Editing Rotational spectroscopy (section)

===Fourier transform microwave (FTMW) spectroscopy===
The theoretical framework<ref>{{cite journal|title=Transient Effects in Microwave Spectroscopy|last=Schwendemann|first=R.H.|journal=Annu. Rev. Phys. Chem.|date=1978|volume=29|pages=537–558|doi= 10.1146/annurev.pc.29.100178.002541
|bibcode = 1978ARPC...29..537S }}</ref> underpinning FTMW spectroscopy is analogous to that used to describe [[NMR spectroscopy|FT-NMR spectroscopy]]. The behaviour of the evolving system is described by optical [[Bloch equations]]. First, a short (typically 0-3 microsecond duration) microwave pulse is introduced on resonance with a rotational transition. Those molecules that absorb the energy from this pulse are induced to rotate coherently in phase with the incident radiation. De-activation of the polarisation pulse is followed by microwave emission that accompanies decoherence of the molecular ensemble. This [[free induction decay]] occurs on a timescale of 1-100 microseconds depending on instrument settings. Following pioneering work by Dicke and co-workers in the 1950s,<ref>{{cite journal|title=Pulse Techniques in Microwave Spectroscopy|last=Dicke|first=R.H.|author2=Romer, R.H. |journal=Rev. Sci. Instrum.|date=1955|volume=26|issue=10|pages=915–928|doi=10.1063/1.1715156 |bibcode = 1955RScI...26..915D }}</ref> the first FTMW spectrometer was constructed by Ekkers and [[Willis H. Flygare|Flygare]] in 1975.<ref>{{cite journal|title=Pulsed microwave Fourier transform spectrometer|last=Ekkers|first=J.|author2=Flygare, W.H. |journal=Rev. Sci. Instrum.|date=1976|volume=47|issue=4|pages=448–454|doi=10.1063/1.1134647|bibcode = 1976RScI...47..448E }}</ref>

====Balle–Flygare FTMW spectrometer====
Balle, Campbell, Keenan and Flygare demonstrated that the FTMW technique can be applied within a "free space cell" comprising an evacuated chamber containing a [[Optical cavity|Fabry-Perot cavity]].<ref>{{cite journal|title=A new method for observing the rotational spectra of weak molecular complexes: KrHCl|last=Balle|first=T.J.|author2=Campbell, E.J. |author3=Keenan, M.R. |author4= Flygare, W.H. |journal=J. Chem. Phys.|date=1980|volume=72|issue=2|pages=922–932|doi=10.1063/1.439210|bibcode = 1980JChPh..72..922B }}</ref> This technique allows a sample to be probed only milliseconds after it undergoes rapid cooling to only a few [[kelvin]]s in the throat of an expanding gas jet. This was a revolutionary development because (i) cooling molecules to low temperatures concentrates the available population in the lowest rotational energy levels. Coupled with benefits conferred by the use of a Fabry-Perot cavity, this brought a great enhancement in the sensitivity and resolution of spectrometers along with a reduction in the complexity of observed spectra; (ii) it became possible to isolate and study molecules that are very weakly bound because there is insufficient energy available for them to undergo fragmentation or chemical reaction at such low temperatures. [[William Klemperer]] was a pioneer in using this instrument for the exploration of weakly bound interactions. While the Fabry-Perot cavity of a Balle-Flygare FTMW spectrometer can typically be tuned into resonance at any frequency between 6 and 18&nbsp;GHz, the bandwidth of individual measurements is restricted to about 1&nbsp;MHz. An animation illustrates the operation of this instrument which is currently the most widely used tool for microwave spectroscopy.<ref>{{cite web|last=Jager|first=W.|title=Balle-Flygare FTMW spectrometer animation|url=http://www.chem.ualberta.ca/~jaeger/misc/ftmw.swf}}</ref>

====Chirped-Pulse FTMW spectrometer====
Noting that digitisers and related electronics technology had significantly progressed since the inception of FTMW spectroscopy, [[Brooks Pate|B.H. Pate]] at the University of Virginia<ref>{{cite web|title=Web page of B.H. Pate Research Group, Department of Chemistry, University of Virginia|url=http://faculty.virginia.edu/bpate-lab/}}</ref>  designed a spectrometer<ref>{{cite journal|title=The rotational spectrum of epifluorohydrin measured by chirped-pulse Fourier transform microwave spectroscopy|last=Brown|first=G.G.|author2=Dian, B.C.|author3=Douglass, K.O.|author4=Geyer, S.M.|author5=Pate, B.H.|journal=J. Mol. Spectrosc.|date=2006|volume=238|issue=2|pages=200–212|doi=10.1016/j.jms.2006.05.003|bibcode = 2006JMoSp.238..200B }}</ref> which retains many advantages of the Balle-Flygare FT-MW spectrometer while innovating in (i) the use of a high speed (>4 GS/s) arbitrary waveform generator to generate a "chirped" microwave polarisation pulse that sweeps up to 12&nbsp;GHz in frequency in less than a microsecond and (ii) the use of a high speed (>40 GS/s) oscilloscope to digitise and Fourier transform the molecular free induction decay. The result is an instrument that allows the study of weakly bound molecules but which is able to exploit a measurement bandwidth (12&nbsp;GHz) that is greatly enhanced compared with the Balle-Flygare FTMW spectrometer. Modified versions of the original CP-FTMW spectrometer have been constructed by a number of groups in the United States, Canada and Europe.<ref>{{cite journal|title=A search accelerated correct intensity Fourier transform microwave spectrometer with pulsed laser ablation source|last=Grubbs|first=G.S.|author2=Dewberry, C.T. |author3=Etchison, K.C. |author4=Kerr, K.E. |author5= Cooke, S.A. |journal=Rev. Sci. Instrum.|date=2007|volume=78|issue=9|pages=096106–096106–3|doi=10.1063/1.2786022|pmid=17902981|bibcode = 2007RScI...78i6106G }}</ref><ref>{{cite journal|title=Two-Dimensional Chirped-Pulse Fourier Transform Microwave Spectroscopy|last=Wilcox|first=D.S.|author2=Hotopp, K.M.|author3=Dian, B.C.|journal=J. Phys. Chem. A|date=2011|volume=115|issue=32|pages=8895–8905|doi=10.1021/jp2043202
|pmid=21728367|bibcode=2011JPCA..115.8895W}}</ref> The instrument offers a broadband capability that is highly complementary to the high sensitivity and resolution offered by the Balle-Flygare design.