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2013-09-20T17:49:20Z

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A '''rotational transition''' is an abrupt change in [[angular momentum]] in [[quantum physics]]. Like all other properties of a quantum [[Elementary particle|particle]], [[angular momentum quantization|angular momentum is quantized]], meaning it can only equal certain discrete values, which correspond to different [[rotational energy]] states. When a particle loses angular momentum, it is said to have transitioned to a lower rotational energy state. Likewise, when a particle gains angular momentum, a positive rotational transition is said to have occurred.

Rotational transitions are important in physics due to the unique [[spectral lines]] that result. Because there is a net gain or loss of energy during a transition, [[electromagnetic radiation]] of a particular [[frequency]] must be absorbed or emitted. This forms [[spectral lines]] at that frequency which can be detected with a [[spectrometer]], as in [[Rotational spectroscopy]] or [[Raman spectroscopy]].

==Diatomic Molecules==
Molecules have [[rotational kinetic energy|rotational energy]] owing to rotational motion of the nuclei about their [[center of mass]]. Due to [[quantization]], these energies can take only certain discrete values. Rotational transition thus corresponds to transition of the molecule from one rotational energy level to the other through gain or loss of a [[photon]]. Analysis is simple in case of [[diatomic molecules]].

===Nuclear Wave Function===
Quantum theoretical analysis of a molecule is simplified by use of [[Born–Oppenheimer approximation]]. Typically, rotational energies of molecules are smaller than [[molecular electronic transition|electronic transition]] energies by a factor of m/M ≈ 10−3 - 10−5 where m is electronic mass and M is typical nuclear mass.<ref>Chapter 10, Physics of Atoms and Molecules, B.H. Bransden and C.J. Jochain, Pearson education, 2nd edition.</ref> From [[uncertainty principle]], period of motion is of the order of [[Planck's constant]] ''h'' divided by its energy. Hence nuclear rotational periods are much longer than the electronic periods. So electronic and nuclear motions can be treated separately. In simple case of a diatomic molecule, radial part of [[Schrodinger Equation]] for nuclear wave function Fs('''R'''), in an electronic state s, is written as (neglecting spin interactions)
: <math>[- \frac{\hbar^2}{2\mu R^2} \frac{\partial}{\partial R} (R^2 \frac{\partial}{\partial R})+ \frac{\langle \Phi_s|N^2|\Phi_s \rangle}{2\mu R^2}+ E_s(R)-E]F_s(\mathbf R) = 0 </math>
where μ is [[reduced mass]] of two nuclei, '''R''' is vector joining the two nuclei, Es(R) is energy [[eigenvalue]] of electronic wave function Φs representing electronic state s and N is orbital [[momentum operator]] for the relative motion of the two nuclei given by
:<math> N^2 = -\hbar^2 [ \frac{1}{sin\Theta} \frac{\partial}{\partial \Theta}(sin \Theta \frac{\partial}{\partial \Theta})+ \frac{1}{sin^2\Theta} \frac{\partial^2}{\partial \Phi^2} ] </math>
The total [[wave function]] for the molecule is
:<math> \Psi_s = F_s(\mathbf R)\Phi_s(\mathbf R,\mathbf r_1, \mathbf r_2, ...., \mathbf r_N)</math>
where '''r'''i are position vectors from center of mass of molecule to ith electron.
As a consequence of Born-Oppenheimer approximation, the electronic wave functions Φs is considered to vary very slowly with '''R'''. Thus Schrodinger equation for electronic wave function is first solved to obtain Es(R) for different values of R. Es then plays role of a [[potential well]] in analysis of nuclear wave functions Fs('''R''').

[[File:Angular Momentum of a Diatomic Molecule.png|thumb|Vector addition triangle for orbital angular momentum of a diatomic molecule with components of orbital angular momentum of nuclei and orbital angular momentum of electrons, neglecting coupling between electron and nuclear orbital motion and spin-dependent coupling.Since angular momentum '''N''' of nuclei is perpendicular to internuclear vector '''R''', components of electronic angular momentum '''L''' and total angular momentum '''J''' along '''R''' are equal.]]

===Rotational Energy Levels===
First term in the above nuclear wave function equation corresponds to [[kinetic energy]] of nuclei due to their radial motion. Term <Φs|N2|Φs>/2μR2 represents rotational kinetic energy of the two nuclei, about their center of mass, in a given electronic state Φs. Possible values of the same are different rotational energy levels for the molecule.

[[angular momentum|Orbital angular momentum]] for the rotational motion of nuclei can be written as
:<math> \mathbf N = \mathbf J - \mathbf L </math>
where '''J''' is the total orbital angular momentum of the whole molecule and '''L''' is the orbital angular momentum of the electrons.
If internuclear vector '''R''' is taken along z-axis, component of '''N''' along z-axis - Nz - becomes zero as
:<math> \mathbf N = \mathbf R \times \mathbf P </math>
Hence
:<math> J_z=L_z </math>
Since molecular wave function Ψs is a simultaneous [[eigenfunction]] of J2 and Jz,
:<math> J^2 \Psi_s = J(J+1) \hbar^2 \Psi_s </math>
where J is called [[quantum number|rotational quantum number]] and J can be a positive integer or zero.
:<math> J_z \Psi_s = M_j\hbar \Psi_s </math>
where -J ≤ Mj ≤ J.

