Angular momentum of a photon

To illustrate some of these principles the angular momentum of a photon will be examined [56]. Suppose a beam of circularly polarized light falls on a perfectly black absorbing surface, which not only heats up (E = hv) but also acquires a torque, on account of the angular momentum it absorbs. Circular polarization means that the probability of an elementary observation 0(P ) = The ratio of energy/torque = w(= 2m/), the angular frequency of... 

When a circularly polarized laser pulse is applied to Mg porphyrin, the spin angular momentum of a photon selects Eu+) or -), and n electrons start to rotate in the clockwise or counterclockwise direction. In other words, a linearly polarized laser pulse, which has no spin angular momentum, cannot induce n-electron rotations in Mg porphyrin. In general, a linearly polarized pulse cannot rotate n electrons in an aromatic ring molecule with degenerate excited states. 

The absolute value of the angular momentum of a photon equals unity. The S... 

Classically, a circularly polarized light beam with angular frequency w(= 2nv) transfers angular momentum at a rate of E/w, where E is the rate of energy transfer. Considered as a beam of photons, E = Nhui/2-n, so that the angular momentum of each photon is h/2n = h. 

The second term s may be called the operator for spin angular momentum of the photon. However, the separation of the angular momentum of the photon into an orbital and a spin part has restricted physical meaning. Firstly, the usual definition of spin as the angular momentum of a particle at rest is inapplicable to the photon since its rest mass is zero. More importantly, it will be seen that states with definite values of orbital and spin angular momenta do not satisfy the condition of transversality. 

It is not possible to ascribe a definite value of the orbital angular momentum to a photon state since the vector spherical harmonic YjM may be a function of different values of . This provides the evidence that, strictly speaking, it... 

The spin of the photon imposes an additional orbital selection rule on absorption. To conserve angular momentum when a photon is absorbed, a change in electronic orbital angular momentum must balance the angular momenrnm provided by the photon. In atomic absorption, this means that the azimuthal quanmm number / must change by either 1 or +1, depending on whether the photon has left nis = +1) or... 

Selection rules for internal conversion largely follow the same rules as for gamma transitions, with one exception EO transitions between two states with angular momentum zero are forbidden for gamma rays, but allowed for internal conversion. This is due to the intrinsic angular momentum of the photon of 1 fi which makes it impossible to fulfill the triangle rule. Electrons can, however, be ejected from the K shell with zero orbital angular momentum. The intrinsic spin of the electron does not enter the equation as the electron is not created in the process but acts as a spectator. 

Atomic and molecular magnetic dipoles have to obey the angular momentum laws of quantum mechanics, since they are proportional to angular momenta. Each dipole can therefore make just a number of orientations with an applied magnetic induction B. Each allowed orientation corresponds to a different potential energy, and absorption of a photon with suitable energy may cause a change in orientation. 

An electron model is needed for the model of the photon in Section V. As a first approximation, let the electron (resp. positron) be a thin disk of mass me and radius re rotating at an average angular velocity ooe. The angular momentum of the disk is then... 

Since photons have angular momentum of +1 or -1, an electronic state absorbing two photons simultaneously may change angular momentum by +2, 0. Two L = +1 photons cause a change of +2 a photon of L = +1 and one of I = -1 cause a change of 0 (A1 = 0, 2, AJ = 0, 2, AL = 0, 2, AS = 0). Thus the selection rules for two-photon absorption allow the excited electron to be either in an s or a d state, states which are of even-to-even parity or odd-to-odd parity such as f-f transitions, which now become allowed. An electron therefore cannot go from an s state... 

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