Tunable diode laser absorption spectroscopy

Absorption spectroscopy differs strongly to the OES discussed above, as instead of passively analysing light emitted from the plasma itself, an external light source is used,

with such sources typically being LEDs or lasers. Photons from these external sources

interact with the plasma by imparting their energy to bound electrons, causing them to excite to higher states within the molecule or atom they are bound to, with the energy difference between these states corresponding to the incident photon’s energy. From detection of photons that pass through the plasma, and use of an appropriate model, factors such as densities and temperatures of species within the plasma can be determined [116].

The specific form of absorption spectroscopy used in this work is tunable diode laser absorption spectroscopy (TDLAS) with an admixture of Argon, Ar. A tunable diode laser is a laser where the output wavelength can be altered by applying a different current to the diode from which photons are emitted, and in some cases by altering the path length of the lasing cavity by means such as piezo-electric motors. TDLAS of Ar for means of determination of gas temperatures is a well accepted technique within plasma physics, as been utilised in a range of conditions, including within low pressure inductive sources applicable to the source utilised in this work [117–119]. However to note in said works the gases used were pure Ar,

not by means to diagnose O2 dominate plasmas such as presented within this work. The

laser used within this work is a Photonics TA Pro, controlled by a Photonics Laser Diode Pro Control Unit, as shown in Figure 3.8, alongside the other apparatus used, with how this is situated on the chamber shown previously in Figure 3.3

Figure 3.8: TDLAS apparatus utilised, (a) situated before laser light enters the chamber, (b) situated after laser light passed through plasma. (See Figure 3.3).

The laser has a continuous output of 150 mW, with to a central wavelength of 810.4 nm, which corresponds to the transition within Ar as outlined in Equation 3.7.

Ar(1s4) + γ(810.4nm) → Ar(2p6) (3.7)

This lower 1s4state is a resonant state of Ar, and therefore known to have lower densities

than other Ar species within RF plasmas [120], yet is present within the low pressure case used here, and shown to be present within low pressure cases in various works, such as Tian et al [121]. Figure 3.9 shows a spectrum taken with a Oceanoptics Maya spectrometer within

790 795 800 805 810 815 0 20 40 60 80 Pure Ar 85 % O 2 : 15 % Ar Wavelength (nm) A b s o l u t e i r r a d i a n c e ( m W / c m 2 / n m ) 2p6 to 1s4 0 1 2 3 4 5 A b s o l u t e i r r a d i a n c e ( m W / c m 2 / n m )

Figure 3.9: Example spectra of pure Ar and O2:Ar ICP, operating 20 Pa, 700 W.

the PE-PLD chamber operating at 20 Pa, 700 W which is a typical case used throughout this work; both a pure Ar plasma is shown, alongside an O2 plasma with an admixture of

Ar used during TDLAS measurements, with this admixture being 15%. It can be seen that in the pure Ar case there is a strong signal corresponding to the decay from the 2p6 to the

1s4 state, with this emission still observed within the O2 plasma, although much weaker due

to affects such as Penning ionisation of the excited Ar. Similar to the admixture of N2 used

in the OES measurements, it is discussed within Chapter 5 that this 15% admixture of Ar did not perturb the oxygen chemistry; In addition it has been shown computationally in the work of Gudmundssson et al, that small admixtures of Ar does not affect the density of many oxygen states or their temperatures [122].

A gas temperature is determined via TDLAS by Equation 3.8, where λ0 is the central

wavelength of the peak, kb the Boltzmann constant, M the mass of the species, and T the

temperature of interest. ∆νD is the measured broadening of the peak determined from the

full width half maximum (FWHM), which can be measured due to the tunable nature of the laser, as during measurements, the wavelength of the laser is scanned across a range of

wavelengths close to the central wavelength. ∆νD = 2 λ0 r 2ln(2)kbT M (3.8)

The broadening of this signal is then assumed to come from Doppler broadening, i.e. the change in wavelength due to the motion of species within the plasma photons interact with, which is caused by the thermal motion of the species. Hence T being able to be determined from this observed absorbance signal, with other broadening effects such as pressure broadening are assumed to be negligible in magnitude compared to that caused by this thermal Doppler effect.

The intensity of the signal observed can be described by the Beer-Lambert law, Equation 2.1, yet written more explicitly for this case in Equation 3.9.

I = I0e−σN l (3.9)

Where I and I0are the observed and initial density as before, N the density of absorbing

species, and l is the path length of the laser within the plasma to be considered; here l is assumed to be 10 cm, the diameter of the electrodes.

The observed signal on the photo-diode, see Figure 3.3, comprises of photons that have passed through the plasma without being absorbed, with a reduction in intensity at a certain time, corresponding to when the wavelength of the scanning laser matches that of the transition in equation 3.7. In order to distinguish this absorbance ”peak” from the rest of the observed signal, Equation 3.10 is utilised.

IP +L(ν) − IP(ν)

IL(ν) − IBG(ν)

= e−σN l (3.10)

Where subscripts P, L, and BG corresponding to emission from plasma, laser, and background, respectively. This results in just the observed difference in intensity caused by absorbance of photons by the plasma. Within this work, signals were obtained in the following order; First the plasma was ignited and the signal of just the plasma obtained (IP), the laser was then turned on and the signal of both (IP +L) obtained. The plasma was

then turned off, and just the laser signal (IL) taken, and lastly the laser turned off the the

and plasma were turned off and on, and therefore minimised any uncertainty caused from such sources as differences in matching of the plasma, or temperature deviation of the laser diode.

A wavelength calibration of the observed signal on the oscilloscope is performed by the following means; Before entering the plasma a beam-splitter is put in the path of the laser (ThorLabs, fused silica plate beam-splitter 9:1), to separate off a fraction of light, which is then passed into a Fabry-P´erot interferometer (FPI). In particular a confocal FPI (ThorLabs SA200-8B) is utilised, which comprises of two concave mirrors at a known distance, as shown in Figure 3.10. A wavelength calibration is achieved, as if the distance between mirrors is an integer multiple of the laser wavelength, constructive interference will occur, resulting in an increase in observed signal transmitted through the FPI. This signal takes the form of sharp peaks, each with a particular spacing between them. This spacing is known as the free spectral range (FSR), with the FPI used here having a FSR of 1.5 GHz. This can be quantified as in Equation 3.11.

Incident

light

Transmitted

light

1,5

2

3

4

d

Figure 3.10: Schematic of a confocal FPI used for wavelength calibration of TDLAS signal.

F SR = c

4d (3.11)

where c is the speed of light, and d is the distance between the mirrors. Note the denominator is a factor of 4 due to four reflections occurring during a photons path through the FPI. Therefore as the timebase of the oscilloscope used is known, and this distance between mirrors set, signals from the photo-diode can be calibrated to a wavelength.