Time-domain spectroscopy

Time-domain spectroscopy is a type of spectroscopy that measures the power of electromagnetic radiation as a function of time.  Normal spectroscopy is frequency-domain spectroscopy.  It measures radiation as a function of frequency or wavelength.  Time-domain spectroscopy usually requires the use of Fourier transforms.


Two monochromatic lights of different frequencies can combine to show interference patterns.  At time t=0, the waves interfere constructively.  This gives a maximum in the added plot.  The waves get more out of phase.  This leads to destructive interference and a minimum in the added plot.  From one peak maximum to the next is a cycle.  So when looking at two monochromatic lights, they can be graphed as two lines in the frequency-domain (this plot shows power, P(ν), as a function of frequency).  The same two lights can be plotted as the sum of their waves in the time-domain (this plot shows power, P(t), as a function of time).

 Getting time-domain spectra 

Spectroscopy done with visible or infrared radiation involves wavelengths of 1012 to 1015 Hz.  Phototransducers (instruments that convert light to information readable by a computer) cannot make measurements that quickly.  What is needed  is a way to make these high-frequency signals into low-frequency signals.  For optical spectroscopy (such as infrared spectroscopy) the most common way to do this is by using a Michelson interferometer.


Most time-domain spectroscopy experiments involve broadband radiation (radiation that consists of a large range of wavelengths). 

 Applications 

Time-domain spectroscopy is used very often in infrared spectroscopy (FT-IR) and NMR spectroscopy(FT-NMR).  It has several advantages over frequency-domain spectroscopy.  In frequency-domain experiments, each wavelength must be measured one at a time.  This means scans can take several minutes.  A single time-domain experiment takes no longer than a minute.  Usually it only takes seconds.  This means that many scans may be taken for a given sample, and that the scans can be averaged.  So not only is the time-domain experiment faster, but it can also have a better signal to noise ratio.

 References 

D.A. Skoog, F.J. Holler, S.R. Crouch. 2007.  Principles of Instrumental Analysis. 6th ed, Thompson and Brooks/Cole: Canada.

Category:Spectroscopy