A.

Definitions and background

According to the IUPAC (International Union of Pure and Applied Chemistry) definition, the luminescence is a “spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment”. Fluorescence is a luminescence generated by photons.

“…not in thermal equilibrium with its environment…” means that we have to provide the energy to the sample with an excitation source in the absorption spectrum of the sample. After interaction with the studied material, the energy is going to be released in different ways. Among several conversions in the material, some photons can be generated by the atom de-excitation. We can collect them and characterize their energy which is directly linked to the energy levels in the material (vibrations, rotations…). This is the fluorescence emission spectrum. Part of the energy of the exciting photon is lost before the emission and there is an energy shift, called the Stokes shift, from short to higher wavelengths between absorption and emission. The Jablonsky diagram and Morse curves [1] [2] [3] both describe the possible quantified energy levels that can be reached in order to excite each species in the material spectrum. In addition, it describes the energy of the photons that can be emitted and therefore the shape of the emission spectrum. For molecules (organic contaminants in our application), there are not only electronic but also vibrational and rotational levels which strongly influence the shape of the spectral bands. The X-ray fluorescence principle means high excitation energy of the electrons (50KeV) localized in the highest energetic levels of the atoms close to the nucleus (K, L, M levels) whereas only the valence electrons are excited in the UV/vis fluorescence of molecules (1 to 5eV). Only covalent and coordinate bond can fluoresce in the UV/vis range of the electromagnetic spectrum. On the contrary, ionized molecules do not fluoresce. There is also a fluorescence behavior in crystals including impurities like ruby and its specific red color, and in some nanoparticles. The emission spectrum of these two examples shows quite narrow bands compared to huge molecules which have numerous complex vibrational and rotational levels. In the UV/vis range (1 to 5eV), luminescence is related to the band gap of the semi-conductors and to the HOMO-LUMO [1] transition in molecules as described in Fig 1.

Fig. 1.

The luminescence is associated to the transition of an electron from the Conduction to the Valence band in semiconductors[3], from the LUMO to the HOMO levels in small emitters [2] and between magnetic sub-levels of the LUMO for Lanthanides[4]

A good example in space applications is the alumina a of CCD detectors package used for Earth observation that fluoresces in a quite narrow red line when irradiated with 450 nm excitation wavelength and the adhesive of the protection window which shows a wide and smooth spectrum at around 550 nm.

The absorption of diluted emitters follows the Beer-Lambert law. In case of fluorescent molecules in solution, the fluorescent flux increases with the density of bright molecules but at high absorbance most of the excitation is absorbed in the first layer (see Fig 2). On the contrary, if fluorescent particles are isolated, they can be discriminated and even counted as it can be seen in Fig 3.

Fig. 2.

Transmission of fluorescein in aqueous solution for high concentration (right) and low concentration (left). On the right cell, the absorption due to the fluorescein is strong and there is a saturation of the fluorescence. On the left cell, the concentration is optimized to balance absorption through the overall cell. The yield is better for this lower concentration.

Fig. 3.

Nanoparticles in MAPSIL213 irradiated respectively from left to right with 365 nm and 450 nm excitation sources. On the left, nanoparticles have a low emission due to their weak absorbance at 365 nm. On the right the excitation is optimum. We can also see the emission of textile particles due to the brightener included in washing powders.

Organic contaminants and particles involve a significant decrease on detectors sensitivity and optical diopters transmission. These contaminants can be due to the outgassing of polymers in vacuum conditions. If these materials have a specific fluorescent behavior, it is obvious that it can be useful for inspection. Furthermore, the fluorescence of the alumina used for CCD package and some doped glasses in dioptric systems can generate an unexpected background in the detection as shown in Fig 4. This phenomenon should be considered in very low flux instruments design.

Fig. 4a.

Picture of the rear side of a mirror taken with a classic reflex camera in visible light (excitation 365 nm). The red fluorescence corresponds to the primer and the blue color to the adhesive.

Fig. 4b.

fluorescence of particles in the bulk of a glass.

Fig. 4c.

