Editing Molecule (section)

== Molecular spectroscopy ==
{{Main|Spectroscopy}}

[[File:Dehydrogenation of H2TPP by STM.jpg|thumb|upright=1.3|Hydrogen can be removed from individual [[Tetraphenylporphyrin|H<sub>2</sub>TPP]] molecules by applying excess voltage to the tip of a [[scanning tunneling microscope]] (STM, a); this removal alters the current-voltage (I-V) curves of TPP molecules, measured using the same STM tip, from [[diode]] like (red curve in b) to [[resistor]] like (green curve). Image (c) shows a row of TPP, H<sub>2</sub>TPP and TPP molecules. While scanning image (d), excess voltage was applied to H<sub>2</sub>TPP at the black dot, which instantly removed hydrogen, as shown in the bottom part of (d) and in the rescan image (e). Such manipulations can be used in [[single-molecule electronics]].<ref>{{cite journal|doi=10.1038/srep08350|pmid=25666850|pmc=4322354|title=N and p type character of single molecule diodes|journal=Scientific Reports|volume=5|page=8350|year=2015|bibcode= 2015NatSR...5.8350Z|last1=Zoldan|first1=V. C.|last2=Faccio|first2=R|last3=Pasa|first3=A.A.}}</ref>]]

'''Molecular spectroscopy''' deals with the response ([[frequency spectrum|spectrum]]) of molecules interacting with probing signals of known [[energy]] (or [[frequency]], according to the [[Planck relation]]). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through [[absorbance]] or [[Emission (electromagnetic radiation)|emission]].<ref name="iupac2">{{GoldBookRef|title=Spectroscopy|file=S05848|accessdate=23 February 2016}}</ref>
Spectroscopy does not generally refer to [[diffraction]] studies where particles such as [[neutron]]s, electrons, or high energy [[X-ray]]s interact with a regular arrangement of molecules (as in a crystal).

[[Microwave spectroscopy]] commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space. [[Infrared spectroscopy]] measures the vibration of molecules, including stretching, bending or twisting motions. It is commonly used to identify the kinds of bonds or [[functional group]]s in molecules. Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible or [[near infrared]] light, and result in colour. [[Nuclear magnetic resonance spectroscopy|Nuclear resonance spectroscopy]] measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule.