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# Bathochromic shift

Bathochromic shift is a change of spectral band position in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a longer wavelength (lower frequency).

This can occur because of a change in environmental conditions: for example, a change in solvent polarity will result in solvatochromism. A series of structurally related molecules in a substitution series can also show a bathochromic shift. Bathochromic shift is a phenomenon seen in molecular spectra, not atomic spectra - it is thus more common to speak of the movement of the peaks in the spectrum rather than lines.

$\Delta\lambda = \lambda^{\mathrm{state 2}}_{\mathrm{observed}} - \lambda^{\mathrm{state 1}}_{\mathrm{observed}}$ where λ is the wavelength of the spectral peak of interest and $\lambda^{\mathrm{state 2}}_{\mathrm{observed}} > \lambda^{\mathrm{state 1}}_{\mathrm{observed}}$

It is typically demonstrated using a spectrophotometer, colorimeter, or spectroradiometer.

Bathochromic shifts are often referred to as red shifts. Although this usage is considered informal [1], it is often used in the literature. It has no relation to Doppler shift or other wavelength-independent meanings of redshift.