SPOT3 detector (glass fiber preform highlighted)

A first research activity on the fluorescence of materials was carried out in 2010 and the results presented at the ISMSE symposium in 2012 [6]. The goal of this study was to synthetize the theory on fluorescence of solid state materials in particular and also to measure the response under irradiation of several common polymer materials found on payloads, generally close to sensitive optics : adhesives: Scotchweld EC2216, Scotchweld DP490, Dynaloy 325; rosin flux; white paints: MAP SG120FD, MAP SG121FD; and varnishes: Solithane 113, MAPSIL213B.

The absorption and the emission spectrum measurement for each excitation wavelength from 200 to 800 nm showed a specific spectral signature for each of them indicating that it is possible to identify at least these 8 materials only with their fluorescence behavior even though a full identification with conventional techniques such as EDS, X-ray fluorescence or FTIR spectroscopy is necessary to go deeper. But standard laboratory techniques cannot really be carried out on satellites without any contact. That is why we are developing a new device to acquire the fluorescence signal, in situ and contactless.

B.

The fluorescence yield

The quantum fluorescence yield is defined as the ratio between the number of emitted photons and the number of absorbed ones (1). It can be related to some fundamental properties of the material. k_f, k_ci and k_cis are respectively the fluorescence rate, the internal conversion and intersystem conversion rate constants.

Fluorescence yield goes from 1E-4 (considered as non-fluorescent species) to 5E-1 (which is considered as a good fluorescence yield). To measure the fluorescence flux, it is important that the molecule have a very low interaction with the others in the bulk. In medicine and forensic science, the fluorescence molecules are chosen for their very good yield and in solution in a particular solvent. For our materials (resins, paints and adhesives…), the yield is strongly affected and can be inhibited by the link between the different molecules in the formulation of the polymers. So, it is really impossible to quantify the fluorescence yield in solid state materials containing a great amount of chemical species which all interact with each other. Moreover, it is difficult to know exactly the nature of the additive products the manufacturers use. Previously and according to the literature [2] we thought that the fluorescence of the materials we use for space applications was due to the conjugated bonds in the polymer itself but the study [6] showed that it is probably the additive molecules that are mainly responsible for the fluorescence of the 8 tested materials. In addition, the non-fluorescent molecules in the bulk can also absorb.

Thanks to our first study [6], we know, that 257 nm is the common excitation wavelength. But the better compromise to cover all the needs with a good yield is to use at least 3 exciting wavelengths from 250 nm to 450 nm. We also have defined the interesting reemission spectrum limits from 300 nm up to 700 nm for the materials we use for space applications. The UV range is the most interesting one and needs a higher resolution that the visible range.

C.

Examples of fluorescent materials used for space applications

The following Fig 5 show the excitation and emission spectra of eight samples measured during the previous study [6]. The UV emission from 300 to 450 nm is significant. Different fluorescent molecules can be identified in EC2216 and Solithane 113. We can notice a spectral width of the two fluorescent molecules in MAP SG121 at 390 nm and 510 nm and the inhibition of the oxygen vacancy in MAP SG120 compared to the next generation MAP SG121. We do not notice modulations due to aromatic bonds in relation to bibliography [2] (these modulations can be inhibited or blurred by interaction between molecules).

Fig. 5.

Absorption and emission spectra of common organic materials for space applications

D.

Needs for contamination inspection in cleanrooms

When an organic deposit is suspected on a sensitive surface of a flight model, any contact is forbidden and no representative samples are available for characterization in a laboratory. A very convenient solution would be to irradiate the area of interest with excitation wavelengths and then to acquire the spectrum from UV to visible wavelengths. It is also important to get information about the location of the organic residues. Therefore a hyperspectral instrument is the solution.

Our ambition was first to build a UV/vis sensitive camera with a catoptric objective, a filter wheel and three excitation sources. This is a quite easy set up for high resolution imaging and location in a reasonable period of time for a preliminary investigation. Then, a long exposure measurement could be carried out to acquire a lower imaging resolution but a continuous spectrum simultaneously. The two following chapters will describe the development on progress for this purpose.