Pump transitions

The SH intensity is proportional to P 2. Experimentally, the oscillatory part of the total SH is so small that one can ignore its second-order term. If coherent surface phonons are created by ISRS, the whole process including excitation and detection is the coherent time-domain analogue of stimulated hyper Raman scattering (y(4) process) [14]. The cross section of the SHG process is then proportional to the product of a Raman tensor in the pump transition and a hyper-Raman tensor dx k/dQn in the probe transition. 

Figure 9.22 Energy levels of most importance in the neodymium laser. The pump transition is from the ground state to the broad 5d 6s band. The main laser transition is between the 4F and 4Ih/2 levels. Internal transitions are marked with dotted lines.

Fig. 1. Energy level diagram for an S = 1/2,1 = 1/2 spin system with an isotropic hfs a, > 2 Ng B0 > 0. The steady state populations shown are for the case where the (A, D) (EPR observer) and the (A, B) (rf pump) transitions are saturated...

Gas-discharge lamps are used to optically pump the metastable helium atoms into a higher excited electronic state, which has a dipole-allowed transition to the ground state. Only He (2 S) can be pumped selectively, thereby producing pure He(23S) beams. For the heavier rare gases, both metastable states are equally pumped by gas-discharge lamps. The use of cutoff filters to selectively pump one state is not adequate because of the temperature dependence of the filter transmission and the low / numbers of the pumping transition. Metastable neon can be selectively pumped by a continuous wave (cw) dye laser,60 whereas Ar, Kr, and Xe have so far only been selectively pumped by pulsed dye lasers.61... 

Note Differential elastic and excitation transfer cross sections have been measured for He(2 S) + Nc and for He(23S) + Ne for energies between 25 and 370 meV (1). Some of the data are shown in Fig. 52. It was possible to measure the differential excitation cross sections for the triplet system, too. A semiclassical two-state calculation was performed for the pumping transition of the red line of the HeNe-laser Hc(2 S)+ Nc— Hc + Ne(5S, lPt), which is the dominant transition for not too high energies (2). A satisfactory fit is obtained to the elastic and inelastic differential cross sections simultaneously, as well as to the known rate constant for excitation transfer. The Hc(215)+ Ne potential curve shows some mild structure, much less pronounced than those shown in Fig. 36. The excitation transfer for the triplet system goes almost certainly over two separate curve crossings. This explains easily the 80 meV threshold for this exothermic process as well as its small cross section, which is only 10% of that of the triplet system. 

Figure 2-1. A schematic diagram of the apparatus used to record photofragment angular distributions of complexes. An F-center laser is used to pump transitions in the parent complex which leads to dissociation. A second F-center laser is used as a probe to state selectively detect the fragments. The electrodes are used to orient the parent molecules prior to excitation.

State pumped Transition observed Solvent Rise time (ps) Decay time (PS) Component... 

The four microwave transitions detected in this double resonance study are listed in table 11.5 the infrared pump transitions are also listed. The rotational transitions are all diagonal in G and G2, which meant that the experiment yielded only the differences in the proton and deuteron Fermi contact constants, and not their absolute values. However, a combination of the microwave and infrared studies [92] has given the absolute values, which are as follows ... 

Figure 11.52. Energy level diagram for typical even-/V and odd-/V rotational levels of the X 2 + state of 14Nj. The principal P-branch optical pumping transitions to the B 2 + excited state, Pi (IV) and P2(Ar)> are shown, as well as the principal radio frequency double resonance transitions in the ground state. The I = 0, F = J levels are not shown, but lie close to the I = 2, F = J levels [99].

Figure 11.54. Microwave/optical double resonance line for CO+, arising from one spin component of the lowest rotational transition in the v = 0 level of the X 2 E+ state [104]. Spectra (a) and (b) correspond to different laser pump transitions (see text).

