Optical emission

A hydrogen plasma is a mixture of neutral molecules, atoms, ions, ro-vibrationally excited atoms, and electrons. A plasma is very dynamic: electrons collide with all kinds of hydrogen species and excited atoms undergo relaxations and thereafter can become excited again. These interactions make that photons will be emitted. This happens for example when an electron is pushed in an outer electron shell of the ion (although for a short time), and the electron falls back to an inner shell. The energy of the photon (and therefore, the wavelength) changes according to

En₂− En₁= hν, (2.9)

where n₁and n₂are the principal quantum numbers, according to the shell the electron is in, with n1< n2. Rydberg’s formula,

in which λvac is the wavelength in vacuum as a result of the electron transition and R is the Rydberg constant (with a numerical value of approximately 1.097 · 10⁷ m^-1). Equation 2.10 is only valid for hydrogen, and it suggests a different series of emission wavelengths for different transitions. These series are the well-known Lyman, Balmer, and Paschen series. The Lyman series describes the wavelengths of the emitted photons for transitions from an arbitrary shell to the most inner shell (n1= 1 and n2= 2, 3, ..., ∞). The Balmer and Paschen series are constructed similarly, but they describe transitions to the second most inner shell (n1 = 2) and third most inner shell (n₁ = 3) respectively. Table 2.1 gives some of the wavelengths corresponding to the three series.

Table 2.1: Wavelengths in vacuum of emitted photons in the Lyman, Balmer and Paschen series [27]. All values are in nanometers.

n₂ Lyman (n₁= 1) Balmer (n₁= 2) Paschen (n₁= 3)

In particular, the Balmer series is the only series that contains (four) wavelengths in the visible range. The spectral line corresponding to the first transition in the Balmer series is called H-α, the second one H-β, and so on. Noting that ν = c/λ and combining this with Equation 2.9, it can be derived that a decrease in wavelength corresponds with an increase in energy difference.

Hence, because the energy of the H-α transition has the smallest energy difference, this will be the spectral line with the highest intensity in the visible range.

In the emission spectrum of atomic hydrogen there will be sharp spectral lines at the wavelengths denotedTable 2.1(as with any other atom), seeFigure 2.7. A molecule can, unlike atoms, be in a rotationally or vibrationally excited state. Because of the symmetry of an atom, it cannot possess such properties. The vibrational states are more energetic, which is why molecules have different rotational states within a vibrational state [28]. For hydrogen molecules, these excited states can, for example, create the so-called Fulcher bands [29], which are also shown inFigure 2.7.

Figure 2.7: An emission spectrum of a hydrogen plasma [30]. Besides the Balmer spec-tral lines, the Fulcher bands originating from ro-vibrationally hydrogen molecules can be observed.

These bands are actually a collection of a large number of small peaks that lie very close to each other, since energy differences between different rotational and vibrational of molecular hydrogen (and every other molecule) states are minimal. When zooming in on the region where the band is located - as shown inFigure 2.8- the individual peaks of the ro-vibrational states become visible.

Figure 2.8: An emission spectrum of the Fulcher bands [31]. To make such a specific and accurate spectrum, a spectrometer with a very high spectral resolution is needed. The wavelength in nanometers is given on the horizontal scale.

AsEquation 2.8suggests, H2O is produced in the reduction process. This water enters the plasma when it is created, whereafter part of it will dissociate into hydroxyl radicals (OH^*). As shown in Figure 2.9radicals has a clear spectral emission line at around 309 nm, although being wider than that of an atom. At around 285 nm, a smaller spectral emission line is located, which corresponds with another vibrational state of the hydroxyl radical. Because OH* is a molecule, it can have multiple ro-vibrational states. Therefore, as with hydrogen, the peak at 309 nm consists of a lot of small peaks as shown inFigure 2.10, which correspond with different rotational states.

Figure 2.9: An emission spectrum of hydroxyl radicals [32]. The blue line shows the measured spectrum, and the red line represents the spectrum which is obtained after the background correction has been applied.

Figure 2.10: An emission spectrum of the OH^*(0-0) peak of Figure 2.9 [33]. It can be seen that such a peak actually consists of different rotational states. The general trend in this peak is depicted by the red line.

The spectrum inFigure 2.10contains multiple higher peaks, which can also be seen inFigure 2.9 as a kind of distortions in the main peak if a closer look is taken. In the zoomed spectrum with higher resolution, the first peak (as a result from a rotational transition, at about 306.5 nm) is high as compared to the spectrum measured below 306.5 nm. At the end of this spectrum, however, the intensity of the peaks decreases gradually. This sudden increase at 306.5 nm is due to the presence of forbidden quantum mechanical states of OH^*. This peak shape is characteristic for hydroxyl radicals. Wavelengths longer than 319 nm are not measured inFigure 2.10. It is expected that these hydroxyl radical peaks will increase during the reduction of the iron oxide from the moment the plasma is discharged.

Experimental set-up

In this chapter, the most important components in the experimental set-up will be given and elucidated. Additionally, relevant calibration test results will be shown and a characterisation of the iron oxide powders (among which their composition) that are used in the experiments will be given.

3.1 Key components

This section will first describe the most relevant components that are used to create a hydrogen plasma and observing it. In addition, it will give an idea of the general overview of the experimental set-up. After, a more detailed description of the cavity (the container in which the plasma is created), the power supply and other measurement devices is given. Next, the working principles of an optical emission spectrometer are explained. Finally, a short note will be made on the reason why a thermogravimetric analyser is used in the experiments. The description of the experimental set-up will be useful especially regarding the first sub-question of this report (What other processes next to the reduction of iron oxide can change the intensity of hydroxyl emission peaks?).