# selection rule for rotational spectroscopy

The selection rule is a statement of when $$\mu_z$$ is non-zero. This proves that a molecule must have a permanent dipole moment in order to have a rotational spectrum. Prove the selection rule for deltaJ in rotational spectroscopy The rotational selection rule gives rise to an R-branch (when ∆J = +1) and a P-branch (when ∆J = -1). Each line of the branch is labeled R (J) or P … A rotational spectrum would have the following appearence. Selection rules. That is, $(\mu_z)_{12}=\int\psi_1^{\,*}\,e\cdot z\;\psi_2\,d\tau\neq0$. $\mu_z(q)=\mu_0+\biggr({\frac{\partial\mu }{\partial q}}\biggr)q+.....$, where m0 is the dipole moment at the equilibrium bond length and q is the displacement from that equilibrium state. The spherical harmonics can be written as, $Y_{J}^{M}(\theta,\phi)=N_{\,JM}P_{J}^{|M|}(\cos\theta)e^{iM\phi}$, where $$N_{JM}$$ is a normalization constant. where $$H_v(a1/2q)$$ is a Hermite polynomial and a = (km/á2)1/2. We can use the definition of the transition moment and the spherical harmonics to derive selection rules for a rigid rotator. Long (1977) gives the selection rules for pure rotational scattering and vibrational–rotational scattering from symmetric-top and spherical-top molecules. Spectra. Separations of rotational energy levels correspond to the microwave region of the electromagnetic spectrum. For a rigid rotor diatomic molecule, the selection rules for rotational transitions are ΔJ = +/-1, ΔM J = 0 . The selection rule for rotational transitions, derived from the symmetries of the rotational wave functions in a rigid rotor, is Δ J = ±1, where J is a rotational quantum number. As stated above in the section on electronic transitions, these selection rules also apply to the orbital angular momentum ($$\Delta{l} = \pm 1$$, $$\Delta{m} = 0$$). The dipole operator is $$\mu = e \cdot r$$ where $$r$$ is a vector pointing in a direction of space. We make the substitution $$x = \cos q, dx = -\sin\; q\; dq$$ and the integral becomes, $-\int_{1}^{-1}x dx=-\frac{x^2}{2}\Biggr\rvert_{1}^{-1}=0$. 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. Selection Rules for rotational transitions ’ (upper) ” (lower) ... † Not IR-active, use Raman spectroscopy! B. • Classical origin of the gross selection rule for rotational transitions. In rotational Raman, for a linear molecule, the selection rule for J is: ΔJ = ±2 (as opposed to ΔJ = ± 1 in pure rotational spectroscopy) If ΔJ = 0 we obtaine Rayleigh line! 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. The result is an even function evaluated over odd limits. The transition dipole moment for electromagnetic radiation polarized along the z axis is, $(\mu_z)_{v,v'}=\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H\mu_z(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$. With symmetric tops, the selection rule for electric-dipole-allowed pure rotation transitions is Δ K = 0, Δ J = ±1. Quantum mechanics of light absorption. Vibration-rotation spectra. We can see specifically that we should consider the q integral. We will study: classical rotational motion, angular momentum, rotational inertia; quantum mechanical energy levels; selection rules and microwave (rotational) spectroscopy; the extension to polyatomic molecules DFs N atomic Linear Molecule 2 DFs Rotation Vibration Rotational and vibrational 3N — 5 3N - 6 N atomic Non-Linear Molecule 3 DFs 15 Av = +1 (absorption) Av = --1 (emission) Vibrational Spectroscopy Vibrationa/ selection rule Av=+l j=ło Aj j=ło 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 Gross Selection Rule: A molecule has a rotational spectrum only if it has a permanent dipole moment. 12. The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Legal. Rotational Raman Spectroscopy Gross Selection Rule: The molecule must be anisotropically polarizable Spherical molecules are isotropically polarizable and therefore do not have a Rotational Raman Spectrum All linear molecules are anisotropically polarizable, and give a Rotational Raman Spectrum, even molecules such as O 2, N 2, H Rotational Spectroscopy: A. In the case of rotation, the gross selection rule is that the molecule must have a permanent electric dipole moment. The harmonic oscillator wavefunctions are, $\psi_{\,v}(q)=N_{\,v}H_{\,v}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}$. 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. 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 only polar molecules will give a rotational spectrum. See the answer. Selection Rules for Pure Rotational Spectra The rules are applied to the rotational spectra of polar molecules when the transitional dipole moment of the molecule is in resonance with an external electromagnetic field. 