Also since electronic wave function Φsis an eigenfunction of Lz,
:<math> L_z \Phi_s = \pm \Lambda\hbar \Phi_s </math>
Hence molecular wave function Ψs is also an eigenfunction of Lz with eigenvalue ±Λ''ħ''.
Since Lz and Jz are equal, Ψs is an eigenfunction of Jz with same eigenvalue ±Λ''ħ''. As |'''J'''| ≥ Jz, we have J ≥ Λ. So possible values of rotational quantum number are
:<math> J = \Lambda, \Lambda +1, \Lambda+2, ...... </math>
Thus molecular wave function Ψs is simultaneous eigenfunction of J2, Jz and Lz.
Since molecule is in eigenstate of Lz, expectation value of components perpendicular to the direction of z-axis (internuclear line) is zero. Hence
: <math> \langle \Psi_s|L_x|\Psi_s\rangle = \langle L_x \rangle = 0 </math>
and
: <math> \langle \Psi_s|L_y|\Psi_s\rangle = \langle L_y \rangle = 0 </math>
Thus
: <math> \langle \mathbf J . \mathbf L \rangle = \langle J_z L_z \rangle = \langle {L_z}^2 \rangle </math>

Putting all these results together,
:<math> \langle \Phi_s |N^2|\Phi_s \rangle F_s(\mathbf R) = \langle \Phi_s |J^2 + L^2 - 2 \mathbf J . \mathbf L|\Phi_s \rangle F_s(\mathbf R) = \hbar^2 [J(J+1)-\Lambda^2]F_s(\mathbf R) + \langle \Phi_s |{L_x}^2 + {L_y}^2|\Phi_s \rangle F_s(\mathbf R)</math>

Schrodinger equation for nuclear wave function can now be rewritten as
: <math>- \frac{\hbar^2}{2\mu R^2}[ \frac{\partial}{\partial R} (R^2 \frac{\partial}{\partial R})- J(J+1)]F_s(\mathbf R)+[{E'}_s(R)-E]F_s(\mathbf R) = 0 </math>
where
:<math> {E'}_s(R)=E_s(R) - \frac{\Lambda^2 \hbar^2}{2\mu R^2} + \frac{1}{2\mu R^2} \langle \Phi_s |{L_x}^2 + {L_y}^2|\Phi_s \rangle </math>
E's now serves as effective potential in radial nuclear wave function equation.

====Sigma States====
Molecular states in which total orbital momentum of electrons is zero are called [[molecular orbitals|sigma states]]. In Sigma states Λ=0. Thus E's(R) = Es(R). As nuclear motion for a stable molecule is generally confined to a small interval around R0 where R0 corresponds to internuclear distance for minimum value of potential Es(R0), rotational energies are given by,
:<math> E_r = \frac{\hbar^2}{2\mu {R_0}^2} J(J+1) = \frac{\hbar^2}{2I_0} J(J+1) = BJ(J+1) </math>
with
:<math> J = \Lambda, \Lambda +1, \Lambda+2, ...... </math>
I0 is [[moment of inertia]] of the molecule corresponding to [[mechanical equilibrium|equilibrium]] distance R0 and B is called '''rotational constant''' for a given electronic state Φs.
Since reduced mass μ is much greater than electronic mass, last two terms in the expression of E's(R) are small compared to Es. Hence even for states other than sigma states, rotational energy is approximately given by above expression.

===Rotational Spectrum===
{{main|Rotational spectroscopy}}
When a rotational transition occurs, there is a change in the value of rotational quantum number J. Selection rules for rotational transition are,
when Λ = 0, ΔJ = ±1 and
when Λ ≠ 0, ΔJ = 0 ,±1 as absorbed or emitted photon can make equal and opposite change in total nuclear angular momentum and total electronic angular momentum without changing value of J.

The pure rotational spectrum of a diatomic molecule consists of lines in the far [[infrared]] or [[microwave]] region. The frequency of these lines is given by
:<math> \hbar \omega = E_r(J+1)-E_r(J)=2B(J+1) </math>
Thus values of B, I0 and R0 of a substance can be determined from observed rotational spectrum.

{{Reflist}}

== References ==
*{{cite book | author=B.H.Bransden C.J.Jochain | title=Physics of Atoms and Molecules | publisher=Pearson Education }}
*{{cite book | author=L.D.Landau E.M.Lifshitz | title=Quantum Mechanics (Non-relativistic Theory) | publisher=Reed Elsvier }}

==See also==
* [[Vibrational transition]]

{{DEFAULTSORT:Rotational Transition}}
[[Category:Chemical physics]]

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