LASERS AND SPECTROSCOPY

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.

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.

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!

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.

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.

STIMULATED ABSORPTION Stimulated absorption occurs, for a two level

system (E1 and E2) when a photon of frequency ν = (E2 - E1)/h is absorbed.

Before absorption After absorption E2

hν

E1

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.

SPONTANEOUS EMISSION Spontaneous emission occurs, for a two level

system (E1 and E2) when a photon of frequency ν = (E2 - E1)/h is emitted.

Before emission After emission E2

hν

E1

STIMULATED EMISSION

STIMULATED EMISSION 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.

STIMULATED EMISSION Stimulated emission occurs, for a two level

system (E1 and E2) when a photon of frequency ν = (E2 - E1)/h is absorbed.

Before absorption After absorption E2

hν hν

hν

E1

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?)

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.

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.

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.

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.

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.)

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”.

TO THE POINT?