Question

What key spectroscopic tool is made possible by population inversion and stimulated emission? Explain.

Answer 1

Let us define the terms Population inversion and stimulated emission.

Population inversion is the redistribution of atomic energy levels so that the ground state population is less than the excited state population. This is opposite to the regular case where the ground state population is always greater. This is a very important phenomenon in Laser physics.

In Stimulated emission photon of a particular frequency interacts with an excited electron causing it to drop to a lower energy level. The energy thus released gets transferred to the electromagnetic field, creating a new photon with a phase, frequency, and polarization identical to the photons of the incident wave. This can lead to amplification.

LASER is an acronym for "light amplification by stimulated emission of radiation”. Here, the principles of stimulated emission and population inversion are applied towards generation of strong coherent pulse of radiation.

In spectroscopy, LASERS act as highly coherent light sources. They have high spectral density compared to ordinary light sources such as Hg and Xe lamps which can result in significantly reduced background noise in the spectrum. LASERS have very low spectral line width which results in a high degree of spectral resolution. Tunablility, the small divergence of laser beams are also advantages. The capability of pulsed or mode-locked lasers to deliver intense and short light pulses with pulse widths down to the sub-picosecond range allows the study of ultrafast transient phenomena such as short spontaneous 1 lifetimes or fast relaxation processes in gases, liquids, or solids.

One such spectroscopic tool that uses ultrashort pulse lasers is called ultrafast laser spectroscopy or Femtosecond spectroscopy which can be used to study processes taking place in sub-picosecond timescales. Ultrafast Laser Spectroscopy of Chemical Reactions was developed by Ahmed Zewail.

Ti-sapphire lasers, tunable lasers which emit red and near-infrared light (700 nm- 1100 nm) are generally used as sources in ultrafast spectroscopy. This technique allows for almost instantaneous excitation of samples, and very high peak powers result in strong signals and sensitivity. The general ultrafast spectroscopic technique utilizes two ultrashort laser pulses (picosecond or femtosecond) in a pump-probe configuration in a molecular beam. In essence, one laser pulse is used to excite the molecule of interest and initiate the reaction. The second pulse is then used to probe for the formation of products (or perturbed fragments) - or the depletion of the reactant - at some later time. Zewail studied a variety of reactions using this technique. Bond breaking in ICN, direct and secondary dissociation in alkyl iodides, state to state unimolecular reactions etc. Ultrafast spectroscopy is also used to monitor electron transfer between molecular donor-acceptor systems.

An example for this involves the quenching of a quantum dot (eg. CdSe) fluorescence by the electron transfer to an electron acceptor molecule such as methyl viologen. Herein, a quantum dot upon excitation leads to the formation of an excited state with an electron residing in the conduction band. Methyl viologen is a molecule with a favourable reduction potential to facilitate electron transfer from the quantum dot to the viologen molecule. The process occurs in picosecond time scale and cannot be monitored by classical steady state spectroscopy.