Energy levels lasers

When a substance is irradiated with light energy, specific ions in this substance are promoted to a higher energy level. When these ions subsequently fall back to a lower energy level, laser light is emitted. 

In the case of measurements in the 2 P multiplet of however, modifications of the apparatus had to be made. Because of the short lifetime of the 2 energy levels, laser excitation from the 2 state... 

The 70 years since these first observations have witnessed dramatic developments in Raman spectroscopy, particularly with the advent of lasers. By now, a large variety of Raman spectroscopies have appeared, each with its own acronym. They all share the conunon trait of using high energy ( optical ) light to probe small energy level spacings in matter. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

Figure B2.3.8. Energy-level sehemes deseribing various optieal methods for state-seleetively deteeting ehemieal reaetion produets left-hand side, laser-indueed fluoreseenee (LIF) eentre, resonanee-enlianeed multiphoton ionization (REMPI) and right-hand side, eoherent anti-Stokes Raman speetroseopy (CARS). The ionization oontinuiim is denoted by a shaded area. The dashed lines indieate virtual eleetronie states. Straight arrows indieate eoherent radiation, while a wavy arrow denotes spontaneous emission.

It has been said tliat anytliing will lase if pumped witli enough energy, but tire efficiency of tire pumping process is important for practical, economical devices. In tliis regard two-level lasers are of little interest because, except under extraordinary pumping conditions, one can only equalize tire populations of tire upper and lower levels. A... 

Figure C2.15.4. (a) A tliree-level laser energy level diagram and (b) tlie mby system.

It was shown above that the normal two-level system (ground to excited state) will not produce lasing but that a three-level system (ground to excited state to second excited state) can enable lasing. Some laser systems utilize four- or even five-level systems, but all need at least one of the excited-state energy levels to have a relatively long lifetime to build up an inverted population. 

In practice the laser can operate only when n, in Equation (9.2), takes values such that the corresponding resonant frequency v lies within the line width of the transition between the two energy levels involved. If the active medium is a gas this line width may be the Doppler line width (see Section 2.3.2). Figure 9.3 shows a case where there are twelve axial modes within the Doppler profile. The number of modes in the actual laser beam depends on how much radiation is allowed to leak out of the cavity. In the example in Figure 9.3 the output level has been adjusted so that the so-called threshold condition allows six axial modes in the beam. The gain, or the degree of amplification, achieved in the laser is a measure of the intensity. 

Figure 9.6 (a) Low-lying energy levels of in ruby, (b) Design for a ruby laser... 

The lasing medium in the titanium-sapphire laser is crystalline sapphire (AI2O3) with about 0.1 per cent by weight of Ti203. The titanium is present as Ti and it is between energy levels of this ion that lasing occurs. 

Laser action fakes place befween excifed levels of fhe neon atoms, in a four-level scheme, fhe helium atoms serving only fo mop up energy from fhe pump source and fransfer if fo neon atoms on collision. The energy level scheme is shown in Figure 9.12. 

Figure 9.12 Energy levels of the He and Ne atoms relevant to the helium-neon laser. The number of states arising from each Ne configuration is given in a box ...

The spectroscopy of ion lasers is generally less well understood than that of neutral atom lasers because of the lack of detailed knowledge of ion energy-level schemes. Indeed, ion lasers were first produced accidentally and attempts to assign the transitions came later. 

Figure 9.1 8 Energy level scheme for a dye molecule showing nine processes important in laser action...

The potential for laser acfivify is nof anyfhing we can demand of any atom or molecule. We should regard if as accidenfal fhaf among fhe exfremely complex sefs of energy levels associated wifh a few atoms or molecules fhere happens to be one (or more) pairs befween which if is possible to produce a population inversion and fhereby create a laser. 

We shall consider just two examples of the use of femtosecond lasers in spectroscopy. One is an investigation of the transition state in the dissociation of Nal and the other concerns the direct, time-based observation of vibrational energy levels in an excited electronic state of I2. 

Laser light is produced from transitions between atomic or molecular energy levels. Generation of light requires two energy levels, E and E separated by the photon energy E of the light that is to be produced. 

A third pumping method (Fig. Ic) uses an electrical discharge in a mixture of gases. It relies on electronic excitation of the first component of the gas mixture, so that those atoms are raised to an upper energy level. The two components are chosen so that there can be a resonant transfer of energy by collisions from the upper level of the first component to level 3 of the second component. Because there are no atoms in level 2, this produces a population inversion between level 3 and level 2. After laser emission, the atoms in the second component return to the ground state by collisions. 

Whereas the gas lasers described use energy levels characteristic of individual atoms or ions, laser operation can also employ molecular energy levels. Molecular levels may correspond to vibrations and rotations, in contrast to the electronic energy levels of atomic and ionic species. The energies associated with vibrations and rotations tend to be lower than those of electronic transitions thus the output wavelengths of the molecular lasers tend to He farther into the infrared. 

---