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 … In solids or liquids the rotational motion is usually quenched due to collisions between their molecules. Rotational spectroscopy is only really practical in the gas phase where the rotational motion is quantized. A transitional dipole moment not equal to zero is possible. Have questions or comments? Note that we continue to use the general coordinate q although this can be z if the dipole moment of the molecule is aligned along the z axis. This condition is known as the gross selection rule for microwave, or pure rotational, spectroscopy. For example, is the transition from $$\psi_{1s}$$ to $$\psi_{2s}$$ allowed? Rotational degrees of freedom Vibrational degrees of freedom Linear Non-linear 3 3 2 3 ... + Selection rules. In order for a molecule to absorb microwave radiation, it must have a permanent dipole moment. $(\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. A selection rule describes how the probability of transitioning from one level to another cannot be zero. $\int_{0}^{\infty}e^{-r/a_0}r\biggr(2-\frac{r}{a_0}\biggr)e^{-r/a_0}r^2dr\int_{0}^{\pi}\cos\theta\sin\theta\,d\theta\int_{0}^{2\pi }d\phi$, If any one of these is non-zero the transition is not allowed. Quantum theory of rotational Raman spectroscopy Polar molecules have a dipole moment. Question: Prove The Selection Rule For DeltaJ In Rotational Spectroscopy This problem has been solved! (1 points) List are the selection rules for rotational spectroscopy. Watch the recordings here on Youtube! Rotational spectroscopy (Microwave spectroscopy) Gross Selection Rule: For a molecule to exhibit a pure rotational spectrum it must posses a permanent dipole moment. $(\mu_z)_{v,v'}=\biggr({\frac{\partial\mu }{\partial q}}\biggr)\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H_v(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$, 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. Once again we assume that radiation is along the z axis. i.e. Some examples. In pure rotational spectroscopy, the selection rule is ΔJ = ±1. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. We can consider selection rules for electronic, rotational, and vibrational transitions. ≠ 0. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. Polyatomic molecules. Describe EM radiation (wave) ... What is the specific selection rule for rotational raman ∆J=0, ±2. Since these transitions are due to absorption (or emission) of a single photon with a spin of one, conservation of angular momentum implies that the molecular angular momentum can change by … This leads to the selection rule $$\Delta J = \pm 1$$ for absorptive rotational transitions. In a similar fashion we can show that transitions along the x or y axes are not allowed either. In an experiment we present an electric field along the z axis (in the laboratory frame) and we may consider specifically the interaction between the transition dipole along the x, y, or z axis of the molecule with this radiation. From the first two terms in the expansion we have for the first term, $(\mu_z)_{v,v'}=\mu_0\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H_v(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$. 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. which is zero. Schrödinger equation for vibrational motion. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Vibrational spectroscopy. Selection rules: a worked example Consider an optical dipole transition matrix element such as used in absorption or emission spectroscopies € ∂ω ∂t = 2π h Fermi’s golden rule ψ f H&ψ i δ(E f −E i −hω) The operator for the interaction between the system and the electromagnetic field is € H" = e mc (r A ⋅ … The gross selection rule for rotational Raman spectroscopy is that the molecule must be anisotropically polarisable, which means that the distortion induced in the electron distribution in the molecule by an electric field must be dependent upon the orientation of the molecule in the field. Substituting into the integral one obtains an integral which will vanish unless $$J' = J + 1$$ or $$J' = J - 1$$. This presents a selection rule that transitions are forbidden for $$\Delta{l} = 0$$. [ "article:topic", "selection rules", "showtoc:no" ], Selection rules and transition moment integral, information contact us at info@libretexts.org, status page at https://status.libretexts.org. /h hc n lD 1 1 ( ) 1 ( ) j j absorption j emission D D D Rotational Spectroscopy (1) Bohr postulate (2) Selection Rule 22. 21. We will prove the selection rules for rotational transitions keeping in mind that they are also valid for electronic transitions. If we now substitute the recursion relation into the integral we find, $(\mu_z)_{v,v'}=\frac{N_{\,v}N_{\,v'}}{\sqrt\alpha}\biggr({\frac{\partial\mu }{\partial q}}\biggr)$, $\int_{-\infty}^{\infty}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}\biggr(vH_{v-1}(\alpha^{1/2}q)+\frac{1}{2}H_{v+1}(\alpha^{1/2}q)\biggr)dq$. Internal rotations. Define vibrational raman spectroscopy. i.e. What information is obtained from the rotational spectrum of a diatomic molecule and how can… $\mu_z=\int\psi_1 \,^{*}\mu_z\psi_1\,d\tau$, A transition dipole moment is a transient dipolar polarization created by an interaction of electromagnetic radiation with a molecule, $(\mu_z)_{12}=\int\psi_1 \,^{*}\mu_z\psi_2\,d\tau$. 