# selection rule for rotational 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. [ "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. Schrödinger equation for vibrational motion. Selection Rules for rotational transitions ’ (upper) ” (lower) ... † Not IR-active, use Raman spectroscopy! Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. $(\mu_z)_{v,v'}=\biggr({\frac{\partial\mu }{\partial q}}\biggr)\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H_v(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$, 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. 21. 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 Explore examples of rotational spectroscopy of simple molecules. We can use the definition of the transition moment and the spherical harmonics to derive selection rules for a rigid rotator. Quantum theory of rotational Raman spectroscopy The dipole operator is $$\mu = e \cdot r$$ where $$r$$ is a vector pointing in a direction of space. 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. 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, $(2J+1)x\,P_{J}^{|M]}(x)=(J-|M|+1)P_{J+1}^{|M|}(x)+(J-|M|)P_{J-1}^{|M|}(x)$. Substituting into the integral one obtains an integral which will vanish unless $$J' = J + 1$$ or $$J' = J - 1$$. 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. We make the substitution $$x = \cos q, dx = -\sin\; q\; dq$$ and the integral becomes, $-\int_{1}^{-1}x dx=-\frac{x^2}{2}\Biggr\rvert_{1}^{-1}=0$. For example, is the transition from $$\psi_{1s}$$ to $$\psi_{2s}$$ allowed? 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. We consider a hydrogen atom. Selection rules: 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 Each line corresponds to a transition between energy levels, as shown. That is, $(\mu_z)_{12}=\int\psi_1^{\,*}\,e\cdot z\;\psi_2\,d\tau\neq0$. In solids or liquids the rotational motion is usually quenched due to collisions between their molecules. From the first two terms in the expansion we have for the first term, $(\mu_z)_{v,v'}=\mu_0\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H_v(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$. 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. What information is obtained from the rotational spectrum of a diatomic molecule and how can… The Raman spectrum has regular spacing of lines, as seen previously in absorption spectra, but separation between the lines is doubled. 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). 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 Polar molecules have a dipole moment. 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. only polar molecules will give a rotational spectrum. Effect of anharmonicity. 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 $\mu_z=\int\psi_1 \,^{*}\mu_z\psi_1\,d\tau$, A transition dipole moment is a transient dipolar polarization created by an interaction of electromagnetic radiation with a molecule, $(\mu_z)_{12}=\int\psi_1 \,^{*}\mu_z\psi_2\,d\tau$. Rotational spectroscopy is only really practical in the gas phase where the rotational motion is quantized. Internal rotations. 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 ⋅ … Prove the selection rule for deltaJ in rotational spectroscopy Rotational Spectroscopy: A. Selection rules specify the possible transitions among quantum levels due to absorption or emission of electromagnetic radiation. Transitions between discrete rotational energy levels give rise to the rotational spectrum of the molecule (microwave spectroscopy). These result from the integrals over spherical harmonics which are the same for rigid rotator wavefunctions. In pure rotational spectroscopy, the selection rule is ΔJ = ±1. This leads to the selection rule $$\Delta J = \pm 1$$ for absorptive rotational transitions. Long (1977) gives the selection rules for pure rotational scattering and vibrational–rotational scattering from symmetric-top and spherical-top molecules. The result is an even function evaluated over odd limits. For electronic transitions the selection rules turn out to be $$\Delta{l} = \pm 1$$ and $$\Delta{m} = 0$$. 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. Watch the recordings here on Youtube! We also see that vibrational transitions will only occur if the dipole moment changes as a function nuclear motion. For a rigid rotor diatomic molecule, the selection rules for rotational transitions are ΔJ = +/-1, ΔM J = 0 . Polyatomic molecules. $(\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. Incident electromagnetic radiation presents an oscillating electric field $$E_0\cos(\omega t)$$ that interacts with a transition dipole. We can see specifically that we should consider the q integral. If we now substitute the recursion relation into the integral we find, $(\mu_z)_{v,v'}=\frac{N_{\,v}N_{\,v'}}{\sqrt\alpha}\biggr({\frac{\partial\mu }{\partial q}}\biggr)$, $\int_{-\infty}^{\infty}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}\biggr(vH_{v-1}(\alpha^{1/2}q)+\frac{1}{2}H_{v+1}(\alpha^{1/2}q)\biggr)dq$. We can consider each of the three integrals separately. Missed the LibreFest? Notice that there are no lines for, for example, J = 0 to J = 2 etc. This condition is known as the gross selection rule for microwave, or pure rotational, spectroscopy. a. The transition moment can be expanded about the equilibrium nuclear separation. See the answer. Once again we assume that radiation is along the z axis. Rotational spectroscopy. If $$\mu_z$$ is zero then a transition is forbidden. ed@ AV (Ç ÷Ù÷­Ço9ÀÇ°ßc>ÏV mM(&ÈíÈÿÃðqÎÑV îÓsç¼/IK~fvøÜd¶EÜ÷GÂ¦HþË.Ìoã^:¡×æÉØî uºÆ÷. Each line of the branch is labeled R (J) or P … A selection rule describes how the probability of transitioning from one level to another cannot be zero. i.e. (2 points) Provide a phenomenological justification of the selection rules. where $$H_v(a1/2q)$$ is a Hermite polynomial and a = (km/á2)1/2. 