Excitation out of the ground state
Fourier Transform Spectometer (FTS) System
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Why use a FTS?As discussed in the section on the near-UV/visible monochromator system, to measure the direct cross section, Qidir, we need to measure not only the apparent cross section Qi, but also all of the optical cross sections that cascade into level-i. As it turns out, for the rare-gases (Ne, Ar, Kr, Xe) many of these cascading levels lie in the infared part of the spectrum.
Photomultipliers (PMT's) do not work well (or at all) much above 1100nm. Photo-diode detectors
(Ge, InGaAs, InSb...) can work in the infared, but have much lower sensitivies than PMTs.
To help compensate for this problem we use the multiplex advantage of Fourier Transform Spectrocopy:
we effectivly measure all the spectral lines simultaneously.
How do we measure the signal?In this experiment, a variable-energy electron beam passes through a vacuum chamber that has been filled with a few mTorr of the gas of interest. The electron beam excites the atoms into higher levels, and when these levels decay, they emit radiation. This light is directed into a weak-emmission FTS fitted with a IR photodiode. A computer reads the output of the detector and computes the inverse Fourier transform to find the spectrum of the emissions leaving the chamber. The computer also records the chamber pressure and the electron current. From this information we can extract optical cross sections.
Absolute calibration of cross sections:To place the results on an absolute scale, we need to know two things: the relative response of the optical system (FTS and detector) as a function of wavelength, and the absolute value of one of the optical cross sections. To determine the optical system's wavelength response, we use a standard lamp whose output as a function of wavelength is well known. We use the PMT/Monochromator system to measure the absolute value of one of the optical cross section in the wavelength range in which the two systems overlap (~ 850 to 1100 nm).
Some experimental specs:
Recent Published Results
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|last updated: Oct-1-2002|