[aeacfm]

what is the use of laser unit in FT-IR ?some body told me it makes alignement to the Ir beam but i cant get it ?can any body help ?

[enahs]

If you are talking about where you put your sample, it is mostly just for you to make sure your sample is positioned correctly. If the last is not on your sample, you need to adjust the position of your sample.


Alignment issues are a whole different thing, and all IRs go through them. Contact the IR manufacture for more help.

[aeacfm]

If you are talking about where you put your sample, it is mostly just for you to make sure your sample is positioned correctly. If the last is not on your sample, you need to adjust the position of your sample.


Alignment issues are a whole different thing, and all IRs go through them. Contact the IR manufacture for more help.

i am not talking about samples or manufacture service

new FT-IR units include laser units . what is the use of this laser unit ?

[enahs]

http://boards.straightdope.com/sdmb/showthread.php?t=345590

[aeacfm]

http://boards.straightdope.com/sdmb/showthread.php?t=345590

cant open it , sorry

[DevaDevil]

n its simplest form, an FTIR instrument has an IR source, a sample cell or chamber, a beamsplitter, a fixed mirror, a moving mirror, and a detector (this is for absorption spectroscopy; for emission spectroscopy, the IR Source is removed and the light from the sample under study is detected instead).

It is crucial to any FTIR experiment to know exactly where the moving mirror is at any given moment. Most modern instruments have this mirror mounted to a voice-coil (similar to that used in an audio loudspeaker). The instrument's resolution is dependent on the "throw" of the mirror, i.e. the distance moved during a given sweep. The instrument's bandwidth is dependent on the sampling rate, i.e. the inverse of the distance that the mirror moves between adjacent samples measured at the detector. For high resolution you need a long-throw voicecoil, and for wide bandwidth you need to be able to measure the mirror position very accurately.

As mentioned above, most commercial FTIR instruments use a Helium-Neon laser to determine the moving-mirror position at any given time. The beam intensity is usually measured at two points in the interferometer. As the mirror moves, the intensity at these two detectors rises and falls due to interferometric enhancement and cancellation of the HeNe beam paths, producing an approximate sinewave of intensity vs mirror position. By counting the number of "fringes" in the sinewave pattern, the instrument knows exacty how far the mirror has moved, and the relative phase of the sinewave at the two HeNe detectors tells the instrument in which direction the mirror is moving. Typically, measurements at the IR detector (the one looking at the sample in absorption or emission) are taken at specific phase-pair points of the two HeNe reference sinewaves; for lower-resolution studies, sampling may take place at every "N" HeNe fringes rather than at every fringe.

Although a given FTIR instrument may be able to be fitted with optics and sources to record both infrared and visible spectra, no single spectrum can extend across the wavelength of the reference laser (usually the HeNe line at 632.8nm). So, in any experiment one can do visible (wavelength<632.8nm) or infrared (wavelength>632.8nm) spectroscopy. [There are ways around this limitation, but they are not available in any commercial instruments AFAIK.]

In some time-resolved studies, in which the chemical species under observation only exists for a short period of time (e.g. nanoseconds, microseconds or milliseconds), Step-Scan FTIR may be used. In this case, the moving mirror of the FTIR instrument is held at a certain position while (typically) a pulsed laser is fired, producing or exciting the species under observation. The transient IR waveform is digitized (at sampling rates greater than 1 Gigasample/second in certain cases), and then the FTIR movable mirror is "stepped" to the next stable location at which the two HeNe reference beams have the selected phase relationship (usually a multiple of 316.4nm, i.e. half of the HeNe wavelength), and so on. [The mirror may stay at a given position for many pulses of the excitation laser, accumulating sufficient transient waveforms to produce the desired signal/noise ratio.] The result, after Fourier Transform, is a 3-D dataset of Intensity vs Time vs Wavelength. Such experiements may take several hours.

Hope this helps!

[Antonius Block, former modifier of FTIR instruments for enhanced time resolution.]