Rovibronic Infrared Spectrum of a Rigid Diatomic Rotor

parameters

fundamental vibrational frequency 

-1

cm



550

rotational constant B 

-1

cm



1

temperature (Kelvin)

100

line width 

-1

cm



0.4

spectrum display

line color
filling
background color
enable peak height
peak height

x axis lower boundary

450

x axis upper boundary

650

This Demonstration lets you study rotationally-resolved infrared band spectra of the

v

0



v

1

vibrational transition in diatomic molecules. Due to quantized vibrational and rotational energy levels and the selection rules

Δv=±1

and

ΔJ=±1

, allowed transitions result in a highly ordered spectrum consisting of a P branch (

ΔJ=-1

, smaller wavenumbers) and an R branch (

ΔJ=+1

, larger wavenumbers) separated by a central gap. Adjacent spectral lines are separated by a spacing of

2B

, and since line intensities depend on the Boltzmann factor for thermal population and the quantum number

J

, each branch monotonically increases and/or decreases. As temperature increases, more lines are observed and the intensities of the lines decrease due to the population being spread over more rotational levels. Interactivity involves the effects of the fundamental vibrational frequency, rotational constant

B

, temperature, and line width on the observed spectrum.