Editing Fourier-transform spectroscopy (section)

==Pulsed Fourier-transform spectrometer==

A pulsed Fourier-transform spectrometer does not employ transmittance techniques{{definition_needed|reason=What is a transmittance technique?|date=August 2016}}.  In the most general description of pulsed FT spectrometry, a sample is exposed to an energizing event which causes a periodic response.  The frequency of the periodic response, as governed by the field conditions in the spectrometer, is indicative of the measured properties of the analyte.

===Examples of pulsed Fourier-transform spectrometry===

In magnetic spectroscopy ([[Electron paramagnetic resonance|EPR]], [[Nuclear magnetic resonance|NMR]]), a microwave pulse (EPR) or a radio frequency pulse (NMR) in a strong ambient magnetic field is used as the energizing event.  This turns the magnetic particles at an angle to the ambient field, resulting in gyration.  The gyrating spins then induce a periodic current in a detector coil.  Each spin exhibits a characteristic frequency of gyration (relative to the field strength) which reveals information about the analyte.

In [[Fourier-transform mass spectrometry]], the energizing event is the injection of the charged sample into the strong electromagnetic field of a cyclotron.  These particles travel in circles, inducing a current in a fixed coil on one point in their circle.  Each traveling particle exhibits a characteristic cyclotron frequency-field ratio revealing the masses in the sample.

===Free induction decay===
Pulsed FT spectrometry gives the advantage of requiring a single, time-dependent measurement which can easily deconvolute a set of similar but distinct signals.  The resulting composite signal, is called a ''free induction decay,'' because typically the signal will decay due to inhomogeneities in sample frequency, or simply unrecoverable loss of signal due to entropic loss of the property being measured.

=== Nanoscale spectroscopy with pulsed sources ===
Pulsed sources allow for the utilization of Fourier-transform spectroscopy principles in [[Near-field scanning optical microscope|scanning near-field optical microscopy]] techniques. Particularly in [[nano-FTIR]], where the scattering from a sharp probe-tip is used to perform spectroscopy of samples with nanoscale spatial resolution, a high-power illumination from pulsed infrared lasers makes up for a relatively small [[Scattering cross section|scattering efficiency]] (often < 1%) of the probe.<ref>{{Cite journal|last1=Hegenbarth|first1=R|last2=Steinmann|first2=A|last3=Mastel|first3=S|last4=Amarie|first4=S|last5=Huber|first5=A J|last6=Hillenbrand|first6=R|last7=Sarkisov|first7=S Y|last8=Giessen|first8=H|title=High-power femtosecond mid-IR sources for s-SNOM applications|url=http://stacks.iop.org/2040-8986/16/i=9/a=094003?key=crossref.3eb2b21f107d58830fc324d0ec18d34e|journal=Journal of Optics|year=2014|volume=16|issue=9|page=094003|doi=10.1088/2040-8978/16/9/094003|bibcode=2014JOpt...16i4003H|s2cid=49192831}}</ref>