Dear Clemente (and others interested),

I recommend that you consider using ZnSe windows.  I still am using my
original windows (I purchased two). They are 2.0 in diameter (0.25 in
thick).  The laser in MILES (Micro-Isotopic Laser Extraction System --
you've got to have acronyms in the government !) is a 25W CO2 laser that I
use in both continuous and pulsed mode.  We use purified F2 (ala Asprey).

For reference:
Beaudoin and Taylor (1994) GCA v. 58,   [system performance and
application], p. 5055-5063.
Taylor and Beaudoin (1993) Current Research of the Geological Survey of
Canada, 1993-D, p. 191-198. [system description]
Beaudoin and Taylor (1993) Current Research of the Geological Survey of
Canada, 1993-D, p. 199-204. [system performance]

Bruce

************************************
Bruce E. Taylor, Ph.D.
Geological Survey of Canada
601 Booth Street
Ottawa, ON  K1A 0E8
CANADA

   Tel:  613-995-4673 (office); 992-4046 (lab)
   FAX: 613-996-9820
   e-mail: [log in to unmask]
************************************

> ----------
> From:         Clemente Recio
> Reply To:     Stable Isotope Geochemistry
> Sent:         Monday, September 27, 1999 12:05 PM
> To:   [log in to unmask]
> Subject:      Teething pains!
>
> Dear Colleagues,
>
> During the last few months I've been building a couple of laser systems; a
> line with a Nd:YAG laser, and another one with a 25W CO2 laser.
>
> Testing of the Nd:YAG system has been successful, and I'm nearly (note
> that
> I say nearly) ready for routine operation.
>
> The CO2 system, however, is giving me some problems, in the sense that I'm
> breaking the BaF2 windows now and again.
>
> Tests performed so far allow me to use about 25% power with no problem.
> I've been firing the laser through the window for periods over 10 minutes
> uninterrupted, with no problem. Both silicates (whole rock powders and Q)
> and carbonates were burnt, although somewhat slowly compared to what I
> remembered from well stablished, well tested lasers: it takes me minutes,
> rather than seconds, to burn out a typical 1 mgr load.
>
> Increasing the power to about 35% results in a broken BaF2 in a couple of
> minutes; 50% power breaks the window in seconds, and powers above that,
> almost instantaneously.
>
> Some colleagues told me to defocus the beam to avoid breaking the BaF2. A
> Vernier spacer in the beam path allows me for about 35 mm focusing range.
> I
> have checked along this range, and no major differences exists as far as
> the BaF2 window is concerned.
>
> After noticing that the broken BaF2 windows were hot, I have contacted the
> manufacturer (in our case, Crystran, from Dorset, U.K.), and to my
> surprise, I was told that BaF2 windows are not advised for the CO2 laser
> wavelenght, that they absorb, thereby heating up and breaking.
>
> However, I know that most of you doing laser silicate work use just this,
> and in fact, the BaF2 windows were manufactured by Crystran to the exact
> specifications of those successfully used at SURRC.
>
> Obviously, I have to be doing something wrong, but after a couple of
> months
> "having fun" breaking the stuff, I cannot work out what.
>
> So far (except for one ocasion I rather forget), all my tests have been on
> "blank" reaction chambers (i.e., with no reagent -ClF3-). I hope you will
> understand that I rather find out what is going on before I get "into the
> real thing".
>
> Anybody can offer some advice? I promish to check all suggestions!
>
> Cheers & thanks,
>
> Clemente
>
>
>
> -----------------------------------------------
> Dr. Clemente Recio
> Stable Isotope Laboratory
> Fac. de Ciencias
> Univ. de Salamanca
> E 37008 - Salamanca
>       Spain
>
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>