Optical Pumping with Lasers

Optical pumping means the selective population or depletion of atomic or molecular levels by absorption of radiation, resulting in a population change 

In this section we restrict the discussion mainly to optical pumping by cw or pulsed lasers with a linewidth larger than the Doppler width of the absorbing transition E E2. In this case all molecules in level E within the thermal velocity distribution N (v) can be pumped by the laser independent of their specific velocity v. 

The degree to which the lower level can be depleted depends on the number of levels involved in the pumping-relaxation cycle and on the corresponding transition probabilities. Under stationary conditions the low level of a two- 

The second aspect of optical pumping refers to the generation of orientation or of alignment in selectively pumped states. Atoms or molecules in a state (J,M) with total angular momentum J and magnetic quantum number M are. called oriented if a nonuniform population of the different M sublevels has been produced. In a classical picture the angular momentum vector J in oriented states has a preferential direction and no longer a random orientation as under thermal equilibrium conditions. 

On a transition J, = 0 polarized light (a polarization) induces AM = +1 transitions and only the 

---

Ducloy, M. (1976). Non-linear effects in optical pumping with lasers. I. General theory of the classical limit for levels of large angular momenta, J. Phys. B At. Mol. Phys., 9, 357-381. 

Optical pumping with lasers may bring an appreciable fraction of all atoms within the volume of a laser beam passing through a vapor cell into an excited electronic state. This allows the observation of collisions between two excited atoms, which lead to many possible excitation channels where the sum of the excitation energies is accumulated in one of the collision partners. Such energy-pooling processes have been demonstrated for Na + Na, where reactions... 

C. Cohen-Tannoudji, Optical pumping with lasers, in Atomic Physics IV, ed. by G. zu Putlitz, E.W. Weber, A. Winnacker (Plenum, New York, 1975), p. 589... 

When the excited state 2) has been selectively populated by optical pumping with laser LI, transitions to still higher levels m) can be induced by a second tunable laser L2 (Fig. 10.14b). This two-step excitation may be regarded... 

When the excited state 2) has been selectively populated by optical pumping with laser LI, transitions to still higher levels m) can be induced by a second tunable laser L2 (Fig. 10.14b). This two-step excitation may be regarded as the special resonance case of the more general two-photon excitation with two different photons hu)2 (Sect. 7.5). Because the upper levels m) must have the same parity as the initial level l), they cannot be reached by an allowed one-photon transition. The one-step excitation by a frequency doubled laser with the photon energy 2hu) = ft(wi+a>2) therefore excites, in the same energy range, levels of opposite parity compared to those levels reached by the two step excitation. 

The large population density Nj which can be achieved in selectively excited levels E by optical pumping with lasers allows one to perform high-resolution spectroscopy of excited molecules with sufficient sensitivity. Many of the techniques applicable to molecules in their ground states (e.g., absorption spectroscopy, optical pumping, LIF, microwave spectroscopy) can be now transferred to transitions between excited states. In the following sections we briefly discuss some of these methods. 

Optical pumping with lasers offers a new and very convenient variation of the conventional Rabi method, which is illustrated in Figs.10.15b and 10.16. The two inhomogeneous magnetic fields A and B are replaced by two parallel laser beams which cross the molecular beam perpendicularly. The first "pump beam" depletes the population N(i) of the lower level i. This is monitored by the resultant decrease of the fluorescence intensity Ip induced by the probe laser. When the rf field induces transitions k i between other levels k and the depleted level i, the population N(i) is increased, resulting in a corresponding increase of Ip. ... 

As soon saturation occurs, those ground-state molecules with p // die out first. This can be detected by observing the corresponding decrease in the fluorescence polarization. Fig. 12 shows the experimental results. This proves that the optical pumping by laser light is even faster than the relaxation rate between molecules of different spatial orientation. 

When optically pumped with a pulsed Ne laser at 540 nm, the rhodamine exhibited laser action at a wavelength determined by the... 

WELLEGEHAUSEN Lasers Optically Pumped with Alkali Molecules 463... 

WELLEGEHAUSEN Lasers Optically Pumped With Alkali Molecules 479... 

Excited target atoms can be prepared in well-defined states by optical pumping with a tunable laser. Specific magnetic substates are excited by polarised light. Momentum distributions are observed for these states by electron momentum spectroscopy. 

A second laser is required to produce fhe Stokes photons for the CARS interaction. This can be achieved with a separate Stokes laser with its own pulse control system or, as is shown in Figure 13.8, some of the laser energy from the pump laser can be used to optically pump a laser suited to the CARS system. The symchro-nization of the timing between the pump and Stokes laser systems must be accurate to within 1 ms to ensure that the two laser beams arrive at the same place at the same time. The advantage of fhe latter laser system is that only one timing circuit is needed to keep the pump... 

---