. Here, chemical science is rich as there are besides dust atoms which will rush up chemical reactions. The circumstellar medium is the part around a star. The part might hold stars which emit high UV radiation that will photodissociate and photoionise most atoms and molecules. It might besides incorporate many dust atoms and dust grain chemical science is diverse if the star has ejected dust during its prostration. The photon-dominated medium is around stars with high UV flux that will disassociate molecules and even ionises the atoms.
2.2 Photochemistry in infinite
Molecules in infinite, particularly those in the photon-dominated parts, are bombarded with photon from UV radiation. They will be excited to a higher energy degree. However, at the higher energy degree, the molecule can be at a larger interatomic distance. When the molecule emits the photons and returns to its land province, the distance at which a bond can organize is smaller. Hence the molecule will fall apart as shown in Figure 2.2.1.
Figure 2.2.1 Photodissociation of H2
( Hartquist & A ; Williams, 1995 )
Photoionisation require a higher energy to happen than photodissociation. Hence, the radiation normally lies in the UV spectrum. This is present around hot stars which have high UV flux of up to 90nm, sufficient to ionize the atoms around the star.
2.3 Dust grain chemical science
Dust grains consists of a Si nucleus surrounded by carbonous atoms which is covered by a bed of ice mantle of volatile organic molecules. Hence, dust grains are where organic chemical science including aminic acerb formation takes topographic point. Compared to gas stage reactions, dust grain surface reaction is much more efficient. This is possible through the extremely self-generated sticking of molecules from the ISM to the surface. This surface assimilation onto the surface may disassociate the molecule into reactive atomic species. The atomic species may so travel on the ice mantle and reacts to organize a new molecule. If the spontaneousness of the new species to lodge to the surface is low, so the new species might be expelled into the ISM, dwelling the ISM with new species. Besides, the energy of the interaction between the molecule and the surface overcomes energy required to interrupt the bonds to organize the passage molecule in gas-phase reactions. Another ground is that radiation may ionize the atoms on the surface, making groups that participate readily in reactions. However, the ice mantle may protect some of the molecules that are closer to the nucleus. Hence, big molecules like amino acid can be in the dust grain. Amino acid wll have a higher opportunity of acquiring photodissociated in the ISM as they have lesser protection against the UV beams.