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. We consider a hydrogen atom. Energy levels for diatomic molecules. which will be non-zero if v’ = v – 1 or v’ = v + 1. Selection rules: The Specific Selection Rule of Rotational Raman Spectroscopy The specific selection rule for Raman spectroscopy of linear molecules is Δ J = 0 , ± 2 {\displaystyle \Delta J=0,\pm 2} . Raman spectroscopy Selection rules in Raman spectroscopy: Δv = ± 1 and change in polarizability α (dα/dr) ≠0 In general: electron cloud of apolar bonds is stronger polarizable than that of polar bonds. This is the origin of the J = 2 selection rule in rotational Raman spectroscopy. Using the standard substitution of $$x = \cos q$$ we can express the rotational transition moment as, $(\mu_z)_{J,M,{J}',{M}'}=\mu\,N_{\,JM}N_{\,J'M'}\int_{0}^{2 \pi }e^{I(M-M')\phi}\,d\phi\int_{-1}^{1}P_{J'}^{|M'|}(x)P_{J}^{|M|}(x)dx$, The integral over f is zero unless M = M' so $$\Delta M =$$ 0 is part of the rigid rotator selection rule. Raman effect. the study of how EM radiation interacts with a molecule to change its rotational energy. In order to observe emission of radiation from two states $$mu_z$$ must be non-zero. Explore examples of rotational spectroscopy of simple molecules. Integration over $$\phi$$ for $$M = M'$$ gives $$2\pi$$ so we have, $(\mu_z)_{J,M,{J}',{M}'}=2\pi \mu\,N_{\,JM}N_{\,J'M'}\int_{-1}^{1}P_{J'}^{|M'|}(x)P_{J}^{|M|}(x)dx$, We can evaluate this integral using the identity, $(2J+1)x\,P_{J}^{|M]}(x)=(J-|M|+1)P_{J+1}^{|M|}(x)+(J-|M|)P_{J-1}^{|M|}(x)$. De ning the rotational constant as B= ~2 2 r2 1 hc = h 8ˇ2c r2, the rotational terms are simply F(J) = BJ(J+ 1): In a transition from a rotational level J00(lower level) to J0(higher level), the selection rule J= 1 applies. Notice that there are no lines for, for example, J = 0 to J = 2 etc. It has two sub-pieces: a gross selection rule and a specific selection rule. For asymmetric rotors,)J= 0, ±1, ±2, but since Kis not a good quantum number, spectra become quite … C. (1/2 point) Write the equation that gives the energy levels for rotational spectroscopy. a. Selection rules specify the possible transitions among quantum levels due to absorption or emission of electromagnetic radiation. 26.4.2 Selection Rule Now, the selection rule for vibrational transition from ! ed@ AV (Ç ÷Ù÷­Ço9ÀÇ°ßc>ÏV mM(&ÈíÈÿÃðqÎÑV îÓsç¼/IK~fvøÜd¶EÜ÷GÂ¦HþË.Ìoã^:¡×æÉØî uºÆ÷. Vibrational Selection Rules Selection Rules: IR active modes must have IrrReps that go as x, y, z. Raman active modes must go as quadratics (xy, xz, yz, x2, y2, z2) (Raman is a 2-photon process: photon in, scattered photon out) IR Active Raman Active 22 A gross selection rule illustrates characteristic requirements for atoms or molecules to display a spectrum of a given kind, such as an IR spectroscopy or a microwave spectroscopy. (2 points) Provide a phenomenological justification of the selection rules. $(\mu_z)_{12}=\int\psi_{1s}\,^{\,*}\,e\cdot z\;\psi_{2s}\,d\tau$, Using the fact that z = r cosq in spherical polar coordinates we have, $(\mu_z)_{12}=e\iiint\,e^{-r/a_0}r\cos \theta \biggr(2-\frac{r}{a_0}\biggr)e^{-r/a_0}r^2\sin\theta drd\theta\,d\phi$. Effect of anharmonicity. Define rotational spectroscopy. Incident electromagnetic radiation presents an oscillating electric field $$E_0\cos(\omega t)$$ that interacts with a transition dipole. It has two sub-pieces: a gross selection rule and a specific selection rule. A selection rule describes how the probability of transitioning from one level to another cannot be zero. Transitions between discrete rotational energy levels give rise to the rotational spectrum of the molecule (microwave spectroscopy). Solution for This question pertains to rotational spectroscopy. If $$\mu_z$$ is zero then a transition is forbidden. Specific rotational Raman selection rules: Linear rotors: J = 0, 2 The distortion induced in a molecule by an applied electric field returns to its initial value after a rotation of only 180 (that is, twice a revolution). Each line corresponds to a transition between energy levels, as shown. The Raman spectrum has regular spacing of lines, as seen previously in absorption spectra, but separation between the lines is doubled. The transition moment can be expanded about the equilibrium nuclear separation. Polyatomic molecules. These result from the integrals over spherical harmonics which are the same for rigid rotator wavefunctions. $\int_{-1}^{1}P_{J'}^{|M'|}(x)\Biggr(\frac{(J-|M|+1)}{(2J+1)}P_{J+1}^{|M|}(x)+\frac{(J-|M|)}{(2J+1)}P_{J-1}^{|M|}(x)\Biggr)dx$. Diatomics. Rotational spectroscopy. This term is zero unless v = v’ and in that case there is no transition since the quantum number has not changed. 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More information contact us at info @ libretexts.org or check out our page! That v = v + 1 ) for absorptive rotational transitions: a gross selection rule how! Motion is quantized occur if the dipole moment, LibreTexts content is licensed CC... That radiation is along the z axis have a rotational spectrum that there are no for...