One possibility, the UV light is degradated. The "PL" actually comes from the light.

Dear Mr. Yoshiki Fukada,

We use a double-gratings monochromator to get the UV excitation light. When we used another sample chamber for room temperature measurements, these five peaks never appeared. Besides, the peaks are identical to the reported PL from pyrene. Therefore we suspect they are from pyrene contamination.

Thank you all the same!

Dear Beichen,

It sounds very strange. Someone used the chamber for excimer experiment and spelled pyrene? I think the window also better to be checked. Thank you.

Dear Yoshiki Fukada,

I am also very confused about the source of pyrene. As far as I know, in our lab no one has ever studied any organic materials. We have only measured inorganic bulk crystals and thin films in this chamber.

I was suspecting two things: adhesive tape used to fix sensor wires in the chamber, and oil in the rotary vane pump. However, I have removed all the tape in the chamber, and the pressure in the chamber is ~10^-3 Pa(there is a turbomolecular pump).

I have cleaned the chamber and windows with cotton pads in ethanol and acetone. Is this cleaning method effective to remove residual pyrene?

Thank you!

Beichen Hu

Hello and sorry to barge in ,it is not exactly my field of expertise but here are my two cents.The datasheet mentions several rubber seals and a vacuum pump- can you check that the pump itself is free of particle sputter and that the TBM pump does not spit oil from its main bearing? Had a nasty case of air contamination in a setup containing one and..it should have been impossible- but yet there was oil mist in the low pressure circuit messing up thermoionic vacuum readouts. Checked the poor meter ,electrodes, contacts, electrical noise, etc when ..a visual inspection showed oil mist condensed on the probe. Source....an aging and yet still silent turbomolecular pump that.. developed a nasty habit of..spitting.Any gloves reused? Any samples handled with bare hands? Skin oils could do that . Maybe indeed the tape had some sort of colorant applied that could explain it. Anyways, the wiki article also says the pyrene is activated even more by the presence of some solvents- you said you tried cleaning with alcohol, but it seems to have made it worse.High quantum yield and persistence it says, and that varies with the various polar solvents so much that it it a used technique to identify solvents. So no, it does not seem to solve the problem..other methods must be found but I can't suggest any ..

Please take a look at the implications of pyrene presence as detailed in the wiki here

https://en.wikipedia.org/wiki/Pyrene

The rotating vane pump should have some oil /moisture traps installed, if not it is a good idea to do so now. Check the tubing ,interconnects, etc. Any recent repairs to the vacuum circuit? Any gaskets that leak, are deteriorated or..some unaware techie put some well-meaning grease to 'seal the deal'?

Please refer to the manufacturer datasheet on how the various parts of such an installation should be cleaned. Also please observe that the high vacuum installation is in accordance with the standards and that filters, oil traps or condensers, etc are installed and see if anywhere some organic matter could have been introduced.

Indeed mr.Fukada is on to something here- the window. Any chance of someone leaving a fingerprint on it for example? It seems that a small quantity of skin oil is all it takes- even if it was visually wiped with a small amount of alcohol..

Please find a suitable solvent - and Ițm not sure if white room conditions apply but it might be a good idea to look at how the equipment is cleaned for that application. Industrial high precision manufacturers should know how to keep optics cleaned for, say, space-going spectrometers and the like- where the cleaning is only done once and well.

Hope it helps and best of luck,

Marius.

Um... "it is produced in a wide range of combustion conditions.".....

cigarette smok?

You already cleaned inside of the chamber. Did you clean outside of the chamber window?

Dear Mr. Panait and Mr. Fukada,

Thank you for your patience to help me, but I still couldn't remove the pyrene contamination. :(

I have cleaned everything of the chamber. Small removable parts were immersed and supersonic-cleaned in acetone, ethanol, and deionized water successively. The walls of the chamber was wiped with cotton pads in ethanol and acetone. I'm not sure whether the adsorbed pyrene is effectively removed.

I have tried two vacuum evacuation systems:

1. Rotary vane pump + turbo molecular pump. This system is able to make the chamber ~5*10^-4 Pa. I guess under this pressure there is not much oil mist diffused to the chamber.

2. Another rotary vane pump with oil trap(activated alumina). And according to the specifications of this pump oil, it's 'PAHs free'.

So I think there must be a source of pyrene inside the chamber. I'm still trying to find it.

Well - confessing that I still don't know the exact machine geometry and components, but assuming it has a basic layout like this- light source- gratings/monochromator- optic port in- test chamber, optic port out, detector - and that the chamber is found to be guilty of contamination - by comparison with measurements made with another sample chamber at room temperatures, as you said- try to replace the (faulty?, dis-integrating maybe) sample chamber with a spare. What happens at normal operating temperatures with the spare chamber? Does pyrene contamination still appear? If so, it might be that the sample chamber itself does not withstand the temperature range well or that some prior cleaning solution that left some residue caused it.

