Before we began on the topic of astrochemistry in detail, let us look at the atomic energy level of an atom. In this section, the example of the hydrogen atom will be used because it is the simplest model of an atom and that 80% of visible matter in the Universe is made up of hydrogen.
The most common perception of a hydrogen atom is that of a proton at the centre while as electron orbits around it. However, based on quantum mechanics, the electron cannot be pictured this way. It is pictured as a wave function in which the probability of finding an electron is found by solving the wave function. Based on this model, an electron can be right next to the nucleus or at the other end of the universe. Also, unlike classical physics that the electron has a range of energy in which a electron can move up or down, quantum mechanics suggests that electron has discrete energy levels as long as proton-electron separation is finite.
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Figure 1.1 Spectra and energy level of an hydrogen atom
Obtained from http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.htmlAs shown in Figure 1.1, Hydrogen atom has a number of energy levels. Electron can move up or down the energy levels by absorbing a photon or emitting a photon respectively. However, since the difference in energy between the energy levels are discrete, only photon of the specific energy can cause the electron to move up or down the energy levels. 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 identification of elements in the ISM (interstellar medium).
1.2 Energies in molecules
Molecules, like atoms also posses energy and the total amount of energy can be calculated by summing the electronic energy, vibrational energy and rotational energy. Like an atom, there are a set of discrete energy levels for each type of energy. The electronic energy in molecules is similar to the electronic energy in an atom where transition between these levels can happen if radiation is absorbed or emitted. However, the forces of attraction between the atoms differ in the different energy levels as electron distribution will be altered. In fact, molecules may even break as it moves through the different electronic states.
Vibrational energy refers to the energy at which the molecule is vibrating. If the vibrational energy is high, then the molecule can vibrate more and vice versa. However a molecule will not stop vibrating even when it is at its lowest vibrational state.
Figure 1.2 Vibrational energy levels
Obtained from: https://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 level at which a molecule is rotating about. Like vibrational energy, rotational energy also have discrete energy bands where a molecule can transit among these bands when radiation is absorped or emitted.
1.2 Cooling and Heating
The universe was created by an event known as the Big Bang. After a period of time, gravity causes matter to move towards each other and collapse to form protogalaxies. As gravity causes the protogalaxies to collapse more and more, most of this gravitational 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 increases, pressure also increases. The pressure created would have retarded the collapse due to gravity and galaxies will not have formed. Thus, a cooling mechanism would have existed.
The first way was that two H atoms collided with each other with enough energy to ionize one of the H atoms. Thus energy was used in ionizing and the movement of the products. When the electron and the ion recombined again, the energy was radiated away. Overall, this causes the energy from the gas to be converted to radiation which could not be utilized by any other atom. This results in a net cooling.
This type of cooling only operated at temperatures above 1000 Kelvin. Below which, another mechanism was used. This time, the H2 molecules are the main coolants. The H2 would collide with other H atoms or H2 molecules and in the process, gain enough energy to raise the molecule to a higher vibrational or rotational level. After which, the molecule would drop back to ground state and emit energy in the form of radiation which cannot be utilised by other matter. Hence, a net cooling results.
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If this is the case, then temperatures would keep on dropping in the universe which is not the case. Heating mechanisms also exists to keep temperature at a constant level. The sources of heating are namely UV radiation and cosmic rays. They heat up gases by ionizing a atom. The electron will now carry the energy from the UV radiation or cosmic ray, and will transfer the energy to neutral gases through collisions. This will result in a net heating.
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, neutral reactions and ion-molecule reactions. Neutral reactions are not very effective because the atoms of molecules need to collide with sufficient energy to overcome their relatively high activation energy in order for reaction to occur. Also, since they are neutral, they must come within influence of each others' electron cloud in order for any interactions to occur. Even then, they might just bounce away if their symmetries or energies are not right. On the other hand, for ion-molecule reactions, it is much more effective. This is because they can occur at a larger distance. The positive ion will polarize the molecule and attract the electrons towards it. This force of attraction is large enough for a bond to form between the positive ion and one of the atoms form the molecule. If the charge density of the positive ion is too high, the molecule might be ionised instead.
2. Interstellar chemistry
2.1 The physical conditions
The interstellar medium(ISM) consist of 4 different medium, namely, the diffuse interstellar medium, giant molecular clouds, circumstellar medium and photon-dominated medium. The diffuse interstellar medium mostly consists of empty space and the atomic density is between 1 to 102 atoms per cm3. Therefore, chemistry in this medium is minimal. In the giant molecular clouds, atomic density is about 106 cm-3 and a temperature of 10K. Here, chemistry is rich as there are also dust particles which will speed up chemical reactions. The circumstellar medium is the region around a star. The region might have stars which emit high UV radiation that will photodissociate and photoionise most atoms and molecules. It might also contain many dust particles and dust grain chemistry is diverse if the star has ejected dust during its collapse. The photon-dominated medium is around stars with high UV flux that will dissociate molecules and even ionises the atoms.
2.2 Photochemistry in space
Molecules in space, especially those in the photon-dominated regions, are bombarded with photon from UV radiation. They will be excited to a higher energy level. However, at the higher energy level, the molecule can exist at a larger interatomic distance. When the molecule emits the photons and returns to its ground state, the distance at which a bond can form is smaller. Hence the molecule will fall apart as shown in Figure 2.2.1.
Figure 2.2.1 Photodissociation of H2
(Hartquist &Williams, 1995)
Photoionisation require a higher energy to occur than photodissociation. Hence, the radiation usually lies in the UV spectrum. This is present around hot stars which have high UV flux of up to 90nm, sufficient to ionise the atoms around the star.
2.3 Dust grain chemistry
Dust grains consists of a silicon core surrounded by carbonaceous particles which is covered by a layer of ice mantle of volatile organic molecules. Hence, dust grains are where organic chemistry including amino acid formation takes place. Compared to gas phase reactions, dust grain surface reaction is much more efficient. This is possible through the highly spontaneous sticking of molecules from the ISM to the surface. This adsorption onto the surface may dissociate the molecule into reactive atomic species. The atomic species may then move on the ice mantle and reacts to form a new molecule. If the spontaneity of the new species to stick to the surface is low, then the new species might be expelled into the ISM, populating the ISM with new species. Also, the energy of the interaction between the molecule and the surface overcomes energy required to break the bonds to form the transition molecule in gas-phase reactions. Another reason is that radiation may ionise the atoms on the surface, creating radicals that participate readily in reactions. However, the ice mantle may protect some of the molecules that are closer to the core. Hence, large molecules like amino acid can exist in the dust grain. Amino acid wll have a higher chance of getting photodissociated in the ISM as they have lesser protection against the UV rays.
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