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 … Separations of rotational energy levels correspond to the microwave region of the electromagnetic spectrum. Selection rules. B. Polyatomic molecules. The harmonic oscillator wavefunctions are, $\psi_{\,v}(q)=N_{\,v}H_{\,v}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}$. 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. As stated above in the section on electronic transitions, these selection rules also apply to the orbital angular momentum ($$\Delta{l} = \pm 1$$, $$\Delta{m} = 0$$). This term is zero unless v = v’ and in that case there is no transition since the quantum number has not changed. Energy levels for diatomic molecules. For asymmetric rotors,)J= 0, ±1, ±2, but since Kis not a good quantum number, spectra become quite … For a symmetric rotor molecule the selection rules for rotational Raman spectroscopy are:)J= 0, ±1, ±2;)K= 0 resulting in Rand Sbranches for each value of K(as well as Rayleigh scattering). 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. With symmetric tops, the selection rule for electric-dipole-allowed pure rotation transitions is Δ K = 0, Δ J = ±1. Rotational degrees of freedom Vibrational degrees of freedom Linear Non-linear 3 3 2 3 ... + Selection rules. 26.4.2 Selection Rule Now, the selection rule for vibrational transition from ! i.e. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. 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 order for a molecule to absorb microwave radiation, it must have a permanent dipole moment. A transitional dipole moment not equal to zero is possible. 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. • Classical origin of the gross selection rule for rotational transitions. the study of how EM radiation interacts with a molecule to change its rotational energy. 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. Diatomics. Vibration-rotation spectra. which will be non-zero if v’ = v – 1 or v’ = v + 1. 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 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 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. Spectra. It has two sub-pieces: a gross selection rule and a specific selection rule. Solution for This question pertains to rotational spectroscopy. $\int_{0}^{\infty}e^{-r/a_0}r\biggr(2-\frac{r}{a_0}\biggr)e^{-r/a_0}r^2dr\int_{0}^{\pi}\cos\theta\sin\theta\,d\theta\int_{0}^{2\pi }d\phi$, If any one of these is non-zero the transition is not allowed. In the case of rotation, the gross selection rule is that the molecule must have a permanent electric dipole moment. ≠ 0. The rotational selection rule gives rise to an R-branch (when ∆J = +1) and a P-branch (when ∆J = -1). 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} . 12. 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. This is the origin of the J = 2 selection rule in rotational Raman spectroscopy. Describe EM radiation (wave) ... What is the specific selection rule for rotational raman ∆J=0, ±2. In a similar fashion we can show that transitions along the x or y axes are not allowed either. A rotational spectrum would have the following appearence. Vibrational spectroscopy. Define vibrational raman spectroscopy. This presents a selection rule that transitions are forbidden for $$\Delta{l} = 0$$. $\int_{-1}^{1}P_{J'}^{|M'|}(x)\Biggr(\frac{(J-|M|+1)}{(2J+1)}P_{J+1}^{|M|}(x)+\frac{(J-|M|)}{(2J+1)}P_{J-1}^{|M|}(x)\Biggr)dx$. Quantum mechanics of light absorption. (1 points) List are the selection rules for rotational spectroscopy. We will prove the selection rules for rotational transitions keeping in mind that they are also valid for electronic transitions. Rotational spectroscopy (Microwave spectroscopy) Gross Selection Rule: For a molecule to exhibit a pure rotational spectrum it must posses a permanent dipole moment. It has two sub-pieces: a gross selection rule and a specific selection rule. Define rotational spectroscopy. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. This proves that a molecule must have a permanent dipole moment in order to have a rotational spectrum. Stefan Franzen (North Carolina State University). $\mu_z(q)=\mu_0+\biggr({\frac{\partial\mu }{\partial q}}\biggr)q+.....$, where m0 is the dipole moment at the equilibrium bond length and q is the displacement from that equilibrium state. The 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. In order to observe emission of radiation from two states $$mu_z$$ must be non-zero. The transition dipole moment for electromagnetic radiation polarized along the z axis is, $(\mu_z)_{v,v'}=\int_{-\infty}^{\infty}N_{\,v}N_{\,v'}H_{\,v'}(\alpha^{1/2}q)e^{-\alpha\,q^2/2}H\mu_z(\alpha^{1/2}q)e^{-\alpha\,q^2/2}dq$. $(\mu_z)_{12}=\int\psi_{1s}\,^{\,*}\,e\cdot z\;\psi_{2s}\,d\tau$, Using the fact that z = r cosq in spherical polar coordinates we have, $(\mu_z)_{12}=e\iiint\,e^{-r/a_0}r\cos \theta \biggr(2-\frac{r}{a_0}\biggr)e^{-r/a_0}r^2\sin\theta drd\theta\,d\phi$. We can consider selection rules for electronic, rotational, and vibrational transitions. The selection rule is a statement of when $$\mu_z$$ is 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 Question: Prove The Selection Rule For DeltaJ In Rotational Spectroscopy This problem has been solved! which is zero. 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. C. (1/2 point) Write the equation that gives the energy levels for rotational spectroscopy. Legal. Have questions or comments? A selection rule describes how the probability of transitioning from one level to another cannot be zero. Some examples. Raman effect. Gross Selection Rule: A molecule has a rotational spectrum only if it has a permanent dipole moment. 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. Thus, we see the origin of the vibrational transition selection rule that v = ± 1.