Notes on the region in front of the spectrometer entrance slit
The radiation source and the optics in all spectrometers are separated by optical planes.
If a stand with SDAR is coupled to an optics system in air, because of the intense UV radiation between the optical plane and the entrance slit, ozone and nitrogen oxides form which alter transparency. This problem can be eliminated by air circulation using a small pump.
If a radiation source with intense UV radiation is coupled to an optics system in vacuum, an internal coating will appear, absorption of which is wavelength-dependent with a maximum at about 193 nm. Pressure in the spectrometer optics tank is generally reduced to 0.01 mbar using a rotary vane pump, taking about 10 min with a clean tank.
Pressure is measured using a vacuum meter. Its mode of operation is based on the relationship between the thermal conductivity of a gas and its pressure. The temperature of an electrically heated wire is a function of pressure. The temperature is measured using a thermocouple and displayed on a μA meter which is calibrated in mbar /96/.
At the ultimate pressure the pump suction rate is nil. Vapour from the pump oil diffuses into the tank and coats the optical components. For the sake of mechanical stability, minimized reflection losses and intensity losses as a result of contamination, the number of optical planes in the optics should be small. Coating of the inner side of the optical interface between the radiation source and the optics is a problem which has remained unsolved since optics systems in vacuum were first produced and is basically responsible for long-term instability. The coating is conspicuous if the optics system is thermostat-controlled to eg. 35℃. The flushing gas flow most often entering in front of the side facing the radiation source may, with short distances between the storage tank and the inlet, be cold because of the Joule-Thomson effect, so that the interior is also cooled and this promotes vapour condensation.
Due to the effect of UV radiation, the vapour polymerises and forms a solid coating. This can lead to intensity loss which eg. in the case of spectrometers with SDAR may amount to 10% per 100 measurements.
The following measures are taken to reduce this coating:
1. An adsorber between the pump and the tank.
2. Meticulous cleaning of the tank interior.
3. No gas-forming materials allowed in the tank.
4. An Ar leak in the tank, set to twice the end pressure of the rotary vane pump. As a result, the suction rate never becomes nil and diffusion of pump oil into the tank is reduced.
5. The optical interface is heated to about 50 ℃ to reduce vapour condsensation. Heating is effective, particularly with thermostat-controlled optics.
6. A shutter between the radiation source and optical interface. This shutter, which can be electromagnegtically or pneumatically operated, only releases the ray path during intensity measurements. As a result, irradiation of the optical interface is several times reduced. This applies both to ICP in which the burn time is usually a multiple of the measurement time, and to SDAR which, during the pre-spark time which is several times longer than the measurement time, still also works with several times higher discharge energy. The shutter also prevents over-irradiation of the CCD. The shutter can have a small opening which allows as much radiation to pass through as enters the optics during the measurements time with the shutter open. A gain in stability is thus obtained and measurement can be taken even during pre-sparking in order to assess the burn-off curve.
By observing the above measures, intensity losses are reduced from about 10%-1% per 100 measurements in SDAR with optics in vacuum. Even then, the intensity loss from inside is still a multiple of the intensity loss from the side facing the radiation source.
Another possible way of avoiding or preventing contamination of the entrance window is to flush the tank with N. The work and expense involved in producing the vacuum can be eliminated, as well as coatings arising from a return oil flow and degasification products from the tank. Transparency is ensured with N down to 120nm. So far, no contamination worthy of mention has been observed in the tank or on optical planes of a N-flushed monochromator which has been in operation for more than two years.
Another possible, much more elegant method is to fill the tank with a transparent gas, generally N. In vacuum tanks with low leakage rates, only a little O and HO-adsorbing penetrate from outside. Using a small membrane pump, the vessel space is pumped out and cleaned, approximately once every 10 min, with O and HO-absorbing reagents( abtainable in airtight one-way tubes). UV-spectrometer optics operated in this way for more than 10000 measurements within 3 months do not show any noticeable contamination (intensity loss) from the interior of the entrance optics. The technical feasibility of gasfilling basically depends on the work involved in opening the tank for servicing.
The path of the rays to the entrance slit must be made “radiation-proof” since the radiation power here is many 1000 of times greater, compared within the optics and exit optics space.
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