If the new sample chamber does not show any contamination then the old one must simply be discarded- sometimes things just get too dirty...if contamination still appears at working temperature, something is deteriorating by action of temperature..I assume source window and optic exit port as well as detector window have been checked for pyrene contamination already.

Best of luck,

Marius

Dear Mr.Panait,

I have added the pictures of the cold head and the spectrometer. Sorry for my carelessness.

Thank you for the clarifications. Well it seems it's time to bring out the big guns.. if simple rinsing with deionized water did not help and the sample chamber along with window, removable section and radiation shield , anything related to the chamber and sample holder that can be removed must be removed and fully immersed in deionized water then some ultrasonic cleaning is in order. Ultrasonic cleaning ( please check that the window can withstand it) is effective because by cavitation it really..micro-scrubs without scratching . If the pyrene is deposited on those surfaces it would be removed mechanically not chemically while the water is just a medium for the transmission of ultrasound waves. That removable lower rad shield has a gasket, is it removable too? if so, dip it with the rest in the ultrasonic bath.. Leave it on for 10 minutes or so but please take care that nothing touches the ultrasonic cleaner tray directly- put the parts on some vibration reducing (plastic?) supports or something (clean!). Then there is the matter of ..drying the whole thing in a controlled, neutral and non-polluted atmosphere- like that in a HEPA-filtered, air conditioned clean room or substitute ( I once used a thoroughly cleaned turned OFF autoclave with the dryer circuit on only). After that, carefully put the device together again if possible in the same controlled air conditioned filtered etc space and try it at first without any sample at all in the chamber. Is it possible that the samples themselves had been priorly contaminated? to measurement You said that even sapphire substrate alone is "dirty". Ok, prior to testing, did the samples experience any polluting atmosphere? Try dipping the whole removable vacuum shroud & other removable parts in ultrasonic bath (use water to avoid further lots of deionized possible contamination) and then try to dry the parts separately in a clean environment. Assemble in the same clean environment and ..try again,

Best luck and curious to hear from you,

Marius.

Dear Mr.Fukada, Mr.Panait, and those who may be interested in this question,

Thank you for helping me with this annoying problem. Finally, at the end of 2017, I found the pollution source of pyrene.

It is the vaccum grease I applied on the rubber o-rings to seal the chamber. After cleaning all the rubber o-rings with ethanol, I put all the dismountable parts in a heating and drying oven for hours to remove residual pyrene.(According to wiki, pyrene volatilizes and decomposes at high temperature.) Then the pyrene luminescence disappeared.

I had suspected this vaccum grease before, and had measured its PL spectrum. The pyrene peaks was not obvious. Now I realize the content of pyrene was so small that the spectra was dominated by the main constituent of vacuum grease.

If someone wants to detect weak luminescence at low temperature, please don't use such kind of vacuum grease.

Best regards,

Beichen Hu.

@Beichen

Dear Beichen

Pyrene is produced by combustion of fossil fuels or burning organic materials is, therefore, a natural ingradient of atmospheric pollution due to vehicular emission. It is strongly physisorbed on the surfaces made of aluminum and its alloys typically used for construction of cryostat. This is not so for stainless steel. Pyrene peaks in your UV excited PL spectra stems from degasing of pyrene from the walls of your cryostat made from Aluminum alloys under cryogenic pumping that occurs in cryostats at low temperatures. Strongly fluorescent Pyrene molecules thus emanating from walls condenses on your sample surfaces as trace impurity which is enough to corrupt your spectrum of weakly fluorescent samples. You can easily verify this by visual observation of strong luminescence from cryostat suraces excited by UV light. Only way to avoid this interference to great extent is to replace the vacuum shrouds of your cryostat with stainless steel ones or atleast electroplate existing ones with nickel.

Dear Beichen

Pyrene is produced by combustion of fossil fuels or burning of organic materials and is, therefore, a natural ingradient of atmospheric pollution due to vehicular emission. It is strongly physisorbed on the surfaces made of aluminum and its alloys typically used for construction of cryostat. This is not so for stainless steel. Pyrene peaks in your UV excited PL spectra stems from degasing of pyrene from the walls of your cryostat made from Aluminum alloys under cryogenic pumping that occurs in cryostats at low temperatures. Strongly fluorescent Pyrene molecules thus emanating from walls condenses on your sample surfaces as trace impurity which is enough to corrupt your spectrum of weakly fluorescent samples. You can easily verify this by visual observation of strong luminescence from cryostat suraces excited by UV light. Only way to avoid this interference to great extent is to replace the vacuum shrouds of your cryostat with stainless steel ones or atleast electroplate existing ones with nickel.