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| The '''rigid rotor''' is a mechanical model that is used to explain rotating systems. | | The title of the author is Nestor. Managing individuals is how she makes cash and she will not change it anytime soon. To perform handball is the factor she enjoys most of all. Delaware has always been my residing place and will never transfer.<br><br>Stop by my page [http://Www.Astracompanies.com/ActivityFeed/MyProfile/tabid/57/UserId/82215/Default.aspx http://Www.Astracompanies.com/] |
| An arbitrary rigid rotor is a 3-dimensional rigid object, such as a [[top]]. To orient such an object in space three angles known as [[Euler angles]] are required. A special rigid rotor is the linear rotor which requires only two angles to describe its orientation. An example of a linear rotor
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| is a diatomic [[molecule]]. More general molecules like water (asymmetric rotor),
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| ammonia (symmetric rotor), or methane (spherical rotor) are 3-dimensional, see [[Rotational spectroscopy#Classification of molecules based on rotational behavior|classification of molecules]].</small>
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| == Linear rotor ==
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| The linear rigid rotor model consists of two point masses located at fixed distances from their center of mass.
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| The fixed distance between the two masses and the values of the masses are the only characteristics of the rigid model. However, for many actual diatomics this model is too restrictive since distances are usually not completely fixed. Corrections on the rigid model can be made to compensate for small variations in the distance. Even in such a case the rigid rotor model is a useful point of departure (zeroth-order model).
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| | |
| === Classical linear rigid rotor ===
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| The classical linear rotor consists of two point masses <math>m_1</math> and <math>m_2</math>
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| (with [[reduced mass]] <math>\mu = \frac{m_1m_2}{m_1+m_2}</math>) each at a distance <math>R</math>. The rotor is rigid if <math>R</math> is independent of time.
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| The kinematics of a linear rigid rotor is usually described by means of [[spherical coordinates|spherical polar coordinates]], which form a coordinate system of '''R'''<sup>3</sup>. In the physics convention the coordinates are the co-latitude (zenith) angle <math>\theta \,</math>, the longitudinal (azimuth) angle <math>\varphi\,</math> and the distance <math>R</math>.
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| The angles specify the orientation of the rotor in space.
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| The kinetic energy <math>T</math> of the linear rigid rotor is given by
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| :<math>
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| 2T = \mu R^2\big [\dot{\theta}^2+(\dot\varphi\,\sin\theta)^2\big]= \mu R^2 \big(\dot{\theta}\;\; \dot{\varphi} \Big)
| |
| \begin{pmatrix}
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| 1 & 0 \\
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| 0 & \sin^2 \theta \\
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| \end{pmatrix}
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| \begin{pmatrix}
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| \dot{\theta}\\ \dot{\varphi}
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| \end{pmatrix}
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| =
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| \mu \Big(\dot{\theta}\;\; \dot{\varphi} \Big)
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| \begin{pmatrix}
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| h_\theta^2 & 0 \\
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| 0 & h_\varphi^2 \\
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| \end{pmatrix}
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| \begin{pmatrix}
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| \dot{\theta}\\ \dot{\varphi}
| |
| \end{pmatrix},
| |
| </math>
| |
| where <math>h_\theta = R\, </math> and <math>h_\varphi= R\sin\theta\,</math> are
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| [[Curvilinear coordinates#Terminology|scale (or Lamé) factors]].
| |
| | |
| Scale factors are of importance for quantum mechanical applications since they
| |
| enter the [[Laplacian]] expressed in [[Curvilinear coordinates#Grad.2C curl.2C div.2C Laplacian|curvilinear coordinates]].
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| In the case at hand (constant <math>R</math>)
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| :<math>
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| \nabla^2 = \frac{1}{h_\theta h_\varphi}\left[
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| \frac{\partial}{\partial \theta} \frac{h_\varphi}{h_\theta} \frac{\partial}{\partial \theta}
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| +\frac{\partial}{\partial \varphi} \frac{h_\theta}{h_\varphi} \frac{\partial}{\partial \varphi}
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| \right]=
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| \frac{1}{R^2}\left[\frac{1}{\sin\theta}
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| \frac{\partial}{\partial \theta} \sin\theta \frac{\partial}{\partial \theta}
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| +\frac{1}{\sin^2\theta}\frac{\partial^2}{\partial \varphi^2}
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| \right].
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| </math>
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| The classical Hamiltonian function of the linear rigid rotor is
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| :<math>
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| H = \frac{1}{2\mu R^2}\left[p^2_{\theta} + \frac{p^2_{\varphi}}{\sin^2\theta}\right].
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| </math>
| |
| | |
| === Quantum mechanical linear rigid rotor ===
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| The linear rigid rotor model can be used in [[quantum mechanics]] to predict the rotational energy of a [[diatomic]] molecule. The rotational energy depends on the [[moment of inertia]] for the system, <math>I </math>. In the [[center of mass]] reference frame, the moment of inertia is equal to:
| |
| | |
| :<math> I = \mu R^2</math>
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| | |
| where <math>\mu</math> is the [[reduced mass]] of the molecule and <math>R</math> is the distance between the two atoms.
| |
| | |
| According to [[quantum mechanics]], the energy levels of a system can be determined by solving the [[Schrödinger equation]]:
| |
| | |
| :<math>\hat H \Psi = E \Psi </math>
| |
| | |
| where <math>\Psi</math> is the [[wave function]] and <math>\hat H</math> is the energy ([[Hamiltonian (quantum mechanics)|Hamiltonian]]) operator. For the rigid rotor in a field-free space, the energy operator corresponds to the [[kinetic energy]]<ref name="Podolsky">{{cite journal| first=B. |last=Podolsky|journal=Phys. Rev.|volume=32|page=812|year=1928|bibcode = 1928PhRv...32...812J |doi = 10.1103/PhysRev.32.812 }}</ref> of the system:
| |
| | |
| :<math>\hat H = - \frac{\hbar^2}{2\mu} \nabla^2</math>
| |
| | |
| where <math>\hbar</math> is [[reduced Planck constant]] and <math>\nabla^2</math> is the [[Laplacian]]. The Laplacian is given above in terms of spherical polar coordinates. The energy operator written in terms of these coordinates is:
| |
| | |
| :<math>\hat H =- \frac{\hbar^2}{2I} \left [ {1 \over \sin \theta} {\partial \over \partial \theta} \left ( \sin \theta {\partial \over \partial \theta} \right ) + {1 \over {\sin^2 \theta}} {\partial^2 \over \partial \varphi^2} \right]</math>
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| | |
| This operator appears also in the Schrödinger equation of the hydrogen atom after the radial part
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| is separated off. The eigenvalue equation becomes
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| :<math>
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| \hat H Y_\ell^m (\theta, \varphi ) = \frac{\hbar^2}{2I} \ell(\ell+1) Y_\ell^m (\theta, \varphi ).
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| </math>
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| The symbol <math>Y_\ell^m (\theta, \varphi )</math> represents a set of functions known as the [[spherical harmonic]]s. Note that the energy does not depend on <math>m \,</math>. The energy
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| :<math> E_\ell = {\hbar^2 \over 2I} \ell \left (\ell+1\right )</math>
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| is <math>2\ell+1</math>-fold degenerate: the functions with fixed <math>\ell\,</math> and <math>m=-\ell,-\ell+1,\dots,\ell</math> have the same energy.
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| | |
| Introducing the ''rotational constant'' ''B'', we write,
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| :<math> E_\ell = B\; \ell \left (\ell+1\right )\quad
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| \textrm{with}\quad B \equiv \frac{\hbar^2}{2I}.
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| </math>
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| In the units of [[reciprocal length]] the rotational constant is,
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| :<math> \bar B \equiv \frac{B}{hc} = \frac{h}{8\pi^2cI}, </math>
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| with ''c'' the speed of light. If [[cgs units]] are used for ''h'', ''c'', and ''I'', <math>\bar B</math> is expressed
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| in [[Wavenumber|wave numbers]], cm<sup>−1</sup>, a unit that is often used for rotational-vibrational spectroscopy.
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| The rotational constant <math>\bar B(R)</math> depends on the distance <math>R</math>. Often one writes <math> B_e = \bar B(R_e) </math> where <math>R_e</math> is the equilibrium value
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| of <math>R</math> (the value for which the interaction energy of the atoms in the rotor has a minimum).
| |
| | |
| A typical rotational spectrum consists of a series of peaks that correspond to transitions between levels with different values of the angular momentum [[quantum number]] (<math>\ell</math>). Consequently, [[Rotational spectroscopy|rotational peaks]] appear at energies corresponding to an integer multiple of <math>2\bar B</math>.
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| | |
| === Selection rules ===
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| Rotational transitions of a molecule occur when the molecule absorbs a photon [a particle of a quantized electromagnetic (em) field]. Depending on the energy of the photon (i.e., the wavelength of the em field) this transition may be seen as a sideband of a vibrational and/or
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| electronic transition. Pure rotational transitions, in which the vibronic (= vibrational plus electronic) wave function does not change, occur in the [[microwave]] region of the electromagnetic spectrum.
| |
| | |
| Typically, rotational transitions can only be observed when the angular momentum [[quantum number]] changes by 1 (<math>\Delta l = \pm 1</math>). This selection rule arises from a first-order perturbation theory approximation of the [[Schrödinger equation|time-dependent Schrödinger equation]]. According to this treatment, rotational transitions can only be observed when one or more
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| components of the [[Dipole#Quantum mechanical dipole operator|dipole operator]]
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| have a non-vanishing transition moment. If ''z'' is the direction of the electric field component of the incoming electromagnetic wave, the transition moment is,
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| :<math>
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| \langle \psi_2 | \mu_z | \psi_1\rangle =
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| \left ( \mu_z \right )_{21} = \int \psi_2^*\mu_z\psi_1\, \mathrm{d}\tau .
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| </math>
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| | |
| A transition occurs if this integral is non-zero. By separating the rotational part of the molecular wavefunction from the vibronic
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| part, one can show that this means that the molecule must have a permanent [[Dipole#Molecular dipoles|dipole moment]].
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| After integration over the vibronic coordinates
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| the following rotational part of the transition moment remains,
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| | |
| :<math>
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| \left ( \mu_z \right )_{l,m;l',m'} = \mu \int_0^{2\pi} \mathrm{d}\phi \int_0^\pi Y_{l'}^{m'} \left ( \theta , \phi \right )^* \cos \theta\,Y_l^m\, \left ( \theta , \phi \right )\; \mathrm{d}\cos\theta .
| |
| </math>
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| Here <math>\mu \cos\theta \, </math> is the ''z'' component of the permanent dipole moment. The moment <math>\mu</math> is the vibronically averaged component of the [[Dipole#Quantum mechanical dipole operator|dipole operator]]. Only the component of the permanent dipole along the axis of a heteronuclear molecule is non-vanishing.
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| By the use of the orthogonality of the [[spherical harmonics]] <math>Y_l^m\, \left ( \theta , \phi \right )</math>
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| it is possible to determine which values of <math>l</math>, <math>m</math>, <math>l'</math>, and <math>m'</math> will result in nonzero values for the dipole transition moment integral. This constraint results in the observed selection rules for the rigid rotor: | |
| | |
| :<math>
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| \Delta m = 0 \quad\hbox{and}\quad \Delta l = \pm 1
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| </math>
| |
| | |
| === Non-rigid linear rotor ===
| |
| | |
| The rigid rotor is commonly used to describe the rotational energy of diatomic molecules but it is not a completely accurate description of such molecules. This is because molecular bonds (and therefore the interatomic distance <math>R</math>) are not completely fixed; the bond between the atoms stretches out as the molecule rotates faster (higher values of the rotational [[quantum number]] <math>l</math>). This effect can be accounted for by introducing a correction factor known as the centrifugal distortion constant <math>\bar{D}</math> (bars on top of various quantities indicate that these quantities are expressed in cm<sup>−1</sup>):
| |
| | |
| :<math> \bar E_l = {E_l \over hc} = \bar {B}l \left (l+1\right ) - \bar {D}l^2 \left (l+1\right )^2</math>
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| | |
| where
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| | |
| :<math> \bar D = {4 \bar {B}^3 \over \bar{\boldsymbol\omega}^2}</math>
| |
| | |
| :<math>\bar{\boldsymbol\omega}</math> is the fundamental vibrational frequency of the bond (in cm<sup>-1</sup>). This frequency is related to the reduced mass and the [[force constant]] (bond strength) of the molecule according to
| |
| | |
| :<math> \bar{\boldsymbol\omega} = {1\over 2\pi c} \sqrt{k \over \mu }</math>
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| | |
| The non-rigid rotor is an acceptably accurate model for diatomic molecules but is still somewhat imperfect. This is because, although the model does account for bond stretching due to rotation, it ignores any bond stretching due to vibrational energy in the bond (anharmonicity in the potential).
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| | |
| == Arbitrarily shaped rigid rotor ==
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| An arbitrarily shaped rigid rotor is a [[rigid body]] of arbitrary shape with its [[center of mass]] fixed (or in uniform rectilinear motion) in field-free space '''R'''<sup>3</sup>, so that its energy consists only of rotational kinetic energy (and possibly constant translational energy that can be ignored). A rigid body can be (partially) characterized by the three eigenvalues of its [[moment of inertia tensor]], which are real nonnegative values known as ''principal moments of inertia''.
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| In [[microwave spectroscopy]]—the spectroscopy based on rotational transitions—one usually classifies molecules (seen as rigid rotors) as follows:
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| * spherical rotors
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| * symmetric rotors
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| ** oblate symmetric rotors
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| ** prolate symmetric rotors
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| * asymmetric rotors
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| This classification depends on the [[Microwave spectroscopy#Classification of molecules based on rotational behavior|relative magnitudes]] of the principal moments of inertia.
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| | |
| === Coordinates of the rigid rotor ===
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| | |
| Different branches of physics and engineering use different coordinates for the description
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| of the kinematics of a rigid rotor. In molecular physics [[Euler angles]] are used almost exclusively. In quantum mechanical applications it is advantageous to use Euler
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| angles in a convention that is a simple extension of the physical convention of [[Spherical coordinate system|spherical polar coordinates]].
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| | |
| The first step is the attachment of a '''right-handed''' orthonormal frame (3-dimensional system of orthogonal axes) to the rotor (a '''body-fixed frame''') . This frame can be attached arbitrarily to the body, but often one uses the principal axes frame—the normalized eigenvectors of the inertia tensor, which always can be chosen orthonormal, since the tensor is [[Hermitian matrix|Hermitian]]. When the rotor possesses a symmetry-axis, it usually coincides with one of the principal axes. It is convenient to choose
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| as body-fixed ''z''-axis the highest-order symmetry axis.
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| | |
| One starts by aligning the body-fixed frame with a '''space-fixed frame'''
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| (laboratory axes), so that the body-fixed ''x'', ''y'', and ''z'' axes coincide with the space-fixed ''X'', ''Y'', and ''Z'' axis. Secondly, the body and its frame are rotated '''actively''' over a '''positive angle''' <math>\alpha\,</math> around the ''z''-axis (by the [[Right-hand rule#Direction associated with a rotation|right-hand rule]]), which moves the <math>y</math>- to the <math>y'</math>-axis. Thirdly, one rotates the body and its frame over a positive angle <math>\beta\,</math> around the <math>y'</math>-axis. The ''z''-axis of the body-fixed frame has after these two rotations the longitudinal angle <math>\alpha \,</math> (commonly designated by <math>\varphi\,</math>) and the colatitude angle <math>\beta\,</math> (commonly designated by <math>\theta\,</math> ), both with respect to the space-fixed frame. If the rotor were cylindrical symmetric around its ''z''-axis, like the linear rigid rotor, its orientation in space would be unambiguously specified at this point.
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| | |
| If the body lacks cylinder (axial) symmetry, a last rotation around its ''z''-axis (which has polar coordinates <math>\beta\,</math> and <math>\alpha\,</math> ) is necessary to specify its orientation completely. Traditionally the last rotation angle is called <math>\gamma\,</math>.
| |
| | |
| The [[Euler angles#Conventions|convention for Euler angles]] described here is known as the <math>z''-y'-z</math> convention; it can be shown (in the same manner as in [[Euler angles#Definition|this article]]) that it is equivalent to the <math>z-y-z</math> convention in which the order of rotations is reversed.
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| | |
| The total matrix of the three consecutive rotations is the product
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| | |
| :<math>
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| \mathbf{R}(\alpha,\beta,\gamma)=
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| \begin{pmatrix}
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| \cos\alpha & -\sin\alpha & 0 \\
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| \sin\alpha & \cos\alpha & 0 \\
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| 0 & 0 & 1
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| \end{pmatrix}
| |
| \begin{pmatrix}
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| \cos\beta & 0 & \sin\beta \\
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| 0 & 1 & 0 \\
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| -\sin\beta & 0 & \cos\beta \\
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| \end{pmatrix}
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| \begin{pmatrix}
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| \cos\gamma & -\sin\gamma & 0 \\
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| \sin\gamma & \cos\gamma & 0 \\
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| 0 & 0 & 1
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| \end{pmatrix}
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| </math>
| |
| | |
| Let <math>\mathbf{r}(0)</math> be the coordinate vector of an arbitrary point <math>\mathcal{P}</math> in the body with respect to the body-fixed frame. The elements of <math>\mathbf{r}(0)</math> are the 'body-fixed coordinates' of <math>\mathcal{P}</math>. Initially <math>\mathbf{r}(0)</math> is also the space-fixed coordinate vector of <math>\mathcal{P}</math>.
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| Upon rotation of the body, the body-fixed coordinates of <math>\mathcal{P}</math> do not change, but the space-fixed coordinate vector of <math>\mathcal{P}</math> becomes,
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| :<math>
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| \mathbf{r}(\alpha,\beta,\gamma)= \mathbf{R}(\alpha,\beta,\gamma)\mathbf{r}(0).
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| </math>
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| In particular, if <math>\mathcal{P}</math> is initially on the space-fixed ''Z''-axis, it has
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| the space-fixed coordinates
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| :<math>
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| \mathbf{R}(\alpha,\beta,\gamma)
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| \begin{pmatrix}
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| 0 \\
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| 0 \\
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| r \\
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| \end{pmatrix}=
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| \begin{pmatrix}
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| r \cos\alpha\sin\beta \\
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| r \sin\alpha \sin\beta \\
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| r \cos\beta \\
| |
| \end{pmatrix},
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| </math>
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| which shows the correspondence with the [[Spherical coordinate system|spherical polar coordinates]]
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| (in the physical convention).
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| | |
| Knowledge of the Euler angles as function of time ''t'' and the initial coordinates <math>\mathbf{r}(0)</math> determine the kinematics of the rigid rotor.
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| | |
| === Classical kinetic energy ===
| |
| | |
| <small> The following text forms a generalization of the well-known special case of the [[rotational energy]] of an object that rotates around ''one'' axis.</small>
| |
| | |
| It will be assumed from here on that the body-fixed frame is a principal axes frame; it diagonalizes the instantaneous [[moment of inertia tensor|inertia tensor]] <math> \mathbf{I}(t)</math> (expressed with respect to the space-fixed frame), i.e.,
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| :<math>
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| \mathbf{R}(\alpha,\beta,\gamma)^{-1}\; \mathbf{I}(t)\; \mathbf{R}(\alpha,\beta,\gamma)
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| = \mathbf{I}(0)\quad\hbox{with}\quad
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| \mathbf{I}(0) =
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| \begin{pmatrix}
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| I_1 & 0 & 0 \\ 0 & I_2 & 0 \\ 0 & 0 & I_3 \\
| |
| \end{pmatrix},
| |
| </math> | |
| where the Euler angles are time-dependent and in fact determine the time dependence of <math>\mathbf{I}(t)</math> by the inverse of this equation. This notation implies
| |
| that at <math>t=0</math> the Euler angles are zero, so that at <math>t=0</math> the body-fixed frame coincides with the space-fixed frame.
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| | |
| The classical kinetic energy ''T'' of the rigid rotor can be expressed in different ways:
| |
| * as a function of angular velocity
| |
| * in Lagrangian form
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| * as a function of angular momentum
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| * in Hamiltonian form.
| |
| | |
| Since each of these forms has its use and can be found in textbooks we will present all of them.
| |
| | |
| ==== Angular velocity form ====
| |
| | |
| As a function of angular velocity ''T'' reads,
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| :<math>
| |
| T = \frac{1}{2} \left[ I_1 \omega_x^2 + I_2 \omega_y^2+ I_3 \omega_z^2 \right]
| |
| </math>
| |
| with
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| :<math>
| |
| \begin{pmatrix}
| |
| \omega_x \\
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| \omega_y \\
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| \omega_z \\
| |
| \end{pmatrix}
| |
| =
| |
| \begin{pmatrix}
| |
| -\sin\beta\cos\gamma & \sin\gamma & 0 \\
| |
| \sin\beta\sin\gamma & \cos\gamma & 0 \\
| |
| \cos\beta & 0 & 1 \\
| |
| \end{pmatrix}
| |
| \begin{pmatrix}
| |
| \dot{\alpha} \\
| |
| \dot{\beta} \\
| |
| \dot{\gamma} \\
| |
| \end{pmatrix}.
| |
| </math>
| |
| The vector <math>\boldsymbol{\omega} = (\omega_x, \omega_y, \omega_z) </math> contains the components of the [[angular velocity]] of the rotor expressed with respect to the body-fixed frame. It can be shown that <math>\boldsymbol{\omega}</math> is ''not'' the time derivative of any vector, in contrast to the usual [[Velocity#Formal description|definition of velocity]].<ref name="Goldstein">Chapter 4.9 of Goldstein, H.; Poole, C. P.; Safko, J. L. (2001). Classical Mechanics (Third ed.). San Francisco: Addison Wesley Publishing Company. ISBN 0-201-65702-3.</ref> The dots over the time-dependent Euler angles indicate [[Newton's notation for differentiation|time derivatives]].
| |
| The angular velocity satisfies equations of motion known as [[Euler's equations]] (with zero applied torque, since by assumption the rotor is in field-free space).
| |
| | |
| ==== Lagrange form ====
| |
| | |
| Backsubstitution of the expression of <math>\boldsymbol{\omega}</math> into ''T'' gives
| |
| the kinetic energy in [[Lagrangian mechanics|Lagrange form]] (as a function of the time derivatives of the Euler angles). In matrix-vector notation,
| |
| :<math>
| |
| 2 T =
| |
| \begin{pmatrix}
| |
| \dot{\alpha} & \dot{\beta} & \dot{\gamma}
| |
| \end{pmatrix}
| |
| \; \mathbf{g} \;
| |
| \begin{pmatrix}
| |
| \dot{\alpha} \\ \dot{\beta} \\ \dot{\gamma}\\
| |
| \end{pmatrix},
| |
| </math>
| |
| where <math>\mathbf{g}</math> is the metric tensor expressed in Euler angles—a non-orthogonal system of [[curvilinear coordinates]]—
| |
| | |
| :<math>
| |
| \mathbf{g}=
| |
| \begin{pmatrix}
| |
| I_1 \sin^2\beta \cos^2\gamma+I_2\sin^2\beta\sin^2\gamma+I_3\cos^2\beta &
| |
| (I_2-I_1) \sin\beta\sin\gamma\cos\gamma &
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| I_3\cos\beta \\
| |
| (I_2-I_1) \sin\beta\sin\gamma\cos\gamma &
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| I_1\sin^2\gamma+I_2\cos^2\gamma & 0 \\
| |
| I_3\cos\beta & 0 & I_3 \\
| |
| \end{pmatrix}.
| |
| </math>
| |
| | |
| ==== Angular momentum form ====
| |
| | |
| Often the kinetic energy is written as a function of the [[Angular momentum#Angular momentum in classical mechanics|angular momentum]] <math>\vec{L}</math> of the rigid rotor. This vector is a conserved (time-independent) quantity.
| |
| With respect to the body-fixed frame it has the components <math>\mathbf{L}</math>, which can be shown to
| |
| be related to the angular velocity,
| |
| :<math>
| |
| \mathbf{L} =
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| \mathbf{I}(0)\;
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| \boldsymbol{\omega}\quad\hbox{or}\quad L_i = \frac{\partial T}{\partial\omega_i},\;\; i=x,\,y,\,z.
| |
| </math>
| |
| Since the body-fixed frame moves (depends on time) these components are ''not'' time independent. If we were
| |
| to represent <math>\vec{L}</math> with respect to the stationary space-fixed frame, we would
| |
| find time independent expressions for its components.
| |
| The kinetic energy is given by
| |
| :<math>
| |
| T = \frac{1}{2} \left[ \frac{L_x^2}{I_1} + \frac{L_y^2}{I_2}+ \frac{L_z^2}{I_3}\right].
| |
| </math>
| |
| | |
| ==== Hamilton form ====
| |
| | |
| The [[Hamiltonian mechanics|Hamilton form]] of the kinetic energy is written in terms
| |
| of generalized momenta
| |
| :<math>
| |
| \begin{pmatrix}
| |
| p_\alpha \\
| |
| p_\beta \\
| |
| p_\gamma \\
| |
| \end{pmatrix}
| |
| \ \stackrel{\mathrm{def}}{=}\
| |
| \begin{pmatrix}
| |
| \partial T/{\partial \dot{\alpha}}\\
| |
| \partial T/{\partial \dot{\beta}} \\
| |
| \partial T/{\partial \dot{\gamma}} \\
| |
| \end{pmatrix}
| |
| = \mathbf{g}
| |
| \begin{pmatrix} \; \,
| |
| \dot{\alpha} \\ \dot{\beta} \\ \dot{\gamma}\\
| |
| \end{pmatrix},
| |
| </math>
| |
| where it is used that the <math>\mathbf{g}</math> is symmetric.
| |
| In Hamilton form the kinetic energy is,
| |
| :<math>
| |
| 2 T =
| |
| \begin{pmatrix}
| |
| p_{\alpha} & p_{\beta} & p_{\gamma}
| |
| \end{pmatrix}
| |
| \; \mathbf{g}^{-1} \;
| |
| \begin{pmatrix}
| |
| p_{\alpha} \\ p_{\beta} \\ p_{\gamma}\\
| |
| \end{pmatrix},
| |
| </math>
| |
| with the inverse metric tensor given by
| |
| :<math>
| |
| {\scriptstyle \sin^2\beta}\;\;\mathbf{g}^{-1}=
| |
| </math>
| |
| | |
| :<math>
| |
| \begin{pmatrix}
| |
| \frac{\cos^2\gamma}{I_1}+\frac{\sin^2\gamma}{I_2} &
| |
| \left(\frac{1}{I_2}-\frac{1}{I_1}\right){\scriptstyle \sin\beta\sin\gamma\cos\gamma}&
| |
| -\frac{\cos\beta\cos^2\gamma}{I_1}-\frac{\cos\beta\sin^2\gamma}{I_2} \\
| |
| \left(\frac{1}{I_2}-\frac{1}{I_1}\right){\scriptstyle \sin\beta\sin\gamma\cos\gamma}&
| |
| \frac{\sin^2\beta\sin^2\gamma}{I_1}+\frac{\sin^2\beta\cos^2\gamma}{I_2} &
| |
| \left(\frac{1}{I_1}-\frac{1}{I_2}\right){\scriptstyle \sin\beta\cos\beta\sin\gamma\cos\gamma}\\
| |
| -\frac{\cos\beta\cos^2\gamma}{I_1}-\frac{\cos\beta\sin^2\gamma}{I_2} &
| |
| \left(\frac{1}{I_1}-\frac{1}{I_2}\right){\scriptstyle \sin\beta\cos\beta\sin\gamma\cos\gamma} &
| |
| \frac{\cos^2\beta\cos^2\gamma}{I_1}+ \frac{\cos^2\beta\sin^2\gamma}{I_2}+\frac{\sin^2\beta}{I_3} \\
| |
| \end{pmatrix}.
| |
| </math>
| |
| This inverse tensor is needed to obtain the [[Laplace-Beltrami operator]], which (multiplied
| |
| by <math>-\hbar^2</math>) gives the quantum mechanical energy operator
| |
| of the rigid rotor.
| |
| | |
| The classical Hamiltonian given above can be rewritten to the following expression, which is needed in the phase integral
| |
| arising in the classical statistical mechanics of rigid rotors,
| |
| :<math>
| |
| \begin{array}{lcl}
| |
| T &=& \frac{1}{2I_1 \sin^2\beta}
| |
| \left( (p_\alpha- p_\gamma\cos\beta)\cos\gamma -p_\beta
| |
| \sin\beta\sin\gamma \right)^2 \\
| |
| &&+ \frac{1}{2I_2 \sin^2\beta}
| |
| \left( (p_\alpha- p_\gamma\cos\beta)\sin\gamma +p_\beta
| |
| \sin\beta\cos\gamma \right)^2 + \frac{p_\gamma^2}{2I_3}. \\
| |
| \end{array}
| |
| </math>
| |
| | |
| === Quantum mechanical rigid rotor ===
| |
| {{see also|Rotational spectroscopy}}
| |
| As usual quantization is performed by the replacement of the generalized momenta
| |
| by operators that give first derivatives with respect to its [[Canonically conjugate coordinates|canonically conjugate]] variables (positions). Thus,
| |
| :<math>
| |
| p_\alpha \longrightarrow -i \hbar \frac{\partial}{\partial \alpha}
| |
| </math>
| |
| and similarly for <math>p_\beta</math> and <math>p_\gamma</math>. It is remarkable that this rule replaces the fairly complicated function <math>p_\alpha</math> of all three Euler angles, time derivatives of Euler angles, and inertia moments (characterizing the rigid rotor) by a simple differential operator that does not depend on time or inertia moments and differentiates to one Euler angle only.
| |
| | |
| The quantization rule is sufficient to obtain the operators
| |
| that correspond with the classical angular momenta. There are two kinds: space-fixed and body-fixed
| |
| angular momentum operators. Both are vector operators, i.e., both have three components
| |
| that transform as vector components among themselves upon rotation of the space-fixed and the body-fixed frame, respectively. The explicit form of the rigid rotor angular momentum operators is
| |
| given [[Wigner D-matrix|here]] (but beware, they must be multiplied with <math>\hbar</math>). The body-fixed angular momentum operators are written
| |
| as <math>\hat{\mathcal{P}}_i</math>. They satisfy [[Wigner D-matrix#Properties of Wigner D-matrix|''anomalous commutation relations'']].
| |
| | |
| The quantization rule is ''not'' sufficient to obtain the kinetic energy operator from
| |
| the classical Hamiltonian. Since classically <math>p_\beta</math> commutes with <math>\cos\beta</math> and <math>\sin\beta</math> and the inverses of these functions, the position
| |
| of these trigonometric functions in the classical Hamiltonian is arbitrary. After
| |
| quantization the commutation does no longer hold and the order of operators and functions in the Hamiltonian (energy operator) becomes a point of concern. Podolsky<ref name="Podolsky" /> proposed in 1928 that the [[Laplace-Beltrami operator#Laplace–Beltrami operator|Laplace-Beltrami operator]]
| |
| (times <math>-\tfrac{1}{2}\hbar^2</math>) has the appropriate form for the quantum mechanical kinetic
| |
| energy operator. This operator has the general form (summation convention: sum over repeated indices—in this case over the three Euler angles <math> q^1,\,q^2,\,q^3 \equiv \alpha,\,\beta,\,\gamma</math>):
| |
| | |
| :<math>
| |
| \hat{H} = - \tfrac{\hbar^2}{2}\;|g|^{-1/2}
| |
| \frac{\partial}{\partial q^i} |g|^{1/2} g^{ij} \frac{\partial}{\partial q^j},
| |
| </math>
| |
| where <math>|g|</math> is the determinant of the g-tensor:
| |
| :<math>
| |
| |g| = I_1\, I_2\, I_3\, \sin^2 \beta \quad \hbox{and}\quad g^{ij} = (\mathbf{g}^{-1})_{ij}.
| |
| </math>
| |
| Given the inverse of the metric tensor above, the explicit form of the kinetic energy operator in terms of Euler angles follows by simple substitution. (Note: The corresponding eigenvalue equation
| |
| gives the [[Schrödinger equation]] for the rigid rotor in the form that it was
| |
| solved for the first time by Kronig and Rabi<ref name="Kronig">{{cite journal| doi=10.1103/PhysRev.29.262| author=R. de L. Kronig and I. I. Rabi| title=The Symmetrical Top in the Undulatory Mechanics|journal= Phys. Rev.|volume= 29|pages= 262–269 |year=1927|bibcode = 1927PhRv...29..262K }}</ref> (for the special case of the symmetric rotor). This is one of the few cases where the Schrödinger equation can be solved analytically. All these cases were solved within a year of the formulation of the Schrödinger equation.)
| |
| | |
| Nowadays it is common to proceed as follows. It can be shown that <math>\hat{H}</math>
| |
| can be expressed in body-fixed angular momentum operators (in this proof one must carefully
| |
| commute differential operators with trigonometric functions). The result has the same appearance
| |
| as the classical formula expressed in body-fixed coordinates,
| |
| :<math>
| |
| \hat{H} = \tfrac{1}{2}\left[ \frac{\mathcal{P}_x^2}{I_1}+ \frac{\mathcal{P}_y^2}{I_2}+
| |
| \frac{\mathcal{P}_z^2}{I_3} \right].
| |
| </math>
| |
| The action of the <math>\hat{\mathcal{P}}_i</math> on the [[Wigner D-matrix#Properties of Wigner D-matrix|Wigner D-matrix]] is simple. In particular
| |
| :<math>
| |
| \mathcal{P}^2\, D^j_{m'm}(\alpha,\beta,\gamma)^* = \hbar^2 j(j+1) D^j_{m'm}(\alpha,\beta,\gamma)^* \quad\hbox{with}\quad
| |
| \mathcal{P}^2= \mathcal{P}^2_x + \mathcal{P}_y^2+ \mathcal{P}_z^2,
| |
| </math>
| |
| so that the Schrödinger equation for the spherical rotor (<math>I=I_1=I_2=I_3</math>)
| |
| is solved with the <math> (2j+1)^2 </math> degenerate energy equal to <math>\tfrac{\hbar^2 j(j+1)}{2I}</math>.
| |
| | |
| The symmetric top (= symmetric rotor) is characterized by <math>I_1=I_2</math>. It is
| |
| a ''prolate'' (cigar shaped) top if <math>I_3 < I_1=I_2 </math>. In the latter
| |
| case we write the Hamiltonian as
| |
| :<math> | |
| \hat{H} = \tfrac{1}{2}\left[ \frac{\mathcal{P}^2}{I_1}+ \mathcal{P}_z^2\Big(\frac{1}{I_3}
| |
| -\frac{1}{I_1} \Big) \right],
| |
| </math>
| |
| and use that
| |
| :<math>
| |
| \mathcal{P}_z^2\, D^j_{m k}(\alpha,\beta,\gamma)^* = \hbar^2 k^2\, D^j_{m k}(\alpha,\beta,\gamma)^*.
| |
| </math>
| |
| Hence
| |
| :<math>
| |
| \hat{H}\,D^j_{m k}(\alpha,\beta,\gamma)^* = E_{jk} D^j_{m k}(\alpha,\beta,\gamma)^*
| |
| \quad \hbox{with}\quad E_{jk}/ \hbar^2 = \frac{j(j+1)}{2I_1} + k^2\left(\frac{1}{2I_3}-\frac{1}{2I_1}\right).
| |
| </math>
| |
| The eigenvalue <math>E_{j0}</math> is <math>2j+1</math>-fold degenerate, for all eigenfunctions
| |
| with <math>m=-j,-j+1,\dots, j </math> have the same eigenvalue. The energies with |k| > 0 are
| |
| <math>2(2j+1)</math>-fold degenerate. This exact solution of the
| |
| Schrödinger equation of the symmetric top was first found in 1927.<ref name="Kronig" />
| |
| | |
| The asymmetric top problem (<math> I_1 \ne I_2 \ne I_3 </math>) is not exactly soluble.
| |
| | |
| ==Direct experimental observation of molecular rotations==
| |
| | |
| For long time, molecular rotations could not be directly observed experimentally. Only measurement techniques with atomic resolution made it possible to detect the rotation of a single molecule.<ref>{{Literatur | Autor = J. K. Gimzewski, C. Joachim, R. R. Schlittler, V. Langlais, H. Tang, I. Johannsen | Titel = Rotation of a Single Molecule Within a Supramolecular Bearing | Sammelwerk = Science | Band = 281 | Jahr = 1998 | Datum = 1998-07-24| Nummer = 5376| Seiten = 531–533| DOI= 10.1126/science.281.5376.531}}</ref><ref name="ReferenceA">{{Literatur | Autor = Thomas Waldmann, Jens Klein, Harry E. Hoster, R. Jürgen Behm | Titel = Stabilization of Large Adsorbates by Rotational Entropy: A Time-Resolved Variable-Temperature STM Study | Sammelwerk = ChemPhysChem | Jahr = 2012 | Datum = 2012| Seiten = n/a–n/a| DOI= 10.1002/cphc.201200531}}</ref> At low temperatures, the rotations of molecules (or part thereof) can be frozen. This could be directly visualized by [[Scanning tunneling microscope|Scanning tunneling microscopy]] i.e., the stabilization could be explained at higher temperatures the rotational entropy.<ref name="ReferenceA"/>
| |
| | |
| ==See also==
| |
| *[[Balancing machine]]
| |
| *[[Gyroscope]]
| |
| *[[Infrared spectroscopy]]
| |
| *[[Rigid body]]
| |
| *[[Rotational spectroscopy]]
| |
| *[[Spectroscopy]]
| |
| *[[Vibrational spectroscopy]]
| |
| *[[Quantum rotor model]]
| |
| | |
| ==References==
| |
| <!-- ----------------------------------------------------------
| |
| See http://en.wikipedia.org/wiki/Wikipedia:Footnotes for a
| |
| discussion of different citation methods and how to generate
| |
| footnotes using the<ref>, </ref> and <reference /> tags
| |
| ----------------------------------------------------------- -->
| |
| {{reflist}}
| |
| | |
| ==General references==
| |
| * {{cite journal|doi=10.1103/RevModPhys.3.280|author=D. M. Dennison|title=The Infrared Spectra of Polyatomic Molecules Part I|journal= Rev. Mod. Physics|volume=3|pages=280–345|year=1931|bibcode = 1931RvMP....3..280D }} (Especially Section 2: The Rotation of Polyatomic Molecules).
| |
| * {{Cite journal| doi=10.1103/RevModPhys.23.213| first=J. H.|last= Van Vleck| title=The Coupling of Angular Momentum Vectors in Molecules|journal= Rev. Mod. Physics| volume=23|pages=213–227 |year=1951|bibcode = 1951RvMP...23..213V }}
| |
| * {{cite book | author=McQuarrie, Donald A | title=Quantum Chemistry | location=Mill Valley, Calif. | publisher=University Science Books | year=1983 | isbn=0-935702-13-X}}
| |
| * {{cite book| isbn= 0-201-65702-3| first1= H. |last1=Goldstein|first2=C. P. |last2=Poole|first3= J. L.|last3= Safko| title=Classical Mechanics|edition=Third |publisher= Addison Wesley Publishing Company|location= San Francisco |year=2001}} (Chapters 4 and 5)
| |
| * {{cite book| isbn=0-387-96890-3| first= V. I.|last= Arnold|title=Mathematical Methods of Classical Mechanics|publisher= Springer-Verlag |year=1989}} (Chapter 6).
| |
| * {{cite book| first=H. W. |last=Kroto| title=Molecular Rotation Spectra|publisher= Dover |location=New York|year=1992}}
| |
| * {{cite book|first1=W.|last1= Gordy |first2= R. L. |last2=Cook|title=Microwave Molecular Spectra|edition=Third| publisher=Wiley|location= New York |year=1984|isbn=0-471-08681-9}}
| |
| * {{cite book| first1=D. |last1=Papoušek |first2= M. T. |last2=Aliev|title=Molecular Vibrational-Rotational Spectra|publisher=Elsevier|location= Amsterdam |year=1982| isbn=0-444-99737-7}}
| |
| | |
| [[Category:Molecular physics]]
| |
| [[Category:Rigid bodies]]
| |
| [[Category:Rotation]]
| |