# selection rule for rotational spectroscopy

Notice that there are no lines for, for example, J = 0 to J = 2 etc. We can consider selection rules for electronic, rotational, and vibrational transitions. Define vibrational raman spectroscopy. A selection rule describes how the probability of transitioning from one level to another cannot be zero. Rotational spectroscopy. Question: Prove The Selection Rule For DeltaJ In Rotational Spectroscopy This problem has been solved! For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Vibrational spectroscopy. Keep in mind the physical interpretation of the quantum numbers $$J$$ and $$M$$ as the total angular momentum and z-component of angular momentum, respectively. Diatomics. For electronic transitions the selection rules turn out to be $$\Delta{l} = \pm 1$$ and $$\Delta{m} = 0$$. A selection rule describes how the probability of transitioning from one level to another cannot be zero. Example transition strengths Type A21 (s-1) Example λ A 21 (s-1) Electric dipole UV 10 9 Ly α 121.6 nm 2.4 x 10 8 Visible 10 7 Hα 656 nm 6 x 10 6 Incident electromagnetic radiation presents an oscillating electric field $$E_0\cos(\omega t)$$ that interacts with a transition dipole. As stated above in the section on electronic transitions, these selection rules also apply to the orbital angular momentum ($$\Delta{l} = \pm 1$$, $$\Delta{m} = 0$$). Separations of rotational energy levels correspond to the microwave region of the electromagnetic spectrum. which will be non-zero if v’ = v – 1 or v’ = v + 1. If we now substitute the recursion relation into the integral we find, $(\mu_z)_{v,v'}=\frac{N_{\,v}N_{\,v'}}{\sqrt\alpha}\biggr({\frac{\partial\mu }{\partial q}}\biggr)$, $\int_{-\infty}^{\infty}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}\biggr(vH_{v-1}(\alpha^{1/2}q)+\frac{1}{2}H_{v+1}(\alpha^{1/2}q)\biggr)dq$. This leads to the selection rule $$\Delta J = \pm 1$$ for absorptive rotational transitions. only polar molecules will give a rotational spectrum. B. We can use the definition of the transition moment and the spherical harmonics to derive selection rules for a rigid rotator. Once the atom or molecules follow the gross selection rule, the specific selection rule must be applied to the atom or molecules to determine whether a certain transition in quantum number may happen or not. $\mu_z=\int\psi_1 \,^{*}\mu_z\psi_1\,d\tau$, A transition dipole moment is a transient dipolar polarization created by an interaction of electromagnetic radiation with a molecule, $(\mu_z)_{12}=\int\psi_1 \,^{*}\mu_z\psi_2\,d\tau$. 12. The dipole operator is $$\mu = e \cdot r$$ where $$r$$ is a vector pointing in a direction of space. The transition dipole moment for electromagnetic radiation polarized along the z axis is, $(\mu_z)_{v,v'}=\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H\mu_z(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$. $(\mu_z)_{J,M,{J}',{M}'}=\int_{0}^{2\pi } \int_{0}^{\pi }Y_{J'}^{M'}(\theta,\phi )\mu_zY_{J}^{M}(\theta,\phi)\sin\theta\,d\phi,d\theta\$, Notice that m must be non-zero in order for the transition moment to be non-zero. In vibrational–rotational Stokes scattering, the Δ J = ± 2 selection rule gives rise to a series of O -branch and S -branch lines shifted down in frequency from the laser line v i , and at We make the substitution $$x = \cos q, dx = -\sin\; q\; dq$$ and the integral becomes, $-\int_{1}^{-1}x dx=-\frac{x^2}{2}\Biggr\rvert_{1}^{-1}=0$. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. Effect of anharmonicity. Each line corresponds to a transition between energy levels, as shown. Selection Rules for rotational transitions ’ (upper) ” (lower) ... † Not IR-active, use Raman spectroscopy! 5.33 Lecture Notes: Vibrational-Rotational Spectroscopy Page 3 J'' NJ'' gJ'' thermal population 0 5 10 15 20 Rotational Quantum Number Rotational Populations at Room Temperature for B = 5 cm -1 So, the vibrational-rotational spectrum should look like equally spaced lines … the study of how EM radiation interacts with a molecule to change its rotational energy. Rotational spectroscopy (Microwave spectroscopy) Gross Selection Rule: For a molecule to exhibit a pure rotational spectrum it must posses a permanent dipole moment. Legal. We will prove the selection rules for rotational transitions keeping in mind that they are also valid for electronic transitions. The transition moment can be expanded about the equilibrium nuclear separation. Some examples. Spectra. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Selection rules: Internal rotations. For example, is the transition from $$\psi_{1s}$$ to $$\psi_{2s}$$ allowed? For asymmetric rotors,)J= 0, ±1, ±2, but since Kis not a good quantum number, spectra become quite … Selection rules. Symmetrical linear molecules, such as CO 2, C 2 H 2 and all homonuclear diatomic molecules, are thus said to be rotationally inactive, as they have no rotational spectrum. Polyatomic molecules. Selection rules for pure rotational spectra A molecule must have a transitional dipole moment that is in resonance with an electromagnetic field for rotational spectroscopy to be used. The rotational spectrum of a diatomic molecule consists of a series of equally spaced absorption lines, typically in the microwave region of the electromagnetic spectrum. $(\mu_z)_{v,v'}=\biggr({\frac{\partial\mu }{\partial q}}\biggr)\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H_v(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$, This integral can be evaluated using the Hermite polynomial identity known as a recursion relation, $xH_v(x)=vH_{v-1}(x)+\frac{1}{2}H_{v+1}(x)$, where x = Öaq. In order to observe emission of radiation from two states $$mu_z$$ must be non-zero. Stefan Franzen (North Carolina State University). Each line of the branch is labeled R (J) or P … The equilibrium nuclear separation status page at https: //status.libretexts.org can be expanded about the equilibrium nuclear separation and! Describe EM radiation ( wave )... † not IR-active, use Raman spectroscopy... + rules... 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