Figure 25F-2 Schematic of a Nd YAG laser (a) and energy levels (b). The pump transitions are in the red region of the spectrum, and the laser output is in the near-infrared, The laser is flashlamp pumped. The region between the two mirrors is the laser cavity.

Pump transition Coupled transition (v = 1 excited state) ... 

The Stimulated Raman laser bears a superficial resemblance to an optically pumped gas phase molecular laser, but with one crucial difference. The Optically Pumped Laser (OPL) is a three level system (Koffend and Field, 1977). The PUMP transition is e v J <— e"v"J" and the lasing transition is e v J — evJ. If the pump laser is monochromatic, it selects a particular velocity in the e v J upper laser level. Thus there is a negligible Doppler width... 

Fig. 3J2. The four-level laser scheme. Pumping transition is I, lasing transition 3, nonradiaiive uansitions 2 and 4. On the right-hand side the level notation for Nd for the case of the 1064 nm laser action is given. H denotes levels above F3/2 further I11/2 cannot be thermally populated since it is about 2000 cm above the ground state...

Figure 14.6 The principle transitions in a four-level laser the laser transition occurs between two intermediate energy levels, E and /q the transition that drives the laser, the pump transition, is between the ground state, Eq, and the energy level 2...

Sequential double resonance excitation requires that both transitions fall within the range of suitable laser sources. However, it is often desirable to use population labelling to assign a single band system where this may not be possible. Figure 13b uses competitive excitation to seek pairs of transitions with a level in common. In this case it is necessary to partially saturate the pump transition in order that it may affect the fluorescence intensity of excitation via the probe laser. With an extended band system and c.w. lasers the pump beam may be chopped, with modulation detected on the fluorescence excited by the unchopped probe, a filter having been used to block the fluorescence from the pump. 

This change ANi of the population density can be probed by a second EM wave, which may be a radio frequency (RE) field, a microwave, or another laser beam. If this probe wave is tuned into resonance with a molecular transition sharing one of the two levels /) or A ) with the pump transition, the pump laser and the probe wave are simultaneously in resonance with the coupled atomic or molecular transitions (Fig. 5.1). This situation is therefore called optical-RF, optical-microwave or optical-optical double resonance. 

In this chapter we shall treat the most important laser double-resonance techniques by illustration with several examples. While the pump transition is always induced by a pulsed or cw laser, the probe field may be provided by any coherent source in the spectral range between the RF region and the ultraviolet. 

Fig. 5.3 (a) Orientation in the upper state produced by optical pumping with light on an R transition J" = 0 J = 1. (b) Classical model of orientation where J precesses around the z-direction with a projection Mh. (c) Orientation in the lower state caused by partial depletion for the example of a P pump transition J" = 2 J = ... 

Note that orientation or alignment can be produced in both the upper state of a pump transition due to a M-selective population as well as in the lower state because of the corresponding M-selective depletion (Fig. 5.3). 

For a quantitative treatment of optical pumping we consider a pump transition between the levels JiMi) /2Af2). Without an external magnetic field all (2/ + 1) sublevels M) are degenerate and their population densities at thermal equilibrium are, without the pump laser... 

The selective population or depletion of a single level simplifies the microwave spectrum considerably. The difference of the microwave spectra with and without optical pumping yields directly those microwave transitions that start from one of the two levels connected by the pump transition. 

The optical-optical double-resonance (OODR) technique is based on the simultaneous interaction of a molecule with two optical waves that are tuned to two molecular transitions sharing a common level. This may be the lower or the upper level of the pump transition. The three possible level schemes of OODR are depicted in Fig. 5.15. 

The second OODR scheme (Fig. 5.15b) represents stepwise excitation of high-lying levels via a common intermediate level 2, which is the upper level of the pump transition but the lower level of the probe transition. This scheme allows the investigation of higher levels (e.g., Rydberg states), where the absorption of the probe can be monitored by either LIF from these levels, or by the ions produced by the absorption of a third photon. 

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