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LASERS AND SPECTROSCOPY . EXCITING MOLECULES  Molecules can be excited using either broadband or monochromatic light. Spectra obtained using monochromatic.

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Presentation on theme: "LASERS AND SPECTROSCOPY . EXCITING MOLECULES  Molecules can be excited using either broadband or monochromatic light. Spectra obtained using monochromatic."— Presentation transcript:

1 LASERS AND SPECTROSCOPY 

2 EXCITING MOLECULES  Molecules can be excited using either broadband or monochromatic light. Spectra obtained using monochromatic light are easier to interpret. Why? In the UV/visible regions a broadband light source and monochromator can be used, but the intensity of light available to produce electronically excited states might be small.

3 LASERS ARE EVERYWHERE  For spectroscopic experiments lasers are often the answer to providing light with the appropriate frequency and intensity.  Lasers are also found in common devices such as DVDs and bar code scanners. We will consider initially the mechanism by which atomic lasers operate.

4 LASERS AND BOLTZMANN  Understanding lasers requires thinking about light, spectroscopy and Boltzmann. For a group of atoms or molecules (at thermal equilibrium at a given T) the Boltzmann expressions allow us to calculate the populations of the atomic and molecular energy levels if the energy level spacings and degeneracies are known!

5 BOLTZMANN AND ATOMS  For atoms, Boltzmann gives particularly simple expressions since there are no rotational or vibrational energies to consider. As well, electronic energy level spacings are so large that essentially all atoms are in the ground state (energy level) at ambient temperature.

6 ATOMIC SPECTROSCOPY  In atomic spectroscopy the movement of electrons between the ground and an excited state(s) is studied. Three mechanisms are important.  1. Stimulated absorption.  2. Spontaneous emission.  3. Stimulated emission.

7 STIMULATED ABSORPTION  Stimulated absorption occurs, for a two level system (E 1 and E 2 ) when a photon of frequency ν = (E 2 - E 1 )/h is absorbed.  Before absorption After absorption  E 2  hν  E 1

8 SPONTANEOUS EMISSION  Electronically excited states are generally not stable. There is a high (well defined) probability that an electron in an excited state will revert (jump) back to the ground state over time. Atoms typically remain excited for short time periods (of the order of 10 ns). Process involves photon emission.

9 SPONTANEOUS EMISSION  Spontaneous emission occurs, for a two level system (E 1 and E 2 ) when a photon of frequency ν = (E 2 - E 1 )/h is emitted.  Before emission After emission  E 2  hν  E 1

10 STIMULATED EMISSION 

11  In stimulated emission a photon of the correct frequency can cause an electron to move from the excited state to the ground state much more quickly than by the spontaneous emission route.  Conservation of energy requires, of course, that two phtons are found after the stimulated emission step.

12 STIMULATED EMISSION  Stimulated emission occurs, for a two level system (E 1 and E 2 ) when a photon of frequency ν = (E 2 - E 1 )/h is absorbed.  Before absorption After absorption  E 2  hν hν  hν  E 1

13 INCOHERENT RADIATION  Spontaneous emission produces incoherent radiation. For a two level system all of the photons have the same frequency but the various photons produced have random phases and propagate in random directions (An incandescent light bulb is, in some respects, a similar example. Why?)

14 COHERENT RADIATION  Stimulated emission produces coherent radiation - photons of the same frequency and phase moving in the same direction. In stimulated absorption the “first” (incident) photon is not absorbed. The two photons available after the stimulated emission can quickly cause other excited atoms to emit photons.

15 STIMULATED EMISSION - LASERS  Stimulated emission causes a “chain reaction” which produces the coherent and intense light beam seen in a laser.  Stimulated emission is an extremely effective mechanism for depopulating excited states. A functioning laser requires an effective means of populating excited states.

16 CLASS HANDOUTS  1. Handout on physical setup and operating conditions of the He/Ne laser  2. Handout on energy levels of He and Ne and the allowed radiative and collisional transitions. Selection rules for atomic spectra are important.

17 POPULATION INVERSION  The efficiency with which electronically excited states can be produced gives rise to a “population inversion” for some of the Ne excited states. The population inversion is critical since (for the 633nm transition shown in the handout) a photon moving through the He/Ne discharge is very likely to be replicated by stimulated emission.

18 POPULATION INVERSION  In fact, photon replication by stimulated emission is more likely than photon absorption by the complementary/”reverse”  Process. (the stimulated emission process is sometimes called a “cloning process” for photons.)

19 FREQUENCY SELECTION  In practice, light of a single wavelength is desirable. In some cases particular frequencies of light can be selected by varying the length of the laser cavity. Analogous to a pipe organ or the PIAB where the particle is a photon and  λ/2 = L/n where L is the length of the “tube”.

20 TO THE POINT?


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