In electronic

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

, constructing a Deslandres table is a useful method to assign

vibronic transition Vibronic spectroscopy is a branch of molecular spectroscopy concerned with vibronic transitions: the simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the appropriate ...

s. In such a table, the frequencies of the lines seen in an electronic spectrum of a

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...

are collected so that the differences in energy between adjacent columns or rows are all the same (within

experimental error Observational error (or measurement error) is the difference between a measured value of a quantity and its true value.Dodge, Y. (2003) ''The Oxford Dictionary of Statistical Terms'', OUP. In statistics, an error is not necessarily a " mistak ...

). Every line seen in the spectrum corresponds to a transition from the lower-lying electronic energy state to an excited electronic state (

molecular electronic transition Molecular electronic transitions take place when electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with this transition provides information on the structure of a molecule and determin ...

): associated with this, there are corresponding transitions between the vibrational levels of the two states, that give rise to many closely spaced lines. The intensity of the lines is governed by the

Franck–Condon principle The Franck–Condon principle (named for James Franck and Edward Condon) is a rule in spectroscopy and quantum chemistry that explains the intensity of vibronic transitions (the simultaneous changes in electronic and vibrational energy levels o ...

. When a Deslandres table is filled correctly, the columns represent the vibrational energy levels of the lower electronic state (''v''''), while the rows represent those for the upper electronic state (''v'''). Clearly, the difference in energy between two successive rows (columns) must remain constant for any column (row) in the table because this represents the energy difference of two given vibrational levels within the same electronic state. Thus, by completing a Deslandres table it is easy to assign the correct vibrational quantum numbers ''v'''' and ''v''' for the transition, allowing important molecular properties to be calculated, such as the

dissociation energy The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical s ...

References

* {{Cite book , last=Hollas , first=J. Michael , year=2003 , edition=4th , title=Modern spectroscopy , publisher=

Wiley Wiley may refer to: Locations * Wiley, Colorado, a U.S. town * Wiley, Pleasants County, West Virginia, U.S. * Wiley-Kaserne, a district of the city of Neu-Ulm, Germany People * Wiley (musician), British grime MC, rapper, and producer * Wiley Mil ...

, isbn=978-0-470-84416-8 , url-access=registration , url=https://archive.org/details/modernspectrosco0000holl_t7j3 Spectroscopy