Excited states laser beams

On the contrary, if we have lasers polarized in perpendicular directions to each other, we point the 2 -axis in the direction of polarization of second laser and consider molecular transitions in a sequence J" = 7 —t J = 8 —> J 2 = 8, then we have molecular axes distribution in the second excited state as shown in Fig. Qh. Now molecular axes are very strongly aligned in the plane that is perpendicular to the axis of quantization. This example demonstrates, how efficiently we can manipulate the molecular axes distribution just by varying angular momentum of the states excited by laser beams and mutual polarization of lasers. 

LIF Laser-induced fluorescence Incident laser beam excites Excited-state processes ... 

At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

All the previous discussion in this chapter has been concerned with absorption or emission of a single photon. However, it is possible for an atom or molecule to absorb two or more photons simultaneously from a light beam to produce an excited state whose energy is the sum of the energies of the photons absorbed. This can happen even when there is no intemrediate stationary state of the system at the energy of one of the photons. The possibility was first demonstrated theoretically by Maria Goppert-Mayer in 1931 [29], but experimental observations had to await the development of the laser. Multiphoton spectroscopy is now a iisefiil technique [30, 31]. 

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

REMPI provides high detection sensitivity for free radicals similar to that of LIF.4 In the REMPI method, one or more photons typically from a focused laser radiation initially excite the free radicals to an intermediate excited electronic state. The radicals are further excited and ionized by another photon in the same laser pulse (one-color REMPI) or by a photon of different wavelength from another laser beam overlapping in space and time... 

Delbriick et al., (1976) have recently sought to determine whether or not the stimulation of Phycomyces involves the excitation of the lowest triplet state of riboflavin. They determined an action spectrum of light-growth response between 575 and 630 nm using a tunable laser beam and taking advantage of the null method described above. This action spectrum was compared with an action spectrum obtained by computer extrapolation of a phototropic action spectrum covering 445—560